Find Relative Humidity Calculator

Calculate Relative Humidity

Enter the air temperature and dew point to calculate the relative humidity.

What is Relative Humidity?

Relative Humidity (RH) is a measure of the amount of water vapor present in the air compared to the maximum amount of water vapor the air can hold at a specific temperature. It is expressed as a percentage. When the air is holding the maximum amount of water vapor possible at that temperature, the relative humidity is 100%. Conversely, if the air contains no water vapor, the relative humidity is 0%. Understanding relative humidity is crucial in meteorology, agriculture, building design, and human comfort.

Anyone interested in weather, comfort levels, or processes affected by air moisture should use a relative humidity calculator or understand relative humidity values. This includes meteorologists, farmers, HVAC technicians, museum curators, and even homeowners managing indoor air quality.

A common misconception is that high relative humidity always means there is a lot of moisture in the air. While it often does, relative humidity is *relative* to temperature. Cold air can have a high relative humidity but hold very little actual water vapor compared to warm air at the same relative humidity. It’s the ratio that matters.

Relative Humidity Formula and Mathematical Explanation

Relative Humidity is calculated using the ratio of the actual vapor pressure (e) to the saturation vapor pressure (e_s) at the given air temperature (T), multiplied by 100:

RH = (e / e_s) * 100%

Where:

• RH is the Relative Humidity.
• e is the actual vapor pressure, which is the saturation vapor pressure at the dew point temperature (T_d).
• e_s is the saturation vapor pressure at the air temperature (T).

The saturation vapor pressure (e_s) can be estimated using various formulas, with the August-Roche-Magnus (or a simplified Tetens’) equation being common:

e_s(T) ≈ 6.112 * exp((17.67 * T) / (T + 243.5)) (when T is in Celsius, e_s is in hPa or mb)

So, the actual vapor pressure (e) is found by using the dew point temperature (T_d) in the same formula:

e = e_s(T_d) ≈ 6.112 * exp((17.67 * T_d) / (T_d + 243.5))

Then, Relative Humidity becomes:

RH ≈ (exp((17.67 * T_d) / (T_d + 243.5)) / exp((17.67 * T) / (T + 243.5))) * 100

This understanding of vapor pressure is key to the Relative Humidity Calculator.

Variable	Meaning	Unit	Typical Range
RH	Relative Humidity	%	0 – 100
T	Air Temperature	°C or °F	-50 to 50 °C
Td	Dew Point Temperature	°C or °F	-60 to T °C
e	Actual Vapor Pressure	hPa (or mb)	0 – 100+
es	Saturation Vapor Pressure	hPa (or mb)	0 – 100+

Practical Examples (Real-World Use Cases)

Example 1: A Warm Humid Day

Imagine a summer day with an air temperature of 30°C and a dew point of 20°C. Using the Relative Humidity Calculator:

• Air Temperature (T) = 30°C
• Dew Point (T_d) = 20°C

The calculator would show a relative humidity of approximately 54-55%. This feels quite humid and can be uncomfortable.

Example 2: A Cool Dry Day

Consider a cool autumn day with an air temperature of 15°C and a dew point of 5°C.

• Air Temperature (T) = 15°C
• Dew Point (T_d) = 5°C

The relative humidity would be around 52-53%. Even though the relative humidity is similar to Example 1, the air feels much drier because the total amount of moisture the air can hold at 15°C is much less than at 30°C.

How to Use This Relative Humidity Calculator

1. Enter Air Temperature: Input the current air temperature into the “Air Temperature” field.
2. Enter Dew Point Temperature: Input the dew point temperature into the “Dew Point Temperature” field. The dew point must be less than or equal to the air temperature.
3. Select Units: Choose whether you are entering temperatures in Celsius (°C) or Fahrenheit (°F) using the dropdown menu.
4. View Results: The calculator will automatically update and display the Relative Humidity in the results section, along with intermediate values like saturation and actual vapor pressures.
5. Reset: Click “Reset” to return to default values.
6. Copy: Click “Copy Results” to copy the main results and inputs to your clipboard.

