Tropical Trio Threatens Coast

Wow, Hanna is still with us after spinning over a week between the Bahamas and Hispanola.

Now she is on the move and will impact the weather along the coast of the Carolinas this weekend.

Although a threat, I think the real danger is coming on her heels with Ike. Currently a category 4 storm, Ike will be near the Bahamas in a few days.

The remains of Gustav will drop moderate rains today from the Gulf Coast to Michigan. The CoCoRaHS maps remain busy tracking the footprint of this storm system.

Today's Lesson

So yesterday we teased today's topic -- convection!

This is the transfer of heat by the mass movement of a fluid (such as air and water). This type of heat transfer takes place within gases and liquids because they can move freely and currents can set up within them.

In the atmosphere, convection happens naturally.

Once the sun rises each day, the process begins as some areas of the earth's surface absorb more heat from the sun than others.

As a result, the surface is heating unevenly and you get currents or pockets of rising air. Some of the pockets or currents rise higher than others.

This is called differential surface heating and it is one reason why your airplane flight can be bumpy.

Ever flown into Vegas or Phoenix during the summer with clear skies but horrible turbulance? Differential surface heating and thermals in the desert are part of the reason for such a rough ride.

The heated air becomes less dense and expands, rising up. Heat is transferred up in this process.

This air expands and spreads out, eventually cooling and then it slowly begins to sink.

The now cooler, heavier air flows toward the surface to replace the rising air. Eventually the cooler air heats and the process starts all over.

This vertical or upward exchange of heat is called convection, and the rising bubbles of warm air are called thermals.

The cycle of rising air, being replaced by the cooled sinking air, only to heat up and rise again is called a thermal cell.