Why Is Florida So Humid?
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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: