NEXRAD Radar & Satellite Maps
Arizona Weather FAQs Page
This page provides some basic weather facts that are displayed on some of the pages as random quotes. This page includes the whole file. Note that they are displayed in random order however.
Most of the severe weather is seen in Arizona, especially in the Phoenix Metro area, is caused by micro bursts--not tornadoes. A "micro burst" 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 strong as 40-60 mph but can exceed 100 mph at times. Micro bursts occur over a rather small space-scale, typically the area affected is less than 2.5 miles in diameter.
Dew point is the temperature to which air must be cooled for saturation (100% relative humidity) to occur. The dew point is an important measurement used to predict the formation of dew, frost, and fog. If dew point and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night. Dew point is also a good indicator of the air's actual water vapor content, unlike relative humidity, which takes the air's temperature into account. High dew point indicates high vapor content; low dew point indicates low vapor content. In addition a high dew point indicates a better chance of rain and severe thunderstorms. You can even use dew point to predict the minimum overnight temperature. Provided no fronts or other weather pattern changes are expected overnight, the afternoon's dew point gives you an idea of what minimum temperature to expect overnight.
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 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 (near Globe) on September 4-5, 1970.
An Acroymn for METeorological Aerodrome Report. It is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. Minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting.
"The most common incorrect reason given for the cause of the Moon's phases is that we are seeing the shadow of the Earth on the Moon! But this cannot be correct: when the Moon passes through the shadow of the Earth, we get a lunar eclipse. Anyone who has seen a lunar eclipse, though, might remember that the Moon actually passes through the Earth's shadow only rarely, so that can't be why the Moon has phases. The real reason for the Moon's phases depends on two things: the Moon is round, and the angle it makes with the Earth and Sun changes over its orbit."
- As Quoted From Bad Astronomy
Humidity or relative humidity measures the amount of water vapor in the air relative to the temperature. It is important in weather because humidity affects how humans feel. A hot, humid day feels hotter because we cannot sweat as effectively. A cool, dry day feels colder because moisture evaporates more easily
Terms such as "slight chance" of rain (10-20%), "chance" of rain (30-50%) or rain "likely" (60-70%) are used when there is uncertainty of receiving measurable precipitation anywhere in the forecast area (such as the Greater Phoenix Area). For instance, 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.
The cloud height on this site is an estimate of cumulus clouds using a formula based on temperature and dew point. Actual measurements of cloud height are made with a Micropulse Lidar (MPL). This device fires a laser into the sky and measures the backscattered signal. Costs for such a device are beyond the scope of weather hobbyists.
Definition Humidex - Over the years, several measures have been proposed to relate various combinations of temperature and humidity into a single number to approximate what hot, humid weather feels like to the average person. Of these, humidex is the one most familiar to Canadians. Humidex was introduced into Canada in 1965. The index is a summer analogue of the wind chill factor in that it is an equivalent air temperature. Air of a given temperature and humidity is equated in comfort to air of a higher temperature that has a negligible moisture content. Comfort is quite subjective and largely dependent on the age and health of the individual. Weather conditions causing prickly heat in an infant may result in heat cramps in a teenager, heat exhaustion in a middle-aged and heat stroke in a senior. Humidex is also limited as an overall hot-weather comfort index because it does not consider other factors such as pressure, wind speed, precipitation, sunshine or pollen.
The air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure. Generally, the more air above an area, the higher the atmospheric pressure. Barometric pressure changes with local weather conditions, making barometric pressure an important and useful weather forecasting tool. High pressure zones are generally associated with fair weather, while low pressure zones are generally associated with poor weather. For forecasting purposes, the absolute barometric pressure value is generally less important than the change in barometric pressure. In general, rising pressure indicates improving weather conditions, while falling pressure indicates deteriorating weather conditions.
Hawley Lake recorded Arizona's coldest temperature of 40° below zero on January 7, 1971. The coldest temperature recorded in Phoenix was 16°, set on January 7, 1913. Phoenix Sky Harbor Airport, where the official temperature for Phoenix is recorded, rarely gets below freezing(32°F).
A common misuse of the term "monsoon" is to refer to INDIVIDUAL thunderstorms as "monsoons" (example - "The east valley was pounded by monsoons this evening!"). The correct statement would be "The east valley was pounded by strong thunderstorms this evening!"
Seasonal wind shifts often bring a dramatic increase in moisture, and associated shower and thunderstorm activity, to the affected region. As the monsoon ends, and the winds shift again, the reverse occurs, with much drier air moving into the area.
The best example of a monsoon on Earth occurs over the Indian sub continent. During the months of April through October, a moist southwest wind brings heavy rains to this region...while a dry northeast wind is prevalent during the remainder of the year.
In North America, a similar situation occurs over much of Mexico. For example, in Acapulco, rainfall averages 51.8 inches during the months of June through October...while only 3.3 inches falls during the remainder of the year.
In the United States, Arizona and New Mexico are located on the northern fringe of the Mexican Monsoon. For most of the year, winds aloft over the southwest U.S. are west to northwest. During the summer, winds turn to a more south to southeast direction, importing moisture from the Pacific Ocean, the Gulf of California and the Gulf of Mexico.
Wind Chill - The wind chill temperature is what the temperature "feels like" to people and animals during cold weather. Wind chill is based on the rate of heat loss from exposed skin caused by wind and cold. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. Once temperatures drop below 10 °F and the wind is gusting, conditions are ripe for cold-related illnesses. Below -5 °F, any wind is a major factor in frostbite and hypothermia.
Weather Station - A weather station is a facility with instruments and equipment to make weather observations by monitoring atmospheric conditions to study the weather. This weather station has a thermometer for measuring temperature; barometer for measuring changes in air pressure; hygrometer for measuring humidity; anemometer for measuring wind speed and wind direction; and rain gauge for measuring precipitation.
The Heat Index (HI) (or apparent temperature) is a measure of relative discomfort due to combined heat and high humidity. It was developed by R.G. Steadman (1979) and is based on physiological studies of evaporative skin cooling for various combinations of ambient temperature and humidity. As temperatures climb above 90 °F and humidity goes above 40 percent, conditions are ripe for heat-related illnesses.
"Measurable rain" refers to a rainfall total of 0.01 inches or greater. When you hear the terms "isolated" showers or "few" showers (10-20%), "scattered" showers (30-50%), or "numerous" showers(60-70%), in the forecast, this refers to the percent of the forecast area covered by measurable rain. For instance, "scattered showers" means that the forecast area WILL receive rain, and approximately 30-50 percent of the area will experience showers.
Hurricane Facts: 1961 was the last time two Category 5 hurricanes formed in the same year. 1960 was the last time two Category 5 hurricanes formed and eventually made U.S. land fall. 2005 has now matched that with Katrina and now Rita.
A statement generally provides additional or follow up information to an existing weather condition. An advisory is for less serious conditions that cause significant inconvenience and, if caution is not exercised, could lead to situations that may threaten life and/or property. A watch is used when the risk of a hazardous weather event has increased significantly, but its occurrence, locations, and/or timing is still uncertain. It is intended to provide advance notice of possible inclement weather. A warning is used for conditions posing an immediate threat to life or property. Depending on the type of warning, you should take immediate, appropriate action.
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° on June 29, 1994. This bests the Phoenix mark of 122°, set on June 26, 1990.
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