Archive for the ‘Meteorology lesson/tutorial’ Category
I am pleased to announce that the Senior Learning Institute (SLI) of the College of Central Florida in Ocala is providing me another opportunity to present a geosciences topic that is near and dear to me.
IMPORTANT SPECIAL UPDATE (5-10-2015): The Senior Learning Institute no longer exists. It has become the non-profit Senior Learners, Inc. and classes are still taught at the College of Central Florida in Ocala. Here is a link:
IDENTIFYING AND UNDERSTANDING CLOUDS will be presented on Feb. 5, 7, 12, 14 (2013) – from 10 until noon (for a total of 8 hours). Click on the following link for my outline which will be distributed at the beginning of the first class meeting.
I have presented a dozen seminars at the SLI since 2006 and thoroughly enjoyed them. Since I taught a 12 hour course on clouds in April, 2007 I have received requests from a number of people who missed it and also from others who wished to do it again as a refresher.
SLI is a membership group composed of some terrific people who seem to consider “learning” to be an integral aspect of their life styles. When I am with them, though my official roll is that of a presenter, I learn so very much. I learn from them and I learn in the processes of preparing and presenting. There are some significant differences between these courses and the courses I taught for 41 years at colleges and universities: 1) the SLI seminars are non-credit courses, 2) they are short in duration compared to most college courses, 3) there are no academic prerequisites to the courses, 4) there are no exams to fret over, 5) there are no grades, 6) all who enroll are there voluntarily and, from what I can tell, gladly and 7) many have a great deal of experience acquired through time and by their sharing are able to enhance the quality of the course.
When my television is turned on I “check out” the Weather Channel fairly often. For the most part I’ve been very impressed by explanations that are given about weather happenings. I’ve never done media weather reporting so can only imagine how frustrating it must be to provide good presentations when time is at such a premium. In the college classroom even though I had an agenda with objectives to cover I had control over the amount of time I spent on individual topics.
Recently I heard a Weather Channel reporter give a less-than-desirable explanation for the cause of lake effect snow events. I imagine that time constraints kept her from being more thorough. This is close to what she said: “The cold, dry air behind this front is moving over the warmer Great Lakes picking up moisture and then dumping snow on the land at the opposite side.”
From my point of view that was far too brief leaving out way too much. But – everything is relative. I suspect that most people would prefer her explanation to an hour lecture on the subject from me. But there would likely be one or two in a large class of meteorology students who would be dissatisfied because I left something out or left them very confused. My dad didn’t have the opportunity to teach me much but one of the things that he tried hard to get through to me was, “You can’t please ’em all.”
So – I’m not complaining about the Weather Channel presentations. I think they do a very nice job. But if I were working for them, here is the minimum that I would insist upon (which is probably one of the reasons why I’m not a suitable candidate to work for them – the brevity necessitated by time constraints would drive me up the wall and my tendency toward long-winded discussions would drive them up the wall – LOL):
Cold, dry air moving over a large, unfrozen lake surface picks up moisture in the form of water vapor made available by evaporation. More often than not unfrozen lake surface water’s temperature is higher than that of the air behind a cold front.
But in most cases clouds that provide precipitation form as a result of air rising and that is most definitely a factor in bringing about lake effect snow events. So – what makes it rise?
The moisture-laden air can rise because of an increase of elevation on the downwind side of the lake but that alone is usually not enough to create snow-producing clouds unless the increase is due to a plateau or mountains. Also, positive buoyancy can cause the air to rise and that can be created in two ways as the air is traveling over the lake water. Picking up heat energy the air can begin to lift (heated air tends to rise); a fair analogy is a hot air balloon. Also, the addition of water vapor to air can increase its buoyancy (as the specific humidity increases, the density increases so long as the temperature of the air does not drop significantly).
The factor most often omitted in a lake effect discussion of why air rises to form snow clouds is this: When air reaches the land it slows down because of the decided increase in friction. When in heavy traffic a car far ahead of you slows down – you, the cars ahead of you, and the cars behind you tend to squeeze closer together. We say the cars are converging (getting closer together). In the case of the fluid air a vertical component of motion is allowed so some of the air “piles up” and therefore moves up. Have you ever heard of cars piling up?
Therefore, even without an elevation increase of the surface over which the air flows and even without an increase in buoyancy, some air on the downwind side of a Great Lake is likely to rise because of convergence. Whether or not snow clouds form is dependent upon a combination of factors.
You’ll have to admit that my explanation is more thorough than “The cold, dry air behind this front is moving over the warmer Great Lakes picking up moisture and then dumping the snow on the land at the opposite side.” The trouble is, it took time, is very “wordy” and you really have to read carefully to pick it all up. Furthermore, as long as my explanation is, it still does not give all of the reasons behind lake effect snows. Also, it makes a blanket statement (as the specific humidity increases, the density increases so long as the temperature of the air does not drop significantly). But it does not elaborate. For most people that sounds like a real paradox – adding moisture to the air can make it less dense and therefore be a contributing factor to its rising! It’s true – and I intend to discuss that paradox on this web-log in the near future. By the way – a friend of mind from way back in high school defines paradox as “two physicians.”
YOU MIGHT HAVE TO CLICK ON THE IMAGE ABOVE TO GET IT TO ENLARGE AND LOOP.
In the radar loop above taken from a small time segment (48 minutes) earlier today (12-6-2010), you see the signature of lake effect snows in Indiana. Lake Michigan’s mean elevation is a bit over 577 ft. above mean sea level. The land between Lake Michigan and Fort Wayne is as much as 250 feet higher but I think it’s unlikely that 250 feet of elevation increase is going to create that much snow. Nor is the land heating the air to make it rise via positive buoyancy. In fact, the land surface temperature is colder than the air flowing over it. I believe that it’s convergence of the type I’ve described in this entry that is responsible for much of the snow.
