Area Climatology — Page 433, Lesson 396

I want to walk you through the summer weather patterns for the area climatology we're studying. We've already covered winter, so now let's look at how things change as the seasons shift. Let's start with the Scandinavian Highs. In summer, these high-pressure systems can persist for a few days, and when they do, they draw air across the North Sea from western Russia. That's an important source region to keep in mind because it determines the character of the air that arrives. Now, cloud and precipitation. The big picture here is that frontal cloud amounts and rain are much less than in winter. Why? Because the associated polar front depressions are fewer, less intense, and they're positioned further north. By summer, the Mediterranean warm fronts are gone entirely. So instead of widespread frontal cloud, the cloud is mainly convective — produced in thunderstorms that develop from thermal lows. Thermal lows form when the ground heats up strongly in summer, creating areas of low pressure that can trigger convection. The rainfall is therefore mainly in the form of heavy showers. But the effect may be increased by orographic lifting in the southern mountains — that means when the air is forced to rise over mountain ranges, it can enhance the shower activity. Let's talk about visibility. Radiation fog is much less likely in summer. It can occur in early spring, but morning insolation — that's the amount of solar radiation received at the surface — will normally ensure quick clearance. So even if it forms, it burns off fast. However, there's a specific situation to watch for: in late spring and early summer, an easterly wind blowing around a Scandinavian High, coming over the North Sea, can often result in extensive advection sea fog along the UK East Coast. In Scotland, this fog is known as haar. It can travel inland some distance, so it's not just a coastal phenomenon. Now, winds. Surface winds are generally westerly, but they're lighter than in winter. Winds may also be modified by sea breezes along coasts — that's a local effect where the land heats up faster than the sea, creating a circulation that can change the wind direction and speed near the shoreline. Upper winds become increasingly westerly with ascent — so the higher you go, the more the wind comes from the west. But the thermal wind component is less than in winter, and upper winds will therefore be less strong. The thermal wind component is the part of the wind change with height that's caused by horizontal temperature gradients. In summer, those gradients are weaker, so the upper winds are weaker. Reduced Speed Polar Front Jets will occur, but they're further north with the summer movement of the polar fronts. The Atlantic subtropical jet will reach the coast near Bordeaux, but due to mountain interference — the Pyrenees — it will not extend overland at jet speeds. It therefore does not affect the region. Let's move to icing. The freezing level will be higher in summer — we'll see the numbers in a moment — and frontal activity is less. But icing in thunderstorms and orographical cloud may still be severe. Orographical cloud is cloud formed when air is forced to rise over mountains, and even in summer, that can produce serious icing conditions. Finally, let's look at the average heights over Central France. In January, the tropopause is at 35,000 feet and the freezing level is at 4,000 feet. In July, the tropopause rises to 39,000 feet — that's 4,000 feet higher. The freezing level in July is not given a specific number in this excerpt, but the key point is that it's higher in summer, which reduces the likelihood of icing in frontal systems, but doesn't eliminate the risk in convective and orographic cloud.