The station set can be reduced by these additional criteria.

• The number of years with data. In the case of monthly data there sometimes is enough metadata to demand this for a specific season. If not, the only guarantee is that there is some data in each of the years.
• Distance between stations. Which stations are chosen depends on the ordering in the database and is therefore approximately random.
• Altitude (elevation): for some analyses it is useful to exclude mountain-top stations.