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October 2, 2020

Humidity – how, when and where!

Misty conditions at sea
This is a short story about humidity

In this article we describe the humidity, how to measure it, how it changes in different weather situations and how location and season will affect it.

Humidity is defined as the amount of moisture in a given air mass. There are several ways to indicate or measure the humidity, but relative humidity is the most common way.
Simplified, relative humidity can be described as the ratio between the actual amount of water vapor to the maximum amount in a given volume of air. At 100% relative humidity, the air is totally saturated with water vapor and cannot hold more moisture.
The absolute amount of water vapor depends on the temperature, as warm air can contain more moisture. Therefore, high relative humidity, can feel very differently depending on the temperature.

Dew point

The dew point is an important parameter to understand in order to predict the humidity. Dew point is the temperature at which air becomes saturated with water vapor. A big difference between the temperature and the dew point, will therefore mean low relative humidity and a small difference a higher relative humidity.
Especially in offshore locations, where the temperature is rather stable due to the slow changing water temperatures, the dew point is a good indicator of the risk of mist/fog.
For more about mist/fog, see this article.


When the relative humidity reaches 100%, clouds will form (or fog if at the surface), and precipitation can then form - depending on the temperature, vertical extent of the cloud and number of aerosols.
However, the relative humidity is not necessary 100% at the surface when it is raining. Actually, the relative humidity can be relatively low when the rain starts in connection with a warm front as the air typically is rather cold and dry ahead of the front. If the air is dry enough, the rain can evaporate before reaching the surface. The evaporation will gradually cause the relative humidity to increase, and the rain droplets will then eventually reach the surface. After a while with rain, the relative humidity will often be close to 100%.

After a passing cold front, colder air with lower dew points will normally prevail. It will often feel like the air is drier (absolute humidity will be lower), but the relative humidity is not always significantly lower than in the warm airmass ahead of the cold front.
However, the highest relative humidity, besides from in passing fronts, is still commonly found in the warm air mass between a warm front and a cold front – also known as the warm sector.

Rain drops
Rain increases the humidity in the air below the rain clouds
The affecting wind

Strong wind speeds near the surface will create turbulence that will mix the airmass near the surface with the air just above. In situations with a high relative humidity and mist/fog, increasing winds will often result in improved visibility near the surface. However, the moisture could sometime be trapped in a temperature inversion a couple of hundred meters above the surface as low clouds – but at least the visibility at the surface will improve significantly and the relative humidity will then be lower.

Winter vs Summer

Humid air during the winter typically means misty weather most of the day, with little variation in temperature and cloud cover. In the summer, a high humidity can be associated with misty weather as well, but also with the possibility of severe thunderstorms.

Inland vs offshore

The relative humidity will in general decrease during spring/early summer as the air temperature increases and then rise again late summer/autumn - but the absolute humidity will increase as warm air can contain more water vapor.
The variation of relative humidity will depend on the location. An inland location will typically have a larger variation between the seasons than a coastal/offshore location.

An offshore wind farm in good visibility
An offshore wind farm in good visibility

In offshore locations, the temperature of the surrounding water will have a big impact on the relative humidity. Cold waters (due to a cold winter or upwelling of cold water) combined with a warm maritime air mass (read this article for more about air masses) will cause a high relative humidity, with risk of fog and mist.
There is also a clear diurnal variation in the relative humidity, especially during the summer when the days are longer. This diurnal variation is more profound over land due to the larger diurnal variation of the air temperature.