Behind cold front region, the same but smaller amounts — Page 422, Lesson 387

I want to walk you through the weather conditions behind a cold front, and then we’ll move into the upper wind patterns over the North Atlantic. This is all part of area climatology for the North Atlantic region. Behind a cold front, the weather is essentially the same as ahead of it, but the amounts are smaller — less cloud, less precipitation, less intensity overall. Let’s start with icing. Icing occurs widely and through great depth in convective cloud and frontal cloud — that means in cumulonimbus and along the fronts themselves. It is frequently moderate to severe. There’s also a specific hazard called rain ice or freezing rain. This happens when rain falls from warm air above a warm front into cold air below it. The rain freezes on contact with surfaces, causing severe clear ice. This can affect airfields, particularly near Washington and New York. Now visibility. Two types of fog are important here. Radiation fog can occur inland, especially in autumn and winter when pressure is high — that’s the classic fog that forms on clear, calm nights. Advection fog can occur when moist mTw air — that’s maritime tropical air — overruns previously cold-soaked inland areas. This is especially common in late winter and early spring. Surface winds in this region: north of the subtropical Bermuda-Azores High, winds are generally westerly. But locally, on the north side of depressions — that is, on the northern side of low-pressure systems — winds can be easterly. There are frequent gales. In the south, the NE trade winds prevail all year. Now let’s move to upper winds. These are generally westerly because their direction is governed by the thermal wind. The thermal wind blows with low temperature on the left — that’s a key rule: in the northern hemisphere, if you stand with your back to the wind, low temperature is on your left. The average winter wind component from London to New York is minus 50 knots — that minus sign means it’s a headwind. Locally, winds can be stronger, and if they exceed 60 knots, they are known as jetstreams. Over the Atlantic, there are two distinct jetstream patterns: the polar front jet and the subtropical jet. Each may reach 200 knots. Let’s focus on the polar front jet. This jet will normally blow from between northwest and southwest, and occasionally outside this range, depending on the surface orientation of the polar front. Remember the thermal wind rule: low temperature on the left. So the warm front jet will normally be from the northwest, and because of the slope of the front, it will be located about 400 nautical miles ahead of the warm front’s surface position. Similarly, the cold front jet will normally blow from the southwest, and it will be about 200 nautical miles behind the surface position of the cold front. The level of the polar front jet is around 300 hPa, which is approximately 30,000 feet. Its average location from southwest Florida to southwest England is shown in Figure 22.6. Figures 22.7 and 22.8 also refer to this. Now, let me walk you through what a pilot flying at high level from east to west across a polar frontal depression would experience in terms of wind and drift. This is shown in Figure 22.9. Initially, winds will be northwesterly, giving strong port drift — that means the wind is pushing the aircraft to the left. About 500 nautical miles ahead of the warm front, you enter the jet axis, with speeds of 100 to 200 knots. This gives increased port drift, and this will last for about 200 nautical miles. Winds remain strong from the northwest until you cross the surface position of the warm front. At that point, winds will back sharply to west or southwest. Backing means the wind direction changes counterclockwise — from northwest to west to southwest. This gives near-zero drift. Above the surface position of the cold front, winds will back again sharply to southwest, giving starboard drift — that’s drift to the right. After 100 nautical miles, you enter the jet axis again, with speeds of 100 to 200 knots, giving increased starboard drift. This will last for 200 nautical miles. Finally, passing out of the jetstream, the southwest winds continue but starboard drift decreases. So the sequence is: strong port drift ahead of the warm front, then near-zero drift as you cross the warm front, then starboard drift as you cross the cold front, then increased starboard drift in the cold front jet, then decreasing starboard drift as you leave the jetstream.