Altitude and the vertical distance measure between one certain point, it's the mean sea level. The altitude and temperature of the place where it is measured are inversely proportional quantities, because when the altitude increases, the ambient temperature decreases by approximately 1 degree Celsius.
The vertical distance between a point on the earth's surface (for example, the top of a mountain) and sea level (sea surface) is known as absolute altitude. Relative altitude is the altitude of a mountain relative to the bottom of a valley or the difference in altitude between two mountains.
The Earth is approximately spherical, with a slight flattening at the poles, and to define the altitude of a point on the Earth, a sphere is defined, with a radius of 6378 km.
Altitude can be divided into orthometric and ellipsoidal. Orthometric altitude is the vertical distance of a point, located on the earth's surface, in relation to a reference geoid, and ellipsoidal altitude is the vertical distance from a point to an ellipsoid of reference. The altitudes indicated by Global Positioning Systems (GPS) receivers, for example, are of the ellipsoidal type.
The main physical harm that high altitudes can cause to human beings are related to cold, oxygenation and the pressure difference between the body and the ambient air. High altitude is a detrimental factor for people who practice sports such as football and climbing, because how much the higher the altitude, the lower the amount of oxygen, which impairs breathing and the performance of athletes.