milder inversions sometimes occur during other seasons. Winter temperature inversions cause cold, foggy and hazy valley conditions, with sunny and somewhat warmer conditions prevailing in the surrounding mountains. Temperature inversions trap valley air pollutants, which often rise to unhealthy levels. Humidity is only high enough and temperatures cold enough for fog to occur during mid-winter, although haze and smog can be found year-round. Inversions occur when strong areas of high pressure remain parked over the Great Basin. It usually takes a cold front to force out or break down the high pressure. Consequently, inversions are rare in spring and fall, when the weather takes on a more progressive pattern, with frequent fronts moving through. Such recurring and persistent patterns of inversions follow a unique timescale of about 30 days, providing a means for long-range weather prediction. Such prediction of persistent inversion up to 30 days has been made operational. Tornadoes are also relatively uncommon. However, there has been at least one damaging tornado in the city. The Salt Lake City Tornado of August 11, 1999, which killed one person and injured 60, resulted in only the second ever fatality from a tornado in Utah, and caused about $500 million in damage. As a result of extended drought patterns, Salt Lake City constructed several reservoirs to hold excess water during flood periods and provide water during drought conditions. Additionally, artesian wells and water siphoned from Utah Lake miles to the south act as alternate water sources for the city. Recent weather events Utah was in a consistent drought from late 1998 through 2004, although experts warn that the drought-breaking conditions experienced in 2004 and 2005 could actually just be a break in a more extended drought pattern. July 2003 set the record for the warmest month ever in Salt Lake City. 2003 overall was the second warmest year on record, and recorded the second-warmest January, August, and October on record, but with the warmest month ever (July). January 2003 saw a record high January temperature of and record low January snowfall (a trace). That winter was the sixth warmest and third least snowy on record. May 2003 also set a record high May temperature of on two consecutive days. July 2002 also saw a record-tying high temperature of , while July 2003 saw a record high minimum temperature () and a record streak of 100 degree weather (July contributed 10 of