Humidity
Humidity is the measure of water vapor present in the air, expressed either as absolute humidity (mass of water per unit volume of air) or relative humidity (percentage of water vapor relative to the maximum possible at that temperature). Warmer air can hold more water vapor, so the same amount of moisture feels "drier" on a hot day than a cool day, which is why relative humidity is the most commonly reported measure. High humidity reduces the body's ability to cool itself through sweating, making heat feel more intense and posing health risks during heatwaves.
Key Formula
RH (%) = (e / e_s) × 100
LaTeX: RH = \frac{e}{e_s} \times 100\%
| Symbol | Meaning | Unit |
|---|---|---|
| RH | Relative Humidity | % |
| e | Actual vapor pressure of air | hPa |
| e_s | Saturation vapor pressure at same temperature | hPa |
Worked Example
Problem
The actual vapor pressure of air at 25°C is 16 hPa. The saturation vapor pressure at 25°C is 31.67 hPa. Calculate the relative humidity.
Solution
Step 1: Identify values: e = 16 hPa, e_s = 31.67 hPa. Step 2: Apply RH formula: RH = (16 / 31.67) × 100%. Step 3: Calculate: RH = 0.505 × 100% = 50.5%.
Answer
Relative Humidity = 50.5%
Humidity Levels and Human Comfort
| RH Range (%) | Sensation | Effect on Comfort | Common Location |
|---|---|---|---|
| 0–20 | Very Dry | Skin and throat irritation | Desert regions |
| 20–40 | Dry | Comfortable in cool weather | Continental interiors |
| 40–60 | Comfortable | Optimal for human health | Temperate climates |
| 60–80 | Humid | Muggy, perspiration inefficient | Coastal tropics |
| 80–100 | Very Humid | Oppressive; heat stroke risk | Equatorial rainforests |
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Related Terms
Dew Point
The dew point is the temperature to which air must be cooled, at constant pressure and water vapor content, for saturation to occur and dew or fog to form. When air cools to its dew point, relative humidity reaches 100% and water vapor begins condensing onto surfaces as liquid water. The dew point is a direct measure of atmospheric moisture content: a higher dew point means more moisture in the air and is used to predict fog, frost, and thunderstorm intensity.
Precipitation
Precipitation is any form of water — liquid or solid — that falls from the atmosphere to the Earth's surface, including rain, snow, sleet, and hail. It forms when water vapor in clouds condenses or freezes around tiny particles called condensation nuclei, growing until droplets or crystals become heavy enough to fall. Precipitation is a critical component of the hydrological cycle, replenishing freshwater supplies in rivers, lakes, and groundwater systems.
Atmospheric Pressure
Atmospheric pressure is the force per unit area exerted by the weight of the overlying column of air in the atmosphere on any surface below it. At sea level, the standard atmospheric pressure is 101,325 Pa (or 1013.25 hPa / 1 atm), and it decreases with altitude as the mass of air above decreases. Atmospheric pressure differences drive wind and weather systems: low-pressure zones are associated with storms and clouds, while high-pressure zones bring clear, calm conditions.
From Latin "humiditas," meaning "moisture" or "dampness," derived from "humidus" (moist) and ultimately from "humus" (earth, soil). The term has been used in meteorology since the 17th century.