LEFT CLICK ON THE IMAGE ABOVE
AND THEN WAIT PATIENTLY FOR AN ANIMATION.
I RECOMMEND YOU READ THE INTRO. BELOW FIRST.
I’m posting this on the afternoon of Wednesday, 9-15-2010. What you will be looking at as you view the animation above is, to my mind, fantastic. I would have loved to have had such a tool to use in the college classroom when I was a full-time meteorology professor. Even though this is jerky, it gives a wonderful view of things which I and my students could only imagine back before my retirement from the profession. The stream will quickly get to mid-day (of Monday, Sept. 13) and the sun will quickly reach the western horizon marking sunset. If you focus upon the eye in the afternoon you will see the shadow created by the wall cloud’s western margin as it creeps eastward. Also watch the boiling cumuliform tops in various places. I was fascinated by the way the clouds moved within the eye of the storm as it rotated. ENJOY!
SPECIAL NOTE: For those of you who understand hurricane circulation in more detail than most, notice the lower level clouds converging cyclonically (counterclockwise in the Northern Hemisphere) and look hard enough at the more diffuse high clouds and you will detect the anticyclonic divergence (clockwise in the Northern Hemisphere). The latter is easiest to see on the west side of the storm where you can envision the feathered cirrus moving toward the north or northwest. If they seem to you to be standing still that is because the ice crystals are sublimating at the leading edges of the clusters (turning from solid to gas) whereas deposition (gas to solid) is occurring at the trailing edges.
Watch the digital clock at the bottom margin of the image and you will note that after the initial spurt of one frame per 15 minutes, it settles down to a nicer one frame per minute.
* * * * * * *
This hurricane is a very strong one and potentially dangerous – particularly for the 65,000 or so people in Bermuda. Let’s hope that they escape unharmed. I’m hoping that Igor takes a surprisingly sharper right turn than anticipated in order to spare those fine people.
However, no matter the outcome, it’s difficult not to be in awe of this beautiful beast. It’s also important, I think, to recognize that there are some good things about this storm especially when coupled with the impending effects of Julia which is positioned further to the east. A certain amount of energy MUST be transformed over the Atlantic in order that it not be all released at once. An analogy: It’s better to have one tiny earthquake per year along an active fault than wait for 100 years before all of that stored energy is released as one gigantic earthquake.
The fact that Igor and Julia are both releasing huge amounts of latent heat into the atmosphere is good – particularly when that is happening over relatively uninhabited places. Generally such long fetches over such long periods of time will move warm, tropical water such that it is replaced from below by cooler upwelling water. That is good because the next system to move by is less likely to have as much oceanic heat to stoke it.
At the time I’m writing this (about 5 pm EDT, 9-15-2010) Igor is a category 4 hurricane and Julia is a category 3. However, not many hours ago Julia was also a 4 and she might intensify to that category again. According to my sources, this is only the second time in history that we have had two category 4 storms in the Atlantic at the same time.
Dr. Jeff Masters of WeatherUnderground.com wrote of that fact in his weblog today. Rather than mimic what he has said, I’m placing his well-written statement below in blue.
T. Ansel Toney
e-mail = ProfToney@gmail.com
“The Atlantic hurricane season of 2010 kicked into high gear this morning, with the landfall of Tropical Storm Karl in Mexico, and the simultaneous presence of two Category 4 hurricanes in the Atlantic, Igor and Julia. Tropical Storm Karl’s formation yesterday marked the fifth earliest date that an eleventh named storm of the season has formed. The only years more active this early in the season were 2005, 1995, 1936 and 1933. This morning’s unexpected intensification of Hurricane Julia into a Category 4 storm with 135 mph winds has set a new record–Julia is now the strongest hurricane on record so far east. When one considers that earlier this year, Hurricane Earl became the fourth strongest hurricane so far north, it appears that this year’s record SSTs have significantly expanded the area over which major hurricanes can exist over the Atlantic. This morning is just the second time in recorded history that two simultaneous Category 4 or stronger storms have occurred in the Atlantic. The only other occurrence was on 06 UTC September 16, 1926, when the Great Miami Hurricane and Hurricane Four were both Category 4 storms for a six-hour period. The were also two years, 1999 and 1958, when we missed having two simultaneous Category 4 hurricanes by six hours. Julia’s ascension to Category 4 status makes it the 4th Category 4 storm of the year. Only two other seasons have had as many as five Category 4 or stronger storms (2005 and 1999), so 2010 ranks in 3rd place in this statistic. This year is also the earliest a fourth Category 4 or stronger storm has formed (though the fourth Category 4 of 1999, Hurricane Gert, formed just 3 hours later on today’s date in 1999.) We’ve also had four Cat 4+ storms in just twenty days, which beats the previous record for shortest time span for four Cat 4+ storms to appear. The previous record was 1999, 24 days (thanks to Phil Klozbach of CSU for this stat.)”
The total amount of thermal energy at the surface in the Western Caribbean is high and wind shear aloft is relative low so it is anticipated that Ida will intensify before striking the Yucatan Peninsula. The Yucatan does not have the type of topography that we associate with significant weakening of a storm due to friction. But, read what Dr. Jeff Masters says this morning about the fate of Ida after she enters the Gulf of Mexico:
“Once Ida crosses into the Gulf of Mexico on Sunday night, the storm will encounter much cooler SSTs and a strong trough of low pressure that will dump cold air into the storm and bring 40 knots of wind shear. This will cause Ida to lose its tropical characteristics and become a powerful extratropical storm with 45 – 55 mph winds. It is highly unlikely that Ida will hit the U.S. as a tropical storm, but it could still bring tropical storm-force winds of 45 mph to the coast next week as an extratropical storm.”
