How latent heat can become sensible heat

The Stueve algorithm can be used to calculate what happens if a parcel of Φ-saturated air moves from sea level over Italy, up a few thousand m over the Alps and then falls back down almost to sea level over the Swiss lowlands (Fig.007). It expands and loses P and T as it rises up the Italian side of the Alps. The cooling works faster than the pressure loss so that cloud forms, and then rain falls. The LQ remains in the cloud.

When the cloud parcel falls back down on the Swiss side of the Alps, it warms as is recompressed back to almost its sea level P. This compression heats the cloud parcel so that the droplets in it dry to clear air. The bottom of the cloud is higher over the Swiss side of the Alps than that at which rain began to condense out of it on the Italian side. This is because the LQ that remained in the parcel when rain fell out of it became SQ as that parcel was recompressed.

That is how hot, dry 'Foehn' winds develop. A good Foehn can thaw (thawing LQ) warm (SQ), and evaporate (evaporate (LQ) 40cm snow off exposed slopes in a day in the middle of winter. The Foehn effect can create deserts. If a country has a coastal mountain range over which moist sea winds normally blow inland, rain falls out over its seaward slope, its leeward slope and hinterland are hot and dry.

<< Prev - Next >>