Farmers and ranchers understand what relative humidity means. Humidity and hay moisture are critical for proper baling. The right humidity is needed for cranking up the combines for wheat harvest in summer or the cotton stripper in fall. High humidities and mild temperatures make it ideal for fungi to attack plants. Low humidity increases fire danger.
So farmers and ranchers have a good feel for humidity. What about dewpoint temperature? Dewpoint temperature is that other “temp” that TV meteorologists have on their graphics. Dewpoint is common to meteorologists. It’s not commonly talked about in ag circles.
The percent of relative humidity is important to agriculture. It is such a good air moisture index for biological organisms and processes. Dewpoint temperature, on the other hand, is an important index of how much water is in the air.
Here is an example, from November 22, 2014, where the relative humidity looks similar across most of Oklahoma.
Looking at a Mesonet Dewpoint map, we get a whole different perspective.
Dryer air was crossing the state, indicated by the sharp color change and much lower dewpoint temperature values. Why did the relative humidity fool us about this change? Because the dryer air with lower dewpoints was also colder air. The relative humidities stayed high, because the air temperatures and the dewpoint temperatures were close to each other.
Dewpoint is all about water. Not water on the ground, but water in the air. We can see water on the ground and when it wets the soil or pavement. We can see it in the air as clouds or fog, but we can’t see it in the air around us.
Water, like all substances, exists in three forms, gas, liquid and solid. Dewpoint is an index used to monitor the amount of water when it is a gas in the air as water vapor.
Dewpoint is the temperature at which water vapor, suspended in the air, will condense to become liquid. Condensation occurs when the air is cooled to the dewpoint temperature. Last fall the Mesonet recorded an excellent example of how air cooling raises relative humidity.
Here is a Mesonet Dewpoint Temperature map on a day, October 22, 2014, when the dewpoints were generally consistent across most of the state, in the upper fifties to low sixties. That narrow spread of dewpoint temperatures meant that the quantity of water vapor in the air across the state was relatively uniform.
The relative humdity for the same date and time was much different. The blue area on the map below had humidities in the upper nineties, while in the green areas humidities were close to sixty.
The driver of the variation between relative humidity and dewpoint on October 22, 2014 was the air temperature. Where the humidity was high, the air temperatures were 10 degrees cooler.
When the dewpoint temperature and air temperature are equal, the humidity is 100%. As the air temperature rises above the dewpoint temperature, the humidity percentage drops lower and lower. As the air temperature cools, getting closer to the dewpoint temperature, the relative humidity percentage climbs higher.
The winter of 2014 has given us some good examples of how relative humidity and dewpoint temperatures are related to each other.
So what is a simple definition of dewpoint temperature? It is the temperature at which water in the air condenses into water droplets. It is the temperature, that if air cools to, dew forms.
[Air Temperature maps are available on the Mesonet Full Site, mesonet.org, under the Weather section. Relative Humidity and Dewpoint temperature maps are also in the Weather section.]
[On Mesonet Apps for Android and iPhone, Air Temperature, Relative Humidity and Dewpoint temperature maps can be accessed by selecting Maps on the Action Bar.]
[On the Mesonet Mobile website, m.mesonet.org, Air Temperature, Relative Humidity and Dewpoint temperature maps can be accessed from the Home Page menu.]