2. Consider a strong wind aloft that forces air to flow over the mountains from A to E, as in the diagram below. Assume the surface temperature at A is 21C and the dew point temperature is 5C; and that any moisture that condenses out of the air falls as precipitation and is left behind. The elevations are: A – 0 m; B – 3000 m; C – 1000 m; D – 5000 m; E – 500 m.
(i) What are the temperatures and dew point temperatures at B, C, D and E?
(ii) Identify where clouds are formed (hint: this is important to part (i).
(iii) How would your answer for E be different if there was no valley at C; that is, if the air flowed upward all the way from A to D?
To answer this question, we need to understand the concept of adiabatic cooling and the lapse rate. Adiabatic cooling refers to the cooling of air as it rises due to a decrease in atmospheric pressure. The lapse rate is the rate at which the temperature decreases with an increase in altitude. The average lapse rate is around 6.5°C per kilometer.
(i) Starting with the given information:
- A: Surface temperature = 21°C, Dew point temperature = 5°C
1. Temperature at B:
Since B is at an elevation of 3000 m above A, we need to account for the lapse rate. Using the average lapse rate, we can calculate the temperature at B as follows:
Temperature at B = Temperature at A - (Lapse rate * Elevation difference)
Temperature at B = 21°C - (6.5°C/km * 3 km) = 21°C - 19.5°C = 1.5°C
2. Dew point temperature at B:
Since the dew point temperature is a measure of humidity, it does not change significantly with altitude. Therefore, the dew point temperature at B will be the same as at A: 5°C.
3. Temperature at C:
C is at an elevation of 1000 m above B. Using the same calculations as before:
Temperature at C = Temperature at B - (Lapse rate * Elevation difference)
Temperature at C = 1.5°C - (6.5°C/km * 1 km) = 1.5°C - 6.5°C = -5°C
4. Dew point temperature at C:
Again, the dew point temperature does not change significantly, so it remains at 5°C.
5. Temperature at D:
D is at an elevation of 5000 m above C, and we can use the lapse rate calculation again:
Temperature at D = Temperature at C - (Lapse rate * Elevation difference)
Temperature at D = -5°C - (6.5°C/km * 4 km) = -5°C - 26°C = -31°C
6. Dew point temperature at D:
Like before, the dew point temperature at D remains at 5°C.
7. Temperature at E:
Lastly, E is at an elevation of 500 m above D:
Temperature at E = Temperature at D - (Lapse rate * Elevation difference)
Temperature at E = -31°C - (6.5°C/km * 4.5 km) = -31°C - 29.25°C = -60.25°C
(ii) Cloud Formation:
Clouds form when the air reaches its dew point temperature and condenses. Looking at the temperatures and dew point temperatures calculated, we can identify where clouds will form by comparing the values. In this case, clouds will form at altitudes where the temperature is equal to or below the dew point temperature. So clouds will form at E, D, C, and possibly B, depending on the actual vapor content.
(iii) Impact of No Valley at C:
If there was no valley at C and the air flowed upward all the way from A to D, the temperature at E would be different. Without the cooling effect of the downward flow in the valley, the lapse rate would be less significant, and the temperature at E would be warmer than calculated earlier.