The Layers Of The Atmosphere










FIGURE 1.1 Layers of the atmosphere.

about 217 K (-56°C). The troposphere can be divided into the planetary boundary layer, extending from the Earth's surface up to about 1 km, and the free troposphere, extending from about 1 km to the tropopause.

As air moves vertically, its temperature changes in response to the local pressure. For dry air, this rate of change is substantial, about 1°C per 100 m (the theory behind this will be developed in Chapter 16). An air parcel that is transported from the surface to 1 km can decrease in temperature from 5 to 10°C depending on its water content. Because of the strong dependence of the saturation vapor pressure on temperature, this decrease of temperature of a rising air parcel can be accompanied by a substantial increase in relative humidity (RH) in the parcel. As a result, upward air motions of a few hundreds of meters can cause the air to reach saturation (RH = 100%) and even supersaturation. The result is the formation of clouds.

Vertical motions in the atmosphere result from (1) convection from solar heating of the Earth's surface, (2) convergence or divergence of horizontal flows, (3) horizontal flow over topographic features at the Earth's surface, and (4) buoyancy caused by the release of latent heat as water condenses. Interestingly, even though an upward moving parcel of air cools, condensation of water vapor can provide sufficient heating of the parcel to maintain the temperature of the air parcel above that of the surrounding air. When this occurs, the parcel is buoyant and accelerates upward even more, leading to more condensation. Cumulus clouds are produced in this fashion, and updraft velocities of meters per second can be reached in such clouds. Vertical convection associated with cumulus clouds is, in fact, a principal mechanism for transporting air from close to the Earth's surface to the mid- and upper-troposphere.

The stratosphere, extending from about 11 km to about 50 km, was discovered at the turn of the twentieth century by the French meteorologist Léon Philippe Teisserenc de Bort. Sending up temperature-measuring devices in balloons, he found that, contrary to the popular belief of the day, the temperature in the atmosphere did not steadily decrease to absolute zero with increasing altitude, but stopped falling and remained constant at 11 km or so. He named the region the stratosphere from the Latin word stratum meaning layer. Although an isothermal region does exist from about 11-20 km at midlatitudes, temperature progressively increases from 20 to 50 km, reaching 271 K at the stratopause, a temperature not much lower than the average of 288 K at the Earth's surface. The vertical thermal structure of the stratosphere is a result of absorption of solar ultraviolet radiation by 03.

1.4 PRESSURE IN THE ATMOSPHERE 1.4.1 Units of Pressure

The unit of pressure in the International System of Units (SI) is newtons per meter squared (N m 2), which is called the pascal (Pa). In terms of pascals, the atmospheric pressure at the surface of the Earth, the so-called standard atmosphere, is 1.01325 x 105 Pa. Another commonly used unit of pressure in atmospheric science is the millibar (mbar), which is equivalent to the hectopascal (hPa) (see Tables A.5 and A.8). The standard atmosphere is 1013.25 mbar.

Because instruments for measuring pressure, such as the manometer, often contain mercury, commonly used units for pressure are based on the height of the mercury column (in millimeters) that the gas pressure can support. The unit mm Hg is often called the torr in honor of the scientist, Evangelista Torricelli. A related unit for pressure is the standard atmosphere (abbreviated atm).

We summarize the various pressure units as follows:

1 Pa = lNnT2 = lkgnrV2 1 atm = 1.01325 x 105 Pa 1 bar = 105 Pa 1 mbar = 1 hPa = 100 Pa 1 torr = ImmHg = 134 Pa

Standard atmosphere: 1.01325 x 105Pa = 1013.25 hPa = 1013.25 mbar = 760 torr

The variation of pressure and temperature with altitude in the standard atmosphere is given in Table A.8. Because the millibar (mbar) is the unit most commonly used in the meteorological literature, we will use it when discussing pressure at various altitudes in the in the atmosphere. Mean surface pressure at sea level is 1013 mbar; global mean surface pressure, calculated over both land and ocean, is estimated as 985.5 mbar. The lower value reflects the effect of surface topography; over the highest mountains, which reach an altitude of over 8000 m, the pressure may be as low as 300 mbar. The 850 mbar level, which as we see from Table A.8, is at about 1.5 km altitude, is often used to represent atmospheric quantities, such as temperature, as the first standard meteorological level above much of the topography and a level at which a considerable quantity of heat is located as well as transported.

1.4.2 Variation of Pressure with Height in the Atmosphere

Let us derive the equation governing the pressure in the static atmosphere. Imagine a volume element of the atmosphere of horizontal area dA between two heights, z and z + dz. The pressures exerted on the top and bottom faces are p(z + dz) and p(z), respectively. The gravitational force on the mass of air in the volume = pg dA dz, with p(z) > p{z + dz) due to the additional weight of air in the volume. The balance of forces on the volume gives

Dividing by dz and letting dz —> 0 produce dp(z)

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