Hygrometers and Humidity

What is a Hygrometer?

Hygrometers are a common and basic tool used to measure the amount of humidity in the atmosphere. There are two common types of hygrometer: the simplest is a dry and wet bulb psychrometer (you can make one of these at home), or the slightly more complex version is called a mechanical hygrometer.

What is Humidity?

Humidity is the amount of water vapor (water in gas form) in the atmosphere. It is most commonly measured as a percentage, known as relative humidity – the ratio of moisture in the atmosphere to the maximum moisture the atmosphere can hold, but it can also be measured as absolute humidity - the amount of water vapor in a unit volume of air. Most of this water vapor in the air comes from evaporation of water from the sea and land.

How do Hygrometers Work?

The simplest type of hygrometer is the ‘wet and dry bulb psychrometer’. This consists of two standard mercury thermometers positioned side by side, one with the bulb at the base dry, and the other with the bulb inside a small muslin sleeve sitting inside a container of water. The dry bulb thermometer measures the current air temperature (known as ambient temperature), whilst the evaporation from the water on the wet bulb thermometer causes the temperature reading to sit slightly lower.

Using the readings from the two thermometers you can calculate the relative humidity using a calculation table. Alternatively, most weather stations will have a humidity sensor that will take these measurements and calculate and display the relative humidity level for you.

The mechanical hygrometer is slightly more complex, and is based on one of the very first hygrometers designed by Horace Bénédict de Saussure in 1783. The mechanical hygrometer uses human hair to help gauge humidity levels because it significantly responds to altering humidity levels by expanding and contracting. The hair is held in slight tension by a spring which is linked to a needle gauge - it expands or contracts depending on the humidity, causing the needle gauge to move and point to the current humidity level.

How does Humidity Affect us?

The amount of water vapor in the atmosphere has a significant effect on the weather. It is the cause of all condensation which produces clouds, fog, and rain. It also helps warm the air by absorbing energy from the sun and is source of energy for storms.

Humidity is important for our comfort and our health. It is what gives you that horrible sticky feeling on a hot day. Humidity (and that horrible sticky feeling) has been linked to sleepiness, lethargy, lower observation skills and irritability. Click here to read more about the weather and its effects on human health.

The humidity can also significantly effect our possessions. Excess humidity causes the air to be very dry, causing problems with electrical equipment and drying out and damaging furniture – this can be combated by install a humidifier in your home. Too little humidity can also cause problems, causing condensation, moisture stains and mould, likewise, this can be combated using a dehumidifier.

Calibrating your Hygrometer

To ensure that you are getting the most accurate results possible from your hygrometer, you should calibrate it at least once a year.

To calibrate, place the hygrometer inside a sealed container with small container of salt water, and place it in a room where the temperature will remain relatively constant throughout the day (e.g. not by a fireplace or front door), then leave it to sit for 10 hours. At the end of the 10 hours, the hygrometer should display the standard relative humidity level of 75% - if not, you need to adjust the display.

Hygrometers are a great tool for helping to keep your family and possessions healthy – and they also make a great gift! Check out our Weather Station Store to find more great tools to help you forecast and monitor your neighbourhood weather.