Now, why does this matter operationally — Page 167, Lesson 142

I want to walk you through the diurnal variation of wind and then into land and sea breezes — these are practical effects that directly affect your flying, especially near coasts. Let's start with the summary of diurnal variation. During the day, at 1500 feet, the wind decreases and backs — that means it slows down and turns anticlockwise in direction. At the surface during the day, the wind increases and veers — it speeds up and turns clockwise. At night, the pattern flips: at 1500 feet the wind increases and veers, while at the surface it decreases and backs. Now, why does this matter operationally? The diurnal variation of the surface wind aids the formation of radiation fog at night and early morning, and its dispersal by day. That's a direct link to your pre-flight planning — if you see conditions favourable for radiation fog, you know the surface wind changes through the day will help form it overnight and clear it out after sunrise. Over the sea, the diurnal effect is small because the diurnal variation of sea temperature is small. Water doesn't heat and cool as quickly as land, so the wind over the sea stays more constant through the day-night cycle. Now let's move to land and sea breezes. These are local wind circulations driven by temperature differences between land and sea. A sea breeze develops on a sunny day, particularly in an anticyclone with a light pressure gradient force. Here's the mechanism: the land heats quickly. The air in contact with the warm land is warmed, rises, and expands. At about 1000 feet, this rising air creates a region of higher pressure over the land compared to the same level over the sea. That pressure difference causes a drift of air from over the land to over the sea at about 1000 feet. As that air drifts away, the surface pressure over the land falls, and the surface pressure over the sea rises. The result is a flow of air from sea to land at the surface — that's the sea breeze. On average, sea breezes extend 8 to 14 nautical miles either side of the coast, and the speed is about 10 knots. In the tropics, the speed is 15 knots or more and the inland extent is greater. The direction of the sea breeze is more or less at right angles to the coast initially, but after some time it will veer under the influence of the Coriolis force — that's the Earth's rotation effect that turns moving air to the right in the Northern Hemisphere. Now the land breeze. From mid-afternoon, the land starts to cool, and this process accelerates after sunset. Overnight, the situation reverses: pressure becomes higher on land than over the sea because the land is colder. This gives rise to a wind blowing from land to sea — the land breeze. The land breeze can be expected within about 5 nautical miles of the coastline, with a maximum speed of about 5 knots. Now let's look at the practical coastal effects that matter to you as a pilot. First, the direction of take-off and landing can reverse with the change from sea breeze to land breeze. During the day, you might be landing into a sea breeze coming from the sea toward the land. At night, that reverses to a land breeze blowing from land to sea, so your approach direction may need to flip. Second, fog at sea can be blown inland by the day sea breeze, affecting coastal airfields. If there's a fog bank sitting offshore, the sea breeze can push it right over your runway. Third, the lifting of air over land with the sea breeze can cause small clouds to form. These clouds are a good navigational feature of the coastline — if you're flying along the coast and see a line of small clouds just inland, that's the sea breeze lifting air and forming them. So to tie it all together: diurnal variation changes wind speed and direction at 1500 feet and the surface differently by day and night. Land and sea breezes are local circulations driven by the temperature difference between land and sea, with the sea breeze stronger and extending further than the land breeze. And these effects directly impact your operations — runway direction, fog, and even cloud formation for navigation.