The UK posted its highest temperature ever recorded — crossing 40°C.  Parts of France, Spain and Portugal recorded temperatures between 42 and 46 degrees.

Why in news?

• Dozens of towns and regions across Europe reeled under what has been described as a “heat apocalypse”, which has caused widespread devastation this year.
• Wildfires caused by a combination of extreme heat and dry weather have destroyed 19,000 hectares of forest in southwestern France.

What is a Heatwave and when is it declared?

• Heatwaves occur over India between March and June.
• IMD declares a heatwave event when the maximum (day) temperature for a location in the plains crosses 40 degrees Celsius.
• Over the hills, the threshold temperature is 30 degrees Celsius.

How are they formed?

• Heatwaves form when high pressure aloft (3,000–7,600 metres) 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 upper layers of the atmosphere, is the high pressure area.
• Summertime weather patterns are generally slower to change than in winter. As a result, this upper level high pressure also moves slowly.
• Under high pressure, the air subsides (sinks) toward the surface, warming and drying adiabatically, inhibiting convection and preventing the formation of clouds.
• Reduction of clouds increases shortwave radiation reaching the surface.
• A low pressure at the surface leads to surface wind from lower latitudes that brings warm air, enhancing the warming.
• Alternatively, the surface winds could blow from the hot continental interior towards the coastal zone, leading to heat waves.

Following criteria are used to declare a heatwave:

To declare heatwave, the below criteria should be met at least in 2 stations in a Meteorological subdivision for at least two consecutive days and it will be declared on the second day.

a) Based on Departure from Normal

• Heat Wave: Departure from normal is 4.5°C to 6.4°C
• Severe Heat Wave: Departure from normal is >6.4°C

b) Based on Actual Maximum Temperature (for plains only)

• Heat Wave: When actual maximum temperature ≥ 45°C
• Severe Heat Wave: When actual maximum temperature ≥47°C

How long can a heatwave spell last?

• A heatwave spell generally lasts for a minimum of four days.
• On some occasions, it can extend up to seven or ten days.

Impact of Heat Waves

• Heat Strokes: The very high temperatures or humid conditions pose an elevated risk of heat stroke or heat exhaustion.
• Healthcare crisis: Effects from extreme heat are also associated with increased hospitalisations and emergency room visits, increased deaths from cardio-respiratory and other diseases, mental health issues, adverse pregnancy and birth outcomes, etc.
• Productivity loss: Extreme heat also lessens worker productivity, especially among the more than 1 billion workers who are exposed to high heat on a regular basis.
• Risk of Wildfires: The heat domes act as fuel to wildfires, which destroys a lot of land area every year in countries like the US.
• Prevents Cloud Formation: The condition also prevents clouds from forming, allowing for more radiation from the sun to hit the ground.
• Effect on Vegetation: The trapping of heat can also damage crops, dry out vegetation and result in
• Increased Energy Demands: The sweltering heat wave also leads to rise in energy demand, especially electricity, leading to pushing up rates.
• Power Related Issues: Heat waves are often high mortality disasters.
• Infrastructure failure: Avoiding heat-related disasters depends on the resilience of the electrical grid, which can fail if electricity demand due to air conditioning use exceeds supply.

What is behind the extreme heat waves in Europe?

Ans. Climate change, but exactly how

• Scientists are near-unanimous that the heat waves are a result of climate change caused by human activity.
• Global temperatures have already risen by more than 1°C , and studies in the UK had shown that a one degree rise in temperature raises the probability of the country witnessing 40°C by ten times.
• The rising global temperature, which this year led to deviations above the normal by as much as 15 degrees in Antarctica, and by more than 3 degrees in the north pole.

Major factor: Changes in old wind patterns

• In the case of the US, the record temperatures are being linked to changes in the jet stream — a narrow band of westerly air currents that circulate several km above the earth’s surface.
• While a conventionally strong jet stream would bring cooler air from the northern Atlantic, in recent years the jet stream has weakened and split into two.
• This has led to intense and more frequent heat waves over parts of the American continent.

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