Introduction to Humidity

Humidity, an important component of Earth’s hydrological cycle, facilitates water change through processes such as evaporation, condensation, and precipitation. These dynamic processes contribute to the formation of dew, frost, fog, clouds, and precipitation, shaping the planet’s climate and weather patterns.

Definition of Humidity

Humidity refers to the presence of water vapor in the air, which is an important element of the atmosphere. Its concentration ranges from 0 to 4%, with the highest levels found in the lower 10 km of the atmosphere. Hygrometers measure humidity, and when the moisture content equals the humidity capacity, the air is considered saturated.

Factors Affecting Humidity

Understanding wettability involves recognizing its direct proportionality with temperature. As the temperature increases, the air’s ability to hold water vapor increases, thereby increasing the humidity capacity. The point at which the air becomes saturated with moisture is known as the dew point. Water in the atmosphere originates from two primary sources: water bodies through evaporation and plants through transpiration.

Types of Humidity

1. Absolute Humidity
Absolute humidity refers to the weight of water vapor per unit volume of air. Unlike relative humidity, absolute humidity remains constant even as the air expands or contracts. Its variation is affected by temperature, hotter air has greater capacity to hold water vapour.

2. Specific Humidity
Specific humidity reflects the ratio of the weight of water vapor to the weight of air. This metric is directly proportional to moisture pressure and inversely related to air pressure, providing insight into the actual amount of moisture present in the air.

3. Relative Humidity
Relative humidity is expressed as a percentage, showing the amount of moisture in the atmosphere compared to its full capacity at a given temperature. Changes in air temperature affect the ability to retain moisture, causing fluctuations in relative humidity. Typically, it is high over oceans and low over continents.

Distribution of Relative Humidity

The distribution of relative humidity exhibits both regional and seasonal patterns. Zonally, relative humidity is highest at the equator and decreases toward the poles, with variations influenced by the apparent movement of the Sun. Seasonally, average relative humidity is higher in summer between 30°N and 30°S, while in higher latitudes, it is higher in winter due to cooler land temperatures.

Phases of Water

Water, which is primarily in the liquid state, can change into gaseous form (water vapor) through evaporation or into solid form (ice) through freezing. These phase changes, facilitated by temperature variations, contribute to the diverse weather conditions experienced globally.


Evaporation, the process of changing water into water vapor, is driven primarily by heat. Latent heat of vaporization refers to the temperature at which water begins to vaporize. The major factors affecting evaporation include humidity, temperature and wind speed. Specifically, the continents have maximum evaporation between 10°N to 10°S latitudes, while the oceans see this phenomenon between 10 to 20 latitudes in both hemispheres.

Factors Controlling Evaporation

Three primary factors affect the rate of evaporation: humidity, temperature, and wind speed. Higher dryness, indicating lower humidity, increases the air’s ability to absorb and retain more moisture, resulting in faster evaporation. Hot water experiences evaporation faster than cold water, because when heated from below, lower layers of cold air become unstable and promote turbulence, facilitating evaporation. Wind speed further affects the rate of evaporation, replacing moist layers with dry layers.


Condensation, the change of water vapor into liquid or solid form, occurs when heat is lost. This process involves three important conditions: the presence of nuclei (fine particles of matter), a drop in air temperature to or below the dew point, and enough water vapor molecules in the air. Additionally, condensation depends on the temperature gradient and the relative humidity of the air.

Forms of Condensation

1. Dew:
Dew, which forms as water droplets on cold surfaces such as stones, grass blades and plant leaves, requires specific conditions such as clear skies, calm air, high relative humidity and cool, long nights. For dew to form the dew point must be above the freezing point.

2. Frost:
Frost consisting of tiny ice crystals forms on cold surfaces when the dew point is at or below the freezing point. Similar to dew, it requires conditions of clear sky, calm air, high relative humidity and low temperatures.

3. Fog:
Fog results from the radiation, movement, and mixing of hot and cold air masses near the Earth’s surface. Different types of fog, such as radiation fog, convection fog, mountain fog and frontal fog, form under different atmospheric conditions.

4. Mist:
Mist, which contains more moisture than fog, occurs when rising warm air meets the cold surface prevailing in mountainous areas.

5. Haze:
Smog, the combination of fog and smoke, contributes to air pollution and poses risks to respiratory health and visibility. The smog that forms before fog persists for a long time.

Ultimately, delving deeper into the complexities of humidity and condensation processes provides a broader understanding of the factors that shape weather patterns and climatic conditions. From the basic concepts of humidity to the diverse forms of condensation, this exploration provides insight into the fascinating dynamics of Earth’s atmospheric phenomena.

Humidity Basics
Humidity is essential to Earth’s water cycle, which affects evaporation and precipitation.
This results in the formation of dew, frost, fog, clouds and rain, creating climate and weather.

Major Humidity Points
As measured by a hygrometer, saturation occurs when the moisture content of the air equals the capacity.
Temperature directly affects humidity; Higher temperature means higher capacity.
Water in the atmosphere is produced by evaporation in water bodies and transpiration of plants.

Types of Humidity
1. Absolute Humidity: The weight of water vapor per unit volume remains constant regardless of the amount of air.
2. Specific Humidity: The ratio of water vapor mass to air mass as affected by humidity and air pressure.
3. Relative Humidity: Indicating the percentage of moisture compared to full capacity, affected by temperature changes.

Relative Humidity Distribution
Highest at the equator, decreasing towards the poles.
Seasonal variations: greater in summer between 30°N and 30°S, and greater in winter in higher latitudes.

Water Phases and Processes
Temperature fluctuations cause water to transition between liquid, gaseous (water vapor) and solid (ice) states.
Evaporation: A heat-driven process affected by humidity, temperature, and wind speed.
Condensation: The change of water vapor into liquid or solid form due to loss of heat.

Forms of Condensation
1. Dew: Water droplets on cold surfaces under specific conditions.
2. Frost: Small ice crystals on cold surfaces under specific conditions.
3. Fog: Resulting from the movement and mixing of hot and cold air masses near the Earth’s surface.
4. Mist: Moist fog occurs when warm air meets cooler surfaces in mountainous areas.
5. Smog: The combination of fog and smoke contributes to air pollution and health risks.

Understanding humidity and condensation processes provides insight into how Earth’s atmospheric dynamics influence weather patterns and climate conditions.

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