The altitude of aircraft can vary due to a number of factors. It could depend which airport the flight is going to, or from and whether it is an arrival or departure. Arrivals are typically following a steady descent to the runway whilst departures are typically following a steeper path as they climb away from the runway.
A core function of Air Traffic Control (ATC) is safety, and one of the primary ways in which this is achieved is by maintaining either lateral or vertical distances between aircraft. When close to airports lateral separation is used, but when at higher altitudes, vertical separation is used with aircraft typically separated by around 1,000ft. This can sometimes be seen in the overflight data for your area.
This chart shows the height of aircraft overflying your area in 1,000ft increments. Mouse over it for more figures and detail. A dropdown menu allows you to view historical months for comparison and trends. The data is refreshed monthly, generally in the middle of the month.
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