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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:
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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What a terrific way to start the new year in this weblog – with an image of nearly the whole earth showing cloud patterns over both the daylight half and the darkness half. The two rows of extratropical cyclones (one over the middle latitudes of each hemisphere) are striking. Since a very high percentage of the over 6.7 billion people on earth live in the middle latitudes, and since these cyclonic systems and their cold fronts commonly extend into the lower latitudes, you might very well be under the influence of one of these systems this very moment. They are marching generally from west to east in both hemispheres followed by cold (or cooler) anticyclones. For example, I live in the low latitudes at 28.972 degrees north and we get several frontal passages.
The Intertropical Convergence Zone is apparent, especially over Africa. Notice how it is further south now over that continent than during the North Atlantic hurricane season.
HERE’S WISHING YOU ALL A HAPPY NEW YEAR. That’s just the beginning because my wishes for you are many. That: You “know” love and feel both loved and lovable, you are never bored, you have a long gratitude list, happiness is not illusive, you feel as good as possible for your circumstances, life’s pros far outweigh the cons, and you experience peace and good will always.
Tonie A. Toney (Cloudman23)
= = = = = = = = = = = = = =
NOTES: Extratropical means “outside of the tropics.”
Middle latitudes are strictly defined as the regions between 30 degrees and 60 degrees latitude (both hemispheres).
Extratropical cyclones differ from tropical cyclones in the following ways. ET cyclones are “cold core” lows while T cyclones are “warm core” lows. ET cyclones generally have fronts associated with them, T cyclones do not. ET cyclones originate mostly in the middle latitudes while T cyclones originate in the low latitudes (0 degrees to 30 degrees). ET cyclones are asymetrical with decided wind direction changes and measurable temperature changes on either side of the fronts, while T cyclones are more nearly circular.
The Intertropical Convergence Zone is where the Northeast Trades and the Southeast Trades converge. Years ago it was referred to as the Doldrums and also the Equatorial Low. Those two outdated terms are still found in the literature and even on line. Generally, the ITCZ migrates northward during the northern hemisphere warm season and southward during the northern hemisphere cold season.
Never in my wildest dreams during my 41 years of teaching college/university meteorology did I ever think that I would be able to sit in my recliner at home (or anywhere else for that matter) with a personal computer on my lap allowing me to gaze at color images of our beautiful earth from near space in nearly real time! Nor did I ever imagine being able to electronically transfer that image to a web-log for hundreds of interested (and interesting) people who visit the site.
The only thing about all of this that disappoints me is my not having been able to do similar things in the classroom for the nearly 25,000 students who took my courses. I feel very fortunate, however, to have a wonderful following of Senior Institute participants at Central Florida Community College in Ocala. In the classroom where I meet with them I am able to project on-line images on a large screen. That they seem to enjoy my use of the technology in the classroom is icing on the cake. I know how lucky I am to be able to continue after retirement, teaching and learning more and more about subjects I love.
Please take a look at this beautiful image. Enlarge it as much as you are able. I suggest right-clicking on the image and saving it so that you can study it using an image viewer of your choice; do that, ONLY after getting the image as large as you are able following the instructions immediately below.
TWO INDEPENDENT LEFT CLICKS SHOULD GIVE YOU
A VERY LARGE IMAGE WHICH WILL ALLOW YOU TO SEE
DETAIL MUCH BETTER SO LONG AS YOU SCROLL
UP AND DOWN, RIGHT AND LEFT.
PLEASE BE PATIENT.
DEPENDING UPON YOUR CONNECTION SPEED,
LOADING MAY TAKE A WHILE.
This image was completed at 3:45 PM EST, November 10, 2008; the time stamp is at the upper left corner but is easy to read only when you enlarge. The satellite that did this, GOES 12, is in geosynchronous orbit. This simply means that it completes one orbit (revolution) in the same period of time the earth makes one rotation; that period of time is one day. Also, it orbits within the equatorial plane. Therefore, as the satellite travels rapidly though space it stays over the same point above earth (about 22,300 miles from the earth’s surface). The distance between the satellite and earth’s surface is almost three earth diameters – so “high” that full disk images of earth can be captured.
