Page 229 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 7 Water and Atmospheric Moisture 193
(a) Unstable 3000
2000 1000 0
–20° –10° 0°
Temperature (°C)
(b) Conditionally unstable 3000
Surrounding air temperature
–11°C
1°C
13°C
25°C Environmental lapse rate 12 C°.1000 m-1
Condensation, cloud formation, and possible precipitation
10° 20° 30°
Surface
–5°C
5°C
15°C
25°C
DAR of rising parcel of air
cools at 10 C°.1000 m-1
Air parcel
is warmer and less dense than surrounding atmosphere
2000 1000 0
–20° –10°
(c) Stable 3000
2000 1000 0
–20° –10° 0°
Temperature (°C)
Parcel resists upward movement
15°C
Parcel is unstable if saturated
13°C
19°C
0° 10° 20° 30° Temperature (°C)
25°C
25°C
Surface
Surrounding air temperature
4°C 11°C
18°C
25°C
Environmental Unsaturated air Saturated air
lifting (DAR) lifting (MAR)
lapse rate parcel begins parcel begins
7 C°.1000 m-1
Surrounding air temperature 10°C
Parcel resists upward movement
15°C 25°C
10° 20° 30°
Surface
Animation
Atmospheric Stability
15°C
20°C
25°C Environmental lapse rate
5 C°.1000 m-1
Air parcel is cooler and denser than surrounding atmosphere
Clear skies, stagnant air conditions
▲Figure 7.14 Stability—three examples. Specific examples of (a) unstable, (b) conditionally unstable, and (c) stable conditions in the lower atmosphere. Note the response to these three conditions in the air parcel on the right side of each diagram.
The example in Figure 7.14c shows stable conditions resulting when the ELR is only 5 C° · 1000 m−1. An ELR of 5 C°·1000 m−1 is less than both the DAR and the MAR, a condition in which the air parcel has a lower tempera­ ture (is more dense and less buoyant) than the surround­ ing environment. The relatively cooler air parcel tends to settle back to its original position—it is stable. The denser air parcel resists lifting, unless forced by updrafts or a barrier, and the sky remains generally cloud­free. If clouds form, they tend to be stratiform (flat clouds) or cir­ roform (wispy), lacking vertical development. In regions experiencing air pollution, stable conditions in the atmo­ sphere worsen the pollution by slowing exchanges in the surface air.
If the ELR is somewhere between the DAR and the MAR, conditions are neither unstable nor stable. In Figure 7.14b, the ELR is measured at 7 C° · 1000 m−1. Under these conditions, the air parcel resists upward movement, unless forced, if it is less than saturated. But if the air parcel becomes saturated and cools at the MAR, it acts unstable and continues to rise.
One example of such conditionally unstable air occurs when stable air is forced to lift as it passes over a mountain range. As the air parcel lifts and cools to the dew point, the air becomes saturated and condensation begins. Now the MAR is in effect, and the air parcel behaves in an unsta­ ble manner. The sky may be clear and without a cloud, yet huge clouds may develop over a nearby mountain range.
Environmental lapse rate 12 C°.1000 m-1
MAR 6 C°.1000 m-1
MAR 6 C°.1000 m-1
DAR 10 C°.1000 m-1
DAR 10 C°.1000 m-1
Environmental lapse rate 7 C°/1000 m-1
MAR 6 C°.1000 m-1
Environmental lapse rate 5 C°.1000 m-1
DAR 10 C°.1000 m-1
Altitude (m) Altitude (m) Altitude (m)