|The very large anvil portion of a large cumulus cloud from the Arizona Monsoon season. Often huge anvils like this mean a strong monsoonal storm is coming. Picture from:http://www.wrh.noaa.gov/fgz/science/monsoon.php?wfo=fgz|
What is the Monsoon?
Monsoon season in the Sonoran Desert technically begins June 15th and ends September 30th. During this time period, the normal dry heat of the desert is replaced with humidity from the Pacific Ocean and Gulf of Mexico. Extreme summer heat in the desert causes rising air, creating a low pressure system pulling in this tropical humidity. Wind patterns change as a result of this low pressure, shifting the normal dry western winds to humid winds out of the southeast. This shifting wind pattern is where the monsoon gets its name, and means "wind-shift" in Arabic. In order for rain to fall, high level humidity must be in place from the Gulf of Mexico (called the Bramuda High) and low level humidity must be in place from the Pacific Ocean and Gulf of California.
|An anvil cloud, a type of cumulus cloud that results from hot rising humid air. The flat top is a result of cooler air higher in the atmosphere. There are no obvious cirrus clouds surrounding this cloud. Picture from Wikipedia.|
1. High temperature around 100 to 108 degrees are optimal. These temperatures heat moist air causing it to rise and form rain clouds. This heated rising air is called convention and the resulting clouds are known as convection clouds.
2. High due points above 50 to 55 degrees, the higher the better. This supplies the moisture to form clouds and rain due to rising air.
3. Winds generally coming out of anywhere from the south to east to north. These winds are blowing humid air into low pressure systems and rise to form clouds.
4. Low level cumulus clouds, another indication of lower level atmospheric humidity. These clouds should form with convection causing moist air to rise due to afternoon heat. And of course, you need clouds for rain.
5. Large cumulus clouds that form flat-tops called anvils. The larger the anvil generally the larger the storm. Clouds wider then they are tall bring the most rain. Smaller, or tall skinny clouds with or without anvils and may appear to be leaning to one side, may bring small amounts of rain, or rain may never hit the ground.
6. High level cirrus clouds. These clouds may indicate no rain if scattered. Cirrus clouds are not always present, but a good storm will almost always have cirrus clouds proceeding it around the anvil.
Monsoonal thunderstorms in the Sonoran Desert are quite complex, it takes a lot of different things to be in place for a good storm to happen. For example, if low level moisture is in place but high level moisture is not in place, clouds will form and some rain may fall from these clouds but may never reach the ground. Clouds that form under these conditions often will be small, bent in one direction. If however high and low level moisture are in place, rising humid air will form cumulus clouds lower in the atmosphere that continue to rise until they hit higher level cooler air. This will result in an anvil cloud, which is a type of cumulus cloud with a flat top like an anvil. This rising humid air hitting cooler air can form some pretty amazing storm clouds and at times some pretty amazing rainfall.
Monsoon Thunderstorms in the Desert
The drenching rain, spectacular lightning, and violent winds are something to behold. Experiencing this type of storm will most definitely leave an impact on anyone. Everything in the desert waits longingly for these storms. In the dry hot months prior to monsoon season, the land becomes painfully dry, causing plants to shrivel up and die or go dormant. Watering holes also dry-up, forcing many animals such as javelina and mule deer to stay near more permanent water holes in the mountains. Mountain lions take advantage of these times by stalking prey near these watering holes, making life even worse for these thirsty animals. Other animals and birds simply hide away from the heat, only coming out at night or limiting their activity all together. Everything hides from the heat and dryness, waiting for rain.
All this activity will slowly wane after days to weeks of time if rain doesn't return. With the decline of monsoon season towards the end of September though, life does not return to the dull seemingly lifeless heat. Rather, temperatures have cooled considerably and often more water remains across the landscape as we head into fall.
More information on the Arizona Monsoon: http://geoplan.asu.edu/monsoon.html