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    Summer Downpours: Why Us and Not Them?

    By By Alex Sosnowski, Expert Senior Meteorologist
    July 14, 2011, 8:55:07 AM EDT

    Ever wonder why it rains in some neighborhoods and not others?

    Did you ever wonder why some lawns in parts of the area are nice and green and yours is, well, brown?

    This story, part of a series this week, will attempt to explain some of the chaos of what meteorologists tend to call the "butterfly season."

    During the summer, the most of the nation receives its rainfall from thunderstorms.

    What Is a Thunderstorm?

    Thunderstorms form as rising columns of warm air cooling on their way up. The air condenses, forming towering clouds. If these clouds are violent enough they result in downpours and thunderstorms.

    Many summer days bring puffy fair-weather clouds known as cumulus clouds. These cottony patches of water vapor stop short of bringing rain.

    It is only when there is enough rising momentum of air that is always warmer than its surroundings that these clouds grow into the towering rainmakers.

    In the most violent cases, thunderstorms have been known to produce rainfall on the order of several inches per hour. This type of rain can bust a drought or lead to catastrophic flooding.

    Most storms are carried along by the prevailing winds. However, some storms can grow opposite of that direction, into the air feeding them; new ones can form on the edge of another.

    The result is a complicated mess.

    Size matters?

    Even on days when thunderstorms are forming, not everyone gets rain.

    Every storm, whether it is a large-scale storm system with rain, ice and snow in the winter or the much smaller summertime thunderstorm, must have a beginning and an end.

    Thunderstorms are much smaller in scale and most only take up part of a county. In some cases these storms may only swipe part of a city and not the other. In other cases, monster thunderstorms can take up multiple counties or a significant part of a state.


    300x201_07111856_92399676

    To make it easier to understand, think of a thunderstorm as a shower head a thousand times bigger than the one in your bathroom.

    You don't necessarily need a large thunderstorm to get torrential rainfall. What matters is that you are under the core of the taller rain-producing clouds within the storm.

    Atmospheric Favoritism

    There is no doubt that some areas of your region are rainier than others. This is true, whether you are in Florida, Arizona, Illinois, Washington or Maine.

    Mountains, plateaus or large hilly areas are some favored areas for summer thunderstorms. This is because during the summer these areas get almost the same amount of heat from the sun as other areas with lower elevation. However, since they are higher in elevation, they have an easier time sending warm thermals high into the atmosphere. It is kind of like an atmospheric head start.

    Fronts are a well-known area for thunderstorm formation. Whether a cold front or warm front is moving through, the action of the front lifting the air is often all that is needed to get the rising columns going.

    Sea breezes or lake breezes are a type of front and also can lead to thunderstorm development. These boundaries of slightly cooler, even less humid air meet up with the warm, humid air inland. The breezes force the air upward as they move inland, leading to another head start for thunderstorms. Generally, if you are on the coastal side of the breeze, storms will avoid you, until the breeze breaks down.

    Upsloping air is another favored area for thunderstorms. Upsloping, in the realm of meteorology, is any air that is blowing uphill because of mountains or hills. This windward side is a zone of rising air that can also lead to thunderstorm development.

    Certainly, the heat generated by a large city can produce a rising column of air overhead. These thermals, although smaller than the other effects aforementioned, in theory, can lead to a thunderstorm or enhance its performance. However, it may be tough to prove because of the infrequency of such.

    Despite these known areas, which are factored in to the AccuWeather.com forecast, it is still nearly impossible to say exactly who will get or who will not get drenching rain, when the atmosphere is ripe for thunderstorm formation.

    Time of Day

    The vast majority of thunderstorms form between the hours of 2:00 p.m. and 8:00 p.m. in the summer.


    300x200_07111849_typicalsummer daytstormstemps


    Although not perfectly in sync when the sun's rays are strongest, it is generally the warmest part of the day. It simply takes a while for the sun to heat up the ground which, in turn, then heats up the air.

    Fronts and cool pockets of air moving in aloft can override this timetable.

    So What About Me?

    Given all that we know about the favored spots for thunderstorm formation, we still cannot say with 100 percent certainty that it will or will not rain on you given favored conditions.

    It may be easier to comprehend if you think of a thunderstorm as a living, breathing phenomenon with a relatively short life span of a couple of hours on average.

    Thunderstorms are highly subject to slight changes in atmospheric conditions such as temperature, humidity levels and wind shear. All of these vary by the minute and can mean drastic changes thunderstorm rainfall intensity output and the track of the rainfall on the ground.

    So, if you have been hit by a lot of storms this summer, or missed completely, blame where you live and the nature of the storms themselves and not the weatherman.

    Later this week we will explore some of the problems caused by thunderstorm downpours.


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