Thursday, May 7, 2015

Spring Rain


Spring is here and for the eastern half of the country it has been some winter.  Very cold into the deep south and plenty of snow up north.  The sun is now advancing slowly northward and hot weather will be upon us. Hello to another weather phenomenon, the thunderstorm.  We are all familiar with thunderstorms, heavy rain, gusty winds, lightning, thunder and even sometimes hail.  Let’s take a closer look at a thunderstorm and see what causes it. 

Basically you need heat, cold and moisture.  Specifically, very warm or hot air at the surface, plenty of moisture available and a very important ingredient cold air in the upper atmosphere.  When it is hot at the surface and cold aloft, we say the atmosphere is unstable.  So, here We go…on a hot summer afternoon, surface temperature readings in the 90’s, the surface air is very moist, the air begins to rise.  As long as this Parcel of air is warmer than the air surrounding it, the air will continue to rise.  As this air rises higher in the sky, the upper air surrounding the parcel continues to be colder and colder.  This causes the air to continue to rise.

 The air will rise until the air surrounding this parcel is the same temperature as the parcel.  If, however, the upper atmosphere is cold all the way up to 60,000-70,000 ft. The parcel will rise that high.  Also, remember the surface air contained a great deal of moisture.  Well, air is like a sponge, warm air can hold a large amount of moisture, however, as the air gets colder, it is like squeezing the Sponge.  Water is released,  if you squeeze the sponge quickly, a great deal of water is released at once….a Heavy downpour.  So, in general, rising air causes clouds and precipitation because rising air expands and cools  and can not hold as much moisture as warm air can. That explains the rainfall during a thunderstorm…what about the lightning and thunder?  Let’s forget mathematics etc and explain these features in a very down to earth fashion.   In a thunderhead cloud, also called a cumulonimbus cloud, there is plenty of water vapor.  Water, as you know, is h20

Two hydrogen atoms and one oxygen atom.  Hydrogen has an electrical charge of plus one.  Since there are two hydrogen atoms, it has a plus two Charge.  However, oxygen has a negative charge, a minus 2 charge.  So, when you combine two hydrogen (+2) with one oxygen (-2), you get water.

With a zero charge.  In a thunderhead cloud there are tremendous up and down drafts of air.  Planes flying through a thunderhead would be damaged severely if not destroyed completely.  Anyway, these up and down drafts of air rip the water molecules apart.  The h’s are ripped from the (oh’s).  These particles are now charged.  The (oh) has a negative charge while the “h” has a positive charge.  The positive charges accumulate at the top of the cloud while the negative charges are found at the base of the cloud.  Beneath the cloud, positive charges prevail.  Since opposites attract, the positive and negatives try to interact.  As the storm system moves along, the positive charges on the ground follow along.  Finally, when A high object appears in the storm’s    path, the positive charges climb up the high object, be it a tree, a hill, a tall building.   The negative charge and the positive charge are now closer together and there is less resistance between them. 
A huge spark occurs when the charges comes together.  We call this lightning.  Lightning can go from cloud to ground, we call this cloud to ground lightning.  Sometimes lightning goes from one cloud to another cloud and does not strike the ground.  We call this cloud to cloud lightning.  Next week i will talk more about lightning , a few safety measures and then discuss thunder and hail.