Synoptic and temporal characteristics of the Las Vegas urban heat island

Over the last fifty years, the average minimum temperature at Las Vegas, Nevada increased by about 3ºC more than nearby rural stations due to the urban heat island (UHI) effect. The maximum nightly UHI effect at Las Vegas is approximately 11-13ºC, which is similar to other cities of comparable size. Since 1950, Las Vegas grew from a town of just 48,000 residents to nearly 1.6 million in 2000. With almost 1.2 million new residents (a 240% increase) in the last two decades, Las Vegas was the fastest growing metropolitan area in the 1990 and 2000 US census readings. While the heat island at Las Vegas is not surprising given its population, it is unusual due to the prevailing high wind speeds in this region, which would typically limit urban heat effects. A strong UHI in spite of this suggests that extremely arid regions may be more sensitive to urban climate change than more humid or temperate locations.

While the maximum UHI effect occurs during autumn, the magnitude of urban warming is similar in each of the seasons. In October and November, the average minimum temperature increased by 3.5-3.7ºC more at Las Vegas than the nearby rural Desert National Wildlife Game Range station. In March, a windier and more synoptically active month, the relative minimum temperature increase was still 2.7ºC. Consistent with results from other cities, the UHI effect at Las Vegas is predominantly a nighttime phenomenon. It is hypothesized that increased pollution attenuates incoming radiation during the day thus slightly lowering the maximum temperature leading to an “urban cool island,” while increased heat storage of the urban fabric leads to significantly higher nighttime temperatures. There is evidence that the average summer maximum temperature decreased by approximately 1ºC since 1950 compared to rural locations. There is no indication of a long-term maximum temperature effect in other seasons. Because this is a major tourist destination with distinct weekly and monthly visitation patterns, there are corresponding air pollution cycles that affect the local and regional climate, including the UHI effect.

There are different synoptic controls on the Las Vegas UHI by season. From autumn to early spring, the maximum UHI effect occurs under clear and calm conditions produced by anticyclonic conditions. In summer, a significant decrease in UHI magnitude occurs during the brief monsoon period characterized by relatively moist conditions and higher sea-level pressure. When the thermal low intensifies in summer, dry air and cloudless skies prevail leading to extremely high daytime temperatures and less urban radiational cooling at night compared to adjacent non-urban areas. An inverse correlation between the UHI and wind speed occurs throughout the year. Significant correlations exist between upper and lower level height patterns and the intensity of the Las Vegas UHI.