Extreme weather

A few notes from Dr. Jeff Masters’s Weather Blog:

Ukraine ties its record for hottest temperature in history
On August 1, Ukraine tied its record for hottest temperature in its history when the mercury hit 41.3°C (106.3°F) at Lukhansk. The Ukraine also reached 41.3°C on July 20 and 21, 2007, at Voznesensk. Sixteen of 225 nations on Earth have set extreme highest temperature in history records this year, the most of any year. The year 2007 is in second place, with fifteen such records.

Five major U.S. cities record their warmest month in history during July
July 2010 was the warmest month in history for five U.S. cities:

Las Vegas, NV: 96.2°F (old record: 95.3°F, July 2005).
Atlantic City, NJ: 79.8°F (old record: 78.7°F, July 1983)
Washington, D.C.: 83.1°F (tied with July 1993)
Baltimore, MD: 81.5°F (tied with July 1995)
Trenton, NJ: 80.5°F (tied with July 1955)

Also, in June, Miami, FL recorded its warmest month in history: 85.6°F (old record: 85.4°F in June 1998.)

Commentary
None of the 303 major U.S. cities listed in the records section of Chris Burt’s book Extreme Weather has set a coldest month in history record since 1994 (these 303 cites were selected to represent a broad spectrum of U.S. climate zones, are not all big cities, have a good range of elevations, and in most cases have data going back to the 1880s.) There were just three such records (1% of the 303 major U.S. cities) set in the past twenty years, 1991 – 2010. In contrast, 97 out of 303 major U.S. cities (32%) set records for their warmest month in history during the past twenty years. It is much harder to set a coldest month in history record than a coldest day in history record in a warming climate, since it requires cold for an extended period of time–not just a sudden extreme cold snap.

Are the pattern of U.S. temperature records due to the Urban Heat Island effect?
Is the huge disparity between extreme heat records and extreme cold records in the U.S. due to global warming, or the Urban Heat Island effect? The Urban Heat Island (UHI) effect occurs when development of former natural areas into pavement and buildings allows more heat to be trapped in cities, particularly at night. During the day, the UHI effect often leads to a slight cooling, since it can increase the amount of turbulence, allowing cooler air to get mixed down to the surface. For example, Moreno-Garcia (1994) found that Barcelona, Spain was 0.2°C cooler for daily maxima and 2.9°C warmer for minima than a nearby rural station.

However, temperature records are typically taken in parks and airports removed from the main heat-trapping areas of cities, and are not as strongly affected as one might expect. There are several reasons for this. One is that when tall buildings are present, they tend to block the view to the sky, meaning that not as much heat can escape upwards. In addition, the presence of moist vegetation keeps the atmosphere moister in park-like areas (which include the grassy fields near airports where temperature measurements are taken). This extra moisture helps cool the atmosphere on a local scale of tens of meters, due to latent heat effects (the energy required to convert liquid water to water vapor). Peterson (2003) found that “Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures.” The study used satellite-based night-light detection to identify urban areas. Recent research by Spronken-Smith and Oke (1998) concluded that there was a marked park cool island effect within the Urban Heat Island. They found that parks in typical cities in the U.S. have temperatures 1 – 2°C cooler than the surrounding city–and sometimes more than 5°C cooler. While the Urban Heat Island effect probably has contributed to some of the reduction in record low temperatures in the U.S. in the past decade, research by Parker (2004, 2006) and Peterson (2003) theorizes that Urban Heat Island effect is a factor ten or more less important than rising temperatures due to global warming.

Chris Burt wrote me yesterday about Las Vegas’ all-time warmest month record set in July. He noted that none of the sites nearby Las Vegas’ McCarran Airport (where the official obs are kept) came close to setting a warmest month in history record. McCarran Airport has set new warmest month in history records in 2003, 2005, and now 2010. These two facts make us suspect that in the case of Las Vegas, an urban heat island effect may be contributing to the spate of recent warmest month in history records there. The heat records for Atlantic City, Washington D.C., Baltimore, and Trenton do not appear to have as much of a UHI influence, since record highs were set over such a large area of the mid-Atlantic in July.

Is the Urban Heat Island effect partially responsible for global warming?
Global warming is affecting the entire Earth, including rural areas far from cities, and the 70% of the world covered by ocean. Thus, the Urban Heat Island effect–if not corrected for–can cause only a small impact on the global temperature figures. Since the Urban Heat Island is corrected for, the impact on the observed global warming signal should be negligible. For instance, NASA uses satellite-derived night light observations to classify stations as rural and urban and corrects the urban stations so that they match the trends from the rural stations before gridding the data. Other techniques (such as correcting for population growth) have also been used. Despite these corrections, and the fact that the Urban Heat Island effect impacts only a relatively small portion of the globe, global warming skeptics have persistently used the Urban Heat Island effect to attack the validity of global warming. There are no published peer-reviewed scientific studies that support these attacks.