As for me, I have been favoring the Geophysical Fluid Dynamics Laboratory Model (GFDL) for the path that Ida will take; currently, if I had to depend upon only one of the many models, that would be the one – in most instances anyway. I have no real science to back that up – only my perception based upon experience. Call it a “gut level” good feeling about the model’s past performance if you will. Therefore I expect Ida to eventually curve rightward as the GFDL shows in the plot below. By the time it does I expect it will have lost its tropical characteristics though the winds will still be strong. In other words, it will become extratropical. TO GET INSTRUCTIONS ON OBTAINING THE GFDL ANIMATION CLICK ON THE FOLLOWING LINK: https://cloudman23.wordpress.com/2008/08/30/gfdl-model-a-great-source-for-an-animation/https://cloudman23.wordpress.com/2008/08/30/gfdl-model-a-great-source-for-an-animation/
The prefix, extra, means “outside of” or “beyond.” Extratropical cyclones are sometimes called cold core lows whereas tropical cyclones are warm core lows. When a tropical cyclone draws in cold air (as usually happens when a front interferes with the storm) it becomes extratropical. The majority of the world’s extratropical cyclones develop in the middle latitudes (30 degrees to 60 degrees latitude) and for that reason are often referred to as middle latitude cyclones.
Graphic courtesy of Jonathan Vigh of the Department of Atmospheric Science at Colorado State University
If you wish to see other posts on this web-log
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please click on the “blog” tab
near the top of this page.
If you wish to see other posts on this web-log
but are unable,
please click on the “blog” tab
near the top of this page.
AVERAGE ANNUAL PRECIPITATION FOR FLORIDA
LEFT CLICK ON IMAGE TO ENLARGE
“Why is it so humid in Florida?”
There is no single, simple answer. Here are at least eight reasons, most of which are interrelated. After this listing, amplified explanations are available:
1. Most of Florida is a peninsula which, by definition is bordered by water on three sides. The adjacent sea water is the most important source of moisture for the atmosphere. That part of Florida which is not a peninsula, the panhandle, is also bordered by sea water on the southern boundary.
2. Sea breeze convergence carries moisture over the land. Air that has converged near the surface will rise and under the right conditions will form clouds that provide precipitation. The precipitation is simply distilled sea water. In some parts of Florida sea breeze convergence provides almost two-thirds of the annual precipitation.
3. Florida is located along the eastern margin of the continent where warm waters arrive from the North Atlantic Gyre. Warm waters mean higher evaporation rates and the warmed air is also able to support more water in the vapor state than if it were cooler.
4. The relatively low latitude location of Florida provides for warmer temperatures which in turn give the air more thermal energy necessary to support large amounts of water in the vapor state. The warmer temperatures also provide for significant convectional uplift of air which is a key factor in the development of many of Florida’s rain clouds.
5. Florida’ vegetation transpires large amounts of water vapor (into the air).
6. Numerous fresh water surfaces within the state provide moisture to the air from evaporation.
7. Weather systems moving from east to west with the “Trades” provides moisture to the state – especially during the Atlantic hurricane season.
8. Winds associated with fronts, especially pre-cold frontal winds bring vast amounts of moisture to Florida from components of the south.
1) Most of Florida is a peninsula and by definition it is surrounded by water on three sides. The rest of the state (the Panhandle) is also coastal. The surrounding water is the source of a great amount of moisture for processing through the hydrologic cycle. But, being peninsular is not reason enough for Florida to be humid. As case in point is the Baja Peninsula of Mexico. Look at this comparison of the two peninsulas:
2) The geography and physiography is such that sea breezes of the Atlantic side of the peninsula converge with sea breezes of the Gulf side of the peninsula. The zone or line of convergence is seldom at the “center” because the sea breezes are seldom of the same strength. As a general rule Atlantic side sea breezes of the peninsula are stronger than Gulf side sea breezes of the peninsula. In any case, these sea breezes carry moisture in the vapor form the origin of which is evaporation off the sea surface. When sea water evaporates the dissolved solids stay behind; therefore the cloud droplets formed when the sea breezes converge and then rise are made of fresh water. In simplistic terms I have described one of Natures own distilleries of fresh water. Air that has converged at or near the surface will rise and rising air cools adiabatically. If that cooling air contains ample moisture the dew point temperature will be reached relatively quickly and further cooling cause by continued ascension of the air causes condensation which releases heat. That added heat usually causes the air to become buoyant enough to continue rising to form clouds that provide precipitation. This is akin to a hot air balloon rising through air that is cooler than the air inside the balloon. Cumulonimbus commonly form when this happens.
In South Florida there are years that two-thirds of the annual precipitation is provided during the warmer 6 months and most of that precipitation is due to sea breeze convergence. This is a real paradox to me because the huge amount of precipitation from sea breeze convergence (sometimes 40” or more) is the result of weather circulation systems that do not show up in the isobaric configurations of a national weather map!
Of course, statistics vary from year to year depending partly upon the amount of tropical activity due to tropical disturbances (waves), tropical depressions, tropical storms, and hurricanes. The whole sea breeze convergence process happens most often when the synoptic pressure gradient is weak (synoptic systems are those lows and highs that are seen on the national surface analyses). Except when tropical synoptic systems are dictating the pressure gradient (e.g. hurricanes) the warmer 6 months of the year are when sea breeze convergence is most likely to occur. In the cooler 6 months synoptic systems that migrate generally from west to east across the United States dominated the flow patterns at the surface.
