Meteorology Lab to Accompany Chapter 4 Relative Humidity Relative humidity is a common way to report the amount of moisture in the air. We hear the relative humidity given by weather reporters or meteorologists on the news. What is relative humidity? It is basically the amount of water vapor currently in the air divided by the maximum amount of water vapor the air can hold at the current air temperature times 100 % to get this value in units of percent. One way of describing the amount of water vapor in the air is by using vapor density (which is often grams H 2 O per kilogram air). Below we have a graph showing the vapor density of air (the “y” axis showing “Water in Air”) versus temperature. The red curve represents saturation , which is the point where the air is holding as much water vapor as it can at that temperature. For example, at 40° C, the air can hold a maximum of 50 g water per kg of air (100% relative humidity). (Be sure you can read the graph below to find out how I got this number.)
Usually, the air is not saturated and has less than 100% relative humidity. You can calculate the relative humidity by using the following formula: amount of water in air maximum amount of water in air x 100% = relative humidity Let’s look at an example. Suppose the air at 20° C has a water vapor content of 11 grams water per kilogram air. What is the relative humidity? You can read the graph above. Find 20° C on the “x” axis, go up until you get to the red curve, then read across to the “y” axis, and it looks like the amount of water in saturated air is about 15 g water per kilogram of air. You would then calculate the relative humidity as shown below: 11 g / Kg 15 g / Kg x 100% = 73% relative humidity 1) Now it’s your turn. Fill in the chart below. Temperature in ° C Actual water vapor in air in g/KG Amount of water vapor in air in g/Kg if air is saturated (read off of graph) Calculated relative humidity in % 1a 50 50 95 52.6% 1b 50 20 95 21.1% 1c 25 15 20 75% 1d 25 5 20 25% 2) Now let’s take a look at an important concept regarding relative humidity. Relative humidity is a comparison of how much water vapor the air is currently holding relative to the maximum amount of water vapor the air can hold. The maximum amount of water vapor the air can hold is dependent on the air’s temperature (as you see in the graph above). The warmer the air, the more water vapor the air can hold. To illustrate this concept, fill out the following chart: Temperature in ° C Actual water vapor in air in Amount of water vapor in air in g/Kg if Calculated relative humidity in %
g/KG air is saturated (read off of graph) 2a 40 20 50 40% 2b 35 20 38 52.6% 2c 30 20 28 71.4 2d 25 20 20 100%
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- Spring '17
- John DOe
- Meteorology, Atmospheric thermodynamics, Water vapor, Psychrometrics, g/kg