Heat burst

In meteorology, a heat burst is a rare atmospheric phenomenon characterized by gusty winds and 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 38 °C (100 °F), sometimes rising by 11 °C (20 °F) or more within only a few minutes. More extreme events have also been documented, where temperatures have been reported to exceed 120 °F (49 °C). 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 the late spring and summer seasons, thunderstorms tend to generate day heating and lose their main energy during the evening hours.^[4] Due to a potential temperature increase, heat bursts normally occur at night. In rare cases, heat bursts have been recorded to occur during the daytime as well. 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 rise, 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 up-draft related fuel.^[6] The raindrops begin to evaporate into dry air, which emphasizes the effects of the heat bursts. As the unsaturated air descends, the air pressure increases. The descending air parcel warms at the dry adiabatic lapse rate of approximately 10° Celsius per 1000 meters (5.5° Fahrenheit 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 weakening, 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 fall, which is cooler than the environment. And lastly, temperature conversion mixed with a downdraft momentum continues downward until the air reaches the ground. The air parcels become warmer than its 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 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]

Some documented cases

• Hobart, Oklahoma, 6-7 July 2016: The temperature rose from 80.6 °F (27.0 °C) just before 11:00 pm CDT, July 6 to 105.8 °F (41.0 °C) at 12:15 am CDT, July 7.^[8]
• Calgary, Alberta, 30 July 2014: Between 10:00 pm, July 29, and 12:00 am, the dew point fell from 12 °C (54 °F) to 0 °C (32 °F), with southwest wind gusts of 85 kilometres per hour (53 mph) at the airport. Meanwhile, the mercury rose from 26 °C (79 °F) to a desert-like 29 °C (84 °F). 31 July 2014: A second heat burst began about 9:30 pm; with the wind gusting to 67 kilometres per hour (42 mph), the dew point falling from 10 °C (50 °F) to 0 °C (32 °F), and the temperature climbing from 26 °C (79 °F) to 29 °C (84 °F)^{[9][10][11][12]}
• Melbourne, Victoria, Australia, 14–15 January 2014: Following a very hot day, decaying thunderstorms produced a heat burst centered over the western suburbs of the city, but affecting most of the urban area. At 10:50 pm Laverton recorded a wind gust of 102 kilometres per hour (63 mph), followed by a rise in temperature from 29.9 to 38.9 °C (85.8 to 102.0 °F) in just over an hour,^[13] while Cerberus station recorded a rise from 24.2 to 32.5 °C (75.6 to 90.5 °F) in 30 minutes and later recorded a second rise from 26.6 to 33.6 °C (79.9 to 92.5 °F) in 46 minutes.^[14] The main Melbourne weather station recorded a smaller rise from 33.6 to 36.4 °C (92.5 to 97.5 °F) in 90 minutes.^[15]
• Grand Island, Nebraska, 11 June 2013: Temperature jumped from 74.2 °F (23.4 °C) to 93.7 °F (34.3 °C) in the 15 minutes between 2:57 and 3:12 AM ^[16]
• Dane County, Wisconsin, 15 May 2013: The National Weather Service reported nearly a 10 °F (5.3 °C) temperature boost that coincided with sustained winds.^[17]
• South Dakota, 14 May 2013: Several "heat bursts" or hybrid "heat burst/wake low" induced wind gusts were observed across portions of northeastern South Dakota. Between 7:00 AM CDT and 8:00 AM CDT temperatures rose from 58 °F (14 °C) to 79 °F (26 °C) and strong winds to 57 miles per hour (92 km/h) were reported.^[18]
• Bussey, Iowa, 3 May 2012: The temperatures shot from about 74 °F (23 °C) to about 85 °F (29 °C) degrees while peak wind gusts jumped from around 15 mph to about 60 mph.^[19][20]