With adequate enlargement you can see the aqua-blue of the shallow Bahama Platform. You can also see ice and snow in the Southern Andes, Greenland, the Arctic Ocean, and the Antarctic peninsula. You can see the remnant of what was once hurricane Paloma centered slightly north of Cuba. You can see the bright tops of high clouds and the grey tones of the lower clouds. If you know weather circulation patterns as marked by clouds you will see cyclonic circulation in both hemispheres. In the North Pacific there is a very large cyclonic system approaching B.C. Washington, and Oregon. There is a huge front stretching across the South Pacific. The Intertropical Convergence Zone is very well marked by clouds in the Pacific. There is a large extratropical cyclone over the Middle United States. The list goes on and on.
Being able to see all of this, to my mind, is a miracle.
Tonie Ansel Toney
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A single left click followed by another single left click will enlarge this image significantly.
Altocumulus lenticularis clouds are not rare but, none-the-less, they inspire countless observers and much “camera clicking” occurs when they are present. Forming mostly over mountainous topography, they generally mark the movement of the air at the altitude where they form. Often (as in this case) they form not immediately above the mountain peak (or peaks) themselves but quite some distance higher. In this case it is forming on a “lee wave.” Rising air is what causes most clouds to develop and this is no exception. Envision water flowing quickly in a stream over a boulder that is on the bottom. Not only does the water touching the boulder rise up and over – so does the water above that lowest layer; and if you look closely you might see at least one other standing wave on the downstream side of the boulder. Likewise, a prevailing wind obstructed by a mountain or mountain range is forced to rise up and over and the air above it does the same. If the air aloft contains enough moisture and the lifting cause enough cooling of that air, condensation (and deposition) can occur forming cloud droplets and ice crystals respectively.
I took this photo today (10-29-2008) from the Blue Ridge Parkway. The clouds are forming on the leeward side of the Black Mountain Range, the famous range most notable for the highest peak in the U.S.A. east of the Mississippi, Mt. Mitchell. I was northeast of Mt. Mitchell at the first viewpoint northeast of the intersection of the Parkway with highway 80. The camera faced south-southeast.
The environment was changing quickly while I took several photos. I intend to post more soon in order to demonstrate how quickly the changes were occurring (which was a function of how rapidly the air was moving up there).
In this region the prevailing winds aloft are from west to east (generally speaking) and the Black Mountains trend north-south. The Black Mt. Crest Trail, rated “difficult” runs northward from the top of Mt. Mitchell (which can be reached by roadway) for about 7 miles to Celo Knob. From near the top of Celo Knob the Bowens Creek Trail heads northwestward another 5 miles to the mountain valley below. My youngest daughter hiked the trail with me when she was 11. It was an experience I’ll never forget – nor will she.
The graphic above and the statement below are
Statement as of 8:00 PM Pacific Daylight Time on October 10, 2008
…Outer rainbands of Norbert approaching Baja California…
a Hurricane Warning remains in effect for the West Coast of Baja
California from Puerto San Andresito to Agua Blanca. Preparations
to protect life and property in the Hurricane Warning area should be
rushed to completion.
At 8 PM PDT…0300 UTC…the government of Mexico has changed the
Hurricane Watch and Tropical Storm Warning for the coast of
Mainland Mexico from Topolobampo northward to guaymas to a
Hurricane Warning. A Hurricane Warning is now in effect for the
coast of Mainland Mexico from Topolobampo northward to guaymas.
Preparations to protect life and property in the Hurricane Warning
area should be rushed to completion.
At 8 PM PDT…the government of Mexico has changed the Hurricane
Watch and Tropical Storm Warning for the East Coast of Baja
California from north of La Paz to Loreto to a Hurricane Warning.
A Hurricane Warning is now in effect from north of La Paz to
A Tropical Storm Warning and a Hurricane Watch remain in effect for
the coast of Baja California from south of Agua Blanca southward to
Cabo San Lucas and northward around the peninsula to La Paz.
A Tropical Storm Warning remains in effect for the East Coast of
Baja California from north of Loreto northward to Mulege.
For storm information specific to your area…including possible
inland watches and warnings…please monitor products issued
by your local weather office.
At 800 PM PDT…0300z…the center of Hurricane Norbert was located
near latitude 22.3 north…longitude 113.1 west or about 210 miles
…335 km…west of the southern tip of Baja California and about
180 miles…290 km…south-southwest of Cabo San Lazaro Mexico.
