What is a Monsoon

     When Do is it Occur

     Monsoon Effects

     Bursts and Breaks

     Monsoon Definitions

     Monsoon Facts

     How to be Safe

Welcome to Arizona! Many people think of the state as a dry and dusty place. Although you would be right to conclude this during the months of May or June you might be surprised if you visit during July or August. These months are referred to by locals as the Monsoon months. During these times is not uncommon for this dry and baron desert to receive 2-4 inches of rain in one hour. Needless to say this type of water causes catastrophic problems in the hardened clay soil of the Sonoran Desert.

What is a Monsoon

          A Monsoon or “Mausim” is an Arabic word that means season. The word first began to be used by sailors in the Indian Ocean and Arabian seas. The sailors used the word to describe winds that shifted direction by the season. The wind shifts directions because as the continent warms it creates a large low front over the land. The natural tendency of air is to move from high to low fronts. As the air begins to move towards the land its shifts direction and brings the clouds, moisture, and humidity along with it. This affect occurs very heavily over India and sometimes drops as much as 27 inches of rain within a 24 hour period. The monsoon in Arizona is the result of the same Phenomenon, however because Arizona is not as close to the Ocean it is much more mild.  This is why the dry landscape of Arizona can turn into a lake literally overnight.

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When Does it Occur

          The official wet period of the Arizona Monsoon begins when the daily dew point is 55  or higher for three consecutive days. On average this occurs during the first two weeks of July. However, what most people don’t realize is that this wet season of the monsoon is the result of the heating of the atmosphere which occurs in April, May, and June.  During these months the extreme desert heat, which is known to exceed 110 degrees, is constantly at work heating the Southwestern area of the continent. In fact the Arizona Monsoon is just a section of the much larger Mexican Monsoon. This monsoon affects the entire country of Mexico and has also sent moisture as far North as Wyoming. But no matter where the location the monsoon acts of the heating that has occurred to draw in the moisture. As the land mass continues to heat it begins to draw in the moisture rich tropical air from the south. As this moist air reaches the extremely hot desert it creates a very unstable atmosphere. The unstable atmosphere combined with the extreme heat of the Arizona desert creates the perfect combination for monster thunderstorm cells to build. They usually build during the day when the temperature is at its peak. Then once they have reached their maximum moisture level they begin to drop rain. This usually occurs in the late afternoon and results in heavy flooding in relatively focused areas. This will continue to occur until the atmosphere begins to cool again in September.

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Monsoon Effects

          The most commonly asked question about the Monsoon is how will it affect me? The answer to that is a varying question because it depends on where you are and what nature has in store for that area that day. The Monsoon is very unpredictable because it occurs in a focused area. For example, in a storm that occurred in July 2002 the areas of Central Phoenix and Tempe were shocked with 3 inches of rain in a 3 hour period. However residents of Scottsdale, some 15 miles east, saw little more then trace amounts of rain from this disturbance. This makes a meteorologists job a nightmare during Monsoon season. Generally what happens is a warning will be issued for the entire county. This is done as a preventive caution to ensure that everybody is aware that somewhere a storm is going to strike. This means that everyone in the valley should take steps of caution when planning for the time period of the warning. So the answer to the most asks question is simply, it depends. But there are certain time periods of the year when you are more prone to get caught in monsoon type weather. During the months of May through June, as discussed earlier, the atmosphere is relatively stable and rain is very unlikely to occur. In fact these months are the driest months of the year in Arizona. But during the months of July, August, and early September the wet Monsoon season is in full swing and you should be careful.

          Although the affects of the Monsoon are difficult to predict most of them can be easily avoided by following these simple steps. Remember it is always better to be safe then sorry. Use common sense so that you will be able to enjoy the beautiful scenery that nature has created in Arizona.

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Bursts and Breaks

          As stated previously the Monsoon storms are difficult to predict because of the focus tendency that they have. However, there is another reason why the storms are hard to forecast and this relates to the cyclical behavior of the Monsoon known as Bursts and Breaks. The bursts and breaks only occur during the wet months of the Monsoon (July-September) when the storm tract comes and goes. The Bursts are areas when the tract is directly over Arizona and moisture is likely to occur on almost a daily basis. Breaks are when there is a shift in the moisture either to the east or west and there is relatively no moisture over Arizona to produce thunderstorms. During the breaks it is very unlikely for any measurable rain to occur because the key component required, moisture is not present.  This cycle will continue all the way until the end of the Monsoon in mid to late September when the cooler air arrives.

                           (Burst)                                                                     (Break)

The satellite image above illustrates a burst and break cycle occurring over Arizona. During bursts (left) weak areas of disturbance in the atmosphere will cause thunderstorm activity to focus on a small area of the state for a few days to one week. The breaks (right) are when the southerly winds are minimal and atmosphere becomes more stable making thunderstorms unlikely to develop.

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Monsoon Definitions

Since the Monsoon in, North America, only occurs in the Western United States most people are not familiar with terminology that is commonly associated with Monsoons. I will be the first to admit the first time someone mentioned a “Haboob” to me I thought they were referring to some type of creature. There are other, seemingly strange, words associated with Monsoon terminology. Below you will find a list of words and their definitions that are specific to Monsoon storms.

          Haboob: A Haboob occurs when the descending air from a thunderstorm hits the ground and is forced in front of the storm cell. As the front moves it picks up dust, debris and anything else in its path and creates a massive wall.  This wall continues to be carried across whatever is in it's path until the storm begins to end.. They usually last 2-3 hours with wind speeds ranging from 25-50MPH. Estimations suggest the dust in a Haboob can reach an altitude 3000'. Its easy to see why Haboobs have a long bitter history with Phoenix's Sky Harbor International Airport.
 

          Gustnadoes: are a combination dust devil and tornado. A gustnado is a tornado-like funnel  cloud that seems to start from the ground and tower hundreds of feet into the air. These generally develop along the leading edge of a thunderstorm. They usually last a very short time, but the winds are violent enough to cause cars to flip over. Because of their features, Gustnadoes, are often confused with fires.

 

- A Gustnado over Guadalupe, AZ (1998) -

          Dust Devils: A dust devil, like a tornado, is a spinning vortex of air. Their size can range from a mere 10 feet to greater then 1000 feet. Dust devils have been observed all over the Phoenix Valley. They can cause serious driving hazards by limiting visibility to near zero.

- A harmless (small) Dust Devil strolling across the Arizona Desert -

The chart above is used by the Arizona Storm Chasers in order to judge the velocity of the wind by observing what has occurred to the landscape.  These type of charts are very useful for damage reporting after a storm occurs.

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