Heat burst

Dew point Wikipedia:Link rot Bureau of Meteorology

In meteorology, a heat burst is a rare atmospheric phenomenon characterized by gusty winds along with a rapid increase in temperature and decrease in dew point (moisture). Heat bursts typically occur during night-time and are associated with decaying thunderstorms.[1]

Although this phenomenon is not fully understood, it is theorized that the event is caused when rain evaporates (virga) into a parcel of cold, dry air high in the atmosphere- making the air denser than its surroundings.[2] The parcel descends rapidly, warming due to compression, overshoots its equilibrium level and reaches the surface, similar to a downburst.[3]

Recorded temperatures during heat bursts have reached well above 40 °C (104 °F), sometimes rising by 10 °C (18 °F) or more within only a few minutes. More extreme events have also been documented, where temperatures have been reported to exceed 50 °C (122 °F). However, such extreme events have never been officially verified. Heat bursts are also characterized by extremely dry air and are sometimes associated with very strong, even damaging, winds.

Characteristics

In general, heat bursts occur during the late spring and summer seasons. During these times, thunderstorms tend to generate due to day heating and lose their main energy during the evening hours.[4] Due to a potential temperature increase, heat bursts normally occur at night; however, heat bursts have also been recorded to occur during the daytime. Heat bursts have lasted for times spanning from a couple of minutes to several hours. The rare phenomenon is usually accompanied by strong gusty winds, extreme temperature changes, and an extreme decrease in humidity. They occur near the end of a weakening thunderstorm cluster. Dry air and a low-level inversion are also present during the storm.[5]

Causes

As the thunderstorm starts to dissipate, the layer of clouds start to rise. After the layer of clouds have risen, a rain-cooled layer remains. The cluster shoots a burst of unsaturated air down towards the ground. In doing so, the system loses all of its updraft related fuel.[6] The raindrops begin to evaporate into dry air, which emphasizes the effects of the heat bursts (evaporation cools the air, increasing its density). As the unsaturated air descends, the air pressure increases. The descending air parcel warms at the dry adiabatic lapse rate of approximately 10 °C per 1000 meters (5.5 °F per 1000 feet) of descent. The warm air from the cluster replaces the cool air on the ground. The effect is similar to someone blowing down on a puddle of water. On 4 March 1990, the National Weather Service in Goodland, Kansas detected a system that had weakened, containing light rain showers and snow showers. It was followed by gusty winds and a temperature increase. A heat burst was being observed. The detection proved that heat bursts can occur in both summer months and winter months. The occurrence also proved that a weakening thunderstorm was not needed in the development of heat bursts.

Heat bursts are a variety of downburst.

Forecasting

The first step of forecasting and preparing for heat bursts is recognizing the events that come before heat bursts occur. Rain from a high convection cloud falls below cloud level and evaporates, cooling the air. Air parcels that are cooler than the surrounding environment fall. And lastly, temperature conversion mixed with a downdraft momentum continue downward until the air reaches the ground. The air parcels then become warmer than their environment. McPherson, Lane, Crawford, and McPherson Jr. researched the heat burst system at the Oklahoma Mesonet, which is owned by both the University of Oklahoma and Oklahoma State University. The purpose of their research was to discover any technological benefits and challenges in detecting heat bursts, document the time of day and year that heat bursts mostly occur, and to research the topography of where heat bursts mostly occur in Oklahoma. Scientists and meteorologists use archived data to manually study data that detected 390 potential heat burst days during a fifteen-year period. In studying the archived data, they observed that 58% of the potential days had dry-line passages, frontal passages or a temperature change. The temperature change was due to an increase in solar radiation in the hours of the morning or a daytime precipitation weather system. By studying the archived data, the scientists' have the ability to determine the beginning, peak and end of heat burst conditions. The peak of heat burst conditions is the maximum observed temperature. The beginning of the heat burst occurrence is the time when the air temperature began to increase without decreasing until after the heat burst. The end of the heat burst is when the system ceased to affect the temperature and dew point of the area. In addition to researching the life cycle and characteristics of heat bursts, a group of scientists concluded that the topography of Oklahoma coincided with the change in atmospheric moisture between northwest and southeast Oklahoma. An increase in convection normally occurs over the United States High Plains during the late spring and summer. They also concluded that a higher increase in convection develops if a mid-tropospheric lifting mechanism interacts with an elevated moist layer.[7]

Documented cases

See also

References

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