It is often important to specify the level at which certain parts of a cloud are located. To indicate such a level, two concepts can be used, that of height and of altitude.
The height of a point (for example: the base of a cloud) is the vertical distance between the level of the observation site and the level of that point. It should be noted that the observation point can be found on a hill to a mountain. Instead, the altitude of a point is the vertical distance between mean sea level and the level of that point. Surface observers generally use the concept of height. Aircraft observers, however, generally refer to altitude. The vertical dimension of a cloud is the vertical distance between the level of its base and that of its top.
Clouds are generally located at altitudes between sea level and tropopause level. The level of the tropopause it is variable in space and time; therefore, cloud tops are higher in the tropics than in middle and high latitudes. We remember that the tropopause was the boundary between the troposphere and the stratosphere.
By convention, the part of the atmosphere where clouds usually occur has been divided into three levels called, respectively, high, medium and low. Each floor is defined by the set of levels in which clouds of certain genres occur most frequently. The floors overlap somewhat and their limits vary with altitude.
As an example, in polar regions the high level is between 3 and 8 km while in the equatorial regions this level is between 6 and 18 km.