Archive for the ‘Cold front’ Tag

ANOTHER FREEZE EPISODE IN MUCH OF FLORIDA

LEFT CLICK TO ENLARGE IMAGE

TWO INDEPENDENT LEFT CLICKS TO ENLARGE IMAGE

In the infrared satellite image above the cold front that raced through Florida on Saturday is clearly visible via the cloud pattern over the Atlantic.  It travels through Maine and continues southward toward Hispaniola.  It appears that there is a linear trough over Atlantic waters closer to Florida.

There are freeze warnings for many counties of northern and central Florida tonight.  Please consult your media weather reports or on-line resources  for details.  Expect the coldest temperatures of the week to occur from approximately 6:30 to 7:30 am on the morning of Tuesday, March 3.

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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

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 Coriolis Effect In the Real World – A Tutorial (Part 2) – Cyclones & Anticyclones

Left Click To Enlarge Image

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:

https://cloudman23.wordpress.com/2008/10/02/the-coriolis-effect-in-the-real-world-a-tutorial-part-1/

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 Image
Left Click To Enlarge Image