Death Valley has a subtropical, hot desert climate (Köppen: BWh), with long, extremely hot summers and short, warm winters as well as little rainfall. As a general rule, lower altitudes tend to have higher temperatures. When the sun heats the ground, that heat is then radiated upward, but the dense below-sea-level air acts as a blanket and reflects the heat back. In addition, the high valley walls trap rising hot air and recycle it back down to the valley floor, where it is heated by compression.[7]
This process is especially important in Death Valley as it provides its specific climate and geography. The valley is surrounded by mountains, while its surface is mostly flat and devoid of plants, and so much of the sun's heat can reach the ground, absorbed by soil and rock. When air at ground level is heated, it begins to rise, moving up past steep, high mountain ranges, which then cools slightly, sinking back down towards the valley more compressed. This air is then reheated by the sun to a higher temperature, moving up the mountain again, whereby the air moves up and down in a circular motion in cycles, similar to how a convection oven works. This superheated air increases ground temperature markedly, forming the hot wind currents that are trapped by atmospheric pressure and mountains, thus stays mostly within the valley. Such hot wind currents contribute to perpetual drought-like conditions in Death Valley and prevent much cloud formation from passing through the confines of the valley, where precipitation is often in the form of a virga.[8] Death Valley holds temperature records because it has an unusually high number of factors that lead to high atmospheric temperatures.
The depth and shape of Death Valley influence its summer temperatures. The valley is a long, narrow basin 282 feet (86 m) below sea level, yet is walled by high, steep mountain ranges. The clear, dry air and sparse plant cover allow sunlight to heat the desert surface. Summer nights provide little relief as overnight lows may only dip into the 82 to 98 °F (28 to 37 °C) range. Moving masses of super-heated air blow through the valley creating extremely high temperatures.[9]
The hottest air temperature ever recorded in Death Valley was 134 °F (56.7 °C) on July 10, 1913, at Furnace Creek,[10] which is the hottest atmospheric temperature ever recorded on earth.[4] During the heat wave that peaked with that record, five consecutive days reached 129 °F (54 °C) or above. Some meteorologists dispute the accuracy of the 1913 temperature measurement.[11]
The greatest number of consecutive days with a maximum temperature of 100 °F (38 °C) or above was 154 days in the summer of 2001. The summer of 1996 had 40 days over 120 °F (49 °C), and 105 days over 110 °F (43 °C). The summer of 1917 had 52 days where the temperature reached 120 °F (49 °C) or above with 43 of them consecutive. Four major mountain ranges lie between Death Valley and the ocean, each one adding to an increasingly drier rain shadow effect, and in 1929, 1953 and 1989 no rain was recorded for the whole year.[9] The period from 1931 to 1934 was the driest stretch on record with only 0.64 inches (16 mm) of rain over a 40-month period.[8] On June 30, 2013, during the 2013 extreme heat wave, the mercury reached 129 °F (54 °C) at Furnace Creek station, which is the all-time hottest air temperature recorded for the month of June.
The mean annual temperature for Death Valley (Furnace Creek Weather Station) is 77.2 °F (25.1 °C) with an average high in January of around 67 °F (19 °C) and 116 °F (47 °C) in July.[12] From 1934-1961 the weather station at Cow Creek recorded a mean annual temperature of 77.2 °F (25.1 °C).[13]
The longest number of consecutive days where temperatures reached 90 °F (32 °C) or more was 205 from April to Oct 1992.[14] On average there are 192 days per year in Death Valley where temperatures reach 90 °F (32 °C) or more.[15] Before being moved to Furnace Creek, the weather station at Greenland Ranch averaged 194.4 days annually where temperatures reached 90 °F (32 °C) or more.[16]
View from Badwater Basin
On July 12, 2012, the day's low temperature was 107 °F (42 °C), tying the record for the world's hottest low temperature ever recorded.[17] On the same day, the average temperature was 117.5 °F (47.5 °C), which is the world's hottest temperature averaged over 24 hours on record.[17]
The lowest temperature recorded at Greenland Ranch was 15 °F (−9 °C) in January 1913.[18]
The period from 17–19 July 1959, was the longest string of consecutive days where nighttime low temperatures did not drop below 100 °F (38 °C).[19] The world-record high overnight low temperature recorded in Death Valley is 110 °F (43 °C), recorded on July 5, 1918.[20] As recently as July 12, 2012, the low temperature at Death Valley dropped to just 107 °F (42 °C) after a high of 128 °F (53 °C) on the previous day. The only other location which matches Death Valley's overnight low temperature of 107 °F (42 °C) in recent years is Khasab Airport in Oman, which also recorded a low of 107 °F (42 °C)[citation needed][dubious – discuss] on June 27, 2012. Also on July 12, 2012 the mean 24-hour temperature recorded at Death Valley was 117.5 °F (47.5 °C), which makes it the world's warmest 24-hour temperature on record.[21]
The average annual precipitation in Death Valley is 2.36 inches (60 mm), while the Greenland Ranch station averaged 1.58 in (40 mm).[22] The wettest month on record is January 1995 when 2.59 inches (66 mm) fell on Death Valley.[8] The wettest period on record was mid-2004 to mid-2005, in which nearly 6 inches (150 mm) of rain fell in total, leading to ephemeral lakes in the valley and the region and tremendous wildflower blooms.[23] Snow with accumulation has only been recorded in January 1922, while scattered flakes have been recorded on other occasions.