Atmospheric Circulation and Weather Systems

Water in the Atmosphere

The atmosphere contains water vapour, which varies from zero to four percent by volume and plays a crucial role in weather phenomena. Water exists in the atmosphere in three forms: gaseous (water vapour), liquid (droplets), and solid (ice crystals). This moisture originates from water bodies through evaporation and from plants through transpiration, creating a continuous exchange of water between the atmosphere, oceans, and continents via evaporation, transpiration, condensation, and precipitation. Humidity is the term used to describe water vapour in the air. It is quantitatively expressed in several ways. Absolute humidity is the actual amount of water vapour present, measured as the weight of water vapour per unit volume of air (grams per cubic metre). The capacity of air to hold water vapour depends entirely on its temperature; warmer air can hold more moisture. Relative humidity is the percentage of moisture present in the atmosphere compared to the maximum amount it can hold at a given temperature. It varies with temperature changes and is generally higher over oceans and lower over continents. When air contains moisture to its full capacity at a given temperature, it is said to be saturated. The temperature at which this saturation occurs is called the dew point. At this temperature, the air cannot hold any additional moisture, leading to condensation if more moisture is added or if the temperature drops further.

• Water vapour in the atmosphere varies from 0 to 4% by volume.
• Water exists in gaseous, liquid, and solid forms in the atmosphere.
• Moisture enters the atmosphere through evaporation and transpiration.
• Absolute humidity is the weight of water vapour per unit volume of air.
• Relative humidity is the percentage of moisture relative to air's capacity at a given temperature.
• Saturation occurs when air holds maximum moisture; dew point is the temperature at saturation.
• 📌 Humidity: The amount of water vapour present in the air.
• 📌 Absolute humidity: Weight of water vapour per unit volume of air (g/m³).
• 📌 Relative humidity: Percentage of moisture present compared to maximum capacity at a given temperature.

Evaporation and Condensation

Evaporation and condensation are key processes that regulate the amount of water vapour in the atmosphere. Evaporation is the transformation of water from liquid to gaseous state, driven primarily by heat energy. The latent heat of vaporisation is the amount of heat required to convert a unit mass of liquid water into vapour without changing its temperature. As temperature increases, the air's capacity to absorb and retain water vapour also increases. When moisture content is low, air has a greater potential to absorb moisture. Air movement replaces saturated air layers with unsaturated ones, enhancing evaporation; thus, greater air movement leads to increased evaporation. Condensation is the reverse process, where water vapour transforms back into liquid water due to loss of heat. When moist air cools to its dew point, it can no longer hold all its moisture, and excess water vapour condenses. Condensation occurs around tiny particles called hygroscopic condensation nuclei, such as dust, smoke, and salt particles, which absorb water. Condensation can also happen when moist air contacts a colder surface or when the temperature approaches the dew point. The process depends on cooling, relative humidity, air volume, temperature, and pressure. Condensation takes place under these conditions: (i) temperature reduces to dew point at constant volume; (ii) both volume and temperature reduce; (iii) moisture is added through evaporation. The most favourable condition is a decrease in air temperature. Following condensation, moisture appears as dew, frost, fog, or clouds, depending on temperature and location. Condensation below freezing point forms frost, while above freezing point, it forms dew.

• Evaporation transforms water from liquid to vapour using heat energy (latent heat of vaporisation).
• Higher temperature increases air's moisture absorption capacity.
• Air movement enhances evaporation by replacing saturated air with unsaturated air.
• Condensation occurs when moist air cools to dew point, converting vapour to liquid.
• Condensation requires condensation nuclei like dust, smoke, and salt particles.
• Forms of condensation include dew, frost, fog, and clouds.
• 📌 Evaporation: Conversion of water from liquid to vapour state.
• 📌 Latent heat of vaporisation: Heat required to convert liquid to vapour without temperature change.
• 📌 Condensation: Transformation of water vapour into liquid water.