Monday "Yawn"

Good Morning Augusta Maine.

This morning it is partly sunny. Highs are in the upper 20s with light and variable winds.

Tonight it will be mostly cloudy. Lows in the lower 20s with light and variable winds.

The readings outside right now, taken from my own instrumentation  are:

a relative humidity of75 % with a Dew Point of 12.9ºF.

The temperature is 19.8ºF with almost no wind chill at all.

The wind velocity right now is between 0.0 mph and 1.3 mph out of the Southwest.

Our Barometric pressure is 30.17 and rising. The weather graphic indicates sun. 

We had no measurable precipitation in the past 24 hours in our area.

Visibility is 10.0 miles andovercast to 10,000 feet with some lower clouds.

This time of year we see a lot of weather posts with relative humidity and Dew point shown together. Confused?

Downloaded from USAToday.com

Q: Is dew point or relative humidity a better indication of how humid the air feels? My guess is it's relative humidity. Also is there much variation between the dew point temperature from day to day?

A: Dew point is by far the better measurement of how humid the air feels.

This is the case because dew point is a measurement of how much humidity is in the air, no ifs, ands or buts about it. Relative humidity tells you how much humidity is in the air compared with how much can be in the air at the temperature the air happens to be when you measure it.

This means that the relative humidity goes down as the temperature goes up even though the amount of water vapor in the air (humidity) remains the same.

To give you an idea of how this works, I used the Weather calculator on the Web site of the National Weather Service office in El Paso, to calculate some temperature, dew point, and relative humidity combinations.

If the temperature in the morning is 71 degrees, and the dew point is 70 degrees, the relative humidity would be 97% (rounded off). As the day warms up the dew point would stay the same unless a new air mass arrived with either more humid air or drier air.

Let's assume that this does not happen. If the air warmed to 95 degrees and the dew point stayed at 71, the relative humidity would be 44% and the heat index would be 101 degrees.

On a day like this, you'd feel sticky when you left your air-conditioned house in the morning, and by mid afternoon, people would be walking around saying things like, "Boy, it must be 95 degrees and 95 humidity."

No way.

The only way you'd ever see a temperature and relative humidity both of 95 would be if the dew point were 93 degrees. The only place you would find that high a dew point outside a lab experiment would be some place like the Persian Gulf.

A good general rule of thumb is that when the dew point is between 60 and 70 degrees F most people will think it's humid. When the dew point tops 70, almost everyone will feel uncomfortably humid. When it tops 80, as it rarely does anywhere in the USA, it's likely to set records.

As for day to day variation. The big variations come when a new air mass moves in. Or, during the summer you could have the humidity (thus the dew point) slowly increase if you have a steady wind from warm water, such as the Gulf of Mexico, even though the weather map never shows a warm front actually arriving.