Archive for the ‘Microclimatology’ Tag


Two independent left clicks will enlarge
Two independent left clicks will enlarge

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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Cold Snap Coming to Central Florida

LEFT CLICK ENLARGES - This image is discussed in the text below
LEFT CLICK ENLARGES – A SECOND LEFT CLICK ENLARGES EVEN MORE.  This image is discussed in the text below


Here in Citrus County, Florida an approaching cold front is expected to arrive tonight (Sunday, January 11, 2009).  Then the lowest temperatures will get progressively lower for a few days.  For Hernando a small town nearby which is 23 miles from the Gulf of Mexico, the current 10-day forecast is for the following LOWS shortly after 7:30 AM:

50˚F. Monday, January 12

42˚F. Tuesday, January 13

33˚F. Wednesday, January 14

31˚F. Thursday, January 15

30˚F. Friday, January 16

28˚F. Saturday, January 17

38˚F. Sunday, January 18

45˚F. Monday, January 19

41˚F. Tuesday, January 20

42˚F. Wednesday, January 21

Because of the diverse micro-climatology of this area, expect even colder temperatures in certain areas that cool off very quickly during the night.

Ocala, about 40 miles inland from the Gulf of Mexico, might expect temperatures 4 degrees lower than those listed above.

Interestingly, it is typical for the minimum temperatures of a day to occur 30 minutes or so AFTER sunrise.  This is because during that early portion of daylight the sun is so low on the horizon (thus the intensity of solar radiation is weak) that more heat escapes the surface than is received from the sun.  People who must protect their crops from freezing temperatures know this.

THE IMAGE ABOVE  shows a steam fog over a roof in Central Florida during a cold morning last month.  You are looking southward at the west side of my house.  You can see frost on the roof except in places where the lack of insulation kept it warmer underneath – including the parallel trusses.

“Steam fog” is actually a MISNOMER.  That is because what you see is not steam.  Steam, in the strict scientific sense, is invisible.  YOU HAVE NEVER SEEN STEAM.  What you see rising from a teapot of boiling water is not steam.  By scientific definition, steam is water vapor and water vapor is defined as water in the gaseous state.  There is some water vapor in the air where you are this very moment but you can’t see it.  What you are actually “seeing” and calling steam is liquid water in the form of tiny droplets, not unlike cloud droplets.  That liquid has formed by the condensation of water vapor (the steam which neither you nor anyone else can see) into tiny little spheres of liquid.

NOTE ABOUT STEAM:  You can search for definitions of steam and you will find some alternate ones which will use terms like “mist.” In the non-scientific world there are even alternate understandings of the meaning of vapor.  Please understand that I am talking about steam as defined by the modern physicist, chemist, meteorologist, physical oceanographer, etc.

WHAT WAS HAPPENING ON AND OVER MY ROOF WHEN THIS IMAGE WAS TAKEN was that frost on the south-facing side and on heated edges of the roof was melting, some of that resultant liquid evaporated into water vapor (steam) but the water vapor quickly condensed back into the liquid phase due to the cold air into which it ascended (water vapor generally rises easily in still air because the water molecules are so much lighter than the nitrogen and oxygen molecules making up most of the air).  NOTE:  The only other remote possibility is that the frost was sublimating into water vapor but the air was not nearly dry enough nor was the temperature cold enough for this to be happening; sublimation is the phase change whereby a solid becomes a gas totally bypassing the liquid phase – as does dry ice.  Vapor pressure plays a significant role in sublimation but I’m ignoring that now since that is not what was happening.

Because evaporation is an important component to the conditions leading up to the development of a steam fog, many meteorologist have chosen to refer to them as evaporation fogs.  To be more specific, a steam fog is a type of evaporation fog.

Steam fogs occur when the air is colder than the moist surface.  Perhaps you have seen steam fogs over liquid surfaces like a wet asphalt highway after a heavy, cooling rain, over a heated swimming pool, or over other bodies of water that are warmer than the air above.  In time, more images of steam fogs will be posted on this site.

SPECIAL NOTE ABOUT STEAM BURNS AND ANOTHER NOTE ABOUT CENTRAL AIR CONDITIONERS:  One reason why steam burns are so serious is because not only is the victim injured by the very hot steam (super-heated water vapor) but also by the extra heat given off when that steam condenses.  Condensation (the opposite to evaporation) gives off heat called the latent heat of condensation.  It is the same heat that was taken away from the environment where the water vapor was originally formed from the evaporation of liquid water.  So, evaporation is a cooling process (taking heat from the environment where it’s occurring) and condensation is a heating process (adding heat to the environment where it is occurring).

In home central air conditioning systems the place where the coolant is being condensed by compression will be outside because both compression and condensation raise the temperature.  If there was not a fan to circulate air out there, the compressor unit would “fry.”  The cooling half of the unit, that which is inside, is the evaporator.  A fan blows air through the cold evaporator coils in order to make that air cooler.