Ways to express humidity

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Ways to express humidity

Ways to Express Humidity

There are several ways to describe the humidity of the air. Absolute Humidity expresses the water vapour content of the air using the mass of water vapour contained in a given volume of air. It may be measured in grams of vapour/cubic meter of air. A problem with using absolute humidity is that an air parcel changes volume as the ambient temperature and pressure change. This means that the absolute humidity changes when the volume changes, even though the mass of water vapour has not changed.

Specific Humidity measures the water vapour content of the air using the mass of the water vapour for a given mass of air. It may be measured in grams of water vapour per kilogram of air. The kilogram of air measured includes the water vapour present (compare this to mixing ratio, below). Unlike absolute humidity, specific humidity doesn't change as the air parcel expands or is compressed.

Mixing Ratio also measures the water vapour content using a measure of mass, but it measures the mass of water vapour for a given mass of dry air. It may be measured in grams of water vapour per kilogram of dry air. Notice the difference between mixing ratio and specific humidity: specific humidity includes the water vapour in the air in the denominator, while mixing ratio measures water vapour per mass of dry air. Since water vapour comprises only a few percent of the mass of air, the values for specific humidity and mixing ratio are very close for a given parcel of air. Mixing ratio is not affected by changes in pressure and temperature. This is a commonly used measure by meteorologists. At a temperature of 20 degrees C, at average sea level pressure, the saturation mixing ratio is 14 grams of water per kilogram of dry air.

Vapour pressure measures the water vapour content of the air using the partial pressure of the water vapour in the air. (Pressure may be expressed using a variety of units: in pascals, in millibars, in pounds per square inch, among others). The gases in the atmosphere exert a certain amount of pressure (about 1013 millibars at sea level). Since water vapour is one of the gases in air, it contributes to the total air pressure. The contribution by water vapour is rather small, since water vapour only makes up a few percent of the total mass of a parcel of air. The vapour pressure of the water in the air at sea level, at a temperature of 20 degrees C, is 24 mb at saturation.

Most of these measures of humidity are not easily determined directly. It is actually easier to measure relative humidity.

Relative Humidity: we can compare how much water vapour is present in the air to how much water vapour would be in the air if the air were saturated. For this we use relative humidity. Relative humidity is a ratio that compares the amount of water vapour in the air with the amount of water vapour that would be present in the air at saturation. One ways it can be stated would be as the ratio of the actual mixing ratio to the saturation mixing ratio. Relative humidity is given as a percentage: the amount of water vapour is expressed as a percent of saturation. If 10 grams of water vapour were present in each kilogram of dry air, and the air would be saturated with 30 grams of water vapour per kilogram of dry air, the relative humidity would be 10/30=33.3%.

For example, a parcel of air at sea level, at a temperature of 25 degrees C, would be completely saturated if there were 20 grams of water vapour in every kilogram of dry air. (Question: which measure of humidity are we using here? Answer 1M)( The measure we are using is mixing ratio: grams of water vapour per kilogram of dry air.). If this air actually contained 20 grams of water vapour per kilogram of dry air, we would say that the relative humidity is 100%.
If the parcel of air (at sea level at 25 deg C) actually had 10 grams of water vapour per kilogram of dry air, what is its relative humidity? Answer 2M(The relative humidity would be 50%. 10 grams water vapour/kg dry air compared to 20 grams water vapour/kg dry air is 10/20=50%).
If a parcel of air (at sea level at 25 deg C) had 18 grams of water vapour per kilogram of dry air, what is its relative humidity? Answer 3M (Relative humidity would be 18/20=90%).

Humidity and Temperature

Consider a bowl of water. We've already seen this bowl, and we made water evapourate from it. As you recall, to make the water evapourate, we added heat, which was absorbed by the individual molecules of water. As each molecule absorbs heat, it gets more energetic, and eventually, has so much energy that it breaks the hydrogen bonds holding it to the other water molecules, leaves the liquid water, and floats off on its own, as a molecule of water vapour. In other words, it evapourates.

Even while some water molecules are evapourating, others are condensing, changing from the vapour state to the liquid state, and joining the liquid water in the bowl. At any given temperature, there will eventually be an equilibrium between the number of molecules evapourating, and the number of molecules condensing. When the number of molecules evapourating balances the number of molecules condensing, we say that the air above the liquid water is saturated. The term saturation refers to the maximum amount of water that can be present as a vapour in the atmosphere. If the air above the water bowl is saturated, then for every molecule of water that evapourates, another molecule condenses.

But we know that to make water evapourate, you add heat. So what happens if we raise the temperature of the bowl and the surrounding air? More water will evapourate. For a while, the rate of evapouration will exceed the rate of condensation.

Eventually, though, the balance between evapouration and condensation will stabilize at the new temperature. Once again, evapouration will balance condensation. The air will again be saturated, but there will be more molecules of water vapour present in the air above the bowl. At the higher temperature, more water vapour will be present in the air at saturation. This is a good general rule to remember: the higher the air temperature, the more water vapour will be present in the air at saturation.

What happens if we lower the temperature? As the temperature is lowered, more water molecules return to the liquid state (condense) than evapourate. Eventually, at the new lower temperature, there will again be a balance between the number of molecules evapourating and the number condensing. But there will be fewer molecules of water vapour in the air at the new cooler temperature than were present at higher temperatures. This rule to remember is just a restatement of the previous one: the lower the air temperature, the less water vapour will be present in the air at saturation.

Relative humidity depends on two factors: the amount of moisture available, and on the temperature.So you can have a change in relative humidity in one of two ways:

1) Change the amount of water vapour available; if there is liquid water present, for instance, a lake, you can have an increase in relative humidity by evapouration from the surface of the lake. This is pretty obvious. You’re adding water vapour, so the humidity increases.

2) The other way is to change the temperature of the air, while holding the water vapour constant. Even though there is no water source, and no water vapour is added, a lowering of air temperature results in a rise of relative humidity. This is automatic. The amount of water vapour that could be present at saturation is less at the lower temperature, so the existing amount of water vapour represents a higher percentage of the saturation level of the air. Similarly, a rise in temperature results in a decrease in relative humidity, even though no water vapour has been taken away.

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