Past IEM Features tagged: rh
The weather over the past few days has been near perfection with warm temperatures, low humidities, and a light breeze. One of the measures of humidity is relative humidity (RH). The RH percentage is somewhat difficult to directly interpret as it does not account for temperature. That aside, the featured chart looks at daily climatology of RH based on period of record data for Des Moines. The red line represents the daily frequency of having the minimum RH below 40%, like it was on Wednesday. This line depicts a clear seasonal cycle with frequencies peaking during mid Spring and mid Fall. This is one of the reasons folks tend to like these two times of year as temperatures are generally warm and humidities are lower like it currently is.
Relative Humidity is one of the more commonly expressed weather variables. It represents the percentage capacity that the air is full of water vapor. Since the value is relative, it is difficult to directly compare the value at different air temperatures. While the air certainly feels drier in the winter time, the relative humidity values are just as high as the summer time. The featured chart presents a computed climatology of RH for Ames. The blue bars represent the daily range of RH values for each day this year. The blue, green, and black lines are simple averages of the daily min, avg, and max. The red line represents the frequency of days with the maximum RH at or above 95%. A lot going on with the chart! Of note for this time of year is that relative humidities are dropping. Demand for water and subsequent transpiration by agricultural crops is on the rapid decline. You may have noticed the corn and soybeans are turning brown now! These lower RH values help to dry out the fields and crops to allow for harvest.
Tags: rh humidity
The featured chart displays the average dew point at a given temperature in September for Des Moines. With that dew point, the relative humidity is plotted as well. In general, air temperatures warm faster than humidity levels are able to increase due to evapotranspiration. The net effect is for relative humidity to decrease during the daytime hours. The more arid the climate, the steeper this line will be and you can check that out for yourself by generating this chart for locations out in Arizona, for example.
The featured image presents an hourly climatology of the frequency of having a relative humidity observation below 60 percent. Since relative humidity is a function of temperature, as the air warms during the daytime the rh value will drop. The plot indicates two maximum in the late springtime and fall. These times of year have strong heating and less profilic sources of evapotranspiration. July into August are a time of maximum corn and soy plant transpiration which helps to keep relative humidity values even while temperatures warm.
Tags: climate rh
The fun never stops with these hourly climatology plots. Today's feature is a simple hourly climatology of the average relative humidity observed at Des Moines. This plot shows a strong annual and diurnal signal! While dew point temperature is not a function of air temperature, relative humidity is! While the atmosphere cools during the night, relative humidities rise (air temperature drops, dew point remains nearly constant). This plot also shows the least humid time of year being in April, when temperatures are warming nicely and vegetation is not transpiring yet.
Tags: climate rh