Critical Thinking about Physical Processes - Essay Example

Falling or rising of air pressure signals the approach of stormy or fair weather respectively. Aneroid barometers are usually equipped with a second pointer that serves as a reference marker for setting the current reading. The user can observe the new pressure reading taken at a later time and compare it with the reading earlier set on the second pointer to determine air pressure tendency. The barograph then provides a continuous trace of air pressure variations with time, making it easier to determine air pressure tendency and forecasting weather.

Air is a compressible mixture of gases, so its volume and density are variable. The pull of gravity compresses the atmosphere so that the maximum air density is at the earth’s surface. So, the atmosphere’s gas molecules are closely spaced at the earth’s surface and the spacing between molecules increases with increasing altitude. Thus, the number of gas molecules per unit volume (air density) decreases with increasing altitude.

The average kinetic energy of individual molecules is increased when air is heated by absorption, conduction, convection or phase change of water. The greater activity of the heated molecules increases the spacing between neighbouring molecules decreasing the air density.

Increasing humidity affects air density in the same way as temperature rise. Air is less dense when the water vapour concentration is greater. Water vapour reduces the density of air because the molecular weight of water is less than the average molecular weight of dry air.

During an adiabatic process, no heat is exchanged between an air parcel and its environment. Adiabatic cooling of ascending unsaturated air amounts to 9.8 Celsius degrees per 1000 m of ascent, which is the dry adiabatic lapse rate. Because of the release of latent heat accompanying phase changes of water vapour, an ascending saturated (cloudy) air parcel cools more slowly than an ascending unsaturated (clear) air parcel. That’s why the rising saturated air parcels cool at the lower moist adiabatic lapse rate averaging 6 Celcius degrees per 1000 m compared to the dry adiabatic lapse rate.

Televised weathercasts report pressure in units of length (inches) as most of the watching audience is more familiar with basic instruments such as mercury and aneroid barometers on which the height of the mercury column changes as air pressure changes. Falling temperature allows the mercury column to drop, whereas increasing air pressure forces the mercury column to rise. This is the origin of the common practice of expressing air pressure in units of length (inches) rather than units of pressure (millibars).

Water vapour reduces the density of air because the molecular weight of water is less than the average molecular weight of dry air. So, the perception of sportscasters that baseballs hit to the outfield “will not carry far in the heavy air” is wrong because warm humid air is less dense than dry air and the baseballs hit to the outfield will carry far.

Cold air is denser than warm air so more air molecules are clustered close to the surface of the Earth. As air pressure is the weight of the atmosphere above a certain location, air pressure decreases more rapidly with increasing altitude in cold air than in warm air. That’s why the air pressure typically drops more rapidly with altitude on a cold day in January than on a warm day in July in Minneapolis, MN.

Tropics receive more heat than middle latitudes. Colder air is denser and therefore the column of air is shorter at middle latitudes than at the tropics, so the tropopause is higher over the tropics than middle latitudes. The movement of air masses due to pressure gradient also helps to transfer heat and momentum from the tropics to middle latitudes.