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Average Humidity: Is this the state of Florida?

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Average Humidity: Is this the state of Florida? Louisiana? Hawai’i? None of the above is correct. Alaska is the state with the most humidity throughout the year.

The freezing, frozen north is, in fact, more humid than the warmest, most southern states. Is this some kind of joke? Both yes and no.

The temperature, cloud cover, relative humidity, and precipitation are likely to be displayed when viewing a local newscast or looking at a weather app on your smartphone.

It’s worth noting that the term “relative” comes before the word “humidity.” Herein lays the blunder. Relative humidity is exactly what it sounds like: it’s relative.

It is a measurement of the amount of water vapour in the air compared to the air’s capacity to hold it. It is conveniently expressed as a percentage ranging from 0% to 100%.

We find these quantities soothing because they do not require any specific scientific knowledge to realise that 50% is twice as much as 25%.

This is the end of the easy portion. The ability of air to store more moisture increases as it warms. In contrast, the ability of air to hold moisture decreases as it cools. Consider it like a sponge.

Warmer air acts as a larger sponge, allowing it to store more water. Colder air acts as a smaller sponge, holding less water.

When both sponges are half-filled with water vapour (half = 50%), the warmer air contains significantly more moisture. We remark that the relative humidity is 50% in both circumstances.

In the illustration above, an 80°F airmass that is 50% saturated with water vapour is depicted as a sponge that is much larger than a 50°F airmass that is also 50% saturated.

This exemplifies why relative humidity is a poor metric for measuring surface wetness. [Note that there are exceptions to this rule, such as in the case of severe fire weather.]

Even though they both have a relative humidity of 50%, the warmer airmass contains 2.7 times as much water vapour as the cooler airmass in the preceding case.

Another factor to consider is the (diurnal) temperature trend during the day. The air is often cooler in the morning and warmer in the late afternoon.

The relative humidity changes substantially as the temperature rises and falls if the amount of water vapour in the air remains constant throughout the day.

This results in a situation where, despite no change in the amount of moisture in the air, the same city has a very high relative humidity at one time of day and a very low relative humidity at another time of day.

A More Accurate Measurement?

There are a number of methods for determining the moisture content of the air that do not rely on relativity. The most well-known is a concept known as the dew point.

Dew point is a common measurement for most people, despite the fact that it is not a direct measure of moisture. In summary.

The dew point is the lowest temperature that an airmass can reach given the amount of moisture in the air.

The dew point on the sponge diagram reflects the temperature if the sponges’ dry parts (yellow regions) were removed.

Other moisture measurements that aren’t covered here include specific humidity, mixing ratio, and vapour pressure.

There are some mystical dew point figures that may be used to estimate the amount of moisture in the air.

A dew point of more than 75°F is stifling. It’s the quantity of moisture in your body that causes you to sweat even when you’re not doing anything physically strenuous.

A dewpoint of 70°F or higher indicates that the air is particularly wet.

This is a common value in the southeast half of the United States during the summer months. When the dew point rises above 60°F, it begins to “feel humid.”

Dew points below 60°F are generally considered to be pleasant. Under 30°F, dew points feel particularly dry.

Comparison

Alaska is, as previously stated, the most humid state in terms of relative humidity. This is due to the cold temperatures.

A chilly airmass is simply incapable of retaining a large amount of moisture. Alaska has the lowest average dew point of any of the 50 states. This is a more accurate representation of the moisture situation.

The two maps below depict 1) annual relative humidity and 2) annual dew point averages.

You’d be excused for scratching your head in bewilderment if you simply saw the first map.

The desert Southwest appears to be what most people expect, but not everywhere else. The second map is more in line with our perceptions of “humid” places.

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