VE6AB Technical
Dew Point & Humidity
This photo is of my sling psychrometer and my hand held weather station with the dew point being read on the display. Use of the sling psychrometer and comparing the readings off the chart indicate the relative humidity. Most people know that dew point and humidity have something to do with moisture, but a lot of people don’t really understand what they mean.
Relative humidity is expressed as a percent and measures water vapor relative to the air temperature. It’s a measure of the actual amount of water vapor in the air compared to the amount of water vapor that can exist in the air at its current temperature.
Warm air can hold more moisture than cold air. So air with the same amount of absolute humidity will have a higher relative humidity if the air is cooler and lower relative humidity if the air is warmer. For example a relative humidity of 50% means the air is holding half of the water vapor it’s capable of holding.
Dew point defines the point at which air will become saturated and condense. In other words, the point at which water droplets form. If suspended in air, you’ll see clouds form. If the air temperature and dew point meet on a surface such as a glass or on a leaf, dew will form and you’ll see drops of moisture on the surface. If the air temperature is below freezing, the condensation will appear as frost or ice.
I like to refer to dew point because it really helps define our comfort level. The closer the dew point and temperature, the more humid or muggy it will feel. On this morning using my sling psychrometer the humidity I measured was 57%, and with a dew point of 12.4, it made for a reasonably pleasant morning to be out and about with the temperature 24 C.
Dew point also plays a key role in the formation of fog. If the air temperature and dew point get within a few degrees of each other, the air will condense and a cloud will form. Remember, fog is just a cloud close to the ground. If the air temperature is below freezing, we will see freezing fog that will coat everything with a layer of frost.
I hope I haven't confused you too much with my explanation of how relative humidity and Dew point relate to each other.
