III. Weather Phenomena

9. Mountain-valley breezes

Mountain-valley breezes are formed by the daily difference of the thermo effects between peaks and valleys. In daytime, the mountainside is directly heated by the sun, the temperature is higher, air expands, air pressure reduces, and therefore air will rise up the mountainside from the valley and generate a valley breeze. At around 10 a.m., wind flows from the valley and up along the mountainside, the higher the temperature the stronger the breeze. The valley breeze reaches its maximum force at around 2 p.m. and can be as strong as scale 3. After this time, the breeze decreases in power and come to a complete stop by sunset. By nightfall, the mountainside region is able to dissipate heat more quickly, due to its higher altitude and therefore temperature drops rapidly. Cold air will then travel down the mountainside from the top and flow into the valley, forming a mountain breeze. The valley breeze is the weaker of the two, with a wind speed of about scale 2-3. In daytime, valley breezes carry water vapor to the peak, which will often condense into clouds; these are the commonly seen peak and flag clouds. The wind speed of mountain breezes is higher and can reach scale 5. When the mountain breeze travels down and gathers in the valleys, water vapor will condense. Therefore in valley or basin regions, there are usually clouds and fog before sunrise. During late spring or early autumn, the cold air trapped in the valley and basin will often generate frost. That's why farmers usually plant cold-sensitive plants such as tangerines and coffee on mountainsides. Since cold air doesn't often accumulate on a mountainside at night, the probability of frost is lower.