Weather school: Relative humidity

The term ‘relative humidity’ is probably one of the most misunderstood in weather-speak

The “dew point” refers to the temperature to which we would have to cool the air down for dew to begin forming.

It seems that pretty much every summer I have to stop and explain to different people just how humidity works and why on a hot summer day when the humidity is in the 50-60 per cent range it is humid even though it doesn’t sound like it is. So, in this issue I am going to revisit this topic by looking at exactly what humidity is, how we measure it and how it is reported and interpreted.

Humidity by its simplest definition is the amount of water vapour that is in the air. The warmer the air, the greater the distance between air molecules and therefore, the greater the holding capacity of the air for water vapour. Conversely, when air is cooled, the distance between air molecules decreases, leaving less room for the air to hold water vapour. Because of this relationship, warm air has the capacity to hold much more water than cold air. The question is, by how much?

If we look only at the water-holding capacity of air at different temperatures and measure the amount of water by its mass (weight), we would find that for every 10 C increase in temperature, the holding capacity of the air for water nearly doubles. For example, air that is at 0 C can hold almost four grams of water for every one kilogram of air. If we warm that air up to 10 C the air could hold nearly eight grams, and by 30 C that same one kg of air would have the capacity to hold nearly 28 grams of water. When humidity is measured this way, it is referred to as specific humidity. While this is a useful way to measure humidity, it is not the way that we usually hear it reported.

The most common way humidity is reported is relative humidity. Unfortunately, it is probably one of the most misunderstood terms used in trying to describe the weather. Relative humidity is a ratio of the amount of water vapour that is in the air, compared to the maximum that it could hold under those same conditions, and is expressed as a percentage. Taking one of the examples from above, if we had an air temperature of 10 C and had eight grams of water vapour per kilogram of air, our relative humidity would be 100 per cent, calculated as follows:

Actual amount of water vapour in the air (eight grams) ÷ The holding capacity of the air at the current temperature (air at 10 C has a holding capacity of about 8 grams) x 100 = 100 per cent.

Now, if this same air was warmed up to a temperature of 30 C and the amount of water vapour in the air didn’t change, our relative humidity would be around 29 per cent [(8 grams ÷ 28 grams) x 100].

This is where the misunderstanding begins to develop. When the air temperature was 10 C and the relative humidity was 100 per cent, people would say it was humid out, but once the temperature has warmed up to 30 C and the relative humidity dropped to 29 per cent, people would say it is very dry out — but in reality, the amount of water vapour in the air has not changed, only the temperature has. This is particularly noticeable on humid summer days. In the morning we have had temperatures in the 16 to 18 C range with relative humidities in the 100 per cent range — it’s humid out. By afternoon, with temperatures in the mid- to upper 20s, the relative humidity has dropped down to around 50-60 per cent and people now say it is dry out — but it’s not, it’s humid; the amount of water vapour in the air hasn’t changed!

A better way to measure humidity is by using the dew point temperature, which we simply refer to as the dew point.

This measurement is a fairly simple way of telling us exactly how much moisture is in the air, no matter how the temperature changes during the day. The dew point is the temperature to which we would have to cool the air down for condensation (or dew) to begin forming — in other words, the temperature that the air would have to be to give us 100 per cent relative humidity. In our previous example, the dew point first thing in the morning would have been around 18 C since the air was at its holding capacity. By the afternoon, even though the air had warmed up, the dew point would still have been around 18 C since no additional water vapour was added or removed from the air. How does this relate to overall humidity?

If we have dew points that are less than 10 C, the atmosphere is considered to be fairly dry and dew points in the 10 to 15 C range are fairly comfortable. Once we get into the 15 to 20 C range, the atmosphere is becoming fairly humid. Dew points over 20 C are getting very humid, and it will start to feel very uncomfortable outside. If the dew point goes over 25 C the atmosphere is heavy with moisture and conditions will be very uncomfortable and can even be dangerous. Going back to relative humidity once more, to pound home the difference between this and the dew point: If the dew point was 25 C, we know it is very humid out no matter what the temperature is, but if the temperature was, let’s say, 35 C, the relative humidity would only be around 55 per cent, and I could guarantee at least one person would be saying it’s not that humid out!

In Manitoba, we rarely get dew points above 25 C, our record dew point being in the 28 C range around the Carman/Morden area (Carman did hit a record dew point of 30 C back in 2007), but we often have extended periods with dew points hovering in the low 20s. So remember, if it is a hot summer day with dew points in the low 20s, even if the relative humidity is only 50 per cent — it is still humid out!

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.



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