3) Florida is located along the eastern margin of the continent. As in all continents except Antarctica, the eastern margins are generally more humid than the western margins. The principle reason for this is that the water at the eastern margins is generally warmer than the water at the western margins. The warm boundary currents belonging to the gyres of the respective oceans are on the western margins of the oceans (which is the same as saying the eastern margins of the continents). Part of the North Atlantic gyre circulation (sometimes called the Gulf stream gyre) enters the Caribbean and eventually much of it travels through the Yucatan Strait (the gap between western Cuba and the Yucatan peninsula) to flow into the Gulf of Mexico as the Loop Current. This, then, circulates warmer water and enhances the moisture potential for the air on the Gulf side of Florida. The Florida Current segment of the gyre provides considerable thermal energy along the east coast of Florida.
The warmer the water the higher the evaporation rates and therefore the more moisture gets into the air in the vapor phase. NOTE: Remember, water vapor is invisible so I’m not talking about clouds – but – clouds develop as a result of either the condensation of water vapor (liquid droplets) or the deposition of water vapor (ice crystals).
4) Most of Florida is in the low latitudes defined as that part of the world between the Equator and 30˚ latitude. Downtown Jacksonville which is at the north end of the state’s Atlantic side is at 30.32 degrees north latitute. I have shown the location of the 30th parallel on the map of”idealized air circulation on a homogeneous globe” (above) and on the map of the boundary currents before that.
You might know that the planetary circulation is not as simple as shown above because the earth’s surface is far from being homogeneous (the same all over). The most obvious surface difference is that between land masses and oceans. Furthermore, there is a hemispherical difference in that category – 39% of the northern hemisphere is land but only 19% of the southern hemisphere is land. Because of earth surface heterogeneity (differences) the planetary circulation is not nearly as ordered as shown above nor do the hemispheres mirror each other as perfectly as shown. And – very obvious seasonal differences exist between the continents and the adjacent oceans. (NOTE: All of that is “fuel” for another tutorial topic which is likely to be addressed on this site at some time in the future). There is one aspect of this that I want to mention up front at this time since some of you know about the “Bermuda High.” It is a warm season phenomenon and in effect, the south half represents the northeast trades (over the Atlantic of our hemisphere) and the north half represents the prevailing westerlies (over the Atlantic of our hemisphere).
Since the lower latitudes have higher sun angles and therefore more intense solar radiation than the higher latitudes, lower latitude surfaces (of both the land and water) get warmer. This added warmth not only causes higher evaporation rates over water and moist land but also more convection over the heated land than would exist were it colder.
Convectional uplift of air is a key factor in the development of rain clouds, providing there is an adequate supply of moist air. And – think about this: When air is heated by the surface and then rises due to it’s positive buoyancy it does not leave a vacuum behind. Air must flow in to take it’s place and in Florida that is moist air which, in turn, is heated and rises. Most clouds providing precipitation result from air rising one way or another.
5) The warmth of Florida along with its vegetation allows for high transpiration rates. Transpiration is the process whereby plant leaf surfaces cast water vapor into the air. A mature oak tree in the Summer will put about 500 gallons of water daily into the air in this way; an acre of mature but still green-leafed corn about 2000 gallons a day; an acre of densely distributed invasive species of the melaleuca tree in the everglades is believed by some to transpire four times as much as a comparable area in native saw grass!
6) Florida is a state with numerous surface fresh water features within it that provide high evaporation opportunities.
Two left clicks will enlarge this photo nicely. It is my favorite image of a beautiful lake where I love to sail my little sloop – taken by my photographer son, Colin Toney.
A traveler in the state finds remarkable beauty in glades, lakes, marshes, and rivers. You might find it interesting (and even sad) that before humans began controlling it, the famous everglades was a 40 mile wide river whose water crept generally southward issuing fresh water into Florida Bay. I’ve been told that the rate of movement was so slow that strong winds from the south would temporarily but significantly reduce the discharge into the bay and even sometimes cause the water to flow backwards. Currently, Florida Bay is far more saline than it used to be because so much less fresh water empties into it these days (due to human usage, and interference through water storage and flood control).
7) Florida is downwind of the North Atlantic segment of the global-scale N.E. Trades. The Trades, rather than a specific wind, represent a planetary-scale force that causes weather systems to move from the east toward the west across the low-latitude portions of the oceans. Examples of these weather systems are the array of tropical lows ranging from tropical disturbances on the lower end of the intensity scale to hurricanes on the higher intensity end of the scale. All of these types of lows bring moisture to Florida.
If you want to get a general idea as to why weather moves across most of the United States from west to east and why hurricanes (and the lesser tropical lows) move generally from east to west across the Atlantic just look at the guiding forces on the “homogeneous globe” diagram – the prevailing westerlies and the northeast trades. As for the oft-asked question, “Why don’t the synoptic systems move in the same direction as the arrows showing the westerlies and the trades? – it’s the rightward Coriolis effect in the northern hemisphere that is mostly responsible. Mid-latitude cyclones, air mass anticyclones, and tropical lows act as separate entities which rotate the way they do because of the Coriolis effect but move in translation over the land and the ocean in a direction influenced by the Coriolis Effect. Here are a couple of links on the Coriolis effect:
There is a link within the one above to take you to Part 2 of the Coriolis effect subject if you wish.
8) Air ahead of cold fronts moves almost parallel to those fronts. It responds to the pressure gradient of the middle-latitude systems containing the fronts. In Florida those pre-frontal winds are generally from some component of the south (typically southeast). SPECIAL NOTE: Winds are named in accordance with the direction from which they are moving. In other words, a southeast wind is a wind blowing from the southeast toward the northwest. What that means is that when cold fronts are moving through Florida the pre-frontal winds are carrying relatively warm and humid air from lower latitudes.
