Relative Humidity Calculator
Calculate relative humidity from air temperature and dew point temperature
What is Relative Humidity?
Relative humidity (RH) is a measure of how much water vapour is present in the air compared to the maximum amount the air can hold at a given temperature. It’s expressed as a percentage, with 100% indicating that the air is completely saturated with water vapour.
The relationship between temperature and humidity is crucial for understanding weather patterns, comfort levels, and various industrial applications. Warmer air can hold more water vapour than cooler air, which is why relative humidity changes with temperature even when the absolute amount of moisture remains constant.
How to Calculate Relative Humidity
Relative humidity can be calculated using the Magnus formula, which relates air temperature, dew point temperature, and relative humidity. The formula used in our calculator is:
RH = 100 × {exp[17.625 × Dp/(243.04 + Dp)]/exp[17.625 × T/(243.04 + T)]}
Where:
- RH = Relative Humidity (%)
- T = Air Temperature (°C)
- Dp = Dew Point Temperature (°C)
- 17.625 and 243.04 = Magnus coefficients
Humidity Comfort Levels
Low Humidity
Below 30%
May cause dry skin, static electricity, and respiratory discomfort
Comfortable
30-50%
Ideal range for human comfort and health
High Humidity
Above 60%
Can feel muggy, promote mould growth, and reduce cooling efficiency
Applications of Relative Humidity
Weather Forecasting
Meteorologists use relative humidity measurements to predict weather patterns, fog formation, and precipitation likelihood. High humidity often indicates the potential for rain or thunderstorms.
Indoor Air Quality
Maintaining proper humidity levels indoors is essential for comfort and health. Too low humidity can cause respiratory issues, whilst too high humidity can promote mould growth and dust mites.
Industrial Processes
Many industries, including pharmaceuticals, food processing, and electronics manufacturing, require precise humidity control to maintain product quality and prevent damage.
Agriculture and Horticulture
Plants require specific humidity levels for optimal growth. Greenhouse operators monitor and control humidity to maximise crop yields and prevent plant diseases.
Measuring Humidity in the UK
The UK’s temperate maritime climate means humidity levels vary significantly throughout the year. Summer months typically see higher humidity levels, especially in coastal areas, whilst winter can bring very dry conditions indoors due to heating systems.
Common humidity measurement tools include:
- Digital hygrometers – Electronic devices providing instant readings
- Psychrometers – Traditional wet and dry bulb thermometer combinations
- Hair hygrometers – Mechanical devices using organic materials that expand and contract with humidity
- Capacitive sensors – Modern electronic sensors used in weather stations
Frequently Asked Questions
When relative humidity reaches 100%, the air is completely saturated with water vapour. At this point, condensation begins to form, leading to dew, fog, or precipitation. The air cannot hold any more moisture at that temperature.
High humidity in summer reduces the body’s ability to cool itself through perspiration. When the air is already saturated with moisture, sweat doesn’t evaporate efficiently, making us feel hotter than the actual temperature suggests.
As temperature increases, the air’s capacity to hold water vapour increases exponentially. This means that if the absolute amount of moisture remains constant, relative humidity will decrease as temperature rises, and increase as temperature falls.
Absolute humidity measures the actual amount of water vapour in the air (typically grams per cubic metre), whilst relative humidity compares this amount to the maximum the air could hold at that temperature, expressed as a percentage.
In laboratory conditions or very clean air, relative humidity can briefly exceed 100%, creating a supersaturated state. However, this is unstable, and condensation will quickly occur when particles are present to act as condensation nuclei.
Tips for Managing Indoor Humidity
Maintaining proper indoor humidity levels is crucial for comfort and health, particularly in the UK’s variable climate:
Reducing High Humidity
- Use dehumidifiers in damp areas like basements and bathrooms
- Improve ventilation with extractor fans
- Fix leaks and address sources of excess moisture
- Use air conditioning during humid summer months
Increasing Low Humidity
- Use humidifiers during dry winter months
- Place water bowls near radiators
- Grow houseplants that release moisture
- Avoid over-heating rooms