The Hydrological Cycle – Environmental Encyclopedia

The Earth and the atmosphere around it contain large amounts of water. About 97% of the water on Earth is salty, the rest, 3%, is in the form of ice. Only 0.7% of the Earth’s water is fresh and occurs in the form of lakes, rivers, aquifers and steam.

Surface waters are an essential component of the water cycle in the troposphere. These constitute an environment in which natural resources undergo the physical, chemical and biological processes that govern their physical evolution. Water in the biosphere transports matter and energy, supports the development of life, is part of inert matter in varying proportions, and is an essential and major component of living matter.

The first phase of the hydrological cycle is evaporation. This happens on the surface of the sea, especially in warm areas. This water in the form of vapor gas, passes into the atmosphere causing the formation of clouds which will be responsible for precipitation.

Clouds are condensed forms of atmospheric moisture made up of tiny water droplets or tiny ice crystals. These are the main visible atmospheric phenomena. As such, they represent a transitory, albeit vital, step in the water cycle. This cycle includes the evaporation of moisture from the Earth’s surface, its transport to higher levels of the atmosphere, the condensation of water vapor in cloud masses and the eventual return of water to the earth in the form of rain and snow. .

hydrological cycle

In meteorology, cloud formation due to cooling air causes invisible water vapor to condense into visible droplets or ice particles. The particles that make up the clouds have a size that varies between 5 and 75 microns, 0.0005 cm and 0.008 cm. The particles are so small that they are held in the air by light vertical currents.

Differences in cloud formations stem, in part, from different condensation temperatures. When ice occurs at temperatures below freezing, clouds are often made up of ice crystals. Those that form in warmer air are usually made up of water droplets. However, “supercooled” clouds sometimes contain water droplets at sub-freezing temperatures.

Air movement associated with the development of clouds also affects their formation. Clouds created in calm air tend to appear in layers or stratus. Those that form between the winds or the air with strong vertical currents have a large vertical development.

heat losses

Clouds play a very important role since they modify the distribution of solar heat on the surface of the earth and in the atmosphere. In general, since the reflection from cloud tops is greater than that from the Earth’s surface, the amount of solar energy reflected back to space is greater on cloudy days. Although most solar radiation is reflected by the upper layers of clouds, some radiation penetrates to the Earth’s surface, which absorbs and re-emits it. Cloud bottoms are opaque to this long-wave terrestrial radiation and reflect it back to Earth.

The result is that the lower atmosphere generally absorbs more thermal energy on cloudy days due to the presence of this trapped radiation. In contrast, on a clear day, the Earth’s surface initially absorbs more solar radiation, but this energy is dissipated very quickly by the absence of clouds. Regardless of other related weather effects, the atmosphere absorbs less radiation on a clear day than on a cloudy day.

Cloud cover has a considerable influence on human activities. Rain, vital for the production of food plants, comes from the formation of clouds. In the early days of aviation, visibility was affected by clouds. With the development of instrument flight which allows the pilot to navigate in a large cloud, this obstacle has been alleviated.

The first scientific study of clouds was made in 1803, when British meteorologist Luke Howard devised a method of classifying clouds. This was followed by the publication, in 1887, of a classification system which later served as the basis for the famous International Cloud Atlas of 1896. This atlas is regularly revised and amended and is used worldwide.

Cloud cover can influence human activities

Clouds are generally divided into four main families based on their height: high clouds, medium clouds, low clouds, and vertically developing clouds. The latter can be extended to any height. These four divisions can be subdivided into genus, species and variety, describing in detail the appearance and mode of formation of clouds. More than a hundred different types of clouds are distinguished.

We describe here only the main families and the most important genera:

  • High clouds are composed of ice particles, located at average altitudes of 8 kilometers above the earth. This family contains three main genera:
    • Cirrus clouds are isolated, feathery and threadlike in appearance, often hooked or bushy, and arranged in bands.
    • Cirrostratus appears as a thin whitish veil; sometimes they present a fibrous structure and, when they are located between the observer and the Moon, they give rise to halos.
    • Cirrocumulus clouds form small cotton-like white clumps and globes; They are placed in groups or rows.
    • The shapes of cirrus clouds obey the ice in suspension

  • Medium clouds are composed of water droplets, have a variable altitude, between 3 and 6 kilometers above the ground. This family includes two main genera:
    • The upper strata look like thick gray or blue veils, through which the Sun and Moon are seen only faintly, as through translucent glass.
    • Altocumulus clouds look like dense, cottony, fluffy globes that are slightly larger than cirro cumulus clouds. Sun and Moon passing through them can produce a corona, or colored ring, much smaller in diameter than a halo.
  • Low clouds are also composed of water droplets, they usually have an altitude of less than 1.6 kilometers. This group includes three main types:
    • Stratocumulus clouds are large rolls of clouds, clear in appearance and gray in color. They often cover the entire sky. Because the cloud mass is generally not thick, patches of blue sky often appear in the cloud ceiling.
    • Nimbostrata are thick, dark and shapeless. These are precipitation clouds, from which it almost always rains or snows.
    • Stratus are upper layers of fog. They appear, in the form of a flat white mantle, at altitudes generally below 600 meters. When they are fractured by the action of rising warm air, a clear blue sky is seen.
    • Dense layer of stratocumulus cloud covering the sky of a city

  • Vertically developing clouds reach altitudes ranging from less than 1.6 to 13 kilometers above the ground. This group includes two main types:
    • Cumulus clouds are shaped like domes or skeins of wool. They are usually observed in the middle and at the end of the day, when the solar heat produces the vertical air currents necessary for their formation. The lower part is usually flat and the upper part rounded, resembling a cauliflower.
    • Cumulonimbus clouds are dark and heavy in appearance. They rise high, like mountains, sometimes showing a veil of ice clouds, false cirrus clouds, in the shape of an anvil at their tops. These thunderclouds are often accompanied by violent and intermittent showers.
  • An anomalous cloud group is a group that includes pearly clouds, with altitudes between 19 and 29 kilometers, and noctilescent clouds, with altitudes between 51 and 56 kilometers. These very thin clouds can only be seen between sunset and sunrise at high latitudes.

Wastewater treatment group. Polytechnic University School. University of Seville.

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