Humidity Vs. Relative Humidity

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By Steve MacLaughlin on April 3, 2013, 7:09pm Last modified: April 4, 2013, 1:47pm

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Today was the first time this season that we had a Red Flag Warning...an advisory that prepares people for an elevated wildfire threat due to a combination of gusty winds and very dry conditions. Connecticut does not see Red Flag Warnings as often as other places in the country like Texas, but the cause is the same: low humidity.

But what does humidity actually mean? I thought today would be a good day to go over one of the most important weather fundamentals and also something that many people can easily mix up.

"Humidity" and "Relative Humidity" are not the same thing. Humidity is basically how moist or dry it is. Relative Humidity is the amount of moisture in the air 'relative' to the temperature. And it is that word 'relative' that makes 'relative humidity' not only confusing, but unimportant to meteorologists on most days.

The way we determine relative humidity is by looking at the relationship between the air temperature and the dew point temperature. The dew point is the temperature at which condensation occurs when you cool down the air. The air temperature can never drop below the dew point temperature. When the temperature drops to the dew point, we get saturation...that means a relative humidity of 100% and dew or fog or clouds or precipitation.

So...lets say at 4pm the temperature is 75 degrees and the dewpoint is 55 degrees (a perfect day, by the way). The relative humidity is about 50%. Now, let's say the sun sets and the temperature begins falling to 65 degrees at midnight. The dew point is still 55 degrees, since the dew point very rarely changes much on any given day. The relative humidity is 70%. At 5am, the temperature drops to 55 degrees and the dew point is still 55 degrees. Now we have saturation and on a night like this, maybe dew or fog. Once you hit saturation, it's also tough for the temperature to drop as easily, so the low temperature probably stays at about 55 that morning. Once the temperature starts going back up with sunrise, the relative humidity starts dropping again because the dew point is probably still about 55 degrees.

So...what does all this mean? It means that relative humidity is a number that means very little since it is always changing and is based on the temperature. Whether it was 50% or 70%, who cares? Who can really tell the difference? That doesn't tell us anything about how much moisture is in the air. It matters when and only when the temperature drops to the dew point and we get saturation, which is how we figure out when it will rain or snow.

So when I want to know what the humidity is - I never look at relative humidity - it's always changing - 50% humidity in the middle of summer can be oppressive while 50% humidity in the middle of winter is just average. It doesn't mean anything to me when I'm forecasting.

Meteorologists look at dew point, and here's why: The dew point tells us exactly how humid it is. It doesn't matter what the temperature is. When the dew point is low, it is dry. When the dew point is high, it is moist.

In the middle of summer, a really humid day will have a dew point in the 70s which is about as high as it goes in this part of the country. But the relative humidity is constantly changing all day as the temperature is going up and down. Remember...the dew point doesn't change much on any given day unless a front comes through. But the temperature rises and falls as the sun comes up and down. If the dew point stays the same but the temperature is rising, the relative humidity will go down. That means if your relative humidity is 80% in the morning, it might be 40% in the afternoon. But it doesn't change the amount of moisture in the air - that is based only on the dew point. If the dew point stays at 75 degrees all day, it doesn't matter what time of day it is or if it is sunny or dark. We know exactly how humid it will be because the dew point tells us this. And 75 is humid. Period. Relative humidity only starts to matter if the air temperature is gonna drop to the dew point of 75. When that happens, the relative humidity becomes 100% and condensation or precipitation happens.

Today, our relative humidity was very low in the afternoon...but the real important number that jumps out at me today is the dew point. As of 6pm, dew points across the state are in the single digits. They very rarely get that low except in the dead of winter. I don't care about the temperature or relative humidity. All I care about is dew points in the single digits and very high winds. That combination is why we have the high fire danger.

So...when you are looking to figure out the weather, how much moisture is in the air, how sticky it is outside - basically, when you want to know the humidity - don't look at relative humidity - look at the dew point. Learn to follow the dew point trends and what the dew point does to the weather and you will unlock forecasting. Remember...in this part of the country, our dew point on the most humid, summer day will close in on 75 or 80 degrees. In the dead of winter, it can be easily below zero on our driest days.

Look at the relationship between the dew point and the temperature and how they rise and fall. You will realize that these two numbers can tell you more about how it feels and what the weather is doing and what it is going to do than most anything else in meteorology.

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Steve MacLaughlin

Town: New Haven, CT  

Reporting for WXedge since January 2012.

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