Atmospheric pressure

The spatial maldistribution of global energy drives an equator to pole temperature gradient at the surface. The spatial pattern of temperature is linked dynamically with a vertical temperature gradient in the atmosphere. These thermal patterns in the horizontal and vertical dimensions contribute to definition of variations in atmospheric pressure in both dimensions, and they trigger atmospheric motion with a variety of time and space scales.

The atmosphere conforms to the ideal gas laws as expressed in Equation 3.2 even though it is a combination of gases in an unconfined state. The downward directed force due to the weight of the overlying atmosphere is atmospheric pressure. Mass is one of the fundamental quantities of the atmospheric system and the mass distribution is closely related to the pressure field. We assume the total mass of the atmosphere taken over a year is practically constant, but it varies in time because of changing constituents. The mass of water vapor is an

http://www.cdc.noaa.gov/.)"/>
Fig. 7.7. Global January mean specific humidity. Units in gkg_1. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

180°W 120" 80" 40" 0" 40" 80" 120" 180"E

Fig. 7.8. Global July mean specific humidity. Units in gkg 1. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

180°W 120" 80" 40" 0" 40" 80" 120" 180"E

Fig. 7.8. Global July mean specific humidity. Units in gkg 1. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

important component of the total atmospheric mass, and water vapor is subject to short-term fluctuations (Trenberth and Smith, 2005). Under hydrostatic conditions the surface pressure is a good measure ofthe total atmospheric mass in a unit vertical column.

Vertical differences in pressure complement the horizontal differences depicted by the surface pressure field. Understanding vertical variations is aided by recognizing the relationship between temperature and pressure in the vertical dimension. Since air density decreases as temperature increases, a warmer layer of air must cover a greater geometrical height to embrace the same mass of gas. Consequently, pressure changes more rapidly in cold air than in warm air. The end result is that the height difference between any two given pressure levels, known as the thickness, varies directly with temperature. This accounts for the atmosphere being thicker or deeper over the equator than over the poles. Thus, the tropopause is higher over the equator (16 km) than over the poles (8 km). At any specified height, a pressure gradient exists directed from the equator to the poles. The rapid decrease of atmospheric density and pressure with height is due to the compressibility of the atmosphere, and it contrasts markedly with the oceans where density displays little vertical variation (Peixoto and Oort, 1992).

The mean sea-level pressure map is commonly used for synoptic weather analysis and is probably the most familiar depiction of horizontal pressure differences. Average monthly conditions for January and July are shown in Figures 7.9 and 7.10, respectively, to illustrate seasonal variations. Atmospheric pressure for a specific site results from the mass of the gas molecules extending from the surface to the upper edge of the atmosphere. This is commonly envisioned as a column of air above a unit area. A mixture of gases representative of the standard atmosphere exerts a downward-directed force equivalent to 1013.2 hPa.

General features evident in Figures 7.9 and 7.10 indicate the nature of pressure differences due to the global pattern of net radiation and features related to atmospheric dynamics. An initial examination of atmospheric pressure emphasizing description of the observed features rather than an explanatory analysis of the features permits a focus on space and time variations. A more comprehensive view emerges when atmospheric circulation is discussed in Section 7.7. Also, it is important to note that the depiction of pressure patterns using isobars, or lines joining points of equal pressure, commonly includes references to high pressure and low pressure. Such references are based on relative comparisons and not absolute pressure values. In addition, the common convention is to identify the lines as isobars even when the pressure is expressed in hPa.

Fig. 7.9. Global January mean sea-level atmospheric pressure. Units in hPa. Wind direction shown by arrows, and wind speed by the length of the arrow shaft. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

Fig. 7.9. Global January mean sea-level atmospheric pressure. Units in hPa. Wind direction shown by arrows, and wind speed by the length of the arrow shaft. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

Fig. 7.10. Global July mean sea-level atmospheric pressure. Units in hPa. Wind direction shown by arrows, and wind speed by the length of the arrow shaft. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

Fig. 7.10. Global July mean sea-level atmospheric pressure. Units in hPa. Wind direction shown by arrows, and wind speed by the length of the arrow shaft. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

In January (see Fig. 7.9), a region of nearly continuous low surface pressure is present in the equatorial zone. The low pressure is coincident with the high net radiation and warm temperatures typically found in the low latitudes. The surface pressure near the subtropics at 30° latitude is generally higher with a definite cellular structure. These semipermanent features centered over the oceans are known as subtropical highs. High pressure is present over the continents where land temperatures are cold but at slightly higher latitudes compared to the subtropical highs. The area over Asia displays a circular form similar to the subtropical highs, but the high pressure over North America merges with the Atlantic subtropical high to form an elongated cell that extends to North Africa. The oceanic high-pressure cells in the Southern Hemisphere are clearly defined, and poleward of these cells is an extensive zone of generally low pressure that constitutes the Antarctic low-pressure trough. The Northern Hemisphere counterpart to this low pressure in January is the Aleutian and Icelandic low-pressure cells centered at the northern margins of the Pacific and Atlantic oceans, respectively. The polar regions in both hemispheres display a predominance of high pressure.

