High Altitude Trek What is Altitude – It is the measurement of height gained vertically from the MSL (Mean Sea Level). MSL Â is a marker situated in israel used to mark the level of sea. While your height gain from your current levels the Altitude gain is calculcated based on the MSL. Why is AltitudeÂ important With the rise in Altitude following are the changes to the climate (For a common man) Reduction in vegetation Colder temperature Thinner Air – This is the biggest cause. With thinner air thinner are your chances of availability High exposure to atmosphere daily.