There are three different level of forest fire warnings (yellow, orange and red) and one in grass fire warnings (yellow). They all refer in law to the same thing, that it is forbidden to start open fire. Probability for large and quickly progressive forest fires increase, when it is orange or red warning. A forest fire warning is usually given and cancelled at 6 am. The warnings are specific to each province. Municipality-specific warnings may also be given in Northern Ostrobothnia (mainly for Kuusamo, Taivalkoski and Pudasjärvi) and in Lapland. Forest fire warnings are issued using an index calculated by means of a model that describes moisture conditions in the top soil layer measuring about 6 centimetres in thickness. The input data for the model includes precipitation and air temperature. Most often, the first forest fire warnings of the year are issued in early May. The forest fire warning season ends in September or at the latest in early October.