It is this warm and humid air that provides the moisture for condensation making the lines of clouds ahead of the fronts and along the fronts. The moisture is not being brought down by the cold air but rather, the cold air is forcing the warmer air ahead of it to rise and cool adiabatically just as moisture bearing air does when it is lifted up the windward side of a mountain range. In fact, I envision cold fronts as moving mountains along whose leading surfaces air is forced to rise, often to a level of free convection where it then “rises freely by convection” forming some very powerful lines of thunderstorms. Additionally, the dynamics and temperatures aloft help to create squall lines out ahead of and nearly parallel to many of those cold fronts.
FLORIDA WATER IS NOT AS
ABUNDANT AS IT SEEMS
It is apparent that there are several reasons for Florida’s sometimes notorious humidity. It may seem that the explanation you have just read is more detailed than most but the truth is, I have not covered all aspects, particularly those dealing with the winds aloft. Obviously, a variety of circumstances cause the large amount of moisture in the air over Florida as well as the amount of precipitation, the latter being enough for most of the state to fit within the parameters of the Humid Subtropical climate – though a small part of South Florida is classified as a Tropical Savanna climatic zone. But – some people are shocked when they learn how little of the water raining upon Florida is available to them.
I will use “ball park” numbers I am comfortable with for the sake of simple illustration. To keep it simple, I’ll round off numbers used to illustrate a typical annual water budget for South Florida.
Annual precipitation is about 60 inches.
18” discharge into the sea by surface runoff and groundwater transport
2” remain for all other usage!
People move to Florida and clearly see “water, water everywhere.” But the truth is that very little of that water, probably less than 3% is captured and exploited by humans. It is always a good idea to conserve water in Florida, even during the rainy season.
MORE ON HUMIDITY
The word “humid” is used in a variety of ways by the general population and humidity is expressed in more than one way by meteorologists. “Relative humidity” is a percentage expression of the amount of water vapor in the air compared to the amount that could exist within it at that particular energy level (based upon its temperature). The left column below shows air temperature in both Celsius and Fahrenheit and the right column shows how much vapor, in grams, a kilogram of that air can support in the vapor (gaseous) state at those temperatures. Consider the following illustrations using the chart provided.
Left column is the Temperature in degrees Celsius and (Fahreheit)
Right column is grams of water vapor per kilogram of air representing specific humidity at saturation. Saturation indicates 100% relative humidity.
-40 (-40) 0.1
-30 (-22) 0.3
-20 (-4) 0.75
-10 (14) 2
0 (32) 3.5
5 (41) 5
10 (50) 7
15 (59) 10
20 (68) 14
25 (77) 20
30 (86) 26.5
35 (95) 35
40 (104) 47
Here are four usages of the chart above
to give you a “feel” for humidity.
1. If the specific humidity of 68 degree Fahrenheit air is 3.5 grams per kilogram, the relative humidity is 25%. (3.5 is 25% of 14).
2. If there are 10 grams of water vapor in a kilogram of 77˚F. air near the surface that air has a relative humidity of 50%. Why? The chart tells us the air has the thermal energy to keep 20 grams of water in the vapor phase so if there are only 10 grams in the vapor state that represents one-half (50%). On the other hand, if the temperature where then to drop down to 59˚F. the relative humidity would be 100%! This is because 10 grams of water vapor in 59 degree air represents the saturation level for air at that temperature. In this case, the meteorologist would say that the 77 degree air (at 50% relative humidity) had cooled down to its “dew point” (59 degrees) – the point or temperature where condensation would occur if there were any further cooling. If that cooling occurred on the ground or on your windows overnight dew would form; if it occurred near the surface fog droplets would form; if it occurred further up, cloud droplets would form. NOTE: Actually, fog is no more than cloud close to the surface.
3. If air over the Arctic at -10 degrees Fahrenheit had a relative humidity of 100% a kilogram would have 2 grams of water (per kilogram) in the vapor phase. Yet if the relative humidity of 104˚F. air over Yuma, Arizona was a very low 15%, that air would contain more water in the vapor phase than the 100% relative humidity Arctic air. Why? Because 15% of 47 grams is 7.05. More than 7 grams of water vapor in a kilogram of air is a lot more than 2 grams within a kilogram. Therefore, even though the relative humidity of the Yuma air is a low 15% compared to 100% for the Arctic air, the hot Yuma air has more than 3 ½ times the amount of water vapor in it than the colder Arctic air. So – the Yuma air at 15% relative humidity has more water vapor in it than the 100% relative humidity air of the Arctic location! A meteorologist might say (if he/she is being careful), “The specific humidity of the Yuma air is much higher, more than 3 ½ times higher, than the air at the Arctic site.” Yes – this is a paradox. What I hope you learn from this is that the warmer the air, the more energy it has for keeping water in the vapor state and as the temperature increases the ability to hold water in the vapor state does not increase linearly, but exponentially.
You can see that on the chart. For example, 20˚Celsius air has the ability to keep 14 grams of water per kilogram of air in the vapor state. But double the temperature to 40˚Celsius and the water vapor “capacity” does not double; in fact, it more than triples! The calculator in my computer tells me that it increases 3.3571428571428571428571428571429 times. Please memorize that number. There will be a test question on the midterm! LOL
4. If a kilogram of 77˚F. air was keeping 18 grams of water vapor within it, the relative humidity would be 90%. Why? Because 18/20ths (reduces to 9/10ths) translates to 90%. You may find this hard to believe but many of my college students (particularly in the last 20 of my 41 year teaching career) in pre-testing could not successfully change a fraction into a percent. Divide the numerator by the denominator and then multiply by 100 to get percent. Percent means “parts per 100.” So, 18 divided by 20 (or 9÷10) = 0.90. 0.90 X 100 = 90. Most people instantly recognize that 0.90 is 90 one-hundredths and therefore do not need to multiply by 100.
