Discovery Science: Earth – Atmosphere – Precipitation and Clouds

Earth Science: Atmosphere – Precipitation and Clouds

Cloud formations and patterns have always greatly inspired humans. They are important for the global distribution of water and required for precipitation, which in turn supports life on Earth.

The Earth’s water is in constant motion. Water vapor rises into the atmosphere and can travel over large distances. The colder the air temperature, the less water vapor it can retain. The point of saturation or “dew point” is when the relative humidity is 100 percent, leading to the condensation of water from vapor into liquid.

This process is facilitated by tiny particles in the air, the condensation nuclei, which act as an attachment point for suspended water molecules. Below freezing point, water vapor sublimates into ice crystals. In both cases clouds become visible. Precipitation occurs when cloud particles become so heavy that they lose their buoyancy.

Clouds are categorized according to their shape and the height at which they occur. Today’s international cloud classification system is based on studies published by the British pharmacist Luke Howard in 1803. There are ten main types of clouds, each with several subcategories and special types. The system for describing these cloud types is based on three cloud height levels. The two basic cloud forms are stratus and cumulus, which are found low in the sky.

The horizontal dimensions of stratus clouds are larger than their vertical reach, while cumulus clouds show a large vertical reach and an almost flat lower limit. Cirrus clouds are icy clouds that, in temperate regions, occur only at very high altitudes between 23,000 and 43,000 feet (7,000 and 13,000 m). The names of the clouds situated at a medium altitude—above about 6,500 feet (2,000 m)-begin with the prefix “alto.” Another type of cloud, nimbus clouds, are dense and dark. This is the type that carries a lot of precipitation.


Precipitation is produced as humid air cools to below the dew point. This can occur when the air rises, such as when two fronts meet (cyclonic precipitation) or there is a ground elevation increase (relief rain or orographic rain). Humid air can also rise freely (convective precipitation).

A liquid droplet is specified ac- cording to its diameter: a droplet is rain when its diameter is greater than 0.019 inch (0.5 mm), drizzle when it is less than 0.007 inch (0.2 mm), and fog when it is between 0.0003 and 0.001 inch (0.01 and 0.04 mm). Solid precipitation, which occurs at colder temperatures, includes snow, sleet, and hail.


CUMULONIMBUS, THE THUNDERCLOUD When warm, hu mid air close to the ground rises and forms cumulonimbus clouds, the updraft pulls positively charged droplets upwards inside the cloud, leaving negatively charged droplets near the base.

As the upper regions of the cloud are altered due to Ice formation, electricity is discharged in a lightning flash The surrounding air Is heated and expands, causing the distinctive sound of thunder.