A Teaching Tool - Relationships between temperature, relative humidity, and dew point

The meteograph above is from the Weather Briefing, LC weather station. It graphs temperature, dew point, relative humidity and time during a 48 hour period. Graphs are very useful for visualizing relationships between weather elements. This chart is useful for students to visualize the relationships.

The solid red line is temperature, the dotted green line is dew point, and the dashed magenta/blue line is relative humidity. Time is plotted on the horizontal axis. Temperature is plotted on the left vertical axis and relative humidity is plotted on the right vertical axis.

Temperature and dew point change independently of each other. That is, either one or both can change without affecting the value of the other. On the other hand, relative humidity is determined by the temperature and dew point. If either temperature or dew point (or both) change, the relative humidity will change. This sounds redundant but relative humidity is - relative. It compares how much water is in the air with the amount needed to saturate the air. Therefore, the amount of water in the air at 100% relative humidity is different at different air temperatures. There is much more water in the air at 100% humidity at a temperature of 70 degrees than at 30 degrees.

Temperature is sensible - we feel it change. Dew point is a measure of water vapor in the air. It is also sensible. High dew points make us uncomfortable because our bodies have a harder time keeping cool. Lower dew points occur when the amount of water air in the air is low. The dew point is the temperature at which dew would form if the air cooled. Dew points are higher when there is more water in the air.

For example, if the temperature is 80 and the dew point 70, the relative humidity is 72%. The air “contains” 72% of the moisture it can “hold” at that temperature. If the temperature cooled to 70 degrees the relative humidity would be 100% and moisture would condense as dew or a cloud.

Fog is a cloud that forms near the ground. It indicates the relative humidity is near 100%. If the temperature warmed to 90 degrees and the dew point stayed at 70 the relative humidity would be lower - 52%. Dew or fog would evaporate. It is important to know that relative humidity near the ground is usually different than it is higher in the atmosphere. If a day is sunny with scattered clouds the relative humidity is at or near 100% where clouds have formed. It is lower where clouds have not formed.

Referring to the meteograph, notice the dew point did not change significantly from midnight through about 6:00 a.m. on July 5th. However, the temperature cooled - which raised the relative humidity. When air cools and the dew point is unchanged, relative humidity increases.

The concept of relative humidity may be difficult to understand but knowing the relative humidity is important. Low relative humidity makes it easier for water to evaporate. Dry air dries soil, vegetation, and our skin. Notice how quickly clothes dry outside on a low humidity day. Low relative humidity and strong winds dry out vegetation in a forest. When fuels are dry the danger of wildfires increases.

If you want to delve into the scientific explanation of the relationships between temperature, dew point, and relative humidity click HERE.

NOTE: Air does not “hold” water. Other gases in the air, in order of the amount are: Nitrogen (78.09%), Oxygen (20.95%), Argon (.93%), Carbon Dioxide (.04%) is a trace gas along with a few other gases. Water vapor is a variable gas because it averages about 1% of the air by volume in the lower atmosphere and about .4% in the entire atmosphere. In some locations it can be as high as 4%.

Water vapor is different than the other gases because it is variable. Like all other gases it exerts pressure. If the pressure of the water vapor exceeds its saturations value for the current temperature it condenses. The vapor changes to a liquid (water) or a solid (ice). When water vapor condenses it may form a cloud of water droplets or ice crystals, depending on the temperature. Water may exist as a liquid in the atmosphere down to -32 F.