The Ever-Changing Sky

Time to pack it in

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Thunderstorms are the most dangerous type of local weather. Most injuries caused by weather in the United States are thunderstorm-related. Since the sun's heat energy is a major factor in thunderstorm generation, thunderstorms typically occur in the spring and summer. Winter sunshine simply doesn't deliver enough warmth. For thunderstorms to develop, the air needs to be warm, moist, and unstable—typical spring and summer conditions.

Keep an eye on those small, fleecy, white puff clouds (cumulus) drifting through the sky. They can grow into thunderstorms in a few hours. These clouds have flat bases and rounded tops and are not a threat as long as the cloud tops are less than one cloud's width above the cloud bases. In the morning and early afternoon of a warm, sunny day, these clouds indicate that the atmosphere is unstable enough to generate thunderstorms. If these are the only clouds in the sky by mid- to late afternoon, there probably isn't much risk of thunderstorms developing. When these puff clouds become taller than they are wide, they change to swelling clouds (cumulus congestus) and the chance of thunderstorms rises greatly.

Swelling clouds: Watch these clouds carefully. They are the first stage of thunderstorm growth and can become dangerous in a matter of minutes. These white or gray clouds with flat bases are taller than they are wide. The earlier in the day that swelling clouds appear, the greater the probability that they will develop into thunderstorms. Their growth can usually be seen with the naked eye, and the faster the swelling clouds are growing the more threatening the situation.

Since the weather in the United States generally moves west to east, swelling clouds to your west are much more dangerous than swelling clouds to your east. This west to east airflow doesn't always hold true, especially in mountain ranges, so try and determine if the swelling clouds are headed toward you. If they are, an orderly retreat before the storm develops might be your best option. It's always better to get to safety before a storm hits.