The Science of Thunderstorms and Lightning


Every thunderstorm produces lightning. Lightning is a giant spark that moves within the cloud, between clouds, or between the cloud and the ground. As lightning passes through the air, it heats the air rapidly to a temperature of about fifty thousand degrees Fahrenheit. This causes a rapid expansion of the air near the lightning channel. This rapid expansion causes a shock wave that we hear as thunder.
Thunderstorms will form if there is enough moisture and instability in the atmosphere. As the sun warms the air near the ground, pockets of warmer air begin to rise and cool. Condensation of water vapor causes cumulus clouds to form. Continued heating can cause these clouds to continue to grow upward into the atmosphere. Towering cumulus clouds may be one of
the first indications of a developing thunderstorm. The mature thunderstorm has both an updraft of rising motion and a downdraft of sinking cool air accompanied by rain and sometimes hail.
Thunderstorms grow tens of thousands of feet into the atmosphere. In the cloud, precipitation forms as ice crystals, hail, and rain. Collisions between ice particles causes a charge separation, and positively charged ice crystals are carried by the updraft high in the thunderstorm. The heavier hail gathers a negative charge and falls toward the lower part of the storm. The top of the cloud becomes positively charged and the lower part of the storm becomes negatively charged. 
Normally,  the earth's surface has a slight negative charge.   As the negative charges build up in the lower part of the storm, the ground near the thunderstorm becomes positively charged. As the cloud moves, these induced positive charges on the ground follow the cloud like a shadow. Farther away from the cloud base, but under the positively charged anvil, a stronger negative charge may be induced.
Air normally acts as an insulator. However, when the electrical potential between the positive and negative charges becomes too great, there is a discharge of electricity that we know as lightning.
Cloud-to-ground lightning can either be a negatively charged flash or a positively charged flash. The negative flash usually occurs between the negative charges in the lower part of the storm and the positive charges on the ground under and near the cloud base. Positive flashes usually
occur between the positively-charged upper levels of the storm and the negatively-charged area on the ground surrounding the storm.
In the negative cloud-to-ground flash, an invisible, negatively-charged step leader forms near the cloud base and surges downward toward the ground. As this step leader approaches the ground,  streamers of positive charge move upward from trees, buildings, and other objects on the ground. When these streamers meet the step leader, the connection is completed, and the result is lightning. The entire process takes place in fractions of a second. If you are under a thunderstorm and your hair are in an area where the positive charges are rising up objects towards the storm. It is a dangerous location, because lightning may be about to strike.   
The process for a positive flash is similar except that a positive channel usually originates in the anvil of the storm and surges downward. In this case, streamers of negative charge shoot up to meet the positively-charged channel as it approaches the ground. When a connection is made, a
positive flash of lightning occurs.
While both negative and positive flashes of lightning can be deadly, the positive flashes generally are more destructive and are more apt to catch people by surprise. Positive flashes are infrequent and may strike the ground miles from the main part of the storm. The positive flashes may involve the exchange of a much greater charge and are usually more destructive. Positive flashes also strike well beyond the area where rain is falling, and away from the bulk of the lightning. Consequently, many victims of positive lightning strikes are caught completely off guard.
The best advice in order to minimize your risk of becoming a lightning victim is to get to a safe shelter sooner and to stay there longer. In general, if you can hear thunder you are within striking distance of the storm. Stay in the shelter for 30 minutes after the end of the storm.  
For additional information about lightning or lightning safety, visit NOAA's Lightning Safety web site at: is the U.S. government's official web portal to all federal, state and local government web resources and services.