Archive for the ‘Science Misconceptions’ Category

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.


Some physicists prefer to use the term “Coriolis Effect” over Coriolis Force claiming that it is only an apparent force.  I tend to agree with that.

The Coriolis force is something that just about everyone in school learns about at one time or another.  To be sure, it is a topic in secondary school earth science and physics courses.  A low percentage of students enroll in the latter but a very large number are exposed to the former partly because in many school systems earth and/or environmental science is required.  Non-science majors in college enroll in earth and/or environmental sciences partly because it is perceived to be far easier than some of the other options – e.g. physics or chemistry.

The crux of the Coriolis force with regard to earth is that because our planet is rotating – objects and fluids in motion tend to deflect to the right in the northern hemisphere and to the left in the southern hemisphere.  The larger the circulation system the more there is likely to be an obvious response to the force.  Physicists, by the way, tell us that it is but an “apparent” force and that it is more accurate to call it the Coriolis “effect” which I intend to do from here on. There is no need to debate the term here but if you want to learn more about the Coriolis effect I suggest you use both terms in your search.

Earth’s period of rotation is once per day.  The rotational direction is from west to east.  If you looked at earth from “above” the north pole you would discover that the rotation is counterclockwise, and if you looked at the earth from “below” the south pole you would find a clockwise rotation.  If you have difficulty envisioning that “reversal” I recommend that you pick up an item and rotate it watching the rotation from one end of the axis.  Then continue rotating it in the same direction – don’t stop – but view it from the other end of the axis.  You should observe the reversal; from one end it will be counterclockwise and from the other end it will be clockwise.

SPECIAL NOTE: One of the greatest myths or misconceptions in physics is that the Coriolis effect determines the direction of rotation of water down a toilet or other drain.  That is absolutely untrue.  If you live in the United States and observe the direction the water moves down a toilet in your dwelling, then, crate it up and ship it to New Zealand and have someone install it there, upon flushing the water would go down the same way.

Next, look at the demonstrations shown on Quick Time at the following site.  Before you go there take note of this.  The first boy, wearing the blue headgear is rotating clockwise when the playground device is viewed from atop the axis of rotation so his setup is analogous to the southern hemisphere.  The other two boys (one with red headgear and the other bare-headed) are rotating counterclockwise so their setup is analogous to the northern hemisphere.

Hopefully you saw that the first boy’s ball went to HIS left as would be expected in the southern hemisphere (clockwise) and the other two experienced the opposite (to THEIR right) as would be expected for the northern hemisphere (counterclockwise).  You might want to scroll down a little further on that page and you will find a Quick Time animation of a ball deflecting to the right on a rotating table.  The rotation will not be apparent because the camera was fixed above the table and rotating at exactly the same period.  Since the ball deflects to the right you should correctly deduce that the rotation of the table was counterclockwise like the rotation of earth from the northern hemisphere point of view.

There are many examples of the Coriolis effect here on earth.  Cold air masses in the northern hemisphere rotate clockwise because of the right turn of the air which, after sinking toward the surface flows outward from the domal system’s high pressure core; this is a great example of an anticyclone.  But my favorite example of the Coriolis phenomenon, surprisingly, is not an atmospheric example.  It is the manner in which most of the water being carried by the oceanic gyres turns right in the northern hemisphere, especially when it reaches a continental margin and left in the southern hemisphere especially when it reaches a continental margin.  Observe the image below where I have removed all but the gyre components of oceanic surface circulation.

SPECIAL NOTE:  Though not discussed here, it is the general circulation of the atmosphere at or near the surface that creates these gyres and general circulation is guided by the Coriolis effect.  If you wish to learn more about the “general circulation” of the atmosphere, other terms are global circulation, planetary circulation, and large macroscale circulation.

Left click image to enlarge.

Have you noticed – I have not explained the earth’s Coriolis effect!  I have described it, I have linked you to visual evidence, I have described a meteorological example and shown you an oceanic example via a very generalized map of the 5 oceanic gyres.  But I have not provided an explanation other than indicating that it is caused by rotation of the earth. If you have stuck with me to this point, I want to entice you with an “issue” that has often remained unaddressed/overlooked by some teachers and learners of meteorology.  That is this:  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?  THAT WILL BE THE TOPIC OF MY NEXT TUTORIAL POST AND IT WILL BE COMING SOON.  Now, let’s look at a hurricane.

If you want a head start on understanding a hurricane’s circulation, read this repeat of a 9-9-2008 post using hurricane Ike as an example