than 15 kilometers (50,000 feet). In the United States, squall lines form in spring and early
summer where the maritime tropical (mT) air mass from the Gulf of Mexico meets the
continental polar (cP) air mass from Canada. In the United States, severe thunderstorms
are most common in the Midwest.
Lightningis a huge release of electricity that forms in cumulonimbus clouds (Figure16.24).
As water droplets in the cloud freeze, positive ions line the colder outside of the drop.
Negative ions collect in the warmer inside. If the outside of the drop freezes, the water
inside often shatters the outside ice shell. The small, positively-charged ice fragment rises
in the updraft. The heavier, negatively-charged water droplet falls in the downdraft. Soon
the base of the cloud is mostly negatively-charged and the top is mostly positively-charged.
The negative ions at the base of the cloud drive away negative ions on the ground beneath
it, so the ground builds up a positive charge. Eventually the opposite charges will attempt
to equalize, creating ground to cloud lightning. Only about 20% of lightning bolts strike the
ground. Lightning can also discharge into another part of the same cloud or another cloud.
Figure 16.24: Lightning over Pentagon City in Arlington, Virginia. ( 34 )Lightning heats the air so that it expands explosively. The loud clap isthunder. Light
waves travel so rapidly that lightning is seen instantly. Sound waves travel much more
slowly, about 330 m (1,000 feet) per second. If you were watching a lightning storm, the
difference in the amount of time between seeing a lighting bolt and hearing its thunder clap
in seconds times 1,000 gives the approximate distance in feet of the lightning strike. For
example, if 5 seconds elapse between the lightning and the thunder, the lightning hit about