An earlier posting on the subject of humidity can be found here:
More related topics will appear in the near future, among them, a fundamental presentation on adiabatic processes that form clouds with a high potential as precipitation providers. Adiabatic processes are so important to us that without them, almost all of the land of the world would be desert. If you are interested, click on the following link to a November, 2008 post for a starter:
If you wish to see other posts on this web-log but are unable,
please click on the “blog” tab near the top of this page.
A related post, “Why Is Florida So Humid” has been added.
It can be found here:
About a third of the way into May I noticed that television weather reports and a few of my acquaintances were starting to suggest that “perhaps” Florida’s rainy season had begun. To be sure, before the middle of May many parts of Florida had been experiencing very significant rainfall events, some of those places on a daily basis. One of those places was northeast Citrus County where I live. However, I doubted that those rainfall events signaled the beginning of the “real” rainy season because my experience living much further south in Florida had conditioned me to considered the “true” rainy season to be that time when precipitation was due almost entirely to mesoscale systems, namely sea breezes and sea breeze convergence within the peninsula. And – unless the views were severely obscured by buildings or dense stand of trees, at those times one can detect evidence of thunderstorms within hearing and/or seeing distance on almost a daily basis.
Florida’s rainfall this May was almost entirely due to weather systems of a much larger magnitude than the mesoscale – systems that show up on the national weather maps (middle-latitude cyclones with their associated frontal weather, et. al.). Those systems, along with anticyclones (rotating highs) are often referred to as synoptic systems.
I’ve always found it interesting that the majority of our annual precipitation in peninsular Florida occurs (on the average) as a result of weather systems far smaller in magnitude than either the mid-latitude synoptic systems or the tropical synoptic systems such as hurricanes and tropical storms.
Here are three graphic illustrations of the synoptic nature of our May events followed three more images of today’s weather (June 2, 2009) over the Florida peninsula. Comments labeled A through F follow each illustration:
MOST ALL IMAGES IN THIS WEB-LOG ENLARGE
WITH A LEFT CLICK OR TWO
IN MOST CASES IN ORDER TO ACTIVATE LOOPS
YOU MUST FIRST ENLARGE.
A. In this 70 minute loop (starting at 5:28 PM EST on May 17th notice the cold front that shows up well along a line from eastern Tennessee down to southern Mississippi. If one were to see only the Florida peninsula portion of this image I can see how he/she might immediately assume that this was a sea breeze convergence day. But as you can see, this is pre-cold frontal weather being drawn northward. Not to say the warmer land surface and some convergence did not play a role, it is nonetheless clear that the weather is dominated by the synoptic scale.
B. This 70 minute loop of the same system shows very nicely the pre-frontal nature of Florida’s rainfall by virtue of the fact that it has moved on in accordance with the general motion of the cyclone across the United States from west to east. This loop starts at 11:28 PM EST on May 17th.
C. Here is an impressive array of alternating lows and highs of the synoptic scale on May 26. At this time the movement of the lows was almost perfectly synchonized in the diurnal mode so that each day, with the help of the intense heating of the peninsula, we got significant rainfall in my neighborhood (latitude 29˚North by longitude 80.4 West – to the nearest 10th of a degree). Notice the lows centered off the Georgia coast, south-central Alabama, and Texas – all three with associated troughs. Each of those provided my neighborhood a great deal of rain and certainly cramped my style as I was attempting to spend a lot of time outdoors landscaping and doing my annual manicuring of my woods. But – because of three years of drought here, I was thanking the Great Guy In The Sky for each and every drop and respecting His audible commands to stay safely indoors in the form of lightning hits that were uncomfortably close.
I was surprised to learn recently that the National Weather Service Forecast Office has declared May 11 to be the beginning of the 2009 “rainy season” of Florida. This is a full 9 days ahead of May 20, the mean starting date. Who am I to disagree with the experts? It matters not in the real world I suppose – only in the academic world in which people like me often get lost. The bottom line is that we need the rain and no matter whether May’s events were “true, traditional” rainy season events or not, they were a blessing.
Now lets take a look at weather over the peninsula a little earlier today.
UNLIKE THE FIRST TWO IMAGES –
THIS IS A STILL – NOT A LOOP.
D. Today, June 2, 2009, the radar shortly before 3 pm EST is showing precipitation as a result of sea breeze “fronts” along both sides of the peninsula. I suspect convergence is occurring in the south part as shown by the beginning of development over some of the glades south of Lake Okeechobee. This is more like a Florida “rainy season” day as I have learned to know it but even today – a synoptic system is providing a noticeable influence (see next two images). For those of you who live in my neighborhood, the Crystal River winds at the time of this observation were 7 mph from the west and that is ample to bring in moist air which is rising over the heated land to form the showers that are appearing on this radar image.
E. Later today the thunderstorms became more intense and in the still radar image above you can see a decided concentration toward the western side of the peninsula.
F. And here is a synoptic map showing the low (with its associated fronts) that is influencing Florida’s weather today. There is a “rule of thumb” in meteorology that the air ahead of a front moves more or less parallel to that front. If you will simply extend in your mind’s eye the warm front further toward Florida you will realize that there is a force over most of Florida tending to make smaller weather systems (like mesoscale thunderstorm complexes) move toward the WNW. Apparently the winds aloft are not strong enough to counteract that.
Here are some interesting statistics for two locations in Florida providing some geographical contrasts along the peninsula.
Ocala averages almost 50” of rainfall per year of which nearly two-thirds falls in May through October.
Homestead (south of Miami) averages nearly 60” per year of which over three-fourths falls in May through October.
