In what atmospheric conditions is dew formed?

You may have observed water droplets formed on the surfaces of plants and trees. You might have even felt the grass to be surprisingly wet while taking a morning walk. Sometimes, even in the evenings, places such as parks and your backyard seem to glisten and give off an aura of a rainy day, even though it might not have rained for days. In cold weather, you may wake up to find your windows and windshields drenched in tiny blobs of water. These mischievous water particles also go by the name “dew.”

First off, what is dew? Evidently, it is just tiny water droplets found at random places. But how come we find them after waking up without ever actually observing them being formed? Also, why is it that we only find dew during some parts of the year and not in others? 

Let us start by talking about moisture. Moisture can be felt in the air around us, especially during the early fall season and in places that are nearer to the coast. In relatively hot areas, like those located near a desert or away from water bodies, moisture is less frequent. Simply put, moisture is water in the form of gas – just like the water vapors you see when you boil water in a kettle. When there is greater humidity in the air someplace, it simply means that, at that time, there is more water present in the air.

Just as water changes into vapors when it is boiled, it can be changed back to its liquid form when the vapors are cooled. This process is called condensation. The moisture in the atmosphere can also undergo condensation. That is to say that whenever the air gets cold enough, the water vapors present in the air around us can turn into the liquid state.  The water vapors thus formed from this cooling process are collectively known as dew.

Unless you happen to live in places like New Orleans or Southeast Asia, where most of the time it is dew season, there are a few reasons as to why we don’t see dew every day. For dew to form, there has to be a sufficient amount of water in the air. Therefore, during times such as spring season or in dry regions, dew is unlikely to be formed. However, even if there is enough humidity in the air, temperature plays a great role in the formation of dew. Dew is formed when the temperature drops enough for the water in the air to condense – this exact temperature at which dew is formed is called the dew point.

In colder regions, dew might not form even if the temperature is below the dew point. The reason for this is that when the air is very cold, moisture, instead of changing into a liquid, decides to change into solid! This solid form of water is what we know as frost, and at frost point (the temperature at which frost forms), the moisture changes directly from vapors into solid.

Dew point is affected by the number of water vapors present in the air. Air that is saturated with water is more likely to form dew at higher temperatures as compared to air that is not. This is because as temperatures decrease, the air becomes denser, and there is less room for water vapors in it.

When all these requirements are met, then in general, dew is observed to be formed during late summer and in the beginning of the fall season. Autumn is sometimes considered to be the dew season because the air in autumn is moist with water vapors, and the temperature is also low enough to be below the dew point but not so low that it allows frost to be formed. Besides that, in months such as October and November, the nights tend to be clearer and undisturbed by clouds. This is an important factor as it allows surfaces to give off their heat from the day and cool down.

This is also the reason that autumn nights create the ideal conditions for dew to form. Exposed surfaces such as grass and leaves, and in our homes, glass and steel, cool down enough for dew to accumulate and persist until late morning.

Additional reading:

Dew (Wikipedia)

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