Re: How do planes know the exact altitude ?

There are several ways for an airplane to determine its altitude, including radar altimetry, inertial navigation, and GPS navigation. However, the simplest, most common, and arguably the most reliable, means is barometric altimetry.

As you note, the reading of a barometric altimeter will be affected by both altitude and meteorological conditions. Let's compare the magnitude of the effects:

A barometric altimeter's scale is calibrated in terms of feet or meters above sea level, using a relationship between pressure and altitude and a "standard atmosphere" defined by an international standard. Near the earth's surface air pressure drops by about 1 inch of mercury (Hg) every 1000 feet (that's about 80 mm of Hg per 1000 meters, or 110 millibars per 1000 meters).

The air pressure difference between top and bottom of a 100 story building is about 1 inch Hg. The barometric pressure on a clear sunny day (i.e., weather due to strong anticyclone) might be as high as 30 inches plus a tenth or two. The barometric pressure on a stormy day might be around 29.7 or so, hitting 28 inches or less at the center of powerful cyclones such as hurricanes or typhoons (or even lower for tornadoes, but these are very localized).

So ordinary day-to-day type of weather variations influences an altimeter's reading by a few hundred feet (100 meters or so), hurricanes excepted.

That's enough to be of concern in navigating aircraft, not only during landing, but even enroute, where assigned altitudes differences between aircraft may be as small as 500 feet. Therefore, pilots use procedures to compensate for barometric pressure. One procedure is used for aircraft flying below 18,000 feet above sea level, and another is used for aircraft above 18,000 feet.

All altimeters have an adjustment called the "altimeter setting", whereby the pilot can adjust the altimeter for local barometric pressure.

For flight below 18,000 feet, the altimeter setting is adjusted to correspond to the current local barometric pressure. This is done prior to flight simply by adjusting the altimeter setting so that the altitude reported while the aircraft is parked on the ground is the actual known airport altitude. In flight and upon approaching the destination, the pilot obtains the current barometric pressure from a nearby airport or weather station and adjusts the altimeter setting to this value.

The rationale for this procedure is that even though barometric pressure varies over time and distance, the change in pressure is usually quite small over time scales on the order of an hour or so and distances on the order of 100 miles. Typically, a change of a few hundredths to a tenth of an inch of Hg might be seen, corresponding to errors in reported altitude of less than 100 feet. That's acceptable for enroute navigation, but demands a current update just prior to landing. (Making a final update to the altimeter setting a few minutes just prior to landing is standard procedure for pilots. If you have an aircraft band radio and listen in on tower or approach control communications you will hear the controller or an automatic broadcast system report the current altimeter setting to pilots approaching for landing).

The rationale for the procedure described above is that flight below 18,000 feet tends to be at slow to moderate speeds, so the distance covered in a half hour is perhaps 100 to 150 miles (at most), and updates to altimeter settings need not be more frequent than one or twice an hour while enroute.

Above 18,000 feet, aircraft can cover 100 miles every 10 minutes or so. To preclude having to update the altimeter setting many times an hour, the practice is for all pilots intending to fly above 18,000 feet to adjust their altimeter setting to 29.92 inches of Hg (1013 millibars), the barometric pressure corresponding to seal leval pressure in a standard atmosphere. This setting is used REGARDLESS of the true barometric pressure. As a result, the altimeter no longer reads actual height above sea level, but "pressure altitude" -- the altitude corresponding to the current air pressure in a standard atmosphere. Pilots set their altimeters to 29.92 upon climbing through 18,000 feet and to local barometric pressure upon descending through 18,000 feet.

Using this method, aircraft flights are no longer assigned actual altitudes, but "flight levels". For example, FL250 represents a nominal altitude of 25,000 feet; in reality this pressure altitude is only approximately 25,000 feet above the ground, depending on the local barometric pressure. Having aircraft fly these approximate heights above the ground is not a problem, however, since the ground (mostly) is far away and all other aircraft flying above 18,000 feet also set their altimeters to 29.92 and will be a similar distance above or below 25,000 feet when their altimeters read 25,000. That is, all aircraft will be at identical flight level (FL250) and separated by exactly 1000 feet from those at FL240 and FL260.

One final side note concerning the flight level system: what actual altitude is an aircraft at FL180, the lowest possible flight level? If the local barometric pressure is above 29.92, then FL180 will be above 18,000 feet (reason: an altimeter set to 29.92 will read zero altitude when the when the actual static air pressure outside the aircraft is 29.92. If local barometric pressure at sea level is above 29.92, then at some height above sea level it drops to 29.92. When the aircraft is at this height its altimeter will read zero. Thus actual altitude is above the reported altitude). If the local barometric pressure is below 29.92, then FL180 is actually below 18,000 feet above sea level. This could cause conflict with aircraft assigned to 17,500 feet or lower. This problem is solved simply by not assigning aircraft to FL180 in regions of low barometric pressure.