Area Climatology — Page 428, Lesson 395

I want to walk you through the summer weather patterns over Europe, building on what we've already covered for winter. Let's start with icing, because it's a critical hazard that changes with the seasons. Over the North Atlantic, icing occurs widely and through great depth in frontal cloud, and it is frequently moderate to severe. That's a direct carryover from winter conditions. But there's a specific phenomenon called Freezing Rain — also labelled Rain Ice in the book — and this can cause severe clear ice. The mechanism is this: rain falls from a warm layer aloft into a sub-zero layer near the surface, where it freezes on impact. This happens in cold air under a warm front or warm occlusion. Now, here's the key distinction for you as a pilot: this is a rare occurrence in the UK, but it is more common over Central Europe. Why? Because over Central Europe the ground is generally much colder. Indeed, the freezing level may frequently be on the surface, especially in the east. So if you're flying into Central Europe in summer, you need to be alert for freezing rain even when the UK is reporting none. Now let's move to the summer pressure systems. The book breaks them into three categories: Polar Front Depressions, Thermal Depressions, and Temporary Highs. First, Polar Front Depressions. These will track eastwards, just as they do in winter, but they track further north. That's the seasonal movement — it follows the sun. They will also be less intense, and the reason is the smaller Polar-to-Tropical temperature difference that forms them. In summer, the temperature gradient between the cold polar air and the warm tropical air is weaker, so the depressions that develop along that front are weaker. Second, Thermal Depressions. Strong insolation — that's intense solar heating of the ground — can cause active thermal depressions over France and Southern Germany. These are not frontal systems; they form from local heating. Thunderstorms are common when moist unstable conditions exist in these areas. Then we have the Azores High. This is well established west of Africa at 35°N. An associated ridge across Europe often gives a limited period of fine dry weather. So the Azores High extends a ridge of high pressure into Europe, and that's what brings the settled spells. Third, Temporary Highs. These are temporary ridges or transient anticyclones to the northwest. They are more dominant in summer, and they occur in between the weaker polar front lows. So the pattern in summer is: weaker low-pressure systems moving further north, with short-lived high-pressure ridges building in from the northwest between them. Let me show you the January weather chart for comparison, so you can see the seasonal shift.