The importance of humidity in meteorology

the humidity of the forests in the mornings

Humidity is a fairly important meteorological variable because water vapor is always present in our air. Regardless of the temperature of the air we breathe, it almost always has some water vapor. We are used to seeing humidity especially on the coldest winter days.

Water is one of the main components of the atmosphere and can be found in all three states (gas, liquid, and solid). In this article I am going to explain everything you need to know about humidity as a meteorological variable and what it is for. Do you want to know more about it?

What is humidity? Types of humidity

accumulated moisture on plants

Humidity is the amount of water vapor in the air. This amount is not constant, but will depend on various factors, such as if it has rained recently, if we are near the sea, if there are plants, etc. It also depends on the temperature of the air. That is, as the air decreases its temperature it is capable of holding less water vapor and that is why mist appears when we breathe, or the dew at night. The air becomes saturated with water vapor and is not able to hold as much, so the water becomes liquid again.

It is curious to know how desert airs are capable of holding more humidity than polar airs, because hot air is not so quickly saturated with water vapor and is capable of containing more quantity, without it turning into liquid water.

There are several ways to refer to the moisture content in the atmosphere:

  • Absolute humidity: mass of water vapor, in grams, contained in 1m3 of dry air.
  • Specific humidity: mass of water vapor, in grams, contained in 1 kg of air.
  • Rmixing zone: mass of water vapor, in grams, in 1 kg of dry air.

However, the most widely used measure of humidity is called RH, which is expressed as a percentage (%). It is obtained as a result of dividing between the vapor content of the air mass and its maximum storage capacity and multiplying it by 100. It is what I have commented before, the more temperature an air mass has, the more temperature it is capable of holding more water vapor, so its relative humidity can be higher.

When does an air mass become saturated?

when an air mass becomes saturated with water vapor, the mist comes out

The maximum capacity to hold water vapor is called saturating vapor pressure. This value tells us the maximum amount of water vapor that an air mass can contain before transforming into liquid water.

Thanks to the relative humidity, we can have an idea of ​​how close an air mass is to reaching its saturation, therefore, the days that we hear that the relative humidity is 100% are telling us that the air mass is no longer can store more water vapor and from there, any more water additions to the air mass will form water droplets (known as dew) or ice crystals, depending on the environmental conditions. Normally this happens when the air temperature is quite low and that is why it cannot hold more water vapor. As the temperature of the air increases, it is capable of holding more water vapor without becoming saturated and that is why it does not form water droplets.

For example, in coastal places, in summer there is high humidity and a “sticky” heat due to the fact that the drops of the waves on windy days remain in the air. However, due to its high temperatures, cannot form drops of water or become saturated, since the air can store a lot of water vapor. It is the reason why dew does not form in summer.

How can we make an air mass saturate?

humidity is higher in air masses with lower temperatures

In order to understand this in a correct way, we have to think about when we exhale the water vapor from our mouth during winter nights. That air that we exhale when we breathe has a certain temperature and water vapor content. However, when it leaves our mouth and comes into contact with the cold air outside, its temperature drops sharply. Due to its cooling, the air mass loses the capacity to contain vapor, easily reaching saturation. Then the water vapor condenses and forms mist.

Again, I highlight that this is the same mechanism by which the dew that wets our vehicles is formed on cold winter nights. Therefore, the temperature to which a mass of air must be cooled to produce condensation, without varying its vapor content, is called the dew temperature or dew point.

Why do car windows fog up and how do we remove it?

water vapor clouds the windows of cars

To solve this problem that can happen to us in winter, especially at night and on rainy days, we have to think about air saturation. When we get into the car and come from the street, the water vapor content of the vehicle begins to grow as we breathe and, due to its low temperature, it saturates very quickly (its relative humidity reaches 100%). When the air inside the car becomes saturated, it causes the windows to fog up because the air can no longer hold any more water vapor, and yet we continue to breathe and exhale more water vapor. That is why the air becomes saturated and all the surplus is transformed into liquid water.

This happens because we have kept the air temperature constant, but we have added a lot of water vapor. How can we solve this and not cause an accident due to the low visibility of the fogged glass? We have to use the heating. Using the heating and directing it to the crystals, we will increase the temperature of the air making it able to store more water vapor without becoming saturated. In this way, the foggy windows will disappear and we can drive well, without any added risk.

How do you measure humidity and evaporation?

psychrometer to measure humidity

Humidity is usually measured by an instrument called a psychrometer. This consists of two equal thermometers, one of which, called a "dry thermometer", is simply used to obtain the temperature of the air. The other, called a "wet thermometer", has the tank covered with a cloth moistened by means of a wick that puts it in contact with a water tank. The operation is very simple: the water that soaks the web evaporates and for this it takes the heat from the air that surrounds it, whose temperature begins to drop. Depending on the temperature and the initial vapor content of the air mass, the amount of evaporated water will be greater or less and to the same extent there will be a greater or lesser drop in temperature of the wet thermometer. Based on these two values, the relative humidity is calculated using a mathematical formula that relates them. For greater convenience, the thermometer is supplied with double entry tables that directly give the relative humidity value from the temperatures of the two thermometers, without having to perform any calculations.

There is another instrument, more precise than the previous one, called an aspyropsychrometer, in which a small motor ensures that the thermometers are continuously ventilated.

As you can see, when it comes to meteorology and climate science, humidity is quite important.

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  1.   José Alberto said

    Excellent very explanatory article, I congratulate you for the work you do, greetings ..

  2.   Raúl Santillan said

    Excellent article German Portillo, do you know how the moisture contained in a product made from cardboard or paper can be absorbed?

    Or if it cannot be removed, reduce the% humidity!

    Raúl Santillan