How is Relative Humidity Calculated

What is relative humidity?

Relative humidity (RH) refers to the measure of moisture in the air compared to the maximum amount of moisture the air can hold at a particular temperature. It is expressed as a percentage. RH is crucial in evaluating the comfort level for humans, as well as in predicting the likelihood of precipitation and understanding various weather phenomena.

How is relative humidity calculated?

To calculate relative humidity, two parameters are required: the current air temperature and the dew point temperature. The dew point temperature is the temperature at which air becomes saturated, causing condensation or dew to form. When the dew point temperature is close to the air temperature, the relative humidity is high.

The calculation process involves several steps. Here is how relative humidity is calculated:

Step 1: Measure the current air temperature.
The air temperature can be measured using a thermometer. It is vital to ensure that the thermometer is in a shaded area away from direct sunlight or any heat sources that could affect the accuracy of the reading.

Step 2: Measure the dew point temperature.
There are various instruments available to measure the dew point temperature. One commonly used device is a hygrometer, which works based on the cooling effect created when air passes over a cold surface.

Step 3: Determine the difference between the air temperature and the dew point temperature.
Subtract the dew point temperature from the air temperature to find the difference between the two values.

Step 4: Calculate the actual vapor pressure.
Using the difference obtained in the previous step, calculate the actual vapor pressure. This is achieved by multiplying the difference by a constant value called the “psychrometric constant.”

Step 5: Calculate the saturated vapor pressure.
Using the measured air temperature, calculate the saturated vapor pressure. This can be achieved using complex formulas or by consulting vapor pressure tables.

Step 6: Calculate relative humidity.
Divide the actual vapor pressure (found in step 4) by the saturated vapor pressure (found in step 5), and multiply the result by 100 to obtain the relative humidity percentage.

For example, let’s say the current air temperature is 25°C, and the dew point temperature is 22°C. The difference between these two temperatures is 3°C. By applying the psychrometric constant and obtaining the saturated vapor pressure for 25°C, we can calculate the actual vapor pressure. Finally, dividing the actual vapor pressure by the saturated vapor pressure and multiplying by 100, we find that the relative humidity is approximately 70%.

Why is relative humidity important in weather forecasting?

Relative humidity plays a crucial role in weather forecasting as it affects the comfort level for humans and influences various atmospheric processes. High relative humidity generally indicates the potential for precipitation, as the air is already saturated with moisture. It also affects human perception of temperature, making hot and humid days feel even more uncomfortable.

Additionally, relative humidity affects the formation of clouds, condensation, and the growth of thunderstorms. Meteorologists use relative humidity as a vital parameter in models that help predict the likelihood of various weather events, enabling them to issue accurate forecasts and warnings.

In conclusion, relative humidity is a significant factor in understanding weather patterns and forecasting changes in the atmosphere. By measuring the air temperature and dew point temperature and following a series of calculations, we can determine the relative humidity. This information allows us to gain insights into weather conditions, the likelihood of precipitation, and the comfort level for humans.