Glattalp lies in a hollow above Bisisthal in the Canton Schwyz. Due to the special location in a trough without surface runoff, a cold air lake can form under ideal conditions (cloudless, no wind). Under these circumstances, the temperature can drop very low. In winter, temperatures below minus 30 degrees are the rule in inversion weather conditions. On Glattalp, the Swiss minus record of -52.5 °C was recorded on 7 February 1991. However, the Swiss Meteorological Institute (SMA) does not recognise this record as it cannot be verified and Glattalp is uninhabited. La Brévine therefore remains the officially coldest point in Switzerland with -41.8 °C on 12 January 1987.

The natural lake has no surface runoff. Some of the water seeps into the karstic underground, the rest is used by the Sahli power station.