Mean Sea Level Pressure Wind.sp

Thus, at any given pressure, an increase in temperature causes a decrease in density, and vice versa.

I Total pressure

Air is highly compressible, such that its lower layers are much more dense than those above. Fifty per cent of the total mass of air is found below 5 km (see Figure 2.13), and the average density decreases from about 1.2 kg m-3 at the surface to 0.7 kg m-3 at 5000 m (approximately 16,000 ft), close to the extreme limit of human habitation.

Pressure is measured as a force per unit area. A force of 105 newtons acting on 1 m2 corresponds to the Pascal (Pa) which is the Système International (SI) unit of pressure. Meteorologists still commonly use the millibar (mb) unit; 1 millibar = 102 Pa (or 1 hPa; h = hecto) (see Appendix 2). Pressure readings are made with a mercury barometer, which in effect measures the height of the column of mercury that the atmosphere is able to support in a vertical glass tube. The closed upper end of the tube has a vacuum space and its open lower end is immersed in a cistern of mercury. By exerting pressure downward on the surface of mercury in the cistern, the atmosphere is able to support a mercury column in the tube of about 760 mm (29.9 in or approximately 1013 mb). The weight of air on a surface at sea-level is about 10,000 kg per square metre.

Pressures are standardized in three ways. The readings from a mercury barometer are adjusted to correspond to those for a standard temperature of 0°C (to allow for the thermal expansion of mercury); they are referred to a standard gravity value of 9.81 ms-2 at 45° latitude (to allow for the slight latitudinal variation in g from 9.78 ms-2 at the equator to 9.83 ms-2 at the poles);

Figure 2.12 The spread of volcanic material in the atmosphere following major eruptions. (A) Approximate distributions of observed optical sky phenomena associated with the spread of Krakatoa volcanic dust between the eruption of 26 August and 30 November 1883. (B) The spread of the volcanic dust cloud following the main eruption of the El Chichón volcano in Mexico on 3 April 1982. Distributions on 5, 15 and 25 April are shown.

Figure 2.12 The spread of volcanic material in the atmosphere following major eruptions. (A) Approximate distributions of observed optical sky phenomena associated with the spread of Krakatoa volcanic dust between the eruption of 26 August and 30 November 1883. (B) The spread of the volcanic dust cloud following the main eruption of the El Chichón volcano in Mexico on 3 April 1982. Distributions on 5, 15 and 25 April are shown.

Sources: Russell and Archibald (1888), Simkin and Fiske (1983), Rampino and Self (1984), Robock and Matson (1983). (A) by permission of the Smithsonian Institution; (B) by permission of Scientific American Inc.

and they are calculated for mean sea-level to eliminate the effect of station elevation. This third correction is the most significant, because near sea-level pressure decreases with height by about 1 mb per 8 m. A fictitious temperature between the station and sea-level has to be assumed and in mountain areas this commonly causes bias in the calculated mean sea-level pressure (see Note 4).

The mean sea-level pressure (p0) can be estimated from the total mass of the atmosphere (M, the mean acceleration due to gravity (g0) and the mean earth radius (R):

where the denominator is the surface area of a spherical earth. Substituting appropriate values into this expression (M = 5.14 X 1018 kg, g0 = 9.8 ms"2, RE = 6.36 X 106 m), we findp0 = 105 kg ms"2 = 105 Nm"2, or 105 Pa. Hence the mean sea-level pressure is approximately 105 Pa or 1000 mb. The global mean value is 1013.25 mb. On average, nitrogen contributes about 760 mb, oxygen 240 mb and water vapour 10 mb. In other words, each gas exerts a partial pressure independent of the others.

Atmospheric pressure, depending as it does on the weight of the overlying atmosphere, decreases logarithmically with height. This relationship is expressed by the hydrostatic equation:

dp dz

i.e. the rate of change of pressure (p) with height (z) is dependent on gravity (g) multiplied by the air density (p). With increasing height, the drop in air density causes a decline in this rate of pressure decrease. The temperature of the air also affects this rate, which is greater for cold dense air (see Chapter 7A.1). The relationship between pressure and height is so significant that meteorologists often express elevations in millibars: 1000 mb represents sea-level, 500 mb about 5500 m and 300 mb about 9000 m. A conversion nomogram for an idealized (standard) atmosphere is given in Appendix 2.

2 Vapour pressure

At any given temperature there is a limit to the density of water vapour in the air, with a consequent upper limit to the vapour pressure, termed the saturation vapour pressure (es). Figure 2.14A illustrates how es increases with temperature (the Clausius-Clapeyron relationship), reaching a maximum of 1013 mb (1 atmosphere) at boiling-point. Attempts to introduce more vapour into the air when the vapour pressure is at saturation produce condensation of an equivalent amount of vapour. Figure 2.14B shows that whereas the saturation vapour pressure has a single value at any temperature above freezing-point, below 0°C the saturation vapour pressure above an ice surface is lower than that above a

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80 60 40 % TOTAL MASS OF AIR BELOW

80 60 40 % TOTAL MASS OF AIR BELOW

Figure 2.14 Plots of saturation vapour pressure as a function of temperature (i.e. the dew-point curve). (A) The semi-logarithmic plot. (B) shows that below 0°C the atmospheric saturation vapour pressure is less with respect to an ice surface than with respect to a water drop. Thus condensation may take place on an ice crystal at lower air humidity than is necessary for the growth of water drops.

Figure 2.13 The percentage of the total mass of the atmosphere lying below elevations up to 80 km (50 miles). This illustrates the shallow character of the earth's atmosphere.

Figure 2.14 Plots of saturation vapour pressure as a function of temperature (i.e. the dew-point curve). (A) The semi-logarithmic plot. (B) shows that below 0°C the atmospheric saturation vapour pressure is less with respect to an ice surface than with respect to a water drop. Thus condensation may take place on an ice crystal at lower air humidity than is necessary for the growth of water drops.

supercooled water surface. The significance of this will be discussed in Chapter 5D.1.

Vapour pressure (e) varies with latitude and season from about 0.2 mb over northern Siberia in January to over 30 mb in the tropics in July, but this is not reflected in the pattern of surface pressure. Pressure decreases at the surface when some of the overlying air is displaced horizontally, and in fact the air in high-pressure areas is generally dry owing to dynamic factors, particularly vertical air motion (see Chapter 7A.1), whereas air in low-pressure areas is usually moist.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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