Will Fay Be Drought Buster?

Check out the CoCoRaHS national map today. It couldn't be prettier!

Widespread rain fell across the eastern half of Tennessee, northern Georgia and Alabama, and the western Carolinas.

This is the drought-stricken area that although has seen some rain in recent weeks -- has not been blessed with anything widespread and significant for quite some time.

The moisture is all thanks to the remnants of Fay, still lingering over the deep south.

All eyes are now turning the Gustav -- a rapidly growing hurricane in the Caribbean.

The forecast track is being influenced by a weather feature over the Atlantic. As this moves east, high pressure building over the Bahamas is expected to ultimately steer the storm further west than previously expected.

Gustav will likely make it into the Gulf of Mexico but where, when and on what path remains to be seen.

This is definitely one to keep your eye on over the next few days.

Today's Lesson

We talked in an earlier blog about water vapor (which is a gas) and how it changes into two states -- liquid (rain) or solids (ice).

When it changes state, or phases, heat energy is requierd.

This heat energy is called latent heat.

To understand this concept, let's pretend we are looking at a water drop under a microscope.

On the surface of the water drop, molecules are constantly escaping. This is called evaporation.

The molecules that are escaping are the ones that are most energetic, or the fastest moving molecules.

As they leave, the average motion of the molecules left behind in the water drop decrease.

Since temperature is a measure of average molecular motion, the slower motion of the water drop's molecules suggest a lower water temperature -- although the actual temperature of the drop may or may not change even though evaporation is happening.

What this tells us is that evaporation is a cooling process.

Remember we learned that energy is neither created nor destroyed.

So the energy that left the water drop we were looking at under the microscope can be thought of as being carried away and "locked up" in the newly formed water vapor molecule.

This energy is now stored, or hidden, because the temperature of the the water drop changing from liquid to vapor is still the same.

The heat energy will reappear as sensible heat (heat we can feel and measure) when the vapor condenses back into liquid water -- this teaches us that condensation is therefore a warming process.

Condensation is the opposite of evaporation.

So here are some new terms for you.

The heat energy released when water vapor condenses to form liquid droplets is called latent heat of condensation.

And the heat energy used to change liquid into water vapor is called latent heat of evaporation.