Down in the Southern Hemisphere, July is winter — Page 377, Lesson 348

I want to walk you through the July temperature patterns and then into the daily and upper-air temperature changes that matter for flight planning. In July, over the Northern Hemisphere land masses, the land heats up more than the ocean. This means that over the continent, the isotherms — the lines of equal temperature — actually reverse direction compared to what you'd expect from latitude alone. For example, the land becomes warmer than the Gulf Stream-warmed Atlantic, so the isotherms are reversed. However, the contrasts between land and sea are less pronounced in July than they are in January. Down in the Southern Hemisphere, July is winter. The seasonal cooling of the ocean shows up in the isotherms, but even so, the isotherms still approximately follow lines of latitude. Now, let's move beyond seasonal variation to daily changes. Temperature also changes from day to night — that's the diurnal change. The diurnal range is greatest over land masses, because that's where the sun's heating effect is strongest. Over water, the temperature doesn't swing as much from day to night. Just as surface temperatures change more as you move away from the Equator, the same is true for temperatures aloft. At the geographical equator, the freezing level sits at 16 000 feet. Locally, it can be as high as 18 000 feet in July, when the heat equator lies overland in Southeast Asia. Because the freezing level is that high, hail that forms in thunderstorms over that region would melt before it reaches mean sea level. Elsewhere, in both hemispheres, the freezing level changes more widely with the seasons, and this seasonal swing is especially large over land areas. Now, here's the key takeaway for global climatology: these topographical temperature variations directly affect surface pressure. They distort the idealized pressure distribution you might have seen earlier. Over the oceans, the climatic pressure zones are maintained. But over land, the pressure patterns — and therefore the winds and weather — are governed much more by surface temperature changes. This effect is especially strong in the Northern Hemisphere, because about two-thirds of the world's land masses are located there. Let's also touch on relative humidity. There's a chart showing how relative humidity varies with latitude and season — the zonal distribution. Finally, let's look at the January pressure patterns. In January, the Southern Hemisphere pattern is close to the idealized circulation. The equatorial low pressure zone lies to the south of the Equator. Subtropical highs are established over oceanic areas. Over the Northern Hemisphere land masses, cold weather highs are established. There are significant pressure areas you need to know: the Iceland Low at 1000 hPa (a statistical low), the Aleutians Low also at 1000 hPa (another statistical low), and a low over Northern Australia at 1005 hPa. On the high-pressure side, we have the Siberian High at 1035 hPa, the North American High at 1020 hPa, the Azores High at 1020 hPa, and the Pacific High at 1020 hPa.