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Dew temperature

Ignacio del Rey
Ignacio del Rey
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The dew temperatureknown to English-speakers as Dew Point, is able to explain why every morning we come back from the field to the cabinet with wet shoes. Why do some mornings we find the field wet? Today we will explain it to you in a simple and technical way: pay attention to the diagrams, they are important to understand!

The dew point is the temperature at which the air in an area is saturated with water vapour, so for dew to occur, the temperature must fall until it reaches the dew point.

The first thing to note is that air contains water vapour in its composition in varying percentages depending on variables such as weather or proximity to bodies of water. Air cannot support an unlimited amount of dissolved water vapour. The amount of solute it can hold depends on temperature. The higher the temperature, the more water it can hold, and vice versa. In the figure below, the Dew Temperature table. This table is a simplification that only considers frequent dry air temperatures, i.e. the temperature of a normal thermometer. The full document considers higher and lower temperatures.

Dew temperature table.

Before proceeding further, we must explain what the relative humidity. Relative humidity is the amount of water vapour that air can hold before it condenses and precipitates. It is measured with a psychrometer. It is a moderately simple instrument consisting of a thermometer with a wet bulb and a thermometer with a dry bulb. With the help of a psychrometric chart the relative humidity point of the air is located.

Psychrometric chart; the point where the blue line crosses the wet bulb temperature and the green dry bulb temperature corresponds to one of the red Relative Humidity curves. For example at 20ºC dry and 15ºC humid we have a Relative Humidity of 60%.
Now that we are clear on the concepts of relative humidity, we can explain precisely what dew is and where it comes from.
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From what we have seen above, we can now understand that if the air is cooled to a certain temperature, which depends on the amount of humidity, it will condense. This is why a glass of cold water, for example, gets wet on the outside. The cold water inside the glass will cool the surface of the glass, which in turn will cool the volume of air around the glass. This cold air reaches its dew temperature and condenses its water on the surface of the glass.

Exactly the same thing happens in the countryside, but on a massive scale. On a normal warm day, water evaporates from all the nearby sources, the ground, the perspiration water from the plants, the water masses .... and this water passes into the surrounding air in the form of vapour. In the evening, all the surfaces that the sun has heated during the day radiate heat into the atmosphere, which causes the ground to start to cool. And as with the example of the glass, the air around the plants will also cool down so it will not be able to hold all the moisture and will condense on the leaves. The temperature at which this happens is called "dew temperature".

If the temperature continues to drop, this dew will freeze and form what is known as frost.

Weather is important, as these phenomena occur more frequently on clear nights, as the clouds act as a barrier to the radiation from the hot ground and keep the heat close to the ground.

Depending on the time of year or the climate of the area, dew can cause problems or benefits in the fields. However, in general, if it is hot, it is a perfect breeding ground for fungi and diseases, so it is advisable to remove it from the most delicate areas. Nowadays, knowing the dew temperature The remote and highly accurate monitoring is easy with weather station and mini-station systems. WatchDog is the state-of-the-art technology capable of forming a network of sensors to keep perfect control of the weather in the field.

WatchDog is also capable of including underground sensors, leaf wetness sensors, radiation sensors or calculating the risk of disease or pest attack.

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2 Responses

  1. Good afternoon I am Daniel Cabrera student of Architecture I have a very big doubt, according to what I read, it can be said that the dew temperature is reached or bine saturating to 100% that temperature of that air, and the ora would be decreasing the temperature of the same by which accepts less water vapour and would condense faster, if I'm wrong correct me.

  2. Correct, either the humidity increases and precipitates water from the atmosphere or the air cools and holds less water vapour and so precipitates. Usually the dew is due to the nocturnal drop in temperature.

    Best regards.

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