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A heat wave is a prolonged period of excessively hot weather, which may be accompanied by high humidity, especially in oceanic climate countries. While definitions vary, a heat wave is measured relative to the usual weather in the area and relative to normal temperatures for the season. The definition recommended by the World Meteorological Organization is when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 °C.
Formation of Heat Waves
Heat waves can form in many ways. They often form when high pressure aloft (from 10,000–25,000 feet (3,000–7,600 meters) strengthens and remains over a region for several days up to several weeks. This is common in summer (in both Northern and Southern Hemispheres) as the jet stream 'follows the sun'. On the equator side of the jet stream, in the middle layers of the atmosphere, is the high pressure area. Summertime weather patterns are generally slower to change than in winter. As a result, this mid-level high pressure also moves slowly. Under high pressure, the air subsides (sinks) toward the surface. This sinking air acts as a dome capping the atmosphere. This cap helps to trap heat instead of allowing it to lift. Without the lift there is little or no convection and therefore little or no convective clouds (cumulus clouds) with minimal chances for rain. The end result is a continual build-up of heat at the surface that we experience as a heat wave.
The heat wave can negatively impact the safety of aviation operations. To mention some of the most significant issues associated with heat waves:
- Aircraft performance decreases at high temperatures.
- High temperatures can damage the airport ground surfaces.
- Extreme heat stresses the vehicle cooling systems which could have negative impact on ground operations, etc.
High temperatures and heat waves in the last decade are widely blamed on climate change that occurred over the last 50 years, amounting to global warming of about 0.5°C, according to the study in the journal Environmental Research Letters. Heath waves are predicted to become harsher and more frequent as the Earth continues to warm over the course of the 21st century.
- Defining heatwaves: heatwave defined as a heat impact event servicing all community and business sectors in Australia, a technical report by J. Nairn and R. Fawcett for the Australian Government Bereau of Meteorology, March 2013
- Heat Index, JetStream – online school for weather, NOAA