SCIENCE2 - Eye on the Weather in Tucson

Last year, in February, I wrote a blog titled, “The Story of Clouds,” all about how clouds form, the different types, and so forth.   Since then I’ve been keenly interested in watching how clouds change every day, even within a day - with weather being a driving cause.

It occurred to me recently, that I didn’t know much about weather, how it changes and why - and its intimate relationship to clouds.  So, I decided to learn a bit and write about weather, its key elements and weather phenomena - with emphasis on weather in and around Tucson, Arizona.

I found three good online sources for some of the basics:  1) How to Read a Weather Map by the National Oceanic and Atmospheric Administration, 2) What is Weather? by the United States Search and Rescue Task Force, 3) Severe Weather 101 by The National Severe Storm Laboratory.  I borrow liberally from these three sources.

Elements of Weather

First, I’m going to talk about weather elements that appear on typical weather maps, namely high and low pressure areas, and warm and cold fronts - as drivers of weather and to establish some basic vocabulary that will help the rest of the discussion.  I’ll be referring to the example weather map below:

Example Weather Map for U.S.

High and Low Pressure Areas.  Earth’s atmosphere is a jacket of gases that surrounds the planet.  Gravity keeps the atmosphere from escaping into space.  The force with which the atmosphere pushes down on a specific location is called atmospheric pressure.

Atmospheric pressure is mainly dependent on two things:  the weight of the atmosphere at the specific location and the temperature of the air.  Generally, we experience higher atmospheric pressure at lower elevations and lower atmospheric pressure at higher elevations.

Warm air can also cause the atmospheric pressure to rise.  When the air is warm, gas molecules move around quickly in the air, pushing out on the volume around them.  This causes higher atmospheric pressure.  In cold air, the gas molecules slow down, causing lower atmospheric pressure.

Water vapor in the atmosphere can also change the atmospheric pressure.  Very moist air that has lots of water vapor is actually less dense and lighter than dry air.  This is because water molecules are lighter than molecules of nitrogen and oxygen, the most abundant gases in our atmosphere.  So, very moist air can lead to low atmospheric pressure and very dry air can lead to high atmospheric pressure.

Atmospheric pressure measurements are collected at many locations across the U.S. by the National Weather Service.  On weather maps, these reading are represented as a blue “H” for high pressure or a red “L” for low pressure.  Note the high and low pressure areas on the example weather map above.

Warm Fronts and Cold Fronts.  A warm front is the boundary zone produced when a mass of warm air moves to replace a mass of cold air.  Warm fronts usually move from southwest to northeast.  Because the warm air is less dense, it slides up and over the colder air.  At first, cirrus clouds might appear.  They may be followed by stratus clouds and some precipitation, either rain or snow.  When the front passes, the sky clears and the air pressure rises.  Temperatures also rise as warm air replaces cold air.  On a weather map, a warm front is usually drawn using a solid red line with half circles pointing in the direction that the warm air is moving.

A cold front is the boundary zone where a mass of cold air moves to replace a mass of warm air.  Cold fronts typically move from northwest to southeast.  Cold air contracts, becomes denser, and thus heavier than warm air, so the cold air pushes underneath the warm air. Cumulus clouds and showers or thunderstorms may be scattered along cold fronts.  As the front passes, the wind changes direction. Skies begin to clear, and the temperature usually drops.  On a weather map, a cold front is usually represented by a solid blue line with triangles pointing in the direction that the cold air is moving.

A stationary front happens when a cold front and warm front meet up, but neither moves out the way and remains in one spot for a considerable time.  Stationary fronts bring long rainy periods.  On a weather map, a stationary front is usually drawn using alternating cold front and warm front symbols.

Cold fronts move faster than warm fronts, and sometimes a cold front catches up to a warm front.  When this happens, it’s called an occluded front.  Occluded fronts usually bring dry air.  Occluded fronts are drawn as a solid purple line with half circles and triangles pointing in the direction the front is moving.

On our example weather map, there are warm fronts in south-central Canada (moving northeastward), and the east-central U.S. (moving to the north).  There are cold fronts off the U.S. west coast (moving eastward), in southern Canada (moving southward), and one about to reach the state of Florida (moving eastward).  There is a stationary front in southwestern Canada and an occluded front in Kentucky/Tennessee (moving northeastward).

Air masses move, so they can be tracked over time. They are clues to the future temperature, moisture level and air pressure of the area into which they are moving.  Note the rain, thunderstorm, and snow forecasts on the example weather map.

