Humidity Converter

Understanding Humidity Measurements

Humidity describes the amount of water vapor present in the air. There are three main ways to express humidity, each useful in different situations: relative humidity, dew point temperature, and absolute humidity. Understanding these measurements helps with weather forecasting, HVAC design, agriculture, and personal comfort.

Relative Humidity (RH):

Relative humidity is the ratio of the current amount of water vapor in the air to the maximum amount the air could hold at that temperature, expressed as a percentage. Warm air has the capacity to hold more moisture than cold air, which is why the same amount of water vapor produces different RH readings at different temperatures.

RH = (actual vapor pressure / saturation vapor pressure) × 100%

Dew Point Temperature:

The dew point is the temperature at which air becomes fully saturated and water vapor begins to condense into liquid droplets. A higher dew point means there is more moisture in the air. The Magnus formula provides a good approximation of dew point:

Td = (b × alpha) / (a - alpha)

Where alpha = (a × T) / (b + T) + ln(RH/100), with constants a = 17.27 and b = 237.7°C, and T is the air temperature in Celsius.

Absolute Humidity:

Absolute humidity is the actual mass of water vapor per unit volume of air, measured in grams per cubic meter (g/m³). Unlike relative humidity, absolute humidity does not change with temperature alone:

AH = (6.112 × e^((17.67 × T)/(T + 243.5)) × RH × 2.1674) / (273.15 + T)

Comfort Levels by Dew Point:

Practical Examples:

• On a 30°C (86°F) day with 50% RH, the dew point is about 18.4°C (65°F), which feels somewhat uncomfortable.
• On a 20°C (68°F) day with 80% RH, the dew point is about 16.4°C (62°F), which is slightly humid.
• Desert air at 40°C (104°F) with 10% RH has a dew point of about 2.6°C (37°F), which is very dry despite the extreme heat.
• Indoor comfort is typically 30-50% RH at room temperature of 20-22°C (68-72°F).

Tips:

• Dew point is a more reliable indicator of comfort than relative humidity because it does not change with temperature fluctuations.
• Fog forms when the air temperature drops to the dew point level.
• High indoor humidity above 60% RH encourages mold growth and dust mites.

Any one of the three is enough. Give the calculator the air temperature and then whichever measure you actually have, and it derives the other two. A weather app gives you relative humidity, a decent hygrometer gives you dew point, and a lab or grow-room sensor may give you grams per cubic metre. They are three descriptions of the same air, and the arithmetic runs in every direction, so there is no need to convert anything by hand before you start.

One caution about absolute humidity: it is mass per unit volume, so it moves when the air expands or contracts even if no water is added or removed. Heat a sealed room and its absolute humidity falls slightly as the air expands, while the dew point stays put. For most practical purposes the dew point is the stable number and the one worth quoting.