Archive for January 21st, 2009|Daily archive page

Central Florida’s Hard Freeze! What Is Happening?

Two independent left clicks should enlarge this image considerably.

Two independent left clicks should enlarge this image considerably.

LIKE MOST POSTING ON THIS WEBLOG, WITH THE EXCEPTION OF THE TUTORIAL PORTION, THIS IS TIME-SENSITIVE.  EVEN ONE DAY CAN MAKE A BIG DIFFERENCE SO IF YOU ARE NOT READING THIS CLOSE TO THE POSTING TIME, PLEASE CONSULT YOUR LOCAL MEDIA OR ON-LINE RESOURCES FOR UPDATES.

I’m posting this on the evening of January 21, 2009 from my home in Citrus County, Florida.  22 miles NE of my location is Ocala; 12 miles SSE is Inverness.

Forecasts for the low tomorrow morning in this part of Central Florida (specifically, the town of Hernando) range from 18 degrees to 24 degrees Fahrenheit (depending upon the source).  Though that may seem to be a broad range it is quite possible to find those two ends of the forecasts both a reality within a very small area – arguably, less than a quarter-section (1/2 mile by 1/2 mile square).  This is due to the highly variable properties of unlike surfaces (heterogeneous surfaces) when it comes to the loss of thermal energy via infrared radiation.

On a larger scale, the satellite image above, completed at 3:45 P.M. E.S.T. today shows Florida virtually cloud free.  This means that all during the daylight hours, even though solar radiation was pouring in, terrestrial (earth) radiation was flowing out freely in the form of infrared – much more freely and abundantly than it would have had the air been humid and had clouds been present.  Tonight, the infrared will continue escaping in its space-bound journey.  The moisture content of the air is low and there will be no clouds though there could be fog (technically speaking, fog is a cloud).

Water vapor (water in the invisible gaseous state) is the most active and abundant of the so-called greenhouse gases.  The presence of clouds suggests that up at that level there is plenty of water vapor (that which resides between and below the cloud droplets that has not condensed into cloud droplets).  So, on a humid, cloudy day one would expect a strong greenhouse effect keeping thermal energy “trapped” at the lower levels.  BUT – TONIGHT THAT IS NOT GOING TO HAPPEN because, as stated, the air is dry and cloud free.  Tonight heat will be escaping rapidly and little will be sent back and none will be pouring in from the sun.  So – the temperature will drop dramatically.

Typically, there is about a 30 minute period after sunrise when the thermal energy escaping earth’s surface at that location exceeds the amount of thermal energy coming in from the sun.  That is why the coldest moment of a 24 hour period is most often after sunrise – about 30 minutes or so.

The satellite image shows how once the cold air coming down from a component of the north leaves the continent to flow over the Atlantic and Gulf of Mexico, moisture is picked up and clouds form.  Notice how they form in short order leaving only a narrow cloud free zone over water near the land.  The fact that the water surfaces are warmer than the continental surfaces at this time of year also play a role in that cloud development.