# How to calculate humidity: 15 steps (with pictures)

## Relative humidity, the amount of moisture in the air relative to how much moisture the air can hold, is an important measure for understanding climate

Humidity can be measured with a hygrometer or other specialized device, but you can also calculate it if you know the air temperature, the dew point, and a few simple, standard equations. For a more practical method, you can build a sling psychrometer (an instrument that can measure relative humidity) using a few cheap supplies and simple tools.

## Steps

### Method 1 of 2: Calculate Relative Humidity Using Dew Point and Temperature

#### Step 1. Convert the air temperature and the dew point temperature to Celsius

If you want to convert degrees Fahrenheit to Celsius, subtract 32 from the temperature in Fahrenheit. Then divide this number by the fraction 5/9. This is what the formula should look like:

• C = 59 ∗ (F − 32) { displaystyle C = { frac {5} {9}} * (F-32)}

• (C){displaystyle (C)}
• = temperatura en Celsius y (F){displaystyle (F)}

= temperatura en Fahrenheit

• Por ejemplo, si la temperatura es de 100 °F, primero debes restarle 32 a 100, con lo que obtienes 68. Luego, multiplicas 68 por 5/9, con lo que obtienes la temperatura 37, 778 °C.
• Por supuesto que, si vives fuera de los EE. UU. y utilizas la escala de Celsius, puedes omitir este paso.
• Asimismo, puedes simplificar este paso usando convertidores de temperatura en línea.

#### Step 2. Calculate the saturated vapor pressure using a formula

Once you have converted the air temperature to Celsius, you must determine the saturated vapor pressure, or the maximum amount of water vapor the air can hold at that temperature. You can use this formula to calculate the saturated vapor pressure:

• is = 6.11 ∗ 10 (7.5 ∗ T237.3 + T) { displaystyle e_ {s} = 6.11 * 10 ({ frac {7.5 * T} {237.3 + T}})}

• (es){displaystyle (e_{s})}
• = presión de vapor estándar y (T){displaystyle (T)}

= temperatura del aire

#### Step 3. Find the actual vapor pressure using the same formula

You can find the actual vapor pressure using the same formula. All you have to do is replace the air temperature in the formula with the dew point.

• e = 6.11 ∗ 10 (7.5 ∗ Td237.3 + Td) { displaystyle e = 6.11 * 10 ({ frac {7.5 * Td} {237.3 + Td}})}

• (e){displaystyle (e)}
• = presión de vapor real y (Td){displaystyle (Td)}

= punto de rocío

• Puedes encontrar el punto de rocío si revisas la página del clima del periódico local o un sitio web en donde se publique información sobre el clima para tu localidad (por ejemplo, weather.gov si vives en los EE. UU.).

#### Step 4. Calculate the relative humidity

After having the saturated vapor pressure and the actual vapor pressure, you can find the relative humidity. Just divide the actual vapor pressure by the saturated vapor pressure and multiply that number by 100. You can use the following equation:

• rh = ees ∗ 100 { displaystyle rh = { frac {e} {e_ {s}}} * 100}

• (rh){displaystyle (rh)}
• = humedad relativa, (e){displaystyle (e)}

= presión de vapor real y (es){displaystyle (e_{s})}

= presión de vapor estándar

#### Step 5. Use an online relative humidity calculator to make it quick and convenient

If math isn't your strong suit, you can use various online calculators to find relative humidity. Just enter the temperature and dew point, and you can get the relative humidity in a fraction of a second.

### Method 2 of 2: Finding Relative Humidity with a Sling Psychrometer

#### Step 1. Buy two identical plastic student thermometers

A plastic student thermometer is an inexpensive tube thermometer that is attached to a white plastic backing. They are typically around 2 inches (5.1 cm) wide and 6 inches (15 cm) long, and have a hole through the top of the plastic backing and a bulb at the bottom of the tube. Also, thermometers should be free of mercury.

### You should be able to find plastic student thermometers at most office supply stores and some major retailers

#### Step 2. Soak a small piece of porous white cloth in water

To start doing the psychrometer, soak a cloth in water. The fabric should be about 2 inches (5.1 cm) long.

### For example, a piece of hollow shoelace would do. Just cut the cord in half to get a single layer of fabric

#### Step 3. Wrap the cloth around the bulb of one of the thermometers

Using a rubber band or piece of string, secure the damp cloth to the bulb, which is the bottom end of the thermometer. This will be the wet bulb thermometer.

### The thermometer that does not have a wet cloth will be the dry bulb

#### Step 4. Attach a washer and thermometer to a 2-inch (5.1-cm) metal screw

Slide the washer over the screw first, and then put the screw through the hole in one of the two thermometers. It doesn't matter which thermometer you put on the screw first.

#### Step 5. Place a plastic spacer, thermometer, and washer over the same screw

After attaching the washer and thermometer to the screw, cut a 0.5-inch (1.5-cm) section from a plastic straw and slide it over them. Then add the other thermometer and another washer.

### You can use a different type of plastic spacer if it is very similar in size

#### Step 6. Tighten the screw into a 6 in (15 cm) long wooden dowel

Using a screwdriver, secure the screw to the dowel. Tighten the screw just enough so that it is secure. Make sure you leave plenty of room for the two thermometers to swing.

#### Step 7. Swing the thermometers for 10-20 seconds

Hold the dowel away from your body and turn it so that the two thermometers rotate around the screw. As you swing the thermometers, the water will evaporate from the wet cloth that you attached to the wet bulb thermometer. This evaporation will cool the bulb and lower the temperature that is recorded on that thermometer.

### Make sure to wear goggles for this step

#### Step 8. Record the temperatures of the two thermometers

Once you have rotated the thermometers for 10-20 seconds, stop and observe the temperatures displayed on them. Write them down in a notebook.

• Read the temperature on the wet bulb thermometer first to get an accurate reading.
• Mark the temperature reading that corresponds to the wet bulb thermometer and the one that corresponds to the dry bulb thermometer.

#### Step 9. Subtract the temperature of the wet-bulb thermometer from that of the dry-bulb thermometer

The temperature of the wet bulb thermometer will be lower than that of the dry bulb thermometer due to evaporation. The drier the air, the lower the temperature of the wet bulb thermometer relative to the dry bulb thermometer.

• Turning the thermometer will cause the moisture in the fabric to evaporate. The evaporation process removes heat from the environment, which causes the wet bulb thermometer temperature reading to be lower.
• When the air is dry, there is more evaporation because dry air has a greater ability to absorb additional moisture than humid air.