Why Does Humidity Increase At Night

I recently moved to another state and lately, I always have felt suffocated during the night never why! So I asked my girlfriend she told me “I don’t know why” maybe its humidity she said. I really never paid attention to humidity before because I lived in a dry warm climate before.

Then I started doing some research on why does humidity increases at night.

So in this article, I will share what I found so that you don’t have to waste time doing research on air and water how they produce humidity.

Looking for a more scientific exploration?

After some digging, I found this great explanation by Paul Jorgensen

The dry bulb temperature generally drops over night. Cooler air has less volume so it has less ability to hold moisture, therefore its relative humidity rises. Warmer daytime air has greater volume, therefore it can hold more moisture, so its relative humidity is lower.


This is a hard concept to visualize, but if you look at a psychrometric chart, created by Willis Carrier, you can plot it on a chart.


This is why dehumidifiers have an evaporator coil that firsts removes latent heat by keeping the evaporator below dew point (condensate) which takes some of the grains down the drain.


Then the same air is used to cool the condenser coil, which warms the air thereby expanding it, which further lowers its humidity.

Mechanical refrigeration is magical!

The sheer volume of humidity really doesn’t increase during the night but it more concentrated toward ground level because of the temperature during the night decrease on the ground level.

A cubic meter of air can’t contain more than 14 grams of water at the world average temperature of 15 degrees Celsius. The relative humidity is then said to be 100%.

If the air cools down then that humidity must condense and create early morning (the coldest part of the night) haze of fog.

As soon as the sun warms up the ground again, the temperature rise and the relative humidity falls.

But the sun doesn’t ‘evaporate’ the humidity. In fact, as the sun warms the soil or the sea, chances are that more humidity in the form of water molecules binds to the air near the surface.

The relative humidity depends not only upon the amount of water vapor actually present in the air but also on the air temperature. In fact, the relative humidity indicates how much is the percentage of saturation of air(with water vapor) for a particular temperature. If it is fully saturated, then the relative humidity is 100 percent.

If air holds some amount of water vapor at a particular temperature and is unsaturated, then, at a lower temperature, the same amount of water vapour may be able to saturate it.

Hence, for the same amount of moisture content in the air, the relative humidity may be less for higher temperatures and more for a lower temperature.

Therefore, naturally, the relative humidity is less during day time and more during night time.

According to Simona Seastrand an Atmospheric Scientist

As the air gets cooler, it gets closer to the dew point. When the temperature=dew point, fog forms! Why does this happen? Well, cold air cannot hold in as much moisture as warm air, and as the temperature cools moisture begins to condense (become liquid from vapor).

This is the same way clouds are formed! As the air rises, it cools, and the moisture condenses and becomes visible in the form of clouds. Click Here To Read Original Post

Why Are Humid Nights More Warmer?

A night when the temperature of the surface drops to surface dewpoint can be called as humid night. A higher dewpoint indicates that there is more water vapor in the air. A night when the temperature drops to a dewpoint which is more than 55 F can be also classified as a true humid night.

Water vapors act as a greenhouse gas in the troposphere. As the amount of water vapor in the air increases, the long-wave radiation in the troposphere increases as well. If the level of water vapor is not high in the air then longwave radiation emitted from the surface of the earth will escape easily without any trouble.

This will result in the cooling of the dewpoint. Clouds are high-density saturated air areas this will result in the formation droplets within the clouds.

Once the temperature of the surface starts to drop to the dewpoint of the surface then the rate of cooling decreases during the night. Once the latent heat of condensation releases temperatures drops to the dewpoint and tend to decrease very little after that.

This is especially true for air at high dewpoints since much more latent heat release occurs with warm and humid air.

The difference between the high and low tends to be much greater on dry clear 24-hour days than on warm cloudy 24-hour days.

This is due to the rate of cooling being greater in dry clear air at night and the rate of warming being greater in dry clear air during sunlight hours.


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