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Relative Humidity

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This activity will look at two of the most fundamental and important elements of weather: temperature and humidity.


1. The students will discover the need for measuring instruments to quantify weather observations. 2. The students will learn how to use a sling psychrometer to determine temperature and relative humidity values. 3. The students will explain and understand the difference between absolute and relative humidity and the relationships between temperature and atmospheric humidity.


Atmospheric Temperature is one of the most frequently considered of the many meteorological elements. This influence of air temperature on the growth and well-being of earth’s life forms is paramount. Temperature is basically a measure of the molecular kinetic energy of a substance. (Or how fast the molecules are moving). Therefore reference points upon which compare molecular activities or temperatures are needed. The freezing point and boiling points of water are used as 32OF or 0OCis the freezing point and 212OF or 100OC is the boiling point.

Water is a basic necessity for the life forms here on Earth. At a given moment, only about 1/100,000 part of the earth’s supply of water is found in the atmosphere, yet it is this atmospheric water that brings moisture to the surface of the earth and makes it habitable. In the hydrologic cycle, water is constantly being moved from the earth’s surface into the troposphere where it gives rise to clouds and precipitation, thereby bringing the water back to the surface again.

Humidity is defined as the amount of water vapor in the atmosphere. It is measured in several ways. In one method, the mass of the water vapor is measured per unit volume, usually a cubic meter (m3). This measure of humidity is referred to as absolute humidity and reported as grams H2O/m3.

The humidity may also be expressed as a mixing ratio. The mixing ratio represents the mass of water vapor in the atmosphere measured in grams per kilogram mass (g/kg) of dry air.

Specific humidity, yet another way to measure and report humidity, is essentially the same as the mixing ratio, differing only in that the sample mass of air includes the mass of H2O vapor contained within it.

While each of the methods just described to report humidity involve measuring the mass of H2O vapor within a sample unit of air, there is yet another way to measure the amount of water vapor in the atmosphere that utilizes a different approach. The concept of partial vapor pressure breaks out the amount of the total atmospheric pressure attributable solely to H2O vapor. For instance if the atmospheric pressure is 1000 millibars, and H2O vapor represents 3% of the mass of the air (which is manifested as air pressure) we could describe the amount of H2O vapor present by reporting the actual vapor pressure as 30 millibars.

Relative humidity, the most widely reported measure of humidity, is not a direct measure of the amount of water vapor in the atmosphere, but rather the ratio of the air’s water vapor content to its water vapor capacity at a given temperature. The capacity is the amount of H2O vapor needed to produce saturation at that particular temperature.

The various formulae for relative humidity are:

amount of water vapor in atmosphere (per unit) X 100 water vapor capacity of atmosphere (per unit)


mixing ratioX 100
saturation mixing ratio


actual vapor pressure X 100
saturation vapor pressure

Humidity can be expressed in several ways. One measure of humidity is vapor pressure. Every gas existing in air is contributing to part of the pressure being exerted by the air. The vapor pressure is the part of the total atmospheric pressure being exerted by the water vapor in the air. Therefore, it is a measure of the actual vapor content or absolute humidity. If the air is holding the maximum amount of vapor capable of holding, it is said to be saturated and the vapor content can be expressed as the saturated vapor pressure. There is a significant relationship between the temperature of the air and its capacity to hold moisture. The higher the temperature the greater of the air to hold moisture. This relationship between temperature and saturated vapor pressure.

Simple Calculations:
Using the formulas stated above in this lab perform the following calculations and answer the questions. You must show your work.

1 If the water vapor capacity of the atmosphere in a given location is 25 grams per kilogram of air sampled, and the actual amount of water vapor present is 5 grams per kilogram of air sampled, what is the relative humidity?

Water vapor/ wv capacity=rh
5g per k/ 25g per k=.20×100=20

2 If the water vapor capacity of the atmosphere in a given location is 12
grams per kilogram of air sampled, and the actual amount of water vapor present is 3 grams per kilogram of air sampled, what is the relative humidity?

3g per k/12g per k=.25×100=25

3 If the mixing ratio of a sample of air is 5 grams/kilogram, and the temperature of the sample is 15 degrees Celsius, yielding a saturation mixing ratio of 10 grams/kilogram, what is the relative humidity of the sample?