Norbert is moving toward the north-northeast near 12 mph…19 km/hr
…And this general motion is expected to continue overnight
followed by a turn toward the northeast and an increase in forward
speed forecast on Saturday. Norbert is forecast to make landfall
along the West Coast of the southern Baja California peninsula on
Saturday morning…then move over the Gulf of California Saturday
afternoon before making a second landfall in northwestern Mexico
Maximum sustained winds are near 105 mph…165 km/hr…with higher
gusts. Norbert is a category two hurricane on the Saffir-Simpson
scale. Little change in strength is forecast before Norbert makes
landfall in Baja California. Some weakening is forecast after that
landfall but Norbert is expected to still be a hurricane as it
makes a second landfall in Mainland Mexico.
Hurricane force winds extend outward up to 35 miles…55 km…from
the center…and tropical storm force winds extend outward up to 140
The estimated minimum central pressure is 970 mb…28.64 inches.
Norbert is expected to produce rainfall accumulations of 4 to 6
inches over southern Baja California as well as portions of
northwestern Mexico…with possible isolated amounts of 10 inches.
Norbert could produce rainfall accumulations of 1 to 2 inches over
portions of the southwestern United States beginning on Saturday.
These rains could result in life-threatening flash floods and mud
Storm surge flooding of 2 to 5 feet above normal tide levels…along
with large and dangerous battering waves…is expected along the
West Coast of the southern Baja Peninsula near and to the southeast
of where Norbert makes landfall.
Repeating the 800 PM PDT position…22.3 N…113.1 W. Movement
toward…north-northeast near 12 mph. Maximum sustained winds…105
mph. Minimum central pressure…970 mb.
An intermediate advisory will be issued by the National Hurricane
Center at 1100 PM PDT followed by the next complete advisory at 200
I suggest that, though there may be some repetition, please read Part 1 first. To go to it quickly, either scroll down or click on this link:
The set of weather maps, provided by NOAA, shows the remains of Ike after it began to head toward the northeast. At this latitude there is a tendency for weather systems in the middle latitudes to travel generally from west to east. Notice the cold fronts which indicate that Ike had changed from tropical to extratropical. The cold fronts represent the leading edge of cooler air being thrown out of the anticyclone (high with rotation) centered over the Eastern Dakotas. That air is coming “down” from some component of the north whereas the air on the “warm” side of the cold fronts is coming up from some component of the south and is being thrown out of the anticyclone centered off Florida. So, we have a cyclone (low with rotation), Ike, between two anticyclones.
The only alterations I have made to the first map are 1) cropping of the original, 2) labeling of the fronts 3) placement of the red L and the two blue H’s, and 4) darkening of two of the isobar values making it easier for you to read.
Isobars are imaginary lines, of course, and plot equal pressure. For example, every point on the 1020 isobar was believed to have had a pressure of 1020 millibars at the time of observation. In many ways isobars are analogous to contour lines on topographic maps. In fact, the two “highs” on a topographic map would be hills and the “low” would be a trough-shaped valley between the hills. Sticking with that analogy, surface runoff water would tend to flow down the hillsides along a stream gradient (or gravity gradient) toward the lower valleys. In the simplest of topographic and geological settings the water would flow down the hills in a radial pattern, just as air would flow out of the highs were it not for the Coriolis effect. Though water flowing down hillsides in stream channels does not respond to the Coriolis effect, air flowing on the scale depicted here does by deflecting to the right of the pressure gradient direction. So, in the second version of the weather map I have drawn blue lines of which two are comparatively long, showing the direction that the air would flow if the earth did not rotate on it’s axis. But, remember, rotation of the globe causes the Coriolis deflection to the right in the northern hemisphere and to the left in the southern hemisphere.
It’s important to note that the deflection is with reference to the object or fluid in motion. For example, if someone driving directly toward you turns right, he/she will have turned to your left. Though that person’s turn would be to your left, it is still a right turn. So, in northern Texas the green line shows that the air is moving to the right of the pressure gradient direction (light blue) – even though that green arrow points toward the left side of the map. Just put yourself in the position of the air in motion and you should not have difficulties with this.
This, then, shows why air in the northern hemisphere moves clockwise around anticyclones and counterclockwise around cyclones.
Near the end of my first tutorial on the Coriolis effect I revealed that the following question had come up often during my teaching career: “If the Coriolis effect is an important influence in large scale weather systems, and since hurricanes are synoptic scale (a type of macroscale) system, why do hurricane winds turn left in the northern hemisphere and right in the southern hemisphere?”
