Understanding humidity

What is humidity?

Water in the atmosphere exists in three states - solid (ice), liquid (water), and gas (vapour) - with vapour making up about 99% of atmospheric water. The air we breathe is mostly nitrogen (78%) and oxygen (21%), but also contains small amounts of carbon dioxide, argon, and water vapour.

Warmer air can hold more water vapour than cooler air, provided there’s enough water available. This is why tropical regions are both warm and humid, while polar regions are cold and dry. Some hot areas, like deserts, remain dry due to a lack of available water for evaporation.

Types of humidity

Humidity can be measured in three ways:

Relative humidity (RH)

The most common measure, RH indicates how close the air is to being saturated with water vapour at a given temperature. If RH is 100%, the air is fully saturated. High RH during warm weather makes us feel uncomfortable because sweat doesn’t evapourate easily, hindering the body’s cooling mechanism. In cold weather, high RH can make us feel colder, as water vapour conducts heat away from the skin more efficiently than dry air.

Specific humidity and the mixing ratio

Both measure the amount of water vapour in the air. Specific humidity is the mass of water vapour per mass of air (including both dry air and water vapour), while the mixing ratio compares the mass of water vapour to the mass of dry air. Both are highest near the equator and lowest in polar regions.

Thermal humidity

Dew point and wet bulb temperatures are also used to assess humidity. Dew point is the temperature at which air becomes saturated and water vapour condenses, while wet bulb temperature is measured by evaporative cooling on a thermometer. If either matches the air temperature, RH is 100%.

Why Is humidity important?

Impact on human health

Humidity plays a significant role in thermal comfort—how hot or cold we feel. High humidity in warm weather makes it harder for the body to cool down, increasing the risk of heat stress, which can lead to serious health issues like heatstroke, heart attacks, or breathing problems. This is especially relevant during summer sporting events, where hydration and rest are vital for athletes and spectators.

After prolonged high humidity, rain can bring relief, making the air feel lighter and even improving mood.

Impact on animal health

Animals, such as dairy and beef cattle, also struggle with high humidity in warm conditions. It can cause stress, reduce food intake, affect reproduction, and increase mortality rates.

Humidity and the environment

Water vapour as a greenhouse gas

Water vapour is the most abundant greenhouse gas in the atmosphere. It allows solar energy to warm the Earth but also traps heat, keeping the planet’s surface much warmer than it would be otherwise. Without greenhouse gases, Earth’s average surface temperature would be around -18°C, compared to the current 14.5°C.

Water vapour also helps move energy around the planet. When the Sun evaporates water, it becomes vapour, which can travel around the atmosphere. As vapour condenses back into liquid, it releases heat, fuelling storms and other weather phenomena.

Humidity and rainfall

Humidity directly affects rainfall. The amount of water vapour in the air determines how much rain can fall, especially during heavy showers. Tropical regions, with higher humidity, often experience more frequent and intense rain than places like the UK.

Measuring humidity

Humidity is measured using various instruments:

• Psychrometers: This is the name for paired thermometers—one web bulb thermometer and one dry bulb thermometer. The wet bulb thermometer cools as water evaporates, allowing calculation of relative humidity (RH) and other humidity variables.
• Capacitive sensors: Modern devices that measure voltage which depends on the amount of water vapour in the air, converting this data into RH or dew point temperature.
• Radiosondes and satellites: Humidity above the surface is measured using similar instruments suspended from helium balloons (radiosondes) or inferred from satellite data. GPS signals can also help estimate water vapour by analysing signal timing which is affected by water vapour. quality.

Understanding humidity is essential for predicting weather and appreciating the complex dynamics of our atmosphere.