Mongolia's weather is characterized by extreme variability and short-term unpredictability in the summer, and the multiyear averages conceal wide variations in precipitation, dates of frosts, and occurrences of blizzards and spring dust storms. Such weather poses severe challenges to human and livestock survival. Official statistics list less than 1% of the country as arable, 8 to 10% as forest, and the rest as pasture or desert. Grain, mostly wheat, is grown in the valleys of the Selenge river system in the north, but yields fluctuate widely and unpredictably as a result of the amount and the timing of rain and the dates of killing frosts.