Archive for the ‘Water vapor’ Tag

Hard Freeze Over Much of Florida Is Due

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LEFT CLICK TO ENLARGE

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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LEFT CLICK TO ENLARGE


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.

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

LIKE MOST POSTING ON THIS WEBLOG, THIS IS TIME-SENSITIVE AND WAS ENTERED DURING THE AFTERNOON OF JANUARY 11, 2009.  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.

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.


THE TROPICS ARE VERY BUSY!

Easterly waves are starting to come off Africa as though a youngster is repeatedly firing a pea-shooter toward the west.  The full-disk infrared image below doesn’t even show it all because Gustav is not visible.  So we now have 4 tropical systems causing various degrees of concern over the water, and four that have not yet left Africa.  Busy periods like this are to be expected for this the middle third (August & September) of the official 6 month long hurricane season.

Tropical waves (or easterly waves) are also called tropical disturbances.  When an African easterly wave gets over the Atlantic it picks up more moisture.  The introduction of water vapor to such a system (through evaporation) has a tendency to lower the pressure.  All other things being equal (like temperature) the more water vapor in the air the less dense it is and therefore the less pressure it exerts. Once rotation near the surface becomes evident the system is then cyclonic and is called a tropical depressions. If the pressure gets low enough it may evolve into a tropical storm (sustained winds = 39 to 73 mph).  The next stage in the possible progression is the hurricane with sustained winds of 74 mph or more.

Here is another sobering thought:  WE ARE NOT YET HALF WAY THROUGH THE OFFICIAL HURRICANE SEASON and we won’t be until the end of Sunday, August 31!

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