When you start heating a pot of water on your stove, the heat is applied by the burner on the bottom of the metal pot, and this hot metal warms the water on the bottom and sides of the pot more than it warms the water near the top. Hot water is less dense than cold water, so the hotter water begins to rise, and the colder water begins to sink. If you watch the bubbles in the water closely, you will see that the water rises along the sides of the pot (near the hotter metal of the pot), and sinks in the center. This "current"of water mixes up the water so that eventually the whole pot of water is warmed up nearly evenly.
In the same way, when the sun warms up the earth, the heat is mainly absorbed by the ground and water at the earth's surface, not by the air that is way up high in the atmosphere. Just like the sides of the pot, the mountains heat up the air along the bottom and "sides" of the valleys, and this warmer air is less dense than the cooler air that is at the same altitude over the middle of the valley. Just like the water in the pot, the warm air starts to rise, and the cold air starts to sink. The warm air rises along the mountains that are on the sides of the valley and the cooler air above the middle of the valley sinks down toward the ground. The air rushing up along the sides of the mountains creates the winds that blow up the mountains and up the river valleys nearly every afternoon. We typically call these valley winds because they blow from the valleys toward the mountains.
Over perfectly flat ground there would be no consistent pattern of winds. Bubbles of warmer air would float up, surrounded by other bubbles of colder air going down in a random pattern. The mountains provide a consistent place where air is warmer than in the middle of the valley and, thus, create the consistent patterns of winds. These consistent winds can bring lots of moisture toward the mountain tops and ridges, and during the summer we are always concerned that all this air rising into the mountains can initiate thunderstorms over the mountains that will later drift out over the valleys.
Once the sun goes down, the ground starts to cool off rapidly. However, the air that was warmed during the day does not cool down as quickly! Just like the water in your pot, the air is slow to warm up, but also slow to cool down. So, as the ground cools off, the air near the ground gets cooler and starts to sink, while relatively warmer air is located above the center of the valley. The whole circulation is reversed, and cool air starts sinking first along the mountain slopes, and trickles down into the valley bottom. If you live near a river or stream, you will probably notice that as soon as the sun goes down, the wind in the river valley starts blowing downhill or downstream. We call these mountain breezes because they blow from the mountains toward the valleys.
When you're boiling a pot of water this summer, watch the pattern of water currents that flow as it heats up. Likewise, when you're out in the mountains this summer, keep your eyes on the winds flowing uphill nearly every afternoon, and look for the thunderstorms that they may spawn. Keep in mind that if you see any hail or experience strong winds from these thunderstorms, we want to hear from you! While we know that these mountain/valley breezes form nearly every afternoon, we rely on spotters like you to tell us about the conditions on the ground near these thunderstorms.
Tim Barker -
Boise Science and Operations Officer