A recipe for storms

 The Conversation -  The essential conditions for storms to form with heavy downpours are moisture and atmospheric instability.

First, in order for a storm to develop, the air needs to contain enough moisture. That moisture comes from water evaporating off oceans, lakes and land, and from trees and other plants.

The amount of moisture the air can hold depends on its temperature. The higher the temperature, the more moisture air can hold, and the greater potential for heavy downpours. This is because at higher temperatures water molecules have more kinetic energy and therefore are more likely to exist in the vapor phase. The maximum amount of moisture possible in the air increases at about 7% per degree Celsius. 

Warm air also supplies storm systems with more energy. When that vapor starts to condense into water or ice as it cools, it releases large amount of energy, known as latent heat. This additional energy fuels the storm system, leading to stronger winds and greater atmospheric instability.

That leads us to the second necessary condition for a storm: atmospheric instability.

Atmospheric instability has two components: rising air and wind shear, which is created as wind speed changes with height. The rising air, or updraft, is essential because air cools as it moves up, and as a result, water vapor condenses to form precipitation.

As the air cools at high altitudes, it starts to sink, forming a downdraft of cool and dry air on the edge of a storm system.

When there is little wind shear, the downdraft can suppress the updraft, and the storm system quickly dissipates as it exhausts the local moisture in the air. However, strong wind shear can tilt the storm system, so that the downdraft occurs at a different location, and the updraft of warm moist air can continue, supplying the storm with moisture and energy. This often leads to strong storm systems that can spawn tornadoes. More