Pressure is a certain force that is applied on a surface unit. As mentioned before, air has weight, and because of it, force is exerted upon earth. This force, when measured by a surface unit, will be called pressure. In order to illustrate this, try to imagine a pillar of air with a surface of one square centimeter, starting from sea level up to the upper edge of the atmosphere. This pillar of air contains air weighing one kilogram. This means, that at sea level, the atmosphere exerts the force of one kilogram on any one square centimeter surface. Pressure is measured in force per unit area. As explained with the air pillar example, the sea level has a pressure level of one kilogram over one square centimeter (marked – 1 kg/1cm2). This pressure unit is called "atmosphere"(atm). To be precise, one may indicate that 1 atm is equal to about 1 kg/1cm2. Environmental conditions change according to climate and weather conditions, so pressure will not always be constant. A pressure of 1 atm is the normal pressure the human body experiences. We were born into it, we breathe air on this pressure level and our body is used to it. At sea level, we don't feel the air pressure. When you go up to high places, it is difficult to feel the pressure lowering. However, when diving, you will definitely feel a rise in pressure, as you will see later on.