When air containing water vapor cools, it can reach the dew point temperature. The dew point temperature is the temperature at which gas condensation can begin. The relative humidity of the air is then 100 percent, and the water vapor becomes saturated. The transition of water vapor into droplets is condensation. Sometimes there is a direct transition from the gaseous to the solid state, in which case the process of sublimation takes place. Lowering the temperature to the dew point is the most common cause of condensation. With a further drop in temperature, excess water vapor condenses and air droplets, or ice crystals, are formed. However, in perfectly clean air, condensation would not occur even when the air is very saturated with water vapor. In order for condensation to begin in air saturated with water vapor, another condition is necessary: condensation nuclei must be present in the air.
These are microscopically sized solid particles that are suspended in the air. On the condensation nuclei settle the condensation products of water vapor in the form of water droplets, or ice crystals. Condensation nuclei can be particles of smoke, ash, or dust. However, the most active particles are sea salt particles, which enter the air due to the splashing of sea waves above the water surface. Also very good condensation nuclei are chemical compounds released by human activity during industrial production. Dusts of organic origin, such as pollen, bacteria and mold, also play a large role. The number of condensation nuclei decreases rapidly as altitude increases. The primary causes of condensation are:
- Cooling of air with increasing altitude (adiabatic transformation)
- Cooling of the air due to the thermal radiation of the Earth’s surface to the higher layers of the atmosphere, the contact of warm and moist air masses with the cooled surface of the Earth
- Air saturation with water vapor, and dew point temperature
- Presence of condensation nuclei
The products of condensation are dew, frost, mists, and clouds. Fogs and clouds are characterized by the same structure, however, their causes of formation and also the extent and altitude at which they occur are different. Fog is a collection of products of condensation of water vapor, suspended in the air reaching to the surface of the Earth at the place of observation. With regard to the causes of formation can be distinguished:
- Radiation fogs, which are caused by strong cooling of the Earth’s surface due to nighttime radiated heat. They measure up to several tens of meters in height. Fogs of this type usually form on sunny and low wind nights and mornings. After sunrise, radiation fogs usually disappear. They are most common in autumn, and in winter. They are not found over the beds of large rivers, where there are turbulent air movements against the warmer water surface at night.
- Advection fogs are caused by the influx of warm and moist air over the cooled surface of the Earth. They reach hundreds of meters in height. They are most often formed with the influx of moist air from lower to higher latitudes in winter from over the warm sea toward the cool land, or in summer from over the warm land over the cooler sea. Advection fogs also form over oceanic waters with different thermal properties, with the influx of air masses from over warmer water areas over cooler ones.
- Evaporation fogs are formed by evaporation from the warmer free water surface, rising water vapor, and its condensation in the cooler air. This type of fog is characteristic of autumn evenings and nights over lakes and large rivers.
- Smog (English: smoke, and fog). This is the commonly accepted English term for a specific type of urban fog, formed by the mixing of ordinary fog with smoke and exhaust fumes. It forms in large industrial centers under unfavorable meteorological conditions. The chemical composition of the air is altered over urbanized areas. Industry and energy found in large cities produce huge amounts of pollution. Dust present in the air can become condensation nuclei for clouds, hence over urbanized areas there are also more favorable conditions for increased cloudiness. Often for this reason, smog is just formed over the city. The phenomenon assumes its greatest proportions in London, the Ruhr, Los Angeles, and other large metropolises with a dominant industrial function.
The area most rich in fogs is the Arctic. They occur there an average of 80 days a year. Regions where fogs are also common are the area around New Finland, California, Western Europe, and Northwest Africa. Relatively few fogs are recorded in the interior of major continents, such as Canada and Siberia. In summer, air masses forming in the interior of the continents are poor in water vapor. In the subtropical latitudes of the southern hemisphere, there are places where the number of days with occurrence of fogs exceeds 80. These are the so-called fog deserts such as the Atacama in South America, or the Namib in southern Africa.
Another product of condensation is clouds. They are huge collections of tiny water droplets, or ice crystals floating in the atmosphere. The sizes of water molecules in clouds are so small that they are practically unaffected by gravity. These particles do not fall to the Earth’s surface but move freely making disorderly movements. Clouds, as a result of horizontal movements, enter areas of different humidity. Therefore, they evaporate and disappear or increase in volume.
Clouds occur in the troposphere at different heights, forming specific floors. Clouds can be divided into:
Due to the altitude of occurrence on:
- high floor clouds
- mid-level clouds
- low floor clouds
Due to the shape, clouds are divided into 4 types :
- feathery cirrus clouds
- stratus clouds
- cumulus clouds
- rain clouds- nimbus
Based on their shape, clouds have been divided into 10 types. These types are a combination of the above types.
Cloud classification
| Latin name | abbreviation | Height. [km] | Characteristics |
| Cirrus | Ci | 5-13 | White, delicate clouds, fibrous, do not give precipitation, but announce a change in the weather |
| Cirrocumulus | Cc | The so-called “rams,” layers composed of tufts, announce a change in the weather | |
| Cirrostratus | Cs | A light whitish veil weakening the intense blue of the sky, covers it entirely, or partially | |
| Altocumulus | Ac | 2-8 | flat white, or gray shoals composed of single, or interconnected lobes and clumps. They shade the sun |
| Altostratus | Ace | 4-6 | gray layers of clouds cover the sky almost entirely, give fleeting rainfall, or snowfall |
| Nimbostratus | Ns | 1-3 | Cover the entire sky with a dark gray layer, cause rainfall |
| Cumulonimbus | Cb | 0,5-10 | Clouds bringing thunderstorms, and violent rain and hail. Highly developed with dark color. They contain powerful atmospheric charges. Their shape resembles an anvil |
| Cumulus | Cu | 1-2 | Single white clouds in the shape of clusters with almost horizontal bases. They signal nice weather |
| Stratocumulus | Sc | 0,2-4 | Shoals, or layers of gray lumps with darker fragments. |
| Stratus | St | 0-2 | A gray layer spread low over the Earth’s surface. Essentially homogeneous, it leads to drizzle precipitation |
Considering the genesis of clouds, they can be divided into:
- Convective clouds: they are formed as a result of unstable thermal equilibrium, which leads to the formation of strong ascending currents with an average speed of 3-6 m/s. The ascending air cools down, and once the condensation level is exceeded, Cumulus clouds begin to form. If the unstable equilibrium continues then these clouds grow vertically and continue to develop often transforming into Cumulonimbus clouds. Clouds of this type develop most intensively in the summer half of the year over land.
- Wave-like clouds: they are formed in solid equilibrium masses. Clouds of this type form when there is intense air flow over mountain ranges, forming so-called orographic clouds. Clouds of this type are Stratus, Stratocumulus, and Altocumulus. Most clouds of this type can be observed at night and in the morning.
- Clouds associated with the passage of a front: due to the sliding of warm air above the cool air, its mass is cooled and the water vapor contained in it is condensed, leading to the formation of extended clouds in different floors.
