1 . Sea level, also referred to as mean(平均的) sea level is the level at which the oceans exist when averaged between high and low tides. It is calculated by measuring the level of the oceans over long periods of time and under all types of weather conditions.     1     For instance, the height of Mount Everest is considered to be 29,028 feet (8,848 meters) above sea level.

    2    Other factors that cause changes include water temperature, air-pressure, seasons of the year, the presence of ice and snow, and the amount of salt in the water.

    3    Therefore, sea level can vary depending on where it is being measured in the world, and it is not a constant(常数).

    4     One of these is for aviation (航空) where it is often important for a pilot to be able to determine his or her height above this level at any given time. Another important function of sea level is to determine how much the sea is rising due to factors such as the melting of the polar ice caps. Mostly, it is calculated in order to determine heights of geological formations (地质建造)and land.

    5     Death Valley, California, for instance, is 282 feet (85.5 meters)below sea level, and there are many other similar locations, throughout the world. The Dead Sea is the lowest of these, measuring an amazing 1,371 feet(418 meters) below sea level.