• Torcy, Seine-et-Marne, 29 April 2012 : while an area of low pressure moved from the southwest of France to the northwest, the wind suddenly increased between 10 pm and midnight in areas to the south of Paris. Sustained winds topped 45 km/h (28 mph) at the station of Torcy (Seine-et-Marne) with gusts of up to 110 km/h (69 mph). At the same time, the temperature rose from 13.4 °C (56.1 °F) at 11 pm to 24 °C (75 °F) at midnight. The vertical temperature profile was similar to that observed during dry downbursts, with a very strong helicity (700 m²/s²) and a strong shear (60 kn) but with only a weak instability (CAPE levels of 100 to 200 J/Kg). No thunderstorms developed over the region, however light rain was reported (due to evaporation in dry low level boundary layer). Other stations in the area also experienced the phenomenon but not as dramatically as in Torcy.^[21]
• Atlantic, Iowa, 23 August 2011: The observation at the Atlantic AWOS at 7:25 pm local time had a temperature of 102 °F (39 °C) and a dew point of 7 °F (−14 °C). Three observations prior to this (6:55 pm), the temperature was 88 °F (31 °C) and the dew point was 64 °F (18 °C). The 7 °F (−14 °C) dew point is considered likely to be incorrect, however, as AWOS stations have been known to have issues with dew points in low humidity environments. Scattered wind damage was also reported in association with the heat bursts, with one wind observation as high as 60 mph (97 km/h).^[22][23][24]
• Indianapolis, Indiana, 3 July 2011: Observations around 1:30 am EDT in the area indicated the temperature rose and the dew point dropped nearly 15 °F (8 °C) in less than an hour, causing the relative humidity to drop nearly 40-50 percentage points. Winds increased rapidly, with gusts to near 50 mph (80 km/h). One NWS Indianapolis employee reported that his neighbor's patio furniture ended up in his backyard. The observation site at Eagle Creek Airpark (KEYE) best observed the temperature, dew point, and pressure changes. The site at Indianapolis International Airport (KIND) observed the strongest wind gusts associated with the heat burst.^[25]
• Wichita, Kansas, 9 June 2011: Temperatures rose from 85 to 102 °F (29 to 39 °C) between 12:22 and 12:42 am. The heat burst caused some wind damage (40–50 mph or 64–80 km/h) and local residents reported the phenomenon to area weather stations.^[26]
• Buenos Aires, Argentina, 29 October 2009: After a day with extremely high and unusual temperatures that peaked over 93.9 °F (34.4 °C) (air temperature 101.6 °F (38.7 °C)), at late midnight temperatures rose from 87.8 to 94.2 °F (31.0 to 34.6 °C) in a matter of minutes with wind gusts over 37 miles per hour (60 km/h)^[27]
• Delmarva Peninsula, 26 April 2009: Temperatures rose from 68 to 87 °F (20 to 31 °C) between 10:00 pm and 2:00 am following a series of heat bursts across the Eastern Shore. Double-digit temperature increases were reported from 1:00 to 2:00 am at Salisbury, Maryland (+13), Ocean City, Maryland (+11), and Wallops Island, Virginia (+10).^[28]
• Edmonton, Alberta 18 August 2008: 23:00 (MST)^[29] In the evening temperatures were cooling off after a high of 34.4 °C (93.9 °F). Thunderstorms had formed to the southwest along the foothills, and were moving to the east-northeast.^[30] By 22:37 the Edmonton City Centre Airport the temperature was 22 °C (72 °F), with dew point at 16 °C (61 °F), light rain from the thunderstorm passing the city.^[29][30] Around 23:00 strong gusts of wind from 37 to 57 kilometres per hour (23 to 35 mph) were recorded at the Airport. The temperatures quickly rose to 31 °C (88 °F), and lowered the dew point to 10 °C (50 °F),^[29][31][32] lasting less than an hour. The burst was caused by the thunderstorms dissipating, North and East of the city.^[33]