Here are the actual numbers (statistical means):
Ocala (in Central Florida) 49.68” annual 31.10” May through October = 62.6%
Homestead (south of Miami) 58.20” annual 45.70” May through October = 78.5%
For further information about Florida’s rainy season here is a safe link in the pdf format from NOAA.
Tonie A. Toney
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THE HYDROLOGIC CYCLE, A CLASSICAL TOPIC
IN NATURAL SCIENCE COURSES
I’ve taught the hydrologic cycle many times in geology, meteorology, physical oceanography and environmental science classes. It’s always been a pleasure but I’ve never had enough time. All of these were college courses and in almost every case the text book covered the subject adequately. However, the manner in which water moves and changes in our natural environment is so very interesting that a few pages in a text with a traditional drawing and an hour lecture from me simply does not do the subject justice. Water is such a remarkable compound – I can’t find the words to explain how very interesting it is and how mysterious it can be at times considering the amount of scientific attention it has received through the years. There is still so much to learn.
So, it is with excitement that I look forward to a 6-hour course that I am scheduled to teach in May to the Senior Institute enrollees at Central Florida Community College. In 37 years of full-time college teaching (and 4 years part-time) I never had the opportunity to devote so much time to the subject. The method I intend to use is my own “idea” but surely it has been done before – that is, to follow water step-by-step as it goes from one phase or one environment to the next. My presentation won’t be a journey without side trips and backtracking. There are multiple manners in which water can transform and/or move with interesting little anomalies along the way. With 6 hours to utilize I will be able to discuss aspects that were only fleetingly mentioned in my previous hour-long presentations e.g.: Capillary action, deposition, glacial calving, influent groundwater movement, juvenile water, super-cooled droplets, and much more.
SUPERCOOLED CLOUD DROPLETS AND ICING
I feel fairly certain that some people who read this have had the experience of having rain freeze upon impact with their vehicle’s windshield. Some would assume that the freezing occurs because the windshield is so very cold. That is usually not the case. Instead, the liquid droplets were probably at a temperature well below “freezing” and the impact with the windshield itself triggered the instant freezing. Hopefully, the “defrosting” vents can keep the windshield warm enough so that the ice can be quickly cleared. Now, imagine what it must be like if the surfaces being iced are the windshield and wings of your aircraft in flight – as well as other aircraft surfaces (e.g. propellers, fuselage, horizontal stabilizers)!
Today, February 15, 2009, the mere thought of super-cooled droplets hauntingly reminds me that in addition to the marvelous beauty of water’s multifaceted journeys and transitions through our natural environment, there are some insidious elements that can become deadly in this modern world. Of course, I’m thinking specifically of the recent terrible aircraft accident responsible for 50 fatalities near Buffalo, New York.
For a short while since the accident it appeared that icing might have been the culprit or perhaps a contributing factor in causing the aircraft to make its sudden rapid descent (apparently almost immediately after the application of flaps). At the time other aircraft in the vicinity were reporting icing. HOWEVER, AT THE TIME OF THIS WRITING, NEWS RELEASES HAVE INDICATED THAT THE NATIONAL TRANSPORTATION SAFETY BOARD CLAIMS THAT ICING APPEARS NOT TO HAVE BEEN A FACTOR. The changing of the airfoil’s shape upon flap engagement might have triggered the rapid descent – an apparent stall leading to a flat spin. That would indicate either insufficient air speed at the time of flap deployment or some type of catastrophic failure. SINCE MANY AVIATION ACCIDENTS HAVE BEEN CAUSED BY ICING – AND IT WILL REMAIN A PROBLEM FOR AIRCRAFT FOR A LONG TIME TO COME, I SHALL CONTINUE.
When icing was being blamed, I suspected that some critical errors might have been made in the cockpit. At best, my notions were intuitive – or, on the other end of the spectrum, unfair during such an early stage in the investigation. Nevertheless, a surprising amount of information has been made available during this embryonic phase – partly due to the fact that the flight recorders are advanced models and they were in very good shape. There is no need for me to dwell on factors that can cause a plane to become unstable when icing occurs – suffice it to say that airfoils lose “lift efficiency” quickly when ice buildup changes their shape and of course the weight of the ice accumulation can also be a huge factor. I do not know what kind of air speed indicators are installed on that type of aircraft but I do know that icing can cause false readings on some types. Icing can also cause problems at air intakes and oil cooler intakes of some aircraft.
IN THE FIRST PLACE?
The cause of the icing is a surprise to most people. Though icing can occur on a plane’s very cold surface when it descends into “warm” clouds whose temperatures are above freezing, the vast amount of problematic icing occurs when the liquid droplets themselves are below what we traditionally consider freezing temperature. These droplets consist of what is called supercooled liquid water (SLW). Water in cloud droplets can get as cold as about negative 40 degrees Celsius (which is the same as negative 40 Fahrenheit) without freezing.
When liquid water freezes (box 3 to 4 in the illustration above) the water molecules align in a crystalline fashion. But in order to do so they need one of two things: 1) either a freezing nuclei whose surface acts as a template to initially “show” the molecules how to (or trigger the molecules to) line up, or 2) some molecules themselves must be jolted (or jiggled) such that for at least an instant they are arranged so they can act as a template or model for the rest to follow. The likelihood of such alignment occurring in undisturbed droplets is slim. This would not be true of most fresh water at the surface, such as in lakes because there are microscopically-sized particles available in the water to act as templates. On the other hand, water that has condensed and remains in the air is very “clean” by comparison.
An aircraft flying though supercooled cloud droplets causes considerable rapid stirring to set the stages for freezing upon impact with that aircraft – just as supercooled raindrops freeze upon impact with trees and suspended wires in those notorious, damaging ice storms.
