Weather - Precipitation

Meteorology versus Meteors

If a meteorologist studies weather, what do we call a person who studies meteors? In the days of ancient Greeks, meteorology was the study of the atmosphere, and anything that came out of it - rain, sleet, snow, hail. So, not too surprising that meteorology became the study of weather. But whatabout meteors? The science of meteors is called Meteorics. Accordingly, a student of meteors is ameteoriticist. A tongue twister to be sure.




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Weather Discussion -September2017

by Richard Schreiber

August Break

I shirked a major responsibility by not producing a Weather Discussion for August. We had departed Portal in mid-August to observe the total solar eclipse in eastern Idaho (the weather was excellent, by the way). But this month we are back in the groove.

Since I also didn't compile a recap and historical comparison for August, here are at least a couple of observations: the high temperature of 92.5F was below the 10-year average and the precipitation of 1.99 inches was low, but not as sparse as 2009 when only 0.52 inch fell during the month.

Monsoon Update

As reported on the NWS Tucson website ( Tucson International Airport registered a paltry 0.03 inches of rain for September, the 11th driest September ever. Across the metro area several sites reported less than a quarter of an inch with only a few readings approaching a half inch, to the southeast of Tucson.

Also, Tucson's monthly average temperature of 83 degrees ranks as the 14th warmest September the city has recorded.

In my September recap below you'll note that our local experience was very similar to Tucson's, showing both low precipitation and high temperatures.

For the monsoon season (6/15 - 9/30) we recorded 6.58 inches of rain, almost 2 inches less than 2016. Tucson had a relatively wet monsoon season with 8.57 inches. Our local rainfall for that period is in the average range.

Let's Talk about Hurricanes

Even if you don't usually watch the nightly TV newscast or catch up on the day's happenings via the internet, it would have been difficult not to have heard about hurricanes Harvey, Irma, and Maria and the devastation created across the Caribbean -- especially Puerto Rico and the US Virgin Islands -- and on the US mainland, targeting Texas, Florida, Georgia and South Carolina. Probably most of us have friends or relatives with ties to those regions and worried about their safety and the incredible consequences of the aftermath.

And it isn't over. As I write this on October 6, Tropical Storm Nate is strengthening in the Caribbean, possibly crossing over the Yucatan Peninsula tonight, and moving towards the Gulf Coast over the weekend. Nate is categorized as a tropical storm at the moment but is predicted to reach hurricane force winds by the time it reaches the vicinity of New Orleans.

Irma and Maria developed over the Atlantic during a period of weeks before aiming for land. But an NWS meteorologist explained that Nate is very different. As a fall storm, Nate formed in the Caribbean and popped up quickly. That doesn't provide much warning. (

Hurricane Formation

NASA-supported Classroom of the Future ( provides a concise description of hurricane formation:

"Hurricanes begin as tropical storms over the warm, moist waters of the Atlantic and Pacific Oceans near the equator. As the moisture evaporates it rises until enormous amounts of heated moist air are twisted high in the atmosphere. The winds begin to circle counterclockwise north of the equator or clockwise south of the equator. The relatively peaceful center of the hurricane is called the eye. Around this center winds move at speeds between 74 and 200 miles per hour.

If a hurricane remains over waters of 79F or warmer, it continues to pull moisture from the surface and grows in size and force. When a hurricane crosses land or cooler waters, it loses its source of power, and its wind gradually slow until they are no longer of hurricane force--less than 74 miles per hour.

Hurricanes over the Atlantic often begin near Africa, drift west on the Trade Winds, and veer north as they meet the prevailing winds coming eastward across North America. Hurricanes over the Eastern Pacific begin in the warm waters off the Central American and Mexican coasts. Eastern and Central Pacific storms are called "hurricanes." Storms to the west of the International Date Line are called "typhoons."

As an aside, the generic term for this type of storm is "cyclone".


Figure 1. Tropical cyclone regions around the world.

hurricane form

Figure 2. A tropical disturbance in the Atlantic can develop into a hurricane.

The Dangerous Aspects of a Hurricane

High Wind -- Along the coast, structures are vulnerable and at high risk as the wind from the water crosses the shore. Inland the risk is reduced somewhat depending on structures, vegetation, and landforms that can reduce wind velocity.


Figure 3. Hurricane wind speeds and the relationship to storm surge.

Storm Surge and Flooding -- An abnormal rise in sea level occurs from a hurricane and can reach heights exceeding 20 feet, span hundreds of miles of coastline, and travel several miles inland, threatening life, structures and the natural environment. Also contributory is storm tide, the rise in water level due to the storm surge and normal tide cycle.

Many of Hurricane Katrina’s 1,500 victims died as a result of storm surge, according to NOAA. Inland flooding from hurricane Katrina’s storm surge in 2005 reached more than 25 feet above normal tide levels along parts of the Mississippi and Louisiana coasts. In addition, torrential rains often accompany these storms causing flash flooding. These rains often exceed 6 inches and are a major cause of loss of life, especially in densely populated urban areas.

Tornadoes -- Inside the hurricane, powerful winds are often created by tornadoes. Although the tornadoes themselves may be relatively weak they can contribute to the extensive damage spawned by a hurricane. Unfortunately tornadoes aren't just observed during the main storm, but can occur for days afterwards.

For comprehensive details see:

So, no danger in our region from hurricanes, right?

Not so fast...Arizona can be impacted by hurricanes that form in the eastern Pacific Ocean and cross into Baja California or Sonora although they usually don't survive long enough to enter the state. Also the prevailing wind pattern steers most storms that form in the eastern Pacific away from northwestern Mexico and the U.S. coastline. Only a small fraction ever enter the U.S.

On the other hand, remnant moisture from such tropical storms can produce heavy rainfall and flooding. And Atlantic sourced hurricanes can on rare occasion contribute rainfall in the eastern part of Arizona as the moisture crosses into this region from Texas and New Mexico. It is estimated that Arizona can expect indirect flash flooding every two years from remnants of tropical cyclones. (source: Wikipedia)

If you were in the area in 2014, you remember vividly the impact from the heavy rains due to remnants of hurricane Odile, as memorialized in this news release (


The monsoon "officially" ended on September 30, but it is evident that the month was not typical. There were only a couple of periods before the middle of the month and towards the end when the dew point temperature exceeded 52 degrees. A long stretch from the 10th through the 27th registered dew points below that 52 degree threshold. That suggests that an important factor was absent for the development of monsoonal rainstorms.

No question about it, September was a hot month, with a maximum temperature within one degree of the 10-year maximum of 95.4 degrees. The mean temperature was the highest we've seen and there were 7 days with temperatures over 90F. The cooling degree days factor was also the highest recorded on Limestone Hill for September.

Was it a dry month? Also no question -- almost not enough to notice, a mere 0.08 inches. YTD precipitation of 9.16 inches is the second lowest amount since 2008 when we started keeping weather records. Compare that with 18.12 inches by this time in 2014.

In September we normally don't experience very high wind gusts compared to other months. A maximum gust of 32 mph occurred and only 2 days in the month had winds over 30mph.

Richard Schreiber

Comments and suggestions appreciated:


Howard Topoff 2011