To study relative humidity we must first know that atmospheric water vapor is an important factor in the climate since it regulates the temperature of the air by absorbing thermal radiation from the Sun and the Earth. The higher the vapor content of the atmosphere, the more energy there will be for the generation of storms and for the processes of condensation and precipitation . Water vapor enters the atmosphere mainly through the evaporation of water from the Earth’s surface and this varies from one place to another depending on the temperature . Relative humidity is the content of water vaporof the air in relation to its saturation . The humidity can be measured in several ways, but the relative humidity is the most common of them. To understand relative humidity , it is also important to know what humidity is , and it is defined as the amount of water that is in the air.

What is relative humidity?

Relative humidity is the amount of water vapor that exists in an environment in relation to the amount of water vapor that could be contained at its maximum point in that air at a given temperature .

  • Definition
  • How to measure relative humidity
  • Unit of measurement
  • What is it for
  • How it differs from absolute humidity
  • Average relative humidity by country


Relative humidity is the relationship between the current absolute humidity and the highest absolute humidity that can be achieved, which depends on the current temperature of the air. A reading of 100% relative humidity means that the air is fully saturated with water vapor and cannot hold any more, creating the possibility of rain . This does not mean that the relative humidity must be 100% for it to rain; It should be 100% where clouds are forming, but the relative humidity near the ground could be much lower.

How to measure relative humidity

It can be measured by means of an instrument known as a hygrometer , and it is used in the area of meteorology to be able to measure the degree of humidity that exists in the air in the atmosphere . The results of the hygrometer measurements are expressed in percentages , and they can also be of two types:

  • Analog : they are very accurate , and can detect changes in humidity in the environment almost immediately, although they need calibration .
  • Digital – Digitals are slightly less accurate but maintenance free , and are ready to use right after purchase.

Calculation of relative humidity

The formula used to calculate the relative humidity is the following:

HR = (  mass of vapor in air /  Maximum mass of vapor that air can hold ) x 100

In other words:

HR = (m vap / m vapmax ) x 100

Unit of measurement

The unit of measurement used to express relative humidity is through the percentages that are represented by the % symbol  This unit of measurement can vary between 0% to identify completely dry air and 100%, which indicates that the air is completely saturated .

What is it for

It is useful to get an idea of how close you can stay a mass of air to reach their maximum saturation . Through this knowledge, we know that when the days have 100% relative humidity , that the air mass no longer has the capacity to store more water vapor and that, from that moment on, any addition of water having more air mass is capable of forming water droplets , which at generally known by the name of dew or crystals of ice , depending on the environmental conditions that exist in a given area.

How it differs from absolute humidity

The absolute humidity is the weight that the water vapor contained per unit volume of air, and which is measured at generally in kg / m3 , whereas the relative humidity is the quotient between the weight of vapor water that is contained in a volume of air and the weight of saturated steam for that same volume, and which is measured in percentages .

Average relative humidity by country

All places in the world have different data and percentages regarding relative humidity, some examples are the following:

  • Spain : the average annual relative humidity ranges between 75% and 80% in the Atlantic and Cantabrian coastal areas , and around 70% in the Mediterranean part. Usually, the annual average humidity decreases in the inside and because of the influence of the sea , the relative humidity is significant in the two islands .
  • England : the dew point changes slowly , which is why even if the temperature drops at night, it is daytime it is usually humid . The perceived level of humidity in the country’s capital, measured by the percentage of time in which the level of humidity comfort is muggy , oppressive or unbearable , varies very little during the year, and remains practically constant at 0%.

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