5gk/10gk=.5×100= 50

4 If the mixing ratio of a sample of air is 2 grams/kilogram, and the temperature of the sample is 25 degrees Celsius, yielding a saturation mixing ratio of 20 grams/kilogram, what is the relative humidity of the sample?

2g per k/20g per k=.10×100=10

5 If the vapor pressure is 10 millibars and the saturation vapor pressure is 40 millibars, what is the relative humidity of the atmosphere?

10 millibars/40 millibars=.25×100=25

6 Why was the relative humidity in question 2 higher than that calculated for question 1, even though the amount of water vapor present in the atmosphere was less?

Maybe because the capacity of the water vapor was a factor and the
temperature outside had an impact.
Dry air will be denser or heavier than a column of air containing any water vapor.

Use of the Sling Psychrometer

The sling psychrometer is a simple, yet effective instrument used to determine relative humidity. It does not, however, give a direct reading of relative humidity. Psychrometric tables must be used to obtain the relative humidity from the temperature data provided by the psychrometer. In this part of the lab, you will use the sling psychrometer and the psychrometric tables as demonstrated by the instructor. Use the data you obtain to complete the chart below: NOTE: Care must be taken with these instruments, as they are very fragile and easily damaged. HUMIDITY DATA TABLE

Temperature inside_70F____ Temperature outside_36F___ Time_8:20pm____

Explain the weather conditions at the present time: It is dark outside, cold, with a slight breeze.

|LOCATIONS: |WET °F |DRY °F |DRY-WET °F |DEW PT °F |% R.H. | | |BULB |BULB | | | | | | | | | | | |Lab-kitchen |62 |76 |14 |51 |44% | |Outdoors |38 |38 |0 |40 |40% | |on sidewalk | | | | | | |Outdoors on a grassy area|32 |36 |4 |25 |63% | |Outdoors under a tree |32 |36 |4 |25
|63% | |Parents Bedroom |56 |62 |6 |49 |68% | |Hallway |52 |60 |8 |45 |58% | |You pick a spot: My |56 |62 |6 |49 |68% | |Bedroom | | | | | |


1 Comment on any differences noted in the relative humidity values you determined for the various locations. Can you give any explanations for the differences?

It is different how the RH compares in the hallway than my bedroom or outside. It seems like the bigger the gap in the dry-wet reading, the lower the relative humidity will be.

2 How does air conditioning alter the relative humidity of a room?

Cooler air cannot hold as much moisture as warmer air. The process of cooling causes the moisture to condense and drain out of the air conditioner.

3 How does heating a room alter the relative humidity?

The relative humidity will go down, if all you do is heat the room. As the temp goes up, so does the ability of the air to hold more water vapor. If you heat the room without adding water vapor, then the air is “drier” compared to what it could be.

4 Can you think of any agents or circumstances that may cause the mixing ratio in a room to change?

Maybe someone is using an air conditioner or a humidifier/ dehumidifier.

5 Given the fact that the molecular weight of water vapor is less than that of dry air, what affect does increasing humidity have on the density of the atmosphere? Does this run counter to what your intuition tells you?

Yes moist air is lighter than dry air. Subconsciously, this seems kind of dumb, because air containing water should be heavier than dry air, but the difference is so great that clouds (contain droplets of moisture as well as water vapor) readily stay in the air.

6 How does relative humidity affect the comfort of people? Can you explain the physiological reasons for this?

It makes people feel uncomfortable because when the humidity is higher, sweat is less able to evaporate, making a person feel stickier and hotter.

7 The diurnal (daily) relationship between temperature and humidity is such that the lowest humidity should occur in the afternoon hours. It is during these times, however, when it often seems to be most humid and uncomfortable. What could be the reason for this perceived inconsistency?

During a warm days, we will feel humid because of the combination of high temperatures mixed with more water vapor in the air. When the temperature is warm, people exposed to this type of heat will sweat. It is the sweating process that we cool down.

8 When dew forms on outdoor objects, can it be assumed that the atmosphere is saturated? If so, why is there often no fog accompanying the dew?

If the atmosphere is the major source of moisture (dewfall), a certain amount of ventilation is needed to replace the vapor that is already condensed. I would think that this can be assumed to be saturated since the moisture forms the dew. There is no fog because the water vapor already is condensed into dew.

Portions of this lab came from the lab “Humidity” from Mississippi State University’s Graduate Meteorology Department with permission.

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