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 what about meteors? The science of meteors is called Meteorics.  Accordingly, a student of meteors is a meteoriticist. A tongue twister to be sure.




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Weather Discussion June 2017 

by Richard Schreiber

The Great Southwest Heat Wave of 2017

If it seemed like the hottest June in recent memory, you are probably right on target.  For the month the temperature at Tucson International Airport averaged 89.7 degrees and the NWS reported that this was their hottest June on record and the seventh hottest month ever.  The hot spell did not subside as we transitioned into July, but an 18-day streak of 105 degrees or higher finally tapered off beginning on July 2. 

This weather scenario was typical of the entire southwestern U.S.  On June 20 or 21, four cities either tied their all-time high temperature records or exceeded them:  Las Vegas, NV, Needles, CA, Ocotillo Wells, CA, and Prescott, AZ.  And on June 20th, Death Valley, CA reached 126.5°, the hottest temperature ever measured in the Western Hemisphere so early in the year. That same day Phoenix registered 119 degrees at Sky Harbor International Airport, just 3 degrees below the 122 degree record set in 1990.

One major airline in Phoenix cancelled around 50 of their regional flights on June 20, because they use smaller aircraft having a maximum operating temperature of 118 degrees.  Larger jets that fly out of Sky Harbor have higher maximum operating temperatures: Boeing, 126 degrees, and Airbus, 127 degrees.   This brings up an interesting theory that evolved from flight cancellations in the 1990 heat wave in Phoenix.

One of the more commonly heard explanations for flights being cancelled during the record temperatures on June 26, 1990 was the following:  the extreme heat caused asphalt on the tarmac to become so soft that planes would have been stuck on taxiways or runways.  Some years later, a photo in the news media of a plane stuck in hot asphalt in Washington may have contributed to that assumption.  But soft asphalt was not the problem in Phoenix in 1990 and no planes got stuck. 

An airport spokesman explained that in 1990, performance charts for most aircraft did not provide operational data for those extreme temperatures.  Since then, performance data and charts have been updated for temperatures into the 120's, allowing pilots to compensate for the higher heat and longer takeoff requirements. 

It’s actually much more demanding for planes to take off in extreme heat.  Air becomes less dense, providing less lift for the wings and requiring more runway to become airborne, as well as a slower climb rate. This was a concern in Iraq during the war and one reason planes took off at night in cooler temperatures.


Causes and effects of a heat wave or "dome"

According to NOAA, heat waves form when high pressure in the range of 10,000 to 25,000 feet strengthens and remains stationary over a region for a period lasting from several days to several weeks. Summer weather patterns evolve more gradually than in winter, and this mid-level high pressure system can move very slowly. The high pressure causes the air to sink toward the ground, increasing in temperature and forming a "dome" that traps heat instead of allowing it to lift.  Similar to winter inversions, little or no convection is possible and therefore very few or no convective clouds (cumulus) can form.  There are consequently minimal chances for precipitation. What transpires is a continual build-up of heat at the surface that we experience as a heat wave. 


Figure 1.  High pressure in the middle layers of the atmosphere acts as a dome or cap allowing heat to build up at the earth's surface.

The old adage about the discomfort and risk of high heat -- "it's not the heat - it's the humidity" -- contains the truth that humidity along with heat is a dangerous combination.  Heat waves kill more people in the U.S. than any other weather phenomenon.  In a typical year there are 124 heat related deaths (10-year average for 2005-2014).

The Heat Index graph shown as Figure 2 below translates how hot weather "feels" to the body. It is based on work by R.G. Steadman and published in 1979 under the title "The Assessment of Sultriness, Parts 1 and 2."  This Index uses relative humidity and temperature to produce the "apparent temperature" which is how the body "feels".  Notice, for example, that with temperatures in the 90's and relative humidity above 75 percent, "apparent temperature" is extreme - around 130 degrees.  The risk of sunstroke is very high. This data is relevant for shaded areas only.  If you are in direct sun, heat index values can increase up to an additional 15 degrees F.

The chart explaining the risk levels of excessive heat is reproduced for legibility as Figure 3. 

heatindex graph

Figure 2.  Apparent temperature based on air temperature and relative humidity.  This provides a guideline for health concerns when exposed to a combination of high temperatures and humidity.

heatindex explan

Figure 3.  This chart explains the risk to the body from continued exposure to the four categories of high apparent temperatures. 

             For more details on Heat Dome and Heat Index

Monsoon Update

As of July 5th, the Tucson NWS dew point tracker data being collected for the 2017 monsoon season shows a definite upward trend with the last three days having dew points above 49.  The monsoon season officially began on June 15, but true monsoon conditions depend on the dew point.  And only when the average dew point exceeds 54 degrees for three consecutive days are conditions appropriate for the onset of monsoon.  Lest we get too impatient for the monsoon, we need to remember that for the past ten years -- at least based on our local data -- there was only a single year (2009) in which the dew point clearly met the above criteria by the end of June.

Our data over the past few days show dew points in the 40 degree range, somewhat below Tucson, but similar in that we have seen a significant increase since late June with its low dew points.

                     Visit the Tucson NWS Dew Point Tracker site:

Monthly Weather Summary

June 2017 recap

Typical of the Southwest, our area experienced much hotter than usual weather as witnessed by the following data:

On Limestone Hill we set a ten-year record high temperature of almost 104 degrees.  We know that at lower elevations in Portal and Rodeo there were even higher temperatures.   

Our monthly mean temperature was at the high end of the range. 

We had the highest number of days (6) with temperatures over 100 degrees in 10 years,  and we had 27 days with temperatures above 90.

Cooling degree days factor was the third highest. This also tells us that it took a lot of power to maintain a comfortable indoor environment.

June was not a windy month with only two days of gusts in excess of 30 mph.  The maximum gust was 36 mph.  But as we are all aware, the lack of rain over the last four months and current low humidity have contributed to some recent nearby wildfires.  Even winds of moderate intensity are inimical to fire control under current drought conditions.  

Although the cumulative precipitation for 2017 is not much below average, the past four months have produced a scant 0.32 inch of rain.  Fortunately January and February together contributed 2.36 inches.  

Richard Schreiber

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© Howard Topoff 2011