While a synoptic chart can give a pilot a good overview of wind directions on a large scale, there are many local factors that can change the wind experienced by a pilot.
Throughout the day, the sun warms the ground but ocean temperatures stay relatively constant. The air sitting over the land is then heated by conduction and begins to rise. This creates an area of lower pressure over the land and a pressure gradient between the warm land and the cold sea develops.
Cooler air over the ocean begins to flow towards the land, creating a sea breeze. The sea breeze is strongest when the temperature difference is greatest – this normally happens in the mid-afternoon on a sunny day.
The cooler air over the sea sinks to replace the air moving over the land, creating higher pressure over the sea. A few thousand feet above this sea breeze, the air begins to move in the opposite direction as the rising air cools and spreads outwards, replacing the sinking air over the ocean.
During the night, this process is reversed. The land cools while the sea again remains at a fairly constant temperature. The air over the land begins to cool and sink, while the air over the sea will begin to rise and a pressure gradient from the land to the sea forms with the wind blowing from the land to the sea.
This is called a land breeze and it is usually weaker than the daytime sea breeze as the temperature difference is smaller.