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Unformatted text preview: cooling when it is the same temperature as the air around it. This is stable
air. Unstable air will always be warmer than the air around it and will keep rising.
The lapse rates we talked about earlier come into play here. Let’s say that we
have an environmental lapse rate of -12°C per 1000 m (recall that the value I
gave before of 6.5°C was just an average–it varies.). Take a parcel of air on the
ground at 30°C and push it up. It cools at the dry adiabatic rate of -10°C per
1000 m. So, at 1000 m, it will be 20°C. The surrounding air will have cooled at
12°C in that 1000 meters, so it will be 18°. The parcel is still warmer and it
continues to rise. At 2000 meters, the parcel is 10° while the surrounding air is
6°. There is nothing to stop that parcel from continuing to rise, so it is unstable.
Now let’s say that the ELR is 6.5°C per 1000 meters. Push up that same parcel
of air and it will never be warmer than the surrounding air, so it will drop back
down. This is stable air. When the ELR is higher than the DALR, the air is
unstable. When ELR is lower than DALR, it is stable, most of the time at least. Atmospheric Moisture:6 Unstable
2000 m 10° 6° 1000 m 20° 18° Stable ground 30°C
Surrounding Air ELR = -12°C/1000 m 20° 23.5° 30°C
Surrounding Air ELR = -6.5°C/1000 m Convectional Precipitation
But what if the air parcel starts out warmer than the surrounding air? Let’s look at
a situation where some of the air is heated more than the air around it. Imaging a
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This document was uploaded on 10/27/2011 for the course GEOG 1401 at Texas Tech.
- Fall '08