How can it be, that on a hot summer day, when it thunders - it's actually snowing like mad overhead?

Most everyone understands that when the temperature is above 0 degrees C at the surface, any precipitation that falls from the sky will do so in the form of rain. But the thing to remember is that temperature decreases rapidly with height, during all seasons, even on the hot days of summer. In fact, temperature decreases at an average rate of 6.5 deg C per km of ascent. By the time you ascend inside thunderclouds to 5 or 6 km, the temperature is below zero - which means that ice particles (snowflakes, graupel, hail) are mixed in with the rain. In fact, at temperatures below -20 C inside a thunderstorm (a height of 9-10 km), most of the precipitation is in the form of ice. As these ice particles bump together in the turbulent updraft and grow larger, they become heavy enough to fall through the cloud. And once they pass below the 0 deg C level, all the ice melts into liquid, and the result is a rain shower.

This is not to say that ALL the rain in a thunderstorm starts as ice...there is abundant condensation of water vapor into liquid water drops that occurs in the lower portion of the cloud, above freezing, and this process does contribute significantly to rain production. Small drops form, collide and coalesce into large raindrops. We call this process the "warm rain process" and the generation of rain from melting ice is known as the "cold cloud process." In summer thunderstorms over the United States, both processes contribute to the rain that falls. In the tropics, however, the cold cloud process is not as active, so much more of the rain derives from collision-coalescence in warm air. In fact, it's interesting that there is very little lightning in heavy rain systems over the tropical oceans. We now know that lightning requires that a significant mass of large ice particles must be present in the middle regions of clouds. This is apparently not the case in heavy tropical showers. Recall that when Hurricane Isabel moved off the ocean and through the Mid Atlantic last September, there were very heavy rains and gusty thunderstorm-like winds, but almost no lightning occurred. This was because of the tropical character of the rain clouds. Most of the rain developed in the lowest few kilometers of these clouds, and there wasn't much large ice in the upper portions.

Next time it thunders, on a warm spring or summer day, just remember...if there is lightning, there is a lot of ice suspended over your location several miles overhead...so much in fact, that it would resemble a blizzard if you flew a plane directly inside!

This week's question is from Dr. Jeffrey Halverson. Dr. Halverson investigates severe storms at the NASA Goddard Space Flight Center, serves as the Education and Outreach Scientist for NASA's Tropical Rainfall Measurement Mission satellite, and teaches courses on meteorology at the University of Maryland Baltimore County. He holds a PhD in Environmental Sciences and writes a column on interesting weather phenomena in the bi-monthly publication Weatherwise. Dr. Halverson is also an avid amateur astronomer and enjoys hiking throughout the Mid Atlantic to better understand the region's complex geology.