Los Alamos has a semiarid continental mountain climate. The average annual precipitation is slightly more than 18 inches, 75 percent of it falling during the months of May through October. This rainfall, much of it released through thunder-shower activity, produces considerable natural vegetation. Shower activity reaches its peak in August, when rainfall of one-tenth inch or more can be expected one day out of four. These showers normally develop in the afternoon or early evening and are usually relatively brief. Most of the winter precipitation falls as snow, with 50 inches descending during an average winter and six inches or more falling in 24 hours. Snow may lie on the ground for several days or some hail may accompany the more severe thunderstorms; no serious hail damage has been reported. Tornadoes have never been observed in this area.
Summers are cool and pleasant. Maximum temperatures reach 90 on an average of only two days a year, with 95 the highest recorded. Summer nights are cool. In July, the hottest month, low readings average in the mid-50's, and it is rare for the temperature to fail to drop to the low 60's before morning. Freezes have been recorded in all months except July and August. Winters are rather cold, although, since this is the drier season, a great many winter days are clear and sunny and daytime warming under cloudless skies is rapid at this elevation. Even in January, the coldest month, daytime shade temperatures in the high 30's can be expected on most days, and an average winter includes only 18 days when the mercury fails to rise above freezing. During winter nights temperatures drop below freezing from November through mid-April, but below zero readings can be expected only once a year.
Humidity, sunshine and wind records are not available for Los Alamos. Relative humidity should be somewhat higher than that in nearby valleys because of the cooler temperatures. However, humid weather is unlikely, and low humidity usually prevails during periods of temperature extremes. This area can expect about 70 percent of the year's possible sunshine, with winter bringing many clear, bright, sunny days. The growing season is approximately five months long, from May 10, the average date of the last freezing temperature in spring, to October 12, the average date of the first freeze in fall. Winds should normally be light, but moderately strong winds, usually paralleling the valley, do occur, especially during late winter and spring.
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