On Monday, thunderstorms with heavy rain around Berlin and on the Baltic Sea caused fallen trees and flooded basements.
On Monday evening in Ahrensfelde near Berlin, 34 liters of rain per square meter were registered within just one hour, in Rostock-Warnemünde it was 31 liters. In Barth, located between Rostock and Stralsund, the rain was so heavy for a short time that 135 liters would have come together within an hour if it had continued to pour down so intensively. That seems almost tropical, but due to the special atmospheric conditions, much heavier downpours are possible there.
Because the sun is high in the sky, much more solar energy reaches the atmosphere and the ground in the tropics than in the higher latitudes of the northern and southern hemispheres. The atmosphere and the oceans with their global flow system are constantly striving to balance this imbalance. Ideally, warm air flows from the tropics to the poles and correspondingly cold air flows back again.
Now we are on a rotating sphere, which makes this transport difficult and complicated. Because the deflection force of the earth’s rotation becomes stronger the further one moves away from the equator. In the middle latitudes, the temperature equalization is thus brought about by migrating low-pressure areas, which make our weather so changeable. A clearer current pattern can develop over the tropical oceans, thanks to the weak deflection caused by the rotation of the earth and the fairly uniform properties of the water surface.
Where the sun is at its zenith at noon, i.e. vertical, evaporation is greatest over the oceans. The warm, humid air masses rise far into the atmosphere and huge thunderstorm towers form. Since there is no vacuum in the atmosphere, air in the lower layers of the atmosphere flows into the inner tropics from the north-east or south-east, deflected by the rotation of the earth. These trade winds blow very constantly and were used by seafarers in the past for passage to the New World.
The confluence from the north and south-east, which is also known as the convergence, supports the rise in the inner-tropical convergence zone. In higher layers of the troposphere, which is up to 18 km thick in the tropics and only about 12 km thick in Europe, the air flows back into extratropical regions and sinks again over a large area in the area of the subtropical high-pressure belt. For this reason, the driest regions on earth, such as the Sahara, are located at about 20 to 40 degrees north and south latitude.
This entire circulatory system is called the Hadley cell after its discoverer. Over the course of the year, the ITC, as the inner-tropical convergence zone is abbreviated, roughly follows the sun’s highest point over the oceans and is located somewhat north of the equator in northern summer and somewhat south in southern summer.
Due to the enormous energy turnover in the tropics and the almost unlimited available moisture over the oceans, the world’s most violent thunderstorms also develop there. Updrafts of over 100 km/h and hailstones the size of tennis balls are the norm in such thunderstorms, and their spatial extent is many times greater. There can also be much heavier downpours. On Guadeloupe in the Caribbean, for example, 38.1 liters of rain per square meter were registered within just one minute. On average, around 15 liters of water are used per minute in the shower.