In July (see Fig. 7.10), seasonal variations in sea-level pressure are most apparent in the Northern Hemisphere. The equatorial low-pressure zone shifts predominantly into the Northern Hemisphere. The large oceanic subtropical highs move toward the pole in the summer hemisphere and are centered near 40° latitude, while in the winter hemisphere they move equatorward to about 30° latitude. The largest seasonal variations are found over the Asian continent where the winter high over Siberia is replaced by a low-pressure system north of the Indian subcontinent. A similar situation takes place over North America, but the pressure change is less intense. The annual variation of surface pressure over Siberia exceeds 25hPa, while the change over North America does not exceed 10hPa. At high latitudes, the low-pressure systems in the Northern Hemisphere are weaker during the summer, whereas in the Southern Hemisphere the almost continuous zonal low-pressure belt around Antarctica displays little seasonal variation.

The seasonal variation of global surface pressure shows a net positive residual during the Northern Hemisphere summer. Although this appears to contradict the basic assumption of a constant atmospheric mass, it is attributed to the additional contribution by the added water vapor associated with variations in the net evaporation minus precipitation at the Earth's surface. Observations of the precipitable water in the atmosphere show an excess of water vapor on the order of 0.2 gkg-1 during the Northern Hemisphere summer due to the higher values ofevaporation and higher surface temperatures over the continents. This excess in water vapor mass is compatible with the observed global pressure difference of 0.2 hPa. The hemispheric values imply the existence of important shifts or exchanges of mass across the equator (Trenberth and Smith, 2005).

Global atmospheric surface pressure reveals a pattern that is sometimes disparate with the global patterns of net radiation and temperature. The deviation is especially evident with regard to the presence and latitudinal shifting of the oceanic subtropical high-pressure cells. High pressure at about 30° latitude implies that the observed pattern ofsurface pressure involves factors other than temperature and may include atmospheric motion as a causal component as well as a resultant. Considering pressure patterns higher in the atmosphere contributes to understanding global atmospheric circulation and its possible role as a causal agent.

The state of the middle-troposphere is indicated by the 500 hPa atmospheric pressure field. This constant pressure surface represents the height above sea level that divides the Earth's atmospheric mass into two approximately equal parts. Typical heights for the 500 hPa surface are around 5500 m, but the actual heights vary depending on thermal and dynamical factors that influence the vertical distribution of the atmospheric mass. Gravity plays a significant role in the processes determining vertical variations in atmospheric mass, and calculation of geopotential heights accounts for variations in gravity due to differences in latitude and elevation. In this way, geopotential heights approximate the actual pressure surface height above mean sea level. For most applications, geopotential heights are used interchangeably with geometric heights so that a constant pressure surface and a constant geopotential height are similar (Wallace and Gutzler, 1981). Figures 7.11 and 7.12 display 500 hPa geopotential heights, but the following discussion uses the 500 hPa geometric surface due to the similarity of the fields and to provide simplicity in referring to the fields.

Thermal influences on the characteristics of air columns are immediately apparent in Figures 7.11 and 7.12. The ideal gas law expressed in Equation 3.2 establishes that atmospheric pressure varies in response to temperature or density differences. Warm air is less dense and occupies a larger space resulting in the height to the 500 hPa surface being higher. Cool air is more dense, occupies a smaller space, and the 500 hPa surface is closer to the Earth's surface. This fundamental difference in the height of air columns underlies the observation that vertical atmospheric pressure changes more rapidly in cold air than in warm air.

Figures 7.11 and 7.12 reveal the 500 hPa surface in both seasons is highest over the equatorial region where air is warmest and lowest over polar regions occupied by cool air. The complex pattern of pressure cells at the surface is replaced in the upper atmosphere by one simple pressure gradient from the equator to the poles.

http://www.cdc.noaa.gov/.)"/>
Fig. 7.11. Global January mean 500 hPa geopotential heights. Units in m. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)
http://www.cdc.noaa.gov/.)"/>
Fig. 7.12. Global July mean 500 hPa geopotential heights. Units in m. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

The equator-to-pole difference in 500 hPa heights is greater in the low-sun hemisphere than in the high-sun hemisphere, and the seasonal difference is greater in the Southern Hemisphere than in the Northern Hemisphere. However, the overall pattern of the height contours displays less seasonal change in the Southern Hemisphere. The height contours for the Southern Hemisphere are more circularly symmetric around the pole due to the dominance of ocean surfaces and the expanse of Antarctica at latitudes above 70°.

The January and July 500 hPa height contours display undulating patterns that are most pronounced in the Northern Hemisphere. Poleward undulations of high heights are called ridges, and equatorward undulations of low heights are called troughs. Cooler air occupies the area under troughs, and warmer air is present under ridges. The ridges correspond to the subtropical highs at the surface and are about the only surface feature evident at the 500 hPa level. Figure 7.11 displays ridges just west of Europe and North America and troughs east of the Asian and American continents. These features are less prominent in Figure 7.12.

Wave patterns are evident in the Southern Hemisphere, but the strong zonal influence reduces the wave amplitude. Ridges along the west coasts of southern South America, southern Africa, and Australia are evident in January but less apparent in July. Troughs in January over the eastern Pacific Ocean and eastern Atlantic Ocean are weaker in July, and the January trough over the eastern Indian Ocean disappears in July.

The surface and upper-air pressure fields provide the initial force that sets the atmosphere in motion. The spatial differences in the atmospheric pressure fields are an indication that we should expect surface winds and upper atmospheric winds to be different. The role of the pressure fields in determining the character of atmospheric circulation and transport of energy and moisture is a central factor at the global scale and at other scales of interest.

Was this article helpful?

0 0
Waste Management And Control

Waste Management And Control

Get All The Support And Guidance You Need To Be A Success At Understanding Waste Management. This Book Is One Of The Most Valuable Resources In The World When It Comes To The Truth about Environment, Waste and Landfills.

Get My Free Ebook


Post a comment