The “obvious” left turning of air within hurricanes causes confusion among many people who are trying to understand air circulation – particularly if they are starting from scratch without knowledge of the Coriolis effect or the pressure gradient force and how the two engage in a tug of war. I understand the confusion because seeing the shape of hurricane rain bands on radar and arcuate cloud band alignment clearly shows how the air turns left as it gets closer and closer to the hurricane’s eye wall.
Not too many years ago I heard a person who should know better, during a television weather report, explain to the viewing audience that hurricanes were so powerful that they did not respond to the Coriolis effect – referring to the “left turns” that she was showing on the satellite loop that was being projected. I don’t know whether or not in some previous weather report she had mentioned the Coriolis effect but it seemed to me that might have been the case. In her honest attempt to educate some of her audience, she gave them information which was entirely incorrect – perhaps because of misinformation given her or maybe some general assumptions she had made. You see, it is the Coriolis effect that forces the counterclockwise rotation in the first place!
In this last illustration (below) you are looking at a satellite image of hurricane Fran (1996). I have drawn blue pressure gradient lines and red air flow lines which clearly show the rightward deflection (in spite of the fact that the air does turn left as it approaches the eye wall. Notice, however, that no matter where pressure gradient lines are placed along the air flow lines, the deflection of the “real wind” is always to the right of the pressure gradient line.
Once again, as in Part 1, I have not truly explained the Coriolis effect; I have merely described it and illustrated it. I have not explored the nitty-gritty. I have implied that it is only an apparent force. You might want to explore other attempts to describe the Coriolis effect – perhaps via an Internet search.
Finally, in the interest of accuracy, I must admit that I have simplified to the point of leaving out some important forces that play a roll in determining the actual direction that air moves (from high toward low) in its quest to reach pressure equilibrium. Among those are friction, centripetal force, and centrifugal force. The conservation of angular momentum is an important consideration and accounts for the increase in wind velocity as the air gets closer to the storm’s center.
Left click to enlarge this image.
At 8 PM EDT this evening Kyle still was maintaining hurricane strength in spite of high wind shear aloft. Generally, a 15 mph wind shear is about the break-off point for being slow enough to allow a hurricane to hold its strength or intensify. It has been greater than that today and is expected to get up to 25 mph tomorrow. However, the winds over the storm are diverging as two cars going down the highway together would diverge a bit if one of them were to move to a lane further from the other car. So, the air over the storm is moving in the same general direction but spreading a bit. When air aloft converges it tends to sink and the opposite happens when air aloft diverges; there tends to be an increase in the amount of air rising from below. This could allow Kyle to maintain hurricane strength tomorrow in spite of the shear. It’s a fine balance and there is some disagreement as to whether it will still be a hurricane tomorrow since at 8 PM the maximum sustained winds were 75 mph and 73 mph would demote it a tropical storm.
It will be interesting to see what happens. Of course, for the sake of the landfall regions and the ships and boats at sea in that area, I hope it weakens quickly.
The storm that is at the North Carolina-South Carolina border may look like a hurricane but it is not. The National Weather Service is calling it a non-tropical cyclone. A more common term for such cyclones is “extratropical cyclone.” “Extra” means “outside of.” This refers to their developing outside of the tropics. Hurricanes are tropical cyclones. Even though in the northern hemisphere they both rotate counterclockwise around a central region of low pressure, tropical cyclones have warm cores and are often referred to as “warm core lows.” Relatively cold air occupies part of most extratropical cyclones and this is most certainly the case with this one. The doublet image of the system that I have prepared which you see (above) shows a visible satellite view of the storm earlier today and compares it with a surface analysis. The two do not represent exactly the same time but it’s close; 44 minutes separate them. So, it’s a near match.
For those of you who know your frontal symbols, notice that there are three different types of fronts, all three representing boundaries between relatively warm air and relatively cool air. An occluded front arcs out from the center of the storm and there is a warm front whose axis runs ENE-WSW, and a stationary front curving down to the south.
In spite of the fact that it is extratropical and therefore un-named, it has many of the characteristics of a tropical storm. People located in the storm’s vicinity should be alert to the potential hazards. Also, there is a strong chance that it will interact with tropical storm Kyle in the interesting Fujiwhara effect. If you are interested in that phenomenon, see the following link and also view the post that followed it (at the next higher post location on the page). To do that you will need to scroll to the top of the page and click on the “blog” tab. That will access you to all entries.