Mountain Weather: Forecasting for the Windward Side

Mountains are obstacles in the eyes of the atmosphere. Impenetrable barriers that drive weather changes in meaningful ways across both local and regional scales. This explainer will consider why you and your neighbor living on the other side of that mountain may experience dramatically different weather.

To show how weather becomes altered by a tall mountain, I will walk through an idealized example. There is a little bit of math involved. Here are the following assumptions:

1) You live on the western base of the mountain at an elevation of 1000 feet (point A).

2) The summit of the mountain is 9,000 feet (point B).

3) Your neighbor is on the opposite eastern side of the mountain at an elevation of 1,000 feet (point C).

4) At your location, weather observations indicate a temperature (T) of 68°F, dewpoint (Td) of 57°F, and relative humidity (RH) of ~67% (so moist, but not saturated since less than 100%).

5) Prevailing wind direction in the troposphere is west to east (i.e., a westerly wind) towards the mountain.

With this information we can calculate the altitude clouds start to develop on your side of the mountain (windward) and the expected T, Td, and RH at points B and C. Ultimately, will you and your neighbor experience and see the same weather is to be determined.

Going up the mountain…

The key to working out the necessary calculations is knowing a fundamental concept that an unsaturated air parcel forced to rise in altitude in the troposphere will decrease in temperature at a rate of 5.5°F per 1,000 feet. So, in this idealized scenario, the air mass at your home (point A) needs to cool 11 more degrees to reach saturation (i.e., T equals Td and RH would be 100%). This does occur, however, after an additional 2,000 foot climb up the mountain slope, meaning at 3,000 feet in elevation on the windward side clouds start to become visible.

An important change in the vertical atmospheric cooling rate occurs at that elevation since the air mass has become saturated. The temperature decrease rate is now moderated (~3.5°F per 1,000 feet) because heat release is introduced into the environment by the condensation phase change!

Discussion continued with Mountain Weather: The Rain Shadow Effect.

Photo Credit: Jonny William Malloy