Formation of a heat wave
A heat wave develops in Europe in summer when a blocking of the westerlies occurs and high-pressure weather conditions prevail. A blocking high (cut-off high) extends over Eastern Europe or Scandinavia and links with the Azores High. These conditions force warm air masses from northern Africa, Russia or the Balkans towards Central Europe. During the transport the air masses are additionally heated due to high radiation and the foehn effect.
High-pressure weather condition
A typical sign for such weather conditions is a clear blue sky. Hot and dry air cause clouds to disappear rapidly and there is only a small breeze or no wind at all. The stability of the atmosphere causes the high-pressure conditions to last for several days thus further heating the air. One calls a heat wave if air temperatures above 30°C are measured over several consecutive days. As an example, the temperature at Basle station reached 35.7°C during the June 2003 heat wave.
Intense solar radiation heats up the landmasses and the lower layers of the atmosphere thus causing air parcels to rise. Together with the advection of cold air masses in high altitudes the atmosphere becomes unstable. A local warm low-pressure system is formed. If humid air is present this can result in thunderstorms.
What damage can occur?
During a heat wave with dry and hot weather there is almost no precipitation. This can result in dryness or even drought. Plants are no longer growing and, in the worst case, they completely wither. Under such conditions agriculture suffers enormous damage and harvest loss.
Intense solar radiation and increased concentration of nitric oxide due to missing exchange of air masses stimulates the forming of ozone in the lowest atmospheric layers. Elevated ozone concentration irritates the mucous membrane and causes illness of the respiratory system. Therefore, physical activity should be avoided. The heat itself can be dangerous for human beings. Heat can cause heatstroke, sunstroke or circulatory collapse. It is necessary to drink enough during hot weather.