Weather Facts



What causes thunderstorms in the Hampton Roads area?

The average number of days with thunderstorms in Hampton Roads is about 40 days annually. Why do thunderstorms occur and what months are we more likely to see one? I will try and answer these questions but keep in mind that there are a great number of complex factors that go into thunderstorm development. I will try to keep it simple so everyone gains an understanding of this process. June, July, and August are the peak months for thunderstorm development here in Southeast Virginia. It is the time of year when humidity levels increase, sunshine is at its peak intensity and as a result, thunderstorms become more numerous. All that is needed is a ětriggerî in the atmosphere. What acts as a trigger? Cold air aloft (upper-level disturbance), a jet max, a frontal boundary, a sea breeze, a low level air stream or low-level jet (night-time only), etc, etc all can act to cause a thunderstorm. Forecasters have a tough time forecasting who will see one on any given day. It is really impossible to determine what part of Hampton Roads will see a storm when they are isolated or scattered in nature, which happens quite frequently. Thunderstorms can be the pop-up or garden variety as they are called. They can come in squall lines, which happens more frequently in spring. The basic cause of thunderstorms is the instability in the atmosphere, which is caused by warm, moist air at the surface rising up into a cold upper atmosphere. Warm, moist air is much lighter than cold air so it rises. The cold air cannot hold as much water vapor in its invisible state as the warm air, so the water vapor condenses into clouds. These clouds are called cumulonimbus clouds. They are large puffy clouds that tower up high into the atmosphere. The water droplets or ice crystals that make up these clouds collide together and grow larger and heavier. They eventually fall in the form or rain. Note that sometimes they fall as hail. (See my weather fact on the hail for more details). The tops of the clouds get blown into an anvil shape by the upper-level winds in mature thunderstorms. Thunderstorms produce very heavy rainfall rates and this is why July and August are Hampton Roadsí wettest months of the year. Sometimes, high winds result from these storms. There are two reasons for the strong winds. The heavy rain and/or hail cools the lower atmosphere rapidly and this cooler, denser air falls towards the ground. When the air hits the ground it spreads out rapidly. That is why so many thunderstorms are preceded by strong winds which are much cooler than the air that was there to begin with. Strangely enough, this cool air that they generate helps to destroy the storm, since thunderstorms need warm, humid air in the lower layers of the atmosphere to sustain themselves. The other reason for the strong winds is that sometimes thunderstorms help to bring down strong upper-level winds to the surface. This happens more frequently in the early spring, when the jet stream is stronger and further south. Of course, I canít forget about the lightning and thunder. Lightning is a very dangerous weather phenomenon. The electrical charges change due to the heavy precipitation and this causes the conditions necessary to cause lightning. Lightning is a killer and it can destroy property. If you hear thunder, look for shelter immediately. I hope that this weather fact has given you a better understanding of thunderstorms.

Dave