• Sioux Falls, South Dakota, 3 August 2008: Temperatures rose rapidly from the lower 70 to 101 °F (21 to 38 °C) in a matter of minutes. Wind speeds also rose with gusts up to 50–60 mph (80–97 km/h).^[34]
• Cozad, Nebraska, 26 June 2008: Wind gusts reached 75 miles per hour (121 km/h), as the temperature rose 20 °F (11 °C)^[35] in a matter of minutes.^[36]
• Midland, Texas, 16 June 2008: At 11:25 pm a wind gust of 62 mph (100 km/h) occurred, and the temperature rose from 71 to 97 °F (21.7 to 36.1 °C) in minutes.^[37] (These measurements were taken from miles away, and theories point to 80–100 mph (130–160 km/h) winds in a 2–3-block perimeter.)^[38]
• Emporia, Kansas, 25 May 2008: Reported temperature jumped from 71 to 91 °F (21.7 to 32.8 °C) between 4:44 and 5:11 am (CDT)^[39] as the result of wind activity from a slow moving thunderstorm some 40 miles (64 km) to the southwest.
• Canby, Minnesota, 16 July 2006: A heat burst formed in Western Minnesota, pushing Canby's temperature to 100 °F (37.8 °C), and causing a wind gust of 63 mph (55 kn; 101 km/h). The dew point fell from 70 to 32 °F (21 to 0 °C) over the course of one hour.^[40]
• Hastings, Nebraska, 20 June 2006: During the early morning the surface temperature abruptly increased from approximately 75 to 94 °F (23.9 to 34.4 °C).^[41][42]
• Sheppard Air Force Base Wichita Falls, Texas, 12 June 2004: During late evening the surface temperature abruptly increased from approximately 83 to 94 °F (28.3 to 34.4 °C) and causing a wind gust of 72 mph (63 kn; 116 km/h). The dew point fell from 70 to 39 °F (21.1 to 3.9 °C)^[43][44]
• Minnesota and South Dakota, 26 March 1998: A temperature increase of 10–20 °F (6-11 °C) was reported in the towns of Marshall, Minnesota, Sioux Falls, South Dakota, Brookings, South Dakota, and Montrose, South Dakota during a two-hour period.^[45]
• Oklahoma, 22–23 May 1996: The temperature in the towns of Chickasha rose from 87.6 to 101.9 °F (30.9 to 38.8 °C) in just 25 minutes, while the temperature at Ninnekah rose from 87.9 to 101.4 °F (31 to 39 °C) in 40 minutes. In addition, wind damage was reported as winds gusted to 95 mph (153 km/h) in Lawton, 67 mph (108 km/h) in Ninnekah, and 63 mph (101 km/h) in Chickasha.^[46]
• Barcelona, Catalonia, Spain: (4–5 August 1993 & 2 July 1994) Heat bursts with temperature rises over 13 °C and wind gusts of more than 44 kn at Barcelona–El Prat Airport, record maximum temperature at the airport and the spread of forest fires.^[47]

Extreme cases

These are cases when temperatures over 56.7 °C (134.1 °F) (the highest officially confirmed in the World, in Death Valley, United States, 1913) were recorded during heat bursts.

• Cherokee, Oklahoma, 11 July 1909: at 3:00 in the morning, a heat burst south of Cherokee, Oklahoma reportedly caused the temperature to rise briefly to 136 °F (57.8 °C), desiccating crops in the area.^[48]
• Kopperl, Texas, United States, 1960: A heat burst sent the air temperature to near 140 °F (60 °C), supposedly causing cotton crops to become desiccated and drying out vegetation.^[49]
• Portugal, 6 July 1949: A heat burst reportedly drove the air temperature from 38 to 70 °C (100.4 to 158.0 °F) within two minutes, in the region of Figueira da Foz and Coimbra, in central Portugal.^[50]
• Abadan, Iran, June 1967: An extreme temperature of 87 °C (189 °F) was recorded during a heat burst.^[51]

See also

• Atmospheric thermodynamics
• Chinook wind
• Wake low
• Downburst

References

External links

• P11.13 THE 22–23 May 1996 HEATBURST: A SEVERE WIND EVENT
• What is a Heat Burst?
• The Texas Heat Burst, Others