Prepared by: Dr. Ahmet KILIÇ

One of the more modern barometers still commonly used today is the aneroid barometer and aneroid barograph. The aneroid is a flexible metal bellow that has been tightly sealed after having some air removed. Higher atmospheric pressures squeeze the metal bellow while lower pressures allow it to expand. The aneroid barograph consists of a slowly rotating cylinder with paper on it that can give up to a week's worth of air pressure records before new paper is needed. As you can see in the graphic above, a pen at the end of a lever attached to the aneroid moves up and down according to pressure changes and records the pressure on the paper wrapped around the cylinder.

UNITS OF AIR PRESSURE

In the U.S., air pressure at the surface is reported in inches of mercury while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists, however, generally use pressures in hectopascals.

The term "hectopascals" is replacing the term "millibars." The hectopascal is a direct measure of pressure, like pounds per square inch, but in the metric system. Since the measurement is in the metric system, 1,000 millibars equal one bar. A bar is a force of 100,000 Newtons acting on a square meter, which is too large a unit to be a convenient measure of Earth's air pressure. Inches of mercury measure how high the pressure pushes the mercury in a barometer.

UNDERSTANDING OF AIR DENSITY  AND ITS EFFECTS

In simple terms, density is the mass of anything divided by the volume it occupies. As you go higher, the air's density decreases. The air's density depends on its temperature, its pressure and how much water vapor is in the air. We'll talk about dry air first, which means we'll be concerned only with temperature and pressure.

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. In the free atmosphere, the air's density decreases as the air is heated.

Pressure has the opposite effect on air. Increasing the pressure increases the density. The density increases as pressure increases.

Altitude and weather systems can change the air's pressure. As you go higher, the air's pressure decreases from around 1,000 millibars at sea level to 500 millibars at around 5 500 m. At 30 000 m above sea level the air's pressure is only about 10 millibars. Weather systems that bring higher or lower air pressure also affect the air's density, but not nearly as much as altitude.

We see that the air's density is lowest at a high elevation on a hot day when the atmospheric pressure is low, say in Denver when a storm is moving in on a hot day. The air's density is highest at low elevations when the pressure is high and the temperature is low, such as on a sunny but extremely cold, winter's day in Alaska.

Humidity and air density. Most people who haven't studied physics or chemistry find it hard to believe that humid air is lighter, or less dense, than dry air. 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.

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

Effects of air density on objects. More dense, or "heavier" air will slow down objects moving through it more because the object has to, in effect, shove aside more or heavier molecules. Such air resistance is called "drag," which increases with air density.

Lower air density penalizes pilots in three ways: The lifting force on an airplane's wings or helicopter's rotor decreases, the power produced by the engine decreases, and the thrust of a propeller, rotor or jet engine decreases. These performance losses more than offset the reduced drag on the aircraft in less dense air.

Air density also affects the performance of automobiles, with lower density decreasing performance in the same way it decreases the performance of aircraft engines.

Effects of lower density on humans . If you go high enough, either by climbing a mountain or going up in an airplane that does not have a pressurized cabin, you will begin feeling the effects of lower air pressure and density.

As air pressure decreases oxygen continues to account for about 21% of the gasses in the air as it does at seal level. But, there is less oxygen because there is less of all of the air's gasses. For instance, by the time you go to 3 500 m the air's pressure is about 40% lower than at sea level. This means that with each breath you are getting about 40% less oxygen than at the lower altitude.

These effects aren't felt in airliners because the cabins are pressurized to keep the air density inside about the same as it would be about 1 800 m or 2 150 m above sea level.

SEA LEVEL PRESSURE

Sea-level pressure, "A pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the Earth's surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of that hypothetical atmosphere are related to conditions observed at the station." To do this, you have to take into account the barometric reading at the station, the elevation above sea level, and the temperature.

The approximate equation is; P (sea level) = P(observed) + (10 mb/100 m) X height above sea level