Ambient Weather WS-108BA Digital Barometer, Altimeter, Compass, Thermometer, Clock The Portable Weather Station includes a digital barometer (relative and absolute with sea-level correction), compass, clock, thermometer, altimeter, and a weather forecaster The handheld Weather Station is big on features and functions, yet small on price and size, and features a convenient carabiner clip for simple carrying and quick access. Includes a powerful backlight for nighttime operation. Perfect for hiking, camping, running, biking, skiing, snowboarding, mountaineering, climbing, boating, kayaking, and outdoor activities, the Keychain Weather Station is a perfect backup to your GPS. IMPORTANT NOTE: Once the unit is calibrated at a known altitude, the sea-level (relative) pressure does not vary and the measured (absolute) pressure fluctuates with the weather and altitude conditions. To determine the sea-level pressure, calibrate the pressure altitude at a known true altitude and the sea-level pressure will be accurate. This behaves differently than a weather station barometer, which assumes a constant altitude, and the measured and sea-level pressures change together with a fixed offset (as determined from the alititude compenstation). IMPORTANT NOTE: This is a general explanation about how altimeters work in general and is applicable to all altimeters, not just this product listing. Altimeters are often misunderstood because they do not measure your actual position on the earth (true altitude) such as a GPS, but measure the air pressure and predict the altitude based on this measurement. This is referred to as Pressure Altitude. Pressure Altitude is profoundly affected by the true altitude and to a lesser extent, changes in the weather, often referred to as barometric pressure changes. Thus, your Pressure Altitude can change without changing your position. These changes can be as great as a few hundred feet (the changes due to weather are more pronounced at higher altitude). To offset the difference between Pressure Altitude and True Altitude, you must calibrate the device at a known altitude, whenever convenient.