Archive for the ‘Air density’ Tag

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!

TWO LEFT CLICKS TO ENLARGE IMAGE TO THE FULLEST