The role of temperature advection in contributing to the pressure or height changes can be misleading. On the one hand, low-level (usually the 1,000- to 500-hPa stratum) warm air advection is frequently cited as responsible for the surface pressure falls ahead of moving surface lows (the converse for cold air advection); on the other hand, warm air advection is frequently associated with rising heights in the upper levels.

The pressure change at the SURFACE is equal to the pressure change at some UPPER LEVEL, plus the change in mass of the column of air between the two. That is, if the pressure at some upper level remains UNCHANGED and the intervening column is replaced with warmer air, the mass of the whole atmospheric column (and consequently the surface pressure) decreases, and so does the height of the 1,000-hPa surface.

As an example, assume that warm air advection is indicated below the 500-hPa surface (5,460 meters) above a certain station. If no change in mass is expected above this level, the height of the 500-hPa level (5,460 meters) will remain unchanged. Suppose the 1,000- to 500-hPa advection chart indicates that the 5,400-meter thickness line is now over the station in question and will be replaced by the 5,490-meter thickness line in a given time interval; that is, warm air advection of 90 meters. The consequence is that the 1,000-hPa surface, which is now 60 meters above sea level, will lower 90 meters to 30 meters below sea level, and the surface pressure will decrease a corresponding amount, about 11 hPa (7.5 hPa approximately equals 60 meters).

Whenever the surface pressure is less than 1,000 hPa, the 1,000-hPa surface is below the ground and is entirely fictitious. In view of the above description of advective temperature changes, the following rules may apply:

. Warm air advection between 1,000 and 500 hPa induces falling surface pressures.

. Cold air advection between 1,000 and 500 hPa induces rising surface pressures.

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