Temperature — Page 72, Lesson 62

I want to walk you through how location and surface type affect temperature — this is critical for understanding the weather patterns you'll encounter as a pilot. Let's start with a striking fact from Figure 5.21. In July, in the Northern Hemisphere, the sea temperature stays cool, while desert land areas in Africa and neighbouring Asia get very warm. Air over snow-covered surfaces is very cold. And here's a specific number: about 80% of solar radiation is reflected from snow surfaces. That means only 20% is absorbed — the snow stays cold and doesn't warm the air above it much. Now, here's an important point about snow: snow does not prevent the earth from radiating its heat. Even with a snow cover, the ground continues to lose heat by radiation into space. So the surface air temperatures over snow will become colder day by day. This is why temperatures in Siberia can reach -72°C after a long cold winter. That's extremely cold air. And that very cold air results in high density — cold air is denser than warm air — and this leads to the development of anticyclones, which are high-pressure systems. Let's move to location effects, starting over land. Air in a valley tends to be more static than air in an exposed position. What does that mean? By night, the air in a valley stays in contact with the ground for a longer time, so it cools more, and the air temperature is lower than on a hill. Additionally, at night, cold air tends to sink from the hills above into the valley, again causing lower temperatures. It is for these reasons that mist and fog tend to form first in valleys — the air gets cold enough for moisture to condense. Now over oceans. The fact that seas tend to have a very small diurnal variation — that's the day-to-night temperature change — has been stated earlier. On a wide scale, this means that in winter, the sea is warmer than the land. So there is a widespread movement of air from land to sea. That's called the monsoon effect. In summer, the opposite tendency occurs — the land warms up more than the sea, so air moves from sea to land. Next, the origin of air supply. Air tends to retain its temperature and humidity for a considerable time. So air from high latitudes — that is, from far north — will bring lower temperatures to the UK. A southerly wind, however, will normally provide an increase in temperature. So when you're planning a flight, knowing where the air is coming from tells you a lot about what temperatures to expect. Let me show you the diagrams that illustrate these effects. shows the July average temperatures I just described. and show diurnal variation and cloud cover by day. shows cloud cover by night. shows diurnal variation over the sea. And is the one that shows the location effect on temperature in valleys versus hills. So to summarise: snow reflects most solar radiation and doesn't stop ground cooling, leading to extreme cold and high-pressure systems. Valleys get colder than hills at night, promoting fog. Oceans have small temperature swings, creating monsoon-like wind shifts between seasons. And the origin of the air mass — whether from the north or south — directly affects the temperature you'll experience.