![]() ![]() Interpreting Relative Humidity, Temperature, and Dew Point Net condensation occurs onto condensation nuclei when there's a little extra cooling that we can't measure with a standard thermometer. Once the temperature falls to the dew point, relative humidity increases to 100 percent, and the measurable cooling ceases as long as dew points don't decrease further. Remember that, by definition, the dew point is the approximate temperature to which the water vapor in the air must be cooled at constant pressure in order for it to condense into liquid water drops. Since the dew point serves as a lower bound for temperature, on clear, calm nights when dew points won't change much, weather forecasters sometimes use the dew point as a guide for what the nighttime low temperature might be. So, for all practical purposes, the temperature does not measurably fall below the dew point and we don't see relative humidity values greater than 100 percent reported. This leads to relative humidity values slightly greater than 100 percent within clouds, for example. Still, in reality, when net condensation is occurring, the dew point is ever so slightly higher than the temperature (even if we can't measure it). But, the standard instruments that we use to make measurements are not precise enough to accurately measure the small difference between dew point and temperature when net condensation is occurring. In other words, the temperature has to fall slightly below the dew point. But, you already know that for net condensation to occur, the condensation rate has to be slightly greater than the evaporation rate. Relative humidity values calculated from standard weather instruments range from as low as near 1 percent when the evaporation rate greatly exceeds the condensation rate (a huge difference between temperature and dew point), to 100 percent when the evaporation rate equals the condensation rate (temperature and dew point are equal). I'll discuss some practical applications for relative humidity shortly, but first I want to mention a little quirk about relative humidity observations. When the condensation rate equals the evaporation rate at equilibrium (the dew point equals the temperature), relative humidity is 100 percent.The smaller the difference, the higher the relative humidity. The larger the difference between dew point and temperature, the lower the relative humidity. Since relative humidity depends on both the condensation rate and the evaporation rate, it depends on both dew point and temperature.The equation for relative humidity, RH = condensation rate evaporation rate × 100%, shows that its essentially a comparison between the condensation rate and the evaporation rate. ![]() Let's review a few important points you've already learned: While relative humidity is not an absolute measure of how much water vapor is present (it doesn't tell us about the concentration of water vapor in the air), it's still an extremely useful variable. ![]() My guess is that you've heard weather forecasters mention relative humidity many times in weather broadcasts or weather-related articles. ![]() But, it's probably not the most commonly cited moisture variable in weather reports. Finally, a historical perspective is given with anecdotes about some of the early work in this field.There's no doubt that dew points can tell a meteorologist (or any weather-savvy person) quite a bit about moisture. Several useful applications of the simple conversion are presented, in particular the computation of the cumulus cloud-base level (or lifting condensation level) as z LCL > (20 + t/5)(100 – RH), where z LCL is in meters when t is in degrees Celcius and RH in percent. This article examines the mathematical basis and accuracy of this and other relationships between the dewpoint and relative humidity. However, there is a very simple rule of thumb that can be very useful for approximating the conversion for moist air (RH > 50%), which does not appear to be widely known by the meteorological community: t d decreases by about 1☌ for every 5% decrease in RH (starting at t d= t, the dry bulb temperature, when RH = 100%). The exact conversion from RH to t d, as well as highly accurate approximations, are too complex to be done easily without the help of a calculator or computer. The relative humidity (RH) and the dewpoint temperature ( t d) are two widely used indicators of the amount of moisture in air. ![]()
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