On the windward side of the mountain,should the relative humidity of the parcel change as it rises from 2000 m to 4000m?
To determine whether the relative humidity of a parcel of air changes as it rises from 2000 m to 4000 m on the windward side of a mountain, you need to consider the processes involved in the formation of clouds and the adiabatic lapse rate.
The relative humidity is affected by two key factors: temperature and water content. As air rises up a mountain slope, it undergoes adiabatic cooling. This means that as the air moves higher, it expands and cools due to decreasing atmospheric pressure.
The adiabatic lapse rate is the rate at which temperature decreases with increasing altitude as a parcel of air rises. It varies depending on the moisture content of the air.
If the air parcel is dry (contains very little water vapor), it cools at a rate of about 10 degrees Celsius per 1000 meters of ascent. This is known as the dry adiabatic lapse rate. In this case, the relative humidity of the parcel would not change significantly because the temperature and moisture content decrease proportionally.
However, if the air parcel is moist (contains a significant amount of water vapor), it cools at a slower rate, known as the moist adiabatic lapse rate, which is closer to 6 degrees Celsius per 1000 meters of ascent. As the air rises, it may reach the point where the temperature drops to the dew point, causing the water vapor to condense and form clouds. This process releases latent heat, which reduces the rate of cooling. The condensed water droplets contribute to higher humidity in the air parcel. Therefore, in this case, the relative humidity of the parcel would increase as it ascends.
In summary, on the windward side of a mountain, if the air parcel is dry (low water vapor content), the relative humidity would remain relatively constant as it rises from 2000 m to 4000 m. However, if the air parcel is moist, the relative humidity would likely increase due to the condensation of water vapor as it rises.