National Weather Service United States Department of Commerce



What you will find here could be considered Meteorological Trivia - things that you wanted to know but were afraid to ask! The questions listed here are mostly meteorological in nature, as compared to the questions in the FAQ.

What is the difference between "Partly Sunny" and "Partly Cloudy"?   The forecast, as written by the National Weather Service, always contains the sky condition, such as "clear" or "mostly cloudy", as one of the required weather elements. The sky condition term is based on the number of tenths of opaque cloud cover expected during the forecast period (e.g. "today" or "tomorrow night").  A forecast of "Cloudy" expects the sky, on average, to be covered with 9/10 to 10/10 of opaque cloud cover. "Opaque" clouds block the sun, unlike thin cirrus, which lets most of the sun, and its heat, through to the ground. A forecast of "Clear", however, expects the sky to be covered with 0/10 to 1/10 of opaque clouds. A forecast of "Partly Cloudy" or "Partly Sunny", is made when between 3/10 and 6/10 of the sky is to be covered with clouds. The difference between these terms is based on the forecast period itself - during the day, either term could be used. However, for the nighttime forecast periods, a forecast of "partly cloudy" would be the only appropriate term, as there is no sunshine at night!

What is the difference between "A Chance for Showers" and "Scattered Showers" in the forecast?  When the National Weather Service anticipates a threat for weather (such as showers or thunderstorms) in the forecast, it must choose an appropriate term to express the degree of the threat.   Terms such as "slight chance" (10-20%), "chance" (30-50%) or "likely" (60-70%) are used when there is uncertainty of receiving measurable precipitation anywhere in the forecast area (such as the Greater Phoenix Area). If there is only a 30-50 percent chance that rain will fall anywhere in the Phoenix Metro area, then the forecast will call for a "chance" of rain. However, there are times when there is nearly 100 percent chance that rain will fall SOMEWHERE within the forecast area; in these cases the terms used describe the amount of area that will be covered by measurable rain. "Measurable rain" refers to a rainfall total of 0.01 inches or greater. When you hear the terms "isolated" or "few" (10-20%), "scattered" (30-50%), or "numerous" (60-70%), in the forecast, this refers to the percent of the forecast area covered by measurable rain. "Scattered Showers" means that the forecast area WILL receive rain, and approximately 30-50 percent of the area will experience showers. The term "scattered" modifies "showers" much in the same way "several" modifies "apples". You would not say "I have a few several apples"; along these lines the phrase" a few scattered showers" is equally incorrect. You either have a few showers, or you have scattered showers.

What exactly is a "microburst" and how is it generated?  Most of the severe weather that we see in Arizona, especially in the Phoenix Metro area, is caused by microbursts (not tornadoes). A "microburst"   is a small area of rapidly descending air beneath a thunderstorm. When the descending air hits the ground, it quickly spreads out in all directions, causing very strong, straight-line winds. These winds are commonly as as strong as 40-60 mph but can exceed 100 mph at times. Microbursts occur over a rather small space-scale, typically the area affected is less than 2.5 miles in diameter. A "macro-burst" is the same thing, physically, as a micro-burst, but over a much larger space scale - it affects a much larger area on the ground. Inside a thunderstorm, water vapor condenses into raindrops, which then fall to the ground. When these raindrops fall through drier air, they start to evaporate. The evaporation process cools the air, causing it to become denser than the air around it. This rain-cooled air, along with the falling raindrops, accelerates downwards; it is this down-rushing air that eventually hits the ground and is forced to spread out in all directions causing the damaging straight-line winds.

What is the hottest location in the state of Arizona?  Although Phoenix residents may feel that their city MUST be the hottest place around at times, top honors go to Lake Havasu City, where the mercury climbed to 128 degrees on June 29, 1994. This bests the Phoenix mark of 122 degrees, set on June 26 1990.

How about the coldest place in Arizona? Obviously, Phoenix cannot touch this mark - Hawley Lake recorded Arizona's coldest temperature of 40 below zero on January 7, 1971. The best (coldest) temperature Phoenix could muster was 16 degrees, set WAY back on January 7, 1913.  Now, on even the coldest of days, Phoenix Sky Harbor struggles to reach, must less break, freezing (32 degrees). This is due to the "heat island" effect caused by urbanization.

How much rain can fall in a 24 period in Arizona? Typically, the heaviest rain falls during the summer thunderstorm season, or Monsoon, in our state. The rain can accumulate very quickly, resulting in flooded streets or washes, and can even cause deaths via flash flooding. In Phoenix, the greatest rainfall in a 24 hour period was 4.98 inches; this fell on July 1-2, 1911. This total is quite a bit less than the Arizona record of 11.4 inches, which fell on Workman Creek (30 NNW of Globe) on September 4-5, 1970. However...this amount is no where near the United States record measured at Waipā Garden on the Hawaiian island of Kauai; on April 14-15 2018, 49.69" of rain fell in one day - that's just over 4 FEET!!

How much rain can fall in an entire calendar year in the Grand Canyon State (Arizona)? The wettest year on record for Phoenix dates way back to 1905, when 19.73 inches fell in the gage. The all-time Arizona mark is 58.92 inches at Hawley Lake - this was set in 1978. Arizona seems bone dry when compared to the United States record holder - Kukui Hawaii, where a whopping 704.83 inches fell in the year 1982! (that's over 58 FEET of rain)

Has Phoenix ever recorded snowfall during the winter? As a matter of fact, yes! The greatest snowfall in one season for Phoenix was 1 inch, set in 1932-1933 and tied in 1936-1937. Top honors for the state go to Sunrise Mountain, where 400.9 inches fell in the winter of 1972-1973. When it comes to the record for the United States, the mark to beat is 1140 inches (95 feet) which fell at the Mount Baker Ski Area in Washington during the 1998-1999 snowfall season.

  • During the four years of 1996 thru 1999, there were more convective events in Maricopa County, AZ (metro Phoenix) than in the 5 county area encompassing Kansas City, MO, KS; 137 vs 115 (source: Storm Data - Stats on Demand).
  • Phoenix is the 17th largest media market in the United States. We also have responsibility for the Yuma/El Centro market (rank 174).
  • In the Western Region, during the period 1996-1999, only Billings and Glasgow have had more severe convective events than Phoenix. The Billings and Glasgow media markets rank 169 and 210, respectively.
  • Sky Harbor Airport is the busiest three runway airport in the United States. It is ranked fifth in the world for airport movements (takeoffs and landings).
  • Violent dust storms (haboobs), which occur routinely during the Arizona monsoon, reduce visibilities from unrestricted to less than 1/4 mile faster than any other meteorological event, presenting an extreme hazard to aviation and vehicular travel.
  • The Arizona severe weather season runs concurrently with the fire weather season in the rest of the western region.
  • Severe Flooding in 1978, 1980, and 1993 produced well over $1 Billion (today's dollars) in damage in Arizona. The vast majority of this damage occurred in the current Phoenix CWA (County Warning Area).
  • Since 1975, the population of the metro Phoenix area has increased by more than 200 people PER DAY.