The direction a hillside faces—its exposure—influences its climate. North of the tropics, the sun is always a little south of being directly overhead. The farther you are from the equator, the farther in the south the sun hangs as it travels from east to west each day. The sun’s rays strike land that slopes to the south more directly than flat land or land sloping in some other direction. South sides of hills are hotter and often drier then north sides or the level land around them.
Air Drainage
As the ground cools in the evening, it cools the air in contact with it. The cooler air is slightly more dense than the warmer air above it, and flows downhill like water. This cool air collects in valleys and hollows, sometimes causing them to be several degrees cooler than the hillsides around them. This is easiest to see on frosty mornings, when these “frost pockets” can be white with frost while the hillsides are untouched.
The areas with the most frost are low pockets. The warmest spots are usually hillsides, from which the cold air drains as quickly as it cools. Tops of hills are often windy, keeping them from being as warm as the hillsides. Small frost pockets can form on a hillside if something, such as a windbreak, house, or fence, blocks the drainage of cold evening air. The cold air can collect behind this “dam,” making a hillside frost pocket.
Different Exposures
South-facing slopes—even small hills and mounds in your garden—warm up earlier in the spring and stay warm later in the fall than other exposures. They are often hot and dry in midsummer. West slopes, which receive the warm afternoon sunshine, are next warmest, followed by east slopes. North slopes are coolest. In arid regions, north slopes are often more heavily vegetated because the summer’s heat is not so intense there. Plants on south or west slopes begin growing and flowering earlier in the year than those on east or north slopes.