A microburst starts with a typical thunderstorm. What is a thunderstorm?
A thunderstorm works like an engine. It pulls moisture and air in and converts it to rain and then pushes wind and rain out. For the thunderstorm to continue; it has to be TILTED. The top of the thunderstorm can NOT be directly over the bottom.
During the later months of Arizona Monsoon (which means a season – like summer is a season), the steering flow in the upper-level of the atmosphere weakens. The UPPER level winds are what tilt storms, such as thunderstorms. The thunderstorm can still form but it will lose the tilt quickly!
This picture is the basics of a thunderstorm. The updrafts and downdrafts are made up of warm air and cooler air.
As warm, humid air rises inside a storm, heavy rain forms and some of it evaporates in the colder air on top. This cooled air then sinks, accelerates and spreads out as it hits the ground, resulting in a localized, wind called a microburst.
These down bursts are put in two categories. A MACRO-burst and MICRO-burst, only difference is the area they are concentrated in.
To understand the difference in the sizes of a Micro-burst and a Macro-burst I included another photo from WVVA TV.
A Microburst Storm is an intense, localized downdraft of air that spreads on the ground causing rapid changes in wind direction and speed. “downburst”
Microbursts are made of winds rushing down to the ground! Wind speeds can be 50- 100 mph, damaging roofs, snapping trees, etc…
Microbursts can happen so quickly here in Arizona and this is one reason why so many warnings are placed regarding flash floods. These intense storms are capable of producing winds of more than 100 mph causing significant damage.
On a positive note: Microbursts replenish the desert with much needed rain.
Most of us have fantasized while admiring clouds; but have you ever wondered why clouds float? As long as the cloud is warmer than the outside air around it, it will float.
The height of the cloud in the atmosphere depends on the temperature and amount of water vapor of the rising air. For example, drier air has to rise higher to cool enough to start condensation.
Cumulus clouds can grow into cumulonimbus clouds which are larger and often spread out in the shape of an anvil or plume. Cumulonimbus may produce heavy rain, lightning, severe and strong winds, hail, microbursts, and even tornadoes.
Clouds are grouped by their shape and by their height in the atmosphere.
The characteristics of clouds are established by the elements available, including amount of water vapor, temperatures at the height, wind, mountains and other air masses.
The names of clouds come from Latin words that describe their characteristics. The main types of clouds are:
Cirrus means “curl” or “fringe”,
Nimbus means “rain-bearing”,
Stratus means “layer”,
Cumulus means “heap” or “pile”
Cumulus clouds are probably the most recognized clouds. These clouds form below 6,000 feet but in some extreme cases they can be in altitudes as high as 39,000 feet! They look like white, fluffy cotton balls. The reason cumulus appear fluffy is because bubbles of air, called thermals, linger in the cloud.
In mountainous areas, clouds may form lines at an angle to the wind. Wave clouds do not move downwind as clouds usually do, but remain fixed in position relative to the obstruction that forms them, for example: mountains.
Lenticular clouds form on the downwind side of mountains and are lens-shaped. Wind blows most types of clouds across the sky, but lenticular clouds seem to stay in one place.
Strato-cumulus clouds form in altitudes below 6,000 feet. Below photo shows a low layer of strato-cumulus clouds spreading the remains of larger cumulus clouds.
Alto-cumulus clouds differ from Strato-cumulus because they are slightly smaller. One easy way to determine if the cloud is alto-cumulus or strato-cumulus is to hold your hand up to the sky, alto-cumulus clouds are about the size of a human thumb nail while strato-cumulus clouds are the size of a fist.
Stratus clouds belong to the low cloud (surface-2000m, below 6,000 ft) group. They are uniformed layered, gray in color and can cover most or all of the sky.
Stratus clouds can look like a fog and are associated with overcast weather. Only drizzle comes from stratus clouds, if heavier rain falls then their title is changed to nimbostratus.
The most common of the high clouds is Cirrus. These clouds are composed of ice and are thin, curly, wispy, feathery clouds.
Cirrus clouds are usually white and predict fair weather even though they are so cold and composed entirely of ice. They are the fastest moving cloud because the wind current is very strong at that high altitude.
Cumulonimbus clouds belong to the thunderstorm clouds or clouds with verticalgrowth group. Reaching heights to 10km, high winds will flatten the top of a cumulonimbus cloud out into an anvil-like shape.
Cumulonimbus clouds, also called Storm Clouds, cause heavy rain, lightning, hail, snow and tornadoes.
Cumulus clouds, which indicate low-level atmospheric moisture often precede storms. In this picture of a Cumulonimbus cloud or thunderstorm cloud, much lightning was occurring with the winds increasing rapidly.
Mammatus clouds are pouches of clouds that hang underneath the base of a cloud. They are usually seen with cumulonimbus clouds that produce very strong storms.
Mammatus clouds look like a field of tennis balls, melons, or like female breasts. That is where the name comes from.
Cirrostratus clouds form in the 18,000 feet and above. The refraction of light by the ice crystals in the Cirrostratus clouds cause a halo around the sun or moon.
You can not see the halo when this happens but the sun or moon will be less visible because the Cirrostratus clouds condense too much for clear visibility. Clouds are usually white and predict fair weather. These clouds often follow Cirrus clouds therefore Cirrostratus clouds are indicators of good weather.
The official start date of the Arizona Monsoon season is June 15. In 2008, the National Weather Service determined the AZ Monsoon starting date along with the ending date of September 30.
What causes monsoon?
Traders sailing the Indian Ocean and Arabian Sea used the word monsoon to describe a system of alternating winds which blow from the northeast during the northern winter and from the southwest, during the northern summer. Therefore, the term monsoonrefers solely to a seasonalwind shift, and not precipitation.
Arizona happens to be located in the area of the United States that experiences a monsoonal circulation. During the summer months, winds shiftfrom a west or northwest direction to a south or southeasterly direction.
This wind shift allows moisture from the Gulf of California and the Gulf of Mexico to stream into Arizona. A monsoonal circulation produces a radical change in moisture conditions throughout the entire state.
Thunderstorm or cumulonimbus clouds are very familiar during monsoon season. These storm clouds cause lightning, hail and heavy rain.
What Arizona experiences during the summer months is only a small part of a larger circulation that encompasses much of the southwestern United States and northwestern Mexico. Thus, it sometimes is also known as the Mexican monsoon. The National Weather Service calls it the North American Monsoon.
This adjustment in wind direction is the result of two meteorological changes:
the movement northward of the huge upper level subtropical high pressure system, known as the Bermuda High,
and the intense heating of the Mohave Desert creating rising air and surface low pressure, called a thermal low
The southerly low-level winds help to bring in moisture from Mexico. When this moisture encounters the higher terrain of Arizonamountain ranges, it gets lifted and forms thunderstorms.
Small driving factors such as: atmosphere interaction, land elevation, soil moisture and vegetation all play a part in what drives the Arizona Monsoon; but it is also why predicting storm intensity is so difficult.
When such high volume rain descends upon the Arizona desert the ground and the surface streets flood. Quite often the rain pools on streets during monsoon storms causing dangerous driving conditions.
The monsoon circulation does not produce thunderstorms every day during the summer months, but rather monsoon storms occur in a pattern known as “bursts” and “breaks”.
This cycle of bursts and breaks will continue from the onset of the monsoon (typically June), until the time when cold fronts begin to move across the state of Arizona (typicallySeptember), which will return our winds to a westerly or northwesterly direction.
We hope you enjoyed this article. Our goal was to make Monsoon Season easier to understand.