The primary result shows the calculated relative humidity. The intermediate values give you the saturation vapor pressure at the air temperature and the actual vapor pressure (which is the saturation vapor pressure at the dew point). These help understand how much moisture the air *could* hold versus how much it *does* hold. High relative humidity (above 60-70%) can feel uncomfortable and promote mold growth, while very low relative humidity (below 30%) can cause dry skin and respiratory irritation. More about comfort levels.

Key Factors That Affect Relative Humidity Results

1. Air Temperature: As air temperature increases, the air’s capacity to hold water vapor increases. If the amount of moisture stays constant, an increase in temperature will decrease the relative humidity.
2. Dew Point Temperature: The dew point is a more direct measure of the actual amount of water vapor in the air. If the dew point remains constant and the air temperature changes, the relative humidity will change.
3. Amount of Water Vapor: If more water vapor is added to the air (e.g., through evaporation from a lake or after rainfall) while the temperature stays the same, the dew point will rise, and so will the relative humidity.
4. Air Pressure: While our calculator doesn’t directly take pressure as an input (it’s implicitly handled in the common formulas for near-surface conditions), changes in air pressure can slightly affect the air’s capacity to hold moisture and thus relative humidity.
5. Proximity to Water Bodies: Areas near large bodies of water tend to have higher moisture content in the air, leading to higher average relative humidity.
6. Time of Day: Relative humidity is often highest in the early morning when temperatures are lowest and lowest in the afternoon when temperatures are highest, assuming the actual moisture content doesn’t change significantly. Daily weather patterns explain this.

Frequently Asked Questions (FAQ)

Q: What is the difference between relative humidity and dew point?
A: Dew point is the temperature to which air must be cooled to become saturated with water vapor (100% relative humidity), assuming constant air pressure and water content. It’s an absolute measure of how much moisture is in the air. Relative humidity is how saturated the air is *at its current temperature*.

Q: Can relative humidity be over 100%?
A: In the atmosphere, it’s very rare and usually short-lived (supersaturation), typically needing very clean air without condensation nuclei. For most practical purposes and weather reporting, relative humidity is capped at 100%. Our Relative Humidity Calculator will show 100% if the dew point equals or exceeds the air temperature.

Q: What is a comfortable relative humidity range?
A: Most people find a relative humidity range of 30% to 60% comfortable indoors.

Q: How does relative humidity affect how hot it feels?
A: High relative humidity slows down the evaporation of sweat from our skin, which is the body’s primary cooling mechanism. This makes the air feel hotter and more oppressive than the actual temperature suggests (this is related to the heat index).

Q: How does relative humidity affect how cold it feels?
A: In cold weather, high relative humidity can make the air feel colder and damper, as the moisture can conduct heat away from the body more effectively, although the effect is less pronounced than with heat.

Q: Why does relative humidity change throughout the day?
A: Assuming the amount of moisture in the air (and thus the dew point) stays relatively constant, relative humidity changes mainly because the air temperature changes. As temperature rises, relative humidity falls, and vice-versa.

Q: Can I use this Relative Humidity Calculator for indoor environments?
A: Yes, if you know the indoor air temperature and dew point, this Relative Humidity Calculator can accurately determine the indoor relative humidity.

Q: What happens if I enter a dew point higher than the air temperature?
A: The dew point cannot be higher than the air temperature under normal conditions. If you enter a higher dew point, the relative humidity will be calculated as 100%, and the calculator will internally cap the dew point at the air temperature for stable calculations.

Related Tools and Internal Resources

• Dew Point Calculator: Calculate the dew point if you know the relative humidity and temperature.
• Heat Index Calculator: Understand how hot it feels based on temperature and relative humidity.
• Wind Chill Calculator: See how cold it feels based on temperature and wind speed.
• Understanding Weather Basics: Learn more about atmospheric conditions.
• Indoor Air Quality Guide: Information on maintaining comfortable and healthy indoor air, including humidity levels.
• Gardening and Microclimates: How humidity affects plant growth.