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Relative Humidity Reading and Lab Exercise

Water is everywhere. It is one of the most ubiquitous substances on earth. It is in the air, under the ground, covers the poles, and it in the air you breathe. The gaseous water which is found in the air is called water vapor. Tiny molecules of it float in between the nitrogen and oxygen which make up most of the troposphere.

This atmospheric water has an enormous ability to control the weather. If you have ever boiled a pot of water for macaroni, you know why. It takes a very long time to boil water because water has an ENORMOUS capacity to take in heat. This property, called specific heat, means that it takes large amounts of energy to raise the temperature of water. It absorbs considerably amounts of heat energy from the environment around it in order to evaporate, and once warm, stays warm for a long time. It is because of this that we study atmospheric water – it basically controls the temperature of the atmosphere.

Humidity is a word which means ‘the amount of water in the atmosphere.’ People often make a mistake and think humid means hot. This is not true. Cold air can be humid, too, although its limit for humidity is much lower than warm air. Cold air has trouble holding water vapor because cold air doesn’t have the heat energy in it to support water as a gas. Because of this, when you cool air down, some of the water vapor condenses and becomes liquid. This is how clouds are formed. The opposite of humid is arid, which means dry.

There are two kinds of humidity: absolute humidity and relative humidity. Absolute humidity is the total amount of water in a given volume of air. If you were reading an absolute humidity measurement, it might look something like this: 3 cm3/m3(volume of water/volume of air.) We don’t often use this kind of measurement because normal people don’t really know what to make of these numbers. Instead, normal folks like us use relative humidity. Relative humidity is a percentage, which represents the percent of water in the air compared to how much it could hold at its current temperature. When you see a relative humidity, you’ll see something like 35% (very dry air), 65% (average air), or 95% (very humid air.) When the relative humidity reaches 100%, it rains/snow/precipitation forms!

There is another kind of variable we measure when it comes to water vapor in the air. This variable is called dew point. Remember how we said earlier that, if you cool air’s temperature down, condensation will happen, and clouds will form? Dew point is the temperature to which you need to cool the air for that to happen. If it suddenly gets colder where you are, and you hit the dew point temperature, it will rain/snow/precipitate. Said another way, if the dew point and air temperature are the same, some kind of water is coming out of the atmosphere. Dew is the name for water which forms on the grass in the morning, right before the sun comes up. It is for this reason dew point is names – it is the point where you’ll get dew.

There is a tool we use to measure these water-related variables. It is called a psychrometer. This might seem like an imposing name, but really, a psychrometer is just two thermometers. One of them is normal, and one has a little wet cloth piece tied to the end. The normal one will give us the normal temperature, also called the dry-bulb temperature. The wet-cloth thermometer will give us a temperature called the wet-bulb temperature. This wet-bulb temperature is affected by the water on the cloth piece. If the air is dry, lots of water will evaporate from the cloth piece. In order to make the phase change from liquid to gas, the water will take heat away from the thermometer bulb. This causes the temperature on the wet bulb thermometer to drop by a significant amount. If the air is already humid, the water on the wet cloth will just stay liquid, no evaporation will happen, and the temperature will not change. To summarize, if the dry-bulb and wet-bulb temperatures are the same, the air is humid, and you will probably get rain soon. If the dry-bulb and wet-bulb temperatures are different, the air is dry, and the sky is probably clear.

Vocabulary

  1. Ubiquitous
  1. Molecule
  1. Specific Heat
  2. Evaporate
  1. Humidity
  1. Arid
  1. Absolute Humidity
  1. Relative Humidity
  1. Dew Point
  1. Dew
  1. Psychrometer

Pre-lab Questions

  1. Why is water such an important substance in controlling the atmosphere?
  1. What is the difference between the two kinds of humidity?
  1. According to the reading, how does a psychrometer work?

Lab Procedure

  1. Get a small cup of water and psychrometer.
  2. Take the temperature in the room and record it. (look at the dry bulb thermometer on the psychrometer)
  3. Dip the psychrometer in the water and let the water soak in to the cloth.
  4. Swing it carefully for 45 seconds.
  5. Quickly take the wet-bulb temperature and record it.
  6. Subtract the dry-bulb and wet-bulb temperatures and record the difference.
  7. Head to the charts on page 12 of your ESRT. The way to use these charts is this: go DOWN the DRY-BULB TEMPERATURE, go ACROSS the DIFFERENCE. Use both charts, and record the relative humidity in the room, and the dew point in the room.
  8. Repeat steps 2 through 7 outside.

Dry-bulb Temperature © / Wet-bulb Temperature © / Difference © / Dew Point © / Relative Humidity (%)

Indoor

Dry-bulb Temperature © / Wet-bulb Temperature © / Difference © / Dew Point © / Relative Humidity (%)

Outdoor

Post lab question

  1. Are the relative humidities the same?
  1. Why would they be different?