Heat Sets 6 Records In 2 Weeks

PhonenixPhoenix has logged six days of record heat since Oct. 24, with temperatures threatening to break more records before the week is over.

The warmest record high in the latest batch of summer-like weather was 97 degrees set on Oct. 28.

The normal high for this time of year is 78.

With temperatures remaining in the 90s in Phoenix this week, forecasters said it is easy for people to begin speculating the reasons behind the unusually warm weather.

// Set DC ad positionif(typeof dcadposition == 'undefined')dcadposition = 1;else dcadposition++;if (typeof segQS == 'undefined')segQS='';if (typeof adid == 'undefined')adid='false';// Add code to define sluser true|falsevar SiteLifeUser = (typeof myCookies != 'undefined' && getCookie('HD') != '')?'true':'false';document.write("");dcmaxversion = 9dcminversion = 6DoOn Error Resume Nextplugin = (IsObject(CreateObject("ShockwaveFlash.ShockwaveFlash." & dcmaxversion & "")))If plugin = true Then Exit Dodcmaxversion = dcmaxversion - 1Loop While dcmaxversion >= dcminversionif ((!document.images && navigator.userAgent.indexOf("Mozilla/2.") >= 0) || navigator.userAgent.indexOf("WebTV")>= 0) {document.write("");document.write("");} Climate change? The urban heat island? Persistent high pressure? The National Weather Service said the answer is a combination of all three. However, meteorologists said there is currently no way of knowing how much each factor contributes to the observed warming. Forecasters said it is well documented that our planet is warming. According to the fourth assessment report published by the international panel on climate change, as the entire spectrum of temperatures shifts warmer, there can be an expected increase in the frequency and intensity of extreme temperatures. Put more simply, hot temperatures will not only occur more often but will also become hotter, NWS said. An additional element of extremely warm temperatures in the Phoenix area is the growth of the urban heat island. The effects of the phenomenon in Phoenix have been well documented in the scientific community for several decades. It has been shown that the greatest impact has been an increase in overnight low temperatures, meteorologists said. Temperature differences from heavily urbanized areas to surrounding rural areas can approach 15 degrees or more during the summer months. The impacts on afternoon high temperatures are less as compared to overnight low temperatures, but do remain, forecasters said. Climate change and the urban heat island help explain why temperatures overall have been increasing. They do not directly address the issue of why it has been particularly warm very recently. The weather pattern across the Southwest U.S. has been dominated by a persistent high pressure system, the NWS said. Under high pressure, air gently descends to the surface in a process known as subsidence. As the air descends, it warms due to compression. This process is further enhanced by the fact that the air under the high pressure system is very dry. Dry air is heavier than moist air, which enhances the sinking motions, meteorologists said. Several days under this type of pattern will inevitably lead to above normal temperatures, forecasters said. It is often said that the weather in Phoenix will cool down by Halloween. How true is this? The NWS looked at when the last 90 degree day has occurred in Phoenix, from 1896 through 2006. On average the last 90 degree day has been Oct. 24. The earliest was Sept. 28 in 1916. The latest was Nov. 15 in 1999. When assessed by decade, there has been an obvious shift toward a later date through the past 50 years, the NWS said. Meteorologists said this is likely due to a combination of climate change, the urban heat island and the short-term weather patterns. Average Last 90-Degree Day 1900s -- Oct. 22 1910s -- Oct. 19 1920s -- Oct. 22 1930s -- Oct. 28 1940s -- Oct. 24 1950s -- Oct. 22 1960s -- Oct. 26 1970s -- Oct. 23 1980s -- Oct. 28 1990s -- Oct. 27 2000s -- Oct. 28 Phoenix Endures Record Heat Here are the new records set the past two weeks, as recorded by the National Weather Service: Oct. 24 -- 96 degrees Oct. 28 -- 97 degrees Oct. 29 -- 95 degrees Nov. 5 -- 93 degrees Nov. 6 -- 94 degrees Nov. 7 -- 92 degrees The record high for Wednesday is 91, set in 1934; for Thursday, 91, set in 1906; and for Friday, 88, set in 2005. High Pollution Advisory Posted For Thursday A high pollution advisory for particulate pollution has been issued for the Phoenix area for Thursday. The Arizona Department of Environmental Quality, which issued the advisory, says course particulate levels are expected to exceed federal health standards. It recommends that children and adults with respiratory problems avoid outdoor activities, that residents car pool or use mass transit to reduce pollution levels, and that people refrain from using gasoline lawn mowers and gas-powered leaf blowers during the advisory.