Who is sick of this heat and humidity already?  I know I am...anybody out there have a swimming pool?  Stay close to that pool this week as summer has made an early entrance into the Midstate.  Today's Questions & Answers focuses on tornadoes and comes from a viewer who wrote to our weather account.

"When a tornado watch is posted, the message usually says that 'conditions are right for a possible tornado' (or something like that).  Could someone tell me just what are the conditions that spawn tornadoes?  Is it temperature, humidity, dewpoint, or a combination of all of them, etc.  Thanks!"

This is a great question because I know many people out there are wondering the same thing.  The exact explanation of what causes a tornado to form is called the conservation of angular momentum.  The classic example of this that I learned about when I was in college are ice skaters.  Think about your favorite ice skater (if you have one) and imagine they have their arms extended away from their body.  As they start to spin, they bring their arms in toward their body and almost instantly, the rate at which they are rotating increases.  Simply put, arms out equals slow rotation, and arms in equals fast rotation.  This is the conservation of angular momentum explained in a fairly comprehensible manner.  This is exactly what occurs in the atmosphere before a tornado forms.  Every single air mass has some spin associated with it, because the earth is always in motion.  As air converges in an updraft during a strong thunderstorm, the rate of rotation increases, similar to the skater as his or her arms converge with their body.  The exact mechanisms of tornado formation are still poorly understood.  Meteorologists do not know whether tornadoes form at the base of the cloud and travel down toward the ground, or start at the ground and work their way upwards.  Maybe tornadoes form from a combination of air traveling both up from the ground and down from the cloud base.  We still do not really have a solid answer, but scientists are studying the formation of tornadoes closely and are getting a better understanding of them each day. 

While the conservation of angular momentum actually causes a tornado, what weather conditions are favorable for their formation?  We are look at several factors.  Temperatures need to be warm and dewpoints need to be on the high side.  Temperature and dew point are plotted at all levels of the atmosphere in order to determine the CAPE of the environment.  CAPE stands for Convective Available Potential Energy.  If the CAPE is on the high side, say between 3000-4000 J/KG, then we start to become concerned for tornadic development.  If other factors are present, the CAPE could even be lower than that.  Another factor we look at is wind shear.  This means how the wind changes with height in the atmosphere in both speed and direction.  If a lot of wind shear is present in the environment, the chances are high that storm clouds will form into rotating funnel clouds.  While there are so many factors that contribute to making tornadoes in a certain environment, the last one I will discuss here is the lifted index.  This is a measure of the instability in the atmosphere.  Typically, once the LI turns negative, we have our guard up for severe weather.  When we see an LI of anywhere between -2 and -5 we become concerned that severe weather is pretty likely.  This past week, Mick O'Hearn and myself were checking the LI values for southern Maryland and Virginia.  We actually saw LI values of -10 and CAPE values of near 4500 J/KG!  Yikes!  That indicated to us that a major severe outbreak was possible just south of here, and sure enough the Washington D.C. metro area got pounded with several rounds of severe weather. 

Thanks for the question and I hoped this helped better inform all of you about some of the things that the National Weather Service and Storm Prediction Center take into consideration when issuing a tornado watch.  If you have a weather question for me, send it to brettsblog@abc27.com or simply post it below!  Thanks for reading and stay cool this weekend!