Humidity

1.0What is Humidity?

Humidity is the amount of water vapour present in the air. It indicates how much moisture the atmosphere contains at a given time and is a major factor influencing our daily weather and comfort.

In simple terms, humidity in weather refers to how “moist” or “dry” the air feels around us. The higher the moisture, the higher the humidity level.

Humidity Definition

Scientifically, humidity is defined as the mass of water vapour per unit mass of dry air in the atmosphere. It is an important meteorological parameter because it affects rainfall, cloud formation, temperature, and human comfort.

When the air holds a large amount of water vapour, it is called humid air, and when it contains little water vapor, it is called dry air.

2.0Importance of Humidity in the Atmosphere

Humidity is important because it influences:

• Cloud formation and rainfall
• Temperature control by absorbing and releasing heat
• Human comfort levels
• Growth of crops and vegetation
  Without humidity, there would be no rain, dew, or fog — all of which are essential for maintaining Earth’s water cycle.

3.0Water Vapour and the Air We Breathe

Formation of Water Vapour

Water vapour enters the atmosphere through evaporation from oceans, lakes, rivers, and soil, and through transpiration from plants. When sunlight heats up water, it changes from a liquid to a gas and mixes with the air.

Role of Evaporation and Condensation

• Evaporation: Converts liquid water into vapour, increasing humidity.
• Condensation: Converts vapour back into liquid droplets when air cools, forming clouds and precipitation.
  These two processes maintain the balance of humidity in the atmosphere and are vital parts of the water cycle.

4.0Types of Humidity

Humidity can be measured in several ways, depending on how water vapor is expressed in the air.

Absolute Humidity

Absolute humidity refers to the actual amount of water vapour present in a given volume of air, usually expressed in grams per cubic meter (g/m³).
For example, if 10 grams of water vapour are contained in 1 cubic meter of air, the absolute humidity is 10 g/m³.
However, it changes with temperature — warmer air can hold more water vapour than cooler air.

Relative Humidity

Relative humidity (RH) is the most commonly used term in weather reports. It compares the current amount of water vapour in the air to the maximum amount it can hold at that temperature.
It is expressed as a percentage (%):

$\text{Relative Humidity}=\frac{\text{Actual Water Vapor}}{\text{Maximum Water Vapor}}\times 100$

• High RH (above 70%) means the air is nearly saturated — feels sticky and uncomfortable.
• Low RH (below 30%) means the air is dry — can cause dry skin and irritation.

Specific Humidity

Specific humidity measures the mass of water vapour in the air compared to the mass of moist air (including both dry air and water vapour).
It remains relatively constant, even with altitude changes, and is often used in meteorological studies.

Formula for humidity ratio:

where,

P_v = vapor pressure of water

P = total atmospheric pressure

5.0Measurement of Humidity

Hygrometer and Psychrometer

Humidity is measured using instruments such as:

• Hygrometer: Directly measures the amount of water vapor in the air.
• Psychrometer: Contains two thermometers — one dry bulb and one wet bulb.
• The difference in readings between the two bulbs helps calculate relative humidity.
• When the air is very humid, the temperature difference between the two bulbs is small.

Dew Point and Its Significance

The dew point is the temperature at which air becomes fully saturated with water vapor, and condensation begins to form dew or fog.

• A high dew point indicates humid air.
• A low dew point indicates dry air.
  Meteorologists often use dew point values to predict rainfall, fog formation, and heat index (how hot it feels).

Humidity Formula and Its Application

There are three main formulas used in meteorology to calculate humidity:

1. Absolute Humidity (AH):
   where m_v​ = mass of water vapor (grams), V = volume of air (m³).
2. Relative Humidity (RH):
   
3. Humidity Ratio (W):
   

These formulas help meteorologists predict weather patterns, cloud formation, and precipitation levels.

6.0Factors Affecting Humidity

Temperature: Temperature is the most important factor influencing humidity.

Warm air can hold more water vapor, increasing humidity levels.

• When air cools, its capacity to hold moisture decreases, leading to condensation and cloud formation.

Geographical Location and Altitude

• Coastal regions usually have high humidity due to proximity to oceans and seas.
• Deserts and high-altitude areas have very low humidity because of dry air and minimal water sources.
• Tropical regions experience consistently high humidity, while polar regions have extremely low humidity.

Wind and Air Movement

Winds affect humidity by transporting moist or dry air from one place to another.

• Moist winds (from oceans) increase humidity.
• Dry winds (from land areas) lower humidity levels.
  Additionally, strong winds promote evaporation, slightly reducing local humidity.

7.0Effects of Humidity on Weather and Climate

Impact on Human Comfort and Health

Humidity directly influences how hot or cold the air feels to humans.

• High humidity reduces the evaporation of sweat from our skin, making us feel warmer than the actual temperature (heat index).
• Low humidity can cause dry skin, sore throat, cracked lips, and dehydration.
  High indoor humidity also promotes mold growth and dust mites, triggering allergies and respiratory issues.

Influence on Rainfall and Cloud Formation

Humidity determines whether clouds will form and rain will fall. When air becomes saturated with water vapor, it cools and condenses into water droplets that form clouds.
If these droplets grow large enough, they fall as rain, snow, or hail, depending on temperature conditions.
Thus, humidity is the driving force behind precipitation.

Agricultural and Environmental Impact

Humidity affects crop growth, soil moisture, and pest development.

• Crops like rice require high humidity, while wheat and maize grow better in moderate humidity.
• Too much humidity may lead to fungal diseases in plants.
  In forests and ecosystems, humidity helps maintain plant transpiration and water balance, vital for biodiversity.

8.0Control and Regulation of Humidity

Artificial Control (Dehumidifiers & Air Conditioners)

In homes, offices, and laboratories, humidity is controlled using:

• Dehumidifiers – Reduce excessive moisture from indoor air.
• Air conditioners – Maintain comfortable humidity levels by cooling and drying the air. Maintaining 40–60% relative humidity indoors is ideal for comfort and health.

Natural Regulation by Plants and Ecosystems

Plants naturally help balance humidity through transpiration — the release of water vapor from leaves. Forests and green spaces act as natural humidifiers, contributing to the moisture balance of the environment.