Weather Imagery

Meteorologists are not entirely sure what causes the somewhat rare weather event called a heat burst, but anyone who has experienced one can tell you they are quite bizarre. In most cases, the air temperature can increase 20°F or more in a matter of minutes, causing ferocious and damaging winds. What’s even more bizarre is that they almost always occur late at night or in the very early morning hours, which usually catches people by surprise.

While the cause of a heat burst isn’t fully understood, it’s believed that a specific set of weather events must all be in place at the right moment. To begin with, the rain from a decaying thunderstorm must fall through a layer of very dry air. This in itself is uncommon, because in order to make thunderstorms, there needs to be moisture. Second, as the rain falls through the drier air, it evaporates and rapidly cools. This creates a pocket of very dense cool air that is much heavier than its surroundings. Because the air is heavier than it’s surroundings, it rapidly descends. As the air descends, it compresses, thus causing it to warm and at the same time gains downward momentum. This momentum causes it to overshoot its atmospheric equilibrium which further increases compression and warming. If the descending air has enough momentum, it will crash into the ground spreading out like a pancake. Those nearby will feel a substantial increase in temperature and a decrease in humidity until the atmosphere stabilizes, which can take up to an hour.

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The following is a list of record low temperatures for each of the 50 states and on what date that temperature occurred. These temperatures are official and don’t take into account wind chill. All these records are based on 2004 data:

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The following is a list of record high temperatures for each of the 50 states and on what date that temperature occurred. These temperatures are official and don’t take into account heat index. All these records are based on 2004 data:

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Tornadoes don’t really hop, jump or skip. They can pull back up into the clouds and come back down sometime later, but this usually occurs over a fairly large distance. On a smaller scale, people tend to believe tornadoes can hop or jump over one house while totally destroying the one next to it. While this is true that a tornado can completely destroy one house and minimally damage another right next to it, the real reason has nothing to do with a tornado jumping or skipping. It has to do with the internal structure and varying intensity of a tornado.

The funnel of a tornado is sometimes composed of two or more vortexes which are just like smaller tornadoes that spin around in a circle (as seen in the picture to the left). This kind of tornado is called a multiple-vortex tornado and is almost always responsible for narrow paths of extreme destruction. We normally can’t see the individual vortexes because condensation and debris obscure the internal structure and give a tornado that wedge shape appearance.

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With today’s land starved metropolitan cities expanding further into their rural surroundings, a strange consequence occurs that directly affects the local weather, in particular temperatures and rainfall. No, this doesn’t have anything to do with global warming. Instead, the principal involved has to do with how much of the sun’s energy is absorbed as compared to how much is reflected on a much smaller scale.

As a city increases in size, the amount of heat it absorbs from the sun increases. Vegetation and trees which cool the air through shade and evaporation are replaced with tar roof tops, dark colored roads, and asphalt parking lots which absorb more energy. The sun’s energy is then converted to heat which in turn heats the surrounding air. This is referred to

as the “urban heat island affect“. It has been well documented that cities like Houston, TX and Atlanta, GA are on average 7°F to 9°F warmer than their surrounding environments. In addition, tall buildings and other structures can alter wind patterns which can force the air to rise. Temperature differentials and rising air currents help to destabilize the atmosphere which can create clouds and help promote rainfall.

NASA and the University of Arkansas have used satellite mapping and ground-based weather station readings to figure out how widespread this phenomenon actually is. Turns out, Houston and Atlanta have quite a few days in which the “urban heat island affect” triggers thunderstorms. During the 1996 Olympic Games in Atlanta, scientists were allowed the opportunity scrutinize data collected by the National Weather Service’s ground-based weather collection equipment used to predict weather for the athletes. After scrutinizing the data, meteorologists Robert Bornstein and Qing Lu Lin of San Jose State University discovered 5 out of 9 days of precipitation were caused by the urban heat island effect.

Air that is warmer than the surrounding air tends to be more buoyant and as a result it wants to rise. Such is the case when a city’s temperature is 8°F warmer than the surrounding environment. As the warm air rises, cooler air fills in the void of the hot rising air. If the air contains sufficient amounts of moisture and has a high enough relative humidity level, clouds may form. As these clouds drift downwind, they can further develop and produce rain. NASA has proven that urban heat islands increase rainfall amounts on the down-wind side of the city:

Using the world’s first space-based rain radar aboard NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite, Shepherd and colleagues found that mean monthly rainfall rates within 30-60 kilometers (18 to 36 miles) downwind of the cities were, on average, about 28 percent greater than the upwind region. In some cities, the downwind area exhibited increases as high as 51 percent.

Can this phenomenon be reduced or eliminated? It’s unlikely it will be eliminated and there could be some argument in trying to reduce it. By creating more rain days, cities are showing signs of being more “green” due to an increase in vegetation. Considering cities will only get larger, it’s best to try and understand as much about the heat-island affect so that agricultural tracts of land can be better positioned and city engineers could design more efficient irrigation systems.