The first three links below show convincing demonstrations of liquid water freezing as a result of hexagonal ice crystal seeding. The ice crystals provide the template which “shows” the liquid water what to do in order to become solid. In the third example when the water freezes and builds up a small mound on the wooden post, I suspect that the split second ideal alignment of some water molecules (while pouring) provoked the freezing.
In this 4th example you will see that a jolt causing a sloshing of the water in the small amount of air space at the top of the bottle allows for enough water movement so that for an instant a hexagonal orientation occurs among some molecules causing a very rapid “follow the leader” freezing all the way down to the bottom of the bottle.
Just as condensation and deposition give off heat, freezing is also exothermic. This is probably why some of the water remains in the liquid state. If the SLW is not very much colder than “freezing” temperature, the heat given off during freezing will cause the remaining liquid to acquire enough heat to teeter over to the liquid side.
Use the search term “supercooled water” on YouTube.com and you will find many other video demonstrations.
WHEN WATER FREEZES IT EXPANDS, BECOMING LESS DENSE. THIS EXPLAINS WHY SOLID WATER FLOATS UPON LIQUID WATER.
If you compare box 3 and 4 in the illustration in this post, you will see why water expands and becomes less dense upon freezing. To establish the hexagonal grid necessary for ice, the molecules can’t be as close together as they were when they were in the cold liquid stage.
Information on supercooled liquid water would have eventually been posted here if the Continental Express Flight 3407 disaster had not occurred. It is regrettable that the accident played a role in my posting this information at this time. I offer my sympathy to all who have broken hearts over the loss of a loved one and all others adversely effected.
Finally, the information in this post about SLW and icing merely scratches the surface compared to that which is known. But, that which is not understood is formidable.
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I recommend that Florida residents who are concerned about tonight’s temperature consult your local media for the forecast in your specific area. On line you can go to http://www.weather.gov/ and at the small white rectangle near the top-left – type in your location or even easier, your 5 digit zip.
The image above shows an almost cloudless Florida earlier today. It is covered by a frigid Arctic air mass. The air is very dry and relatively clean. There is not much water vapor within it to intercept outgoing infrared; the colder the air, the less energy is available to keep water in the vapor state. During the daylight hours the incoming solar radiation exceeds the outgoing infrared but of course at night there will be no incoming solar radiation while terrestrial infrared continues to flow outward. Therefore, it will get even colder.
Some folks in my neighborhood have wells. Freezing at or near well sites is not uncommon. It happened to one of my neighbors during a recent cold spell but fortunately there was no damage. Since water expands by about 9% when it freezes, considerable damage can occur. I run an extension cord out to my well and place a shop lamp on the surface and throw some sheets over the pump and plumbing fixtures to help hold in the heat from the 60 Watt light bulb. In the several freezing episodes during the 43 months I’ve lived in this part of Florida, that method has worked for me without fail. SEE IMAGE BELOW.
A neighbor suggested to me that a slow drip at a faucet inside will also help to prevent a line closure from freezing. I have not tried that.
After tonight a slow warming trend is expected but this is probably not the last of this season’s cold episodes.
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This seems almost like an instant replay! We Floridians are again playing host to a couple of surges of cold air. Florida is once again cloudless and the cold air is relatively dry – therefore the state can’t count on much of a greenhouse effect to slow the loss of heat from the surface.
My neighborhood in northeast Citrus County, Florida can expect freezing temperatures on Wednesday and Thursday morning – and perhaps Friday morning. As is so often the case, the fickle microclimatology of a neighborhood can be manifested by a wider-than-expected range of low (and high) temperatures. For example, during a luncheon today a neighbor reminded me that by virtue of his property being on about the highest ground in the neighborhood, his low temperatures end up being not quite as low as those in other parts of the neighborhood. This is not always the case but it happens the majority of times because on those cold, marginal mornings when the synoptic pressure gradient is weak, the coldest (and therefore densest) air tends to spill downward into the lower vicinities.
My wife and I have given up on covering our ornamentals – deciding a while ago to allow “survival of the fittest” to kick in. But – many of my neighbors have already covered some of their plants.
This is not a mean-spirited criticism but it is a huge paradox to me that so many will go out of their way to protect a plant that isn’t meant to grow here yet some think nothing of killing a native species of harmless snake that dares to stray on to their property. I understand the fear – but not the lethal reaction.
If you are “up north” reading this, I imagine that you’d love to be enjoying our temperatures down here. Everything is relative, is it not? For example. I took my daily 3-mile walk earlier today wearing a light-weight sweater over a T-shirt and at the half-way mark the sweater came off! It has been a delightful day for early February – that’s for sure.
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A neighbor on the other side of our road experienced the surprise of having no water from her well this morning. Depending upon how well insulated a well head is in this part of Florida – freezing can happen in this kind of weather.
Though my well is properly insulated I take an extra precaution. I run an outdoor electric line from an exterior outlet to near the well site and then put a shop light (electric with a conventional light bulb) on the surface and drape old sheets to build a makeshift tent over the well assembly so that the heat from the bulb will help to keep the temperature up. So long as the bulb does not burn out – it provides significant protection.
When water freezes, it expands by about 9%. This exerts tremendous force if the water is confined which can do a lot of damage. Among the things that can be ruined is the pump and hose of a pressure cleaner stored where it gets very cold – e.g. a storage shed. One should be sure to blow out the water from the line leading from the pump to the spray wand. The pump can be ruined by the expansion of water inside as it freezes. There is a simple product sold in hardware stores where a lubricant/anti-freeze chemical (in a pressurized can) can easily be injected into the pump to protect it during the cold season. In my opinion, it’s well worth the peace of mind.
Ordinary garden hoses left outside can split if they are left with water inside and the nozzle at the end of the hose in the closed position. In weather like this I make sure my hoses outside are water free.