BTW - the weather forecast for Tucson on Thursday, May 4, 2017, corresponding to the example weather map, was:

Sunny and hot today with a high near 100.  Clear skies tonight with a low in the upper 60s to near 70.  Sunny and hotter tomorrow with a high near 101.  Breezy and cooler Saturday with a high in the low 90s.  Breezy Sunday low 80s.  Breezy Monday mid-70s.  Scattered showers and thunderstorms possible Tuesday mid-70s. Mostly sunny Wednesday low 80s.

Wind.  Wind is caused when air moves from an area of high pressure to one of low pressure.  The greater the pressure difference between areas, the stronger the wind.  In many areas the wind usually blows from the same direction.  

Winds near the ground are typically stronger and gustier in the afternoon.  This is because, as the sun heats the atmosphere over the day, and the temperature rises, thermal turbulence and instability increase with increasing surface heating.

Wind gusts are caused when wind flows over uneven landscapes and objects.  The wind breaks into a series of irregular, twisting eddies that can influence air flow for hundreds of yards above the surface.  Within each eddy, the wind speed and direction fluctuate rapidly, producing non uniform wind gusts.

Wind speed tends to decrease after sunset because at night the surface of the Earth cools much more rapidly than does the air above it.  The air in close contact with the ground (lowest 300 feet of the atmosphere) then becomes colder than the air above it, making it much harder for fast moving air above the ground to mix down to the surface, where it could appear as a gust of wind.

If there is a low-pressure area or storm in the region, the winds can blow day and night.

Based on ten years of measurements at Tucson International Airport, Tucson’s yearly average prevailing wind direction is from the southwest (SW) at nine miles per hour, with 25 miles per hour gusts.  The monthly average prevailing wind direction changes over the year:  SE (January), W (February, March, April, June), WSW (May, July), S (August, September, December), SW (October), SSE (November).  The monthly average wind speed holds fairly steady:  eight miles per hour (January, October, December), nine miles per hour (February, March, July, August, September, November), ten miles per hour (April, June).

Tucson’s Climate.  Tucson lies in the Sonoran Desert with a subtropical climate.  Global air circulation keeps it dry most of the year.  There are two distinct rainy seasons:  the monsoon season (see below) occurs from mid-June through September, giving us on average 50% of our rain.  Another period of mostly gentle rains occurs in winter, producing on average 20% of Tucson’s rain, with the intervening months much drier. 

On average, Tucson receives about 12 inches of rain per year, spread over 52 days, and occasional sleet or snow.  Tucson enjoys 286 days of sunshine per year.  Spring/summer high temperatures can exceed 100 degrees Fahrenheit and winter lows sometimes (rarely) dip into the 20s.  The all-time high temperature in Tucson since records were kept is 117 degrees which occurred on June 26, 1990.  The all-time low is six degrees Fahrenheit on January 7, 1913.

Tucson’s average humidity is 38.0 %.  December is the most humid month with an average humidity around 50%, and June is the least humid month with an average humidity of 20%.

Weather Phenomena

Now that I’ve covered some of the basics, I’m going to talk about several weather phenomena - some common, some rare - that Tucson has experienced over the years:  the monsoon season, thunderstorms and lightning, tornadoes, hurricanes, dust storms, dust devils, and rainbows.

Monsoon Season.  The word “monsoon” comes from the Arabic “mausim” which means “a season.”  It was first used to describe dramatically changing seasonal winds over the Arabian Sea.  Since then, “monsoon” has been extended to include Europe, Africa, the western coast of Chile, and the southwestern U.S.

In Arizona the monsoon is a summertime phenomenon, characterized by southerly or southeasterly winds and daytime heating that result in heavy rain and thunderstorms.  During the winter, the primary wind flow in Arizona is across the dry desert from the west or northwest - from California and Nevada.  As we move into summer, the winds shift to a southerly or southeasterly direction.  Moisture streams northward from the Sea of Cortez and the Gulf of Mexico.  Once the moisture arrives, our strong summer sun heats the moist air causing the familiar thunderstorm (cumulonimbus) clouds.  The monsoon is most pronounced in southern Arizona and becomes more marginal over northern Arizona.

Prior to 2008, the Arizona monsoon was considered to start when there were three successive days that the dew point averaged 55 degrees Fahrenheit or higher.  Monsoon start dates ranged from June 16^th to July 15^th.  In 2008, the National Weather Service decided to take the guesswork out of it and defined Arizona’s monsoon season as beginning on June 15^th and ending on September 30^th.

Tucson receives half of its annual rain during the monsoon season, and can experience severe thunderstorms, accompanied by spectacular lightning, and even occasional tornadoes.

Thunderstorms and Lightning.  Thunderstorms occur when large air masses rise quickly into the atmosphere, forming huