Atmosphere

Evaporation and condensation are fundamental processes that regulate the amount of water vapour in the atmosphere, facilitating a continuous exchange of water between the atmosphere, oceans, and continents. Evaporation is the process by which water changes from a liquid state to a gaseous state (water vapour). This transformation is primarily driven by heat. The specific amount of heat energy required to convert a unit mass of a liquid into vapour without changing its temperature is known as the latent heat of vaporisation. Evaporation is increased by higher temperatures, lower moisture content in the air (which allows for greater absorption and retention), and increased air movement that replaces saturated air layers with unsaturated ones. Conversely, condensation is the process where water vapour transforms back into a liquid or solid state. This process occurs due to the loss of heat. When moist air cools to a point where its capacity to hold water vapour is exceeded, the excess vapour condenses. In free air, condensation often occurs around very small particles called hygroscopic condensation nuclei, such as dust, smoke, and salt particles from the ocean, which readily absorb water. Condensation can also happen when moist air encounters a colder object or when the temperature drops close to the dew point. The extent of condensation is influenced by the degree of cooling, the relative humidity of the air, the volume of air, temperature, pressure, and overall humidity. The most favorable condition for condensation is a decrease in air temperature. After condensation, the moisture in the atmosphere can take various forms, including dew, frost, fog, and clouds, which are classified based on temperature and location.

Atmospheric visibility phenomena like haze, mist, and fog are elements that significantly affect visibility in the lower atmosphere. Haze is characterized by reduced visibility in regions of lower humidity (less than 75%), typically caused by smoke, dust particles, or unequal light refraction in air of varying densities. Mist, conversely, forms when water vapour condenses into small droplets floating in the air, creating a ground-level cloud, and occurs in wet air with relative humidity exceeding 75%. Fog is a denser form of ground-level cloud, formed by water condensing on dust and other particles, and leads to even lower visibility than mist. Specialized types of fog include smog (thick, smoky fog in industrial areas), hill fog (found on hills, common in mornings), radiation or land fog (resulting from land surface cooling by radiation on clear nights), and sea fog (formed over the sea when cooling occurs or warm and cold ocean currents meet, or cold ocean currents meet hot desert air). These phenomena differ in their causes, humidity conditions, and the extent to which they reduce visibility.

A cloud is defined as a significant mass of minute water droplets or tiny ice crystals, which forms due to the condensation of water vapour in free air at considerable heights above the Earth's surface. When air rises, it is cooled by expansion, and after the dew-point has been reached, this cooling leads to the condensation of water vapour. These tiny droplets, too small to fall as rain or snow (less than 0.01 mm in radius), remain suspended and float as clouds. The formation of clouds at elevated positions leads to their varied shapes. Clouds are primarily classified into four basic types based on their height, expanse, density, and transparency or opaqueness: cirrus, cumulus, stratus, and nimbus. The classification of clouds is also based on a combination of their form, height, and appearance. Cirrus clouds are high-altitude formations, characterized by their thin, detached, and feathery white appearance. Cumulus clouds resemble cotton wool, typically found at medium altitudes, appearing in scattered patches with a flat base. Stratus clouds are layered, covering vast areas of the sky, forming either from heat loss or the mixing of air masses with differing temperatures. Nimbus clouds are dark gray or black, dense, opaque, and can form at middle levels or very close to the ground, appearing as shapeless masses of thick vapour. These four basic types can combine to form more specific cloud classifications, such as high clouds (cirrostratus, cirrocumulus), middle clouds (altostratus, altocumulus), low clouds (stratocumulus, nimbostratus), and clouds with extensive vertical development (cumulus, cumulonimbus).