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.

HT-2



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Weather Discussion - May 2017

by Richard Schreiber


Dust Devil Season


Dust Devils are appropriately named. Many years ago when we lived west of Phoenix, as we were returning home one very hot summer afternoon, we noticed something strange resting on the roof. When we were closer, it dawned on us that the strange object was our patio cover -- fairly sizeable and constructed of wood, it weighed a lot. Neighbors reported that a large dust devil had swept through the neighborhood and someone had heard a loud bang.


Are dust devils -- also known as whirlwinds -- just small tornadoes? They might appear so, but are formed by a different process, and don't get nearly as large nor as destructive. According to the American Meteorological Society they can range in width from about 10 to 100 feet and average 650 feet in height, reaching as high as a half-mile.


Dust devil (1)

A dust devil in Arizona. (wikipedia)

They form in relatively dry, clear weather when the sun is providing strong surface heating and winds are light. Figure 1 below explains the process: convective rolls of air are produced by the heated surface and if winds aloft are stronger than at the ground the convective current is uplifted into a vertical column. To see a dust devil, there needs to be loose soil or debris for it to pick up. Unlike a tornado which contains moisture, the dust devil is a clear air phenomenon. Generally dust devils cannot produce the excessive damage that a tornado is capable of, but nevertheless damage to people and property can be significant. Howard Topoff reported that, while driving on I-10 across western New Mexico several years ago, hewas riding with his arm out of the window. A dust devil swooped across his car and literally "sanded" enough skin to induce bleeding.


dust devil

Figure 1. Dust devil formation


Now just a few words about the formation of tornados to understand how they are distinct from a dust devil. A tornado forms in the presence of a thunderstorm cloud formation and is a violently rotating column of air that is in contact both with the ground and the thunderstorm. Because the funnel is composed of water droplets and stirs up dust and debris, it becomes very visible. Fully understanding tornado formation is far more complex than this, but that is best left for the reader to pursue. Figure 2 describes how tornadoes develop.


Tornadic supercell

Figure 2. Air circulation in a supercell thunderstorm. Tornadoes that develop from a supercell thunderstorm are the most common.


Sources: http://www.nssl.noaa.gov/education/svrwx101/tornadoes/

https://en.wikipedia.org/wiki/Tornado

Monsoon Season Arriving - What to Expect in 2017



In mid-May KVOA TV in Tucson published a story outlining the factors that will influence the 2017 monsoon season and what we might expect. Here are the major points of that discussion:


1. A weak El Nino could mean less moisture in our region. Although temperatures in parts of the Pacific near the coast of Peru and Ecuador are currently much higher than usual, that may not happen in areas of the Eastern Pacific that affect us most. Whether El Nino becomes a major factor is being called a "coin toss" at the moment.


2. The intensity and number of tropical storms and hurricanes in the Eastern Pacific are related to the El Nino pattern. Warmer than normal waters can lead to an active season, providing moisture boosts in Southern Arizona. There has already been one named storm in the Eastern Pacific (Adrian in early May), the earliest one on record. But there isn't reliable data for that region before 1971 so predicting trends isn't easy.


3. Because a large area of southern Arizona was under moderate drought in the winter and spring, high pressure episodes during monsoon season may be more prevalent here than in the Four Corners region. Historically it has been observed that high pressure in the Four Corners is important for fueling monsoon storms. This could leave Arizona with below normal monsoon rainfall since the highs could limit moisture from Northern Mexico.


So it appears that predicting monsoon rainfall at this point is a toss-up.

For more detail see:

http://www.kvoa.com/story/35428889/potential-factors-for-monsoon-2017


Preceded by warmer than usual weather in March and April, May moderated somewhat:

  • Mean and high temperatures for the month were historically very near the average
  • Interestingly, the monthly low temperature of 39.6 was almost a duplicate of the lowest 10-year value of 39.3 degrees.
  • There were 21 days on which the temperature exceeded 80 degrees. The record was 27 days in 2012.
  • Also notable that on only two days did the temperature exceed 90 degrees.
  • Cooling degrees days factor of 218.0 was also close to the historic average for our site.

Here on Limestone Hill, May did not produce very high wind gusts -- we experienced a maximum gust of 36 mph, which would be considered low on the scale for this time of year. Back in 2015 we recorded a 66 mph gust and that was a record. Also average was the number of days with gusts over 30 mph; however, that isn't the complete story. There were periods of sustained winds while not particularly strong were of long duration. This fact, coupled with some very low relative humidity readings during the latter part of the month, elicited several "Red Flag" warnings from the National Weather Service. Fortunately, no significant fires occurred in the immediate vicinity.

Similar to last month, May produced only a trace of moisture. Unfortunately that is the typical scenario, and in only one of the past ten years did monthly precipitation exceed one inch. Cumulative moisture for 2017 now totals a mere 2.58 inches, but that is not the low end of the scale historically. Maybe El Nino will show greater promise as we approach the monsoon season and will help direct moisture our way.


Richard Schreiber

Comments and suggestions appreciated: dschre@att.net


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Howard Topoff 2011