Measuring atmospheric pressure is one of the essential operations in weather-forecasting. Indeed, for over two hundred years, the barometer—which derives directly from the Torricellian tube—has been the basic instrument for forecasting and measuring climatic changes and the resulting variations in temperature. A low-pressure zone pulls in a rush of air at normal pressure from neighboring regions: this results in changeable weather. By contrast, in a high-pressure zone, the weather tends to be stable. Therefore, a fall in the mercury level in the barometer means that variable weather is on its way; a rise in the mercury heralds fine weather. Atmospheric pressure is measured in millibars (mb). At sea level, it averages 1,013 mb. At 3,000 meters, it falls to 700 mb. At 9,000 meters—the cruising altitude of most airliners—the pressure drops to 300 mb, while at 21,000 meters, it is only 40 mb.