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The growing season is short . In the prepara- <br /> tion plant area , the growing season extends from May through <br /> September (170 days) , with around 90 frost-free days . At <br /> the mine portals (all above 10 , 000 ' elevation) , the season <br /> extends from mid-May through mid-September (130 days) with <br /> only 60 days frost free . <br /> Summers are warm and pleasant . Temperature <br /> highs reached 890 in July, 1978 ; 870 in August , 1979 ; 840 in <br /> July and August, 1980 . Lows recorded during the same months <br /> were 360 in 1978 ; 340 in 1979 ; 400 and 320 in 1980 . <br /> During the summer, Coal Basin does not <br /> experience the same heavy precipitation which characterizes <br /> • the winter months . In 1978 , only 2 . 63" of precipitation <br /> were recorded from June through September , the only months <br /> free of snowfall ; in 1979 , precipitation totalled 3 . 66" from <br /> July through September , the snow-free months ; in 1980, <br /> precipitation totalled 3 . 00" from June through September . <br /> Predominant air masses over the area during <br /> the summer are maritime polar and , occasionally , continental <br /> tropical . Local winds are shallow surface currents , moving <br /> up the valleys and mountain slopes during the day , and down- <br /> slope at night . <br /> Data sources are outlined in Appendix III-F- <br /> 1 . <br /> -2- <br />