Precipitation comes in different forms - rain, snow, hail and sleet - each caused by specific atmospheric conditions. For the sake of clarity, fog, on the other hand, is a phenomenon that is not a form of precipitation, but is closely related to the circulation of moisture in the atmosphere.
What types of precipitation are there?
What forms of precipitation do we know and what conditions must be met for this particular form of precipitation to occur?
Rain
The most common form of precipitation is rain. It occurs when atmospheric moisture condenses into water droplets large enough to overcome gravity and fall to the Earth's surface. The basic requirements for rain are a sufficient amount of water vapor in the air and a cooling mechanism, such as air rising in the atmosphere, which leads to condensation.
ice rain
- Warm air layer at altitude: Weather conditions may be such that in higher layers of the atmosphere a layer of warm air lies above a layer of cold air.
- Precipitation in the warm layer: Raindrops form in the warm layer. This layer is warm enough that the rain remains in liquid form even if it later falls through a colder layer.
- Crossing the cold air layer: On their way to the ground, the raindrops pass through a layer of cold air that is close to the ground. However, the cold is not enough to completely freeze the drops. Instead, the drops supercool, meaning they remain liquid even though their temperature is below freezing.
- Impact on surfaces: When these supercooled droplets hit objects on the ground such as trees, roads, cars or buildings that are at or below freezing, they immediately freeze to these surfaces. This leads to the formation of what is known as freezing rain or black ice, which can be very slippery and dangerous.
Snow
Snow forms when air temperatures in the entire layer of the atmosphere, from the ground to the cloud base, are below freezing. Water vapor then condenses directly into ice crystals that grow around aerosol particles. If the crystals are heavy enough, they fall to the ground as snow. An important prerequisite is that the temperature at the ground is also low enough so that the snow does not melt before it reaches the ground.
Hagel
Hail forms in strong thunderstorm clouds, known as cumulonimbus clouds. These clouds have strong updrafts and downdrafts. Water droplets freeze in the high, cold areas of the cloud and then fall to lower layers, where they become larger as more water droplets collect. This can happen several times until the hailstones are too heavy for the updrafts and fall to the ground. Hail mostly occurs during strong thunderstorms and can cause considerable damage.
Sleet
Graupel is similar to hail, but it is made up of smaller ice grains that form when frozen raindrops or snow crystals adhere to other supercooled water droplets. The ice grains have a soft, dimpled surface. Graupel typically forms at temperatures around or slightly above freezing in clouds with strong buoyancy.
And the fog?
Fog is not a form of precipitation, but a band of clouds lying on the ground that causes visibility of less than a kilometer. It is formed when air cools and the moisture it contains condenses into small water droplets. This can happen due to radiation when the sky is clear and the wind is low, or due to contact with a cooling surface. Although fog does not directly cause precipitation, it is an indicator of high humidity, which can lead to various forms of precipitation under certain conditions.
Precipitation therefore occurs in different forms, which are caused by a variety of atmospheric conditions and processes.
Understanding and predicting these conditions are central to meteorological science and have far-reaching implications for agriculture, water management and disaster prevention.