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9.8 Temperature, Pressure and Humidity

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Air Density and its Effects

In simple terms, density is the mass of anything - including air - divided by the volume it occupies. Scientists use the metric system to measure density in terms of kilograms per cubic metre.

The air's density depends on its temperature, its pressure and how much water vapour is in the air. Pressure and density are directly related. Decreased pressure results in decreased density. Temperature and density are inversely related. Increased temperature results in decreased density. (This assumes a constant pressure and increased volume.)

Humidity and density are inversely related. Increased humidity results in decreased density. When looking at dry air we'll be concerned only with temperature and pressure.

Temperature and Air Density: Volume is directly proportional to temperature. At constant force and pressure, the volume of a gas increases proportionately as its absolute temperature increases. If the absolute temperature is doubled, the volume is doubled.

The molecules of nitrogen, oxygen and other gases that make up air are moving around at incredible speeds, colliding with each other and all other objects. The higher the temperature, the faster the molecules are moving. As the air is heated, the molecules speed up, which means they push harder against their surroundings.

If the air is in a balloon, heating it will expand the balloon, cooling it will cause the balloon to shrink as the molecules slow down. If the heated air is surrounded by nothing but air, it will push the surrounding air aside. As a result, the amount of air in a particular "box" decreases when the air is heated if the air is free to escape from the box. In the free atmosphere, the air's density decreases as the air is heated.

Pressure and Air Density: Pressure has the opposite effect on air density. Increasing pressure increases air density. Think of what happens when you press down the handle of a bicycle pump. The air is compressed. The density increases as pressure increases.

Altitude and weather systems can change air pressure. As you ascend, the air pressure decreases from 1,000 hectopascal at sea level to 500 hectopascal at 18,000 feet approximately. At 100,000 feet above sea level the air pressure is about 10 hectopascal. Weather systems that bring higher or lower air pressure also affect the air density, but not nearly as much as altitude.

We find air density is at its lowest with; at high elevations, on hot days and low barometric pressures. The air's density is highest at low elevations when the barometric pressure is high and the temperature is low, such as on a sunny but extremely cold winter's day.

Humidity and Air Density: Most people who haven't studied physics or chemistry find it hard to believe that humid air is lighter, less dense, than dry air. How can the air become lighter if we add water vapour to it?

Scientists have known this for a long time. The first was Isaac Newton, who stated that humid air is less dense than dry air in 1717 in his book, Optics. But, other scientists didn't generally understand this until later in that century.

To see why humid air is less dense than dry air, we need to turn to one of the laws of nature the Italian physicist Amadeo Avogadro discovered in the early 1800s. In simple terms, he found that a fixed volume of gas, say one cubic metres, at the same temperature and pressure, would always have the same number of molecules no matter what gas is in the container.

Avogadro’s Law: At constant temperature and pressure, the volume of a gas increases proportionately as its molecule amount increases. If the molecule amount is doubled, the volume is doubled.

Imagine a cubic metre of perfectly dry air. It contains about 78% nitrogen molecules. Another 21% of the air is oxygen. The final one percent is a mixture of other gases, which we won't worry about. Molecules are free to move in and out of our cubic metre of air. What Avogadro discovered leads us to conclude that if we added water vapour molecules to our cubic metre of air, some of the nitrogen and oxygen molecules would leave. Remember, the total number of molecules in our cubic metre of air stays the same.

The water molecules, which replace nitrogen or oxygen, have a lighter molecular weight. They are lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapour decreases the weight of air within one cubic metres; that is, it's density decreases.

Wait a minute, you might say, "I know water's heavier than air." True, liquid water is heavier, or denser, than air. But, the water that makes the air humid isn't liquid. Water vapour, which is a gas can’t be seen with the human eye, is odourless and lighter than nitrogen or oxygen.

Compared to the differences made by temperature and air pressure, humidity has a small effect on the air's density. However, humid air is lighter than dry air at the same temperature and pressure.

Psychrometer: A psychrometer has two built-in thermometers to determine humidity. One thermometer (the dry bulb) measures the ambient (room) temperature, and the other thermometer (the wet bulb) is wrapped in an absorbent material moistened with distilled water. To obtain a reading, air is passed through the psychrometer to evaporate moisture on the wet bulb.

The rate or speed of evaporation is conditioned by the amount of moisture in the air. When the moisture has evaporated, a reading of the temperature on the wet bulb is taken, as the process of evaporation will have cooled it. The readings on the dry bulb thermometer and the wet bulb thermometer are then compared and used to determine the actual humidity.

There are several types of psychrometer, the most popular for libraries and archives being the sling version, which is the least expensive and easiest to operate. The instrument is rapidly spun for several minutes to impel air through it.

Psychrometers must themselves be calibrated from time to time, and procedures must be followed to the letter in order to obtain reliable information. Misreading can be caused by over-wetting the bulb, under-rotating the sling, exposing the instrument to strong light, and handling the instrument without gloves.