Atmospheric pressure decreases with increasing altitude. This is due to the fact that as altitude increases, the atmospheric density decreases. The atmospheric pressure at sea level is approximately 1013.25 millibars, but this decreases as altitude increases. At an altitude of 10,000 meters, for example, the atmospheric pressure is approximately 200 millibars. The pressure also decreases exponentially with increasing altitude; at a height of 20,000 meters, the atmospheric pressure is only about 10 millibars.