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Dew Point

If you keep track of the weather forecast or you've recently bought a weather station, you'll undoubtedly have been intrigued by the mention of the dew point. Below we take a look at what the dew point is, what it's for and how it's measured.

What is the dew point?

The dew point is used to measure the amount of humidity in the air. In actual fact it's shorthand for the temperature of the dew point. It refers to the temperature at which the water in the ambient air condenses and is converted into water droplets, which collect on cold surfaces such as windows and poorly insulated walls.

The dew point is reached when the relative humidity is 100%, i.e. when the air is saturated and can no longer contain the ambient humidity. This phenomenon is dependent on the temperature, the humidity level and the atmospheric pressure measured in hPa or millibars. The lower the temperature is, the less able the air is to contain water vapour and the more quickly the dew point is reached.

What is the dew point for?

Knowing the dew point or dew point temperature is useful in several situations.

It's of interest to meteorologists because it's a measurement of the air saturated with water vapour, and therefore humidity, which for example causes phenomena such as fog and frost. To establish weather forecasts, you therefore need to know the dew point rather than just the temperature and the relative humidity.

However, knowing the dew point isn't just beneficial in weather forecasting, it's also essential to guaranteeing the integrity of your home and the health of your family.

By collecting as condensation on cold surfaces when the dew point is reached, humidity can promote the appearance and growth of mould and fungi. This is a sign that the ambient humidity is too high inside your home and can usually be seen on cold surfaces. These may be walls that are poorly insulated or exposed to the cold, windows and doors, as well as mirrors or a tiled surface in rooms where there is a high water vapour content and condensation forms daily: the kitchen, bathroom, laundry room, etc.

Unless it's properly vented, this excess humidity will eventually damage the walls, coverings and materials in a home. Meanwhile, mould and fungi indoors are responsible for health problems such as asthma, allergies and respiratory irritation. It's therefore essential to check the humidity level in your home to stop them forming. Discover all our products

How to limit humidity indoors

To limit the effects of condensation indoors, you need to limit the formation of water vapour or ensure that it's quickly vented. There are several ways to restore the balance between temperature, humidity level and pressure. Proper venting is essential for regulating the humidity level, either manually by airing your home or by installing a mechanical ventilation system or a heat recovery ventilator. Manual ventilation is particularly useful after activities that produce a lot of water vapour: condensation from cooking steam, showers, washing hangers, etc.

Another area that can be improved is a home's insulation: this ensures that a consistent temperature is maintained indoors and stops a surface (walls or windows) becoming colder than other surfaces and attracting condensation.

How is the dew point measured?

As explained above, the dew point is determined by the ambient temperature, the atmospheric pressure expressed in hPa and the humidity level (notably the relative humidity). This information can be obtained by consulting a weather forecast or using a thermometer and a hygrometer, or even a weather station.

Armed with the temperature in degrees and the relative humidity level, you can look at measuring tables online to determine the temperature of the dew point. Dew point calculators are also available on the internet: all you have to do is enter the heat and the ambient humidity. The calculator condenses this information using a water vapour saturation pressure table and determines the dew point.

Another way to find out the dew point is to use a mirror dew point hygrometer. This involves a mirror at a controlled temperature, which mists over when the water vapour condenses. An optical measurement of the mirror's reflectiveness, modified by the condensation, determines the dew point.

Modern instruments such as digital weather stations determine the temperature, relative humidity and pressure using sensors and measure the dew point based on the information collected. Smart weather stations therefore act as a reliable indicator of the atmospheric humidity phenomenon. As well as determining the likelihood of rain and giving the temperature, relative humidity level and pressure, they provide information about the dew point and eliminate the need for tedious calculations. They can also produce reports using the information collected and allow the user to view the records of each measurement (temperature, humidity, etc.) in an app.

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