The vertical variation of pressure systems

The air pressure at the surface, or at any level in the atmosphere, depends on the weight of the overlying air column. In Chapter 2B, we noted that air pressure is proportional to air density and that density varies inversely with air temperature. Accordingly, increasing the temperature of an air column between the surface and, say, 3 km will reduce the air density and therefore lower the air pressure at the surface without affecting the pressure at 3 km altitude. Correspondingly, if we compare the heights of the 1000 and 700 mb pressure surfaces, warming of the air column will lower the height of the 1000 mb surface but will not affect the height of the 700 mb surface (i.e. the thickness of the 1000 to 700 mb layer increases).

The models of Figure 7.1 illustrate the relationships between surface and tropospheric pressure conditions. A low-pressure cell at sea-level with a cold core will intensify with elevation, whereas one with a warm core will tend to weaken and may be replaced by high pressure. A warm air column of relatively low density

Vertical Motion Low High Pressure System

Figure 7.1 Models of the vertical pressure distribution in cold and warm air columns. (A) A surface low pressure intensifies aloft in a cold air column. (B) A surface high pressure weakens aloft and may become a low pressure in a cold air column. (C) A surface low pressure weakens aloft and may become a high pressure in a warm air column. (D) A surface high pressure intensifies aloft in a warm air column.

Figure 7.1 Models of the vertical pressure distribution in cold and warm air columns. (A) A surface low pressure intensifies aloft in a cold air column. (B) A surface high pressure weakens aloft and may become a low pressure in a cold air column. (C) A surface low pressure weakens aloft and may become a high pressure in a warm air column. (D) A surface high pressure intensifies aloft in a warm air column.

causes the pressure surfaces to bulge upward, and conversely a cold, more dense air column leads to downward contraction of the pressure surfaces. Thus, a surface high-pressure cell with a cold core (a cold anticyclone), such as the Siberian winter anticyclone, weakens with increasing elevation and is replaced by low pressure aloft. Cold anticyclones are shallow and rarely extend their influence above about 2500 m. By contrast, a surface high with a warm core (a warm anticyclone) intensifies with height (Figure 7.1D). This is characteristic of the large subtropical cells, which maintain their warmth through dynamic subsidence. The warm low (Figure 7.1C) and cold high (Figure 7.1B) are consistent with the vertical motion

Siberian High Pressure Cell

Figure 7.2 The characteristic slope of the axes of low- and high-pressure cells with height in the northern hemisphere.

schemes illustrated in Figure 6.7, whereas the other two types are produced primarily by dynamic processes. The high surface pressure in a warm anticyclone is linked hydrostatically with cold, relatively dense air in the lower stratosphere. Conversely, a cold depression (Figure 7.1A) is associated with a warm lower stratosphere.

Mid-latitude low-pressure cells have cold air in the rear, and hence the axis of low pressure slopes with height towards the colder air to the west. High-pressure cells slope towards the warmest air (Figure 7.2). Thus, northern hemisphere subtropical high-pressure cells are shifted 10 to 15┬░ latitude southward at 3 km, and towards the west. Even so, this slope of the high-pressure axes is not constant through time.

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Responses

  • KRISTIAN
    How pressure variation effect climate?
    6 years ago
  • mirin
    What is vertical variation of pressure?
    12 months ago

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