Clouds are formed from a large number of tiny water droplets and small ice crystals that result from the change of state of water vapor into liquid and solid within a mass of air. The air mass rises and cools until it becomes saturated and condenses into water droplets. When the cloud is laden with water droplets and environmental conditions are favorable, precipitation occurs in the form of ice, snow, or hail.
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How is precipitation formed?
When air at the surface heats up, it rises. The troposphere's temperature decreases with altitude; that is, the higher we go, the colder it gets. Therefore, when the air mass rises, it encounters cooler air and becomes saturated. Upon saturation, it condenses into tiny water droplets or ice crystals (depending on the surrounding air temperature) and forms around small particles less than two microns in diameter called hygroscopic condensation nuclei.
When water droplets cling to condensation nuclei and surface air masses continue to rise, a vertically developing cloud forms. This is because the amount of air that becomes saturated and condenses is so great that it eventually increases in height. This type of cloud, which forms due to atmospheric instability, is called a cumulus humilis . As it develops vertically and reaches a considerable thickness (enough to block almost all solar radiation), it is called a cumulonimbus.
For the vapor present in a mass of air that reaches saturation to condense into droplets, two conditions must be met: the first is that the mass of air has cooled sufficiently , and the second is that there are hygroscopic condensation nuclei in the air on which water droplets can form.
Once clouds have formed, what determines whether or not they produce rain, hail, or snow—that is, any type of precipitation? The tiny droplets that make up the cloud, suspended within it by updrafts, begin to grow by absorbing other droplets as they fall. Two forces act upon each droplet: the drag exerted by the updraft and the droplet's own weight.
When the droplets are large enough to overcome the drag force, they will rush to the ground. The longer the water droplets spend in the cloud, the larger they become, as they add to other droplets and other condensation nuclei. In addition, they also depend on the time the droplets spend ascending and descending in the cloud and the greater the total amount of water that the cloud has.
Types of precipitation
Types of precipitation are determined by the shape and size of the water droplets that fall under the right conditions. These can include drizzle, showers, hail, snow, sleet, rain, etc.
Drizzle
Drizzle consists of light precipitation with very small water droplets that fall evenly. These droplets typically don't wet the ground much and their intensity depends on other factors such as wind speed and relative humidity. If you'd like to learn more about the different types of rain , this link may be helpful.
Showers
Showers are characterized by larger raindrops that typically fall violently and over a short period. They usually occur in areas where atmospheric pressure drops, creating a low-pressure system known as a cyclone. Showers are associated with cumulonimbus clouds that form very quickly, causing the water droplets to grow large and often resulting in torrential rain.
Hail and snowflakes
Precipitation can also occur in solid form. For this to happen, ice crystals must form at the top of the cloud at very low temperatures, around -40°C . These crystals can grow from tiny water droplets at very low temperatures that freeze onto them (the beginning of hail formation) or by joining together with other crystals to form snowflakes. When they reach a suitable size, and due to gravity, they can fall from the cloud, resulting in solid precipitation at the surface, if the environmental conditions are right.
Sometimes snowflakes or hailstones that fall from a cloud, if they encounter a layer of warm air as they descend, melt before reaching the ground, eventually resulting in liquid precipitation. If you're interested in snow, you can learn more about orographic rainfall here.
Forms of precipitation and types of clouds
The type of precipitation depends fundamentally on the environmental conditions in which the cloud forms and the type of cloud that forms. In this case, the most common precipitations are frontal, orographic and convective or stormy types.
Frontal precipitation occurs when clouds are associated with fronts, both warm and cold. The intersection of a warm front and a cold front creates clouds that produce frontal precipitation. A cold front forms when a mass of cold air pushes and displaces a warmer air mass upward. As it rises, it cools, leading to cloud formation. In the case of a warm front, a warm air mass moves over a colder air mass.
When a cold front forms, the cloud type that typically develops is a cumulonimbus or altocumulus . These clouds tend to have greater vertical development and, therefore, produce heavier and more intense rainfall. Furthermore, the raindrops are much larger than those that form during a warm front. If you'd like to learn more about the formation of low-pressure systems, you can do so here.
Clouds that form along a warm front tend to be more layered and are usually nimbostratus , stratus , or stratocumulus . Precipitation along these fronts is typically lighter, often in the form of drizzle.
In the case of precipitation from thunderstorms, also called 'convective systems', the clouds are highly developed ( cumulonimbus ) and therefore produce intense, short-lived , and often torrential rainfall. To learn more about this phenomenon, you can consult the section on convective rainfall.
How precipitation is measured
To measure the amount of rain or snow that has fallen in a certain area and in a given time interval, there is a rain gauge. It is a kind of deep funnel-shaped glass that sends the collected water to a graduated container where the total amount of rain that falls accumulates.
Depending on where the rain gauge is located, there may be external factors that alter the correct measurement of precipitation. These errors can be the following:
- Lack of data: The series can be completed by correlation with other nearby stations that have a similar topographic situation and are in climatologically homogeneous zones.
- Accidental mistakes: random error, a specific data shows an error but it does not repeat itself (some water falls during the measurement, printing errors, etc.). They are difficult to detect although an isolated error will not affect a general study with values ​​of a long period.
- Systematic errors: they affect all station data during a certain time interval and always in the same direction (for example, bad station location, use of inappropriate probes, change of station location, change of observer, bad state of the apparatus).
To prevent raindrops from splashing onto the outer edge of the rain gauge, it is constructed with beveled edges. It is also painted white to reduce the absorption of solar radiation and minimize evaporation . If the channel through which the water falls into the container is made narrow and deep, the amount of water that evaporates is reduced, making the measurement of total precipitation as close as possible to the actual amount.
In mountain areas, where it is common for precipitation to be in solid form (snow) or for temperatures to drop below the freezing point of water, some type of product is usually included in the deposit (usually anhydrous calcium chloride) whose function is to reduce the value of the temperature at which the water would solidify.
It must be taken into account that the position of the rain gauge can affect its measurement. For example, if we place it near buildings or near trees.
The volume of rainwater collected is measured in liters per square meter (l/m2) or, equivalently, in millimeters (mm). This measurement represents the height, in millimeters, that a layer of water would reach if it covered a horizontal surface of one square meter.
With this information you will be able to know more about the rains, types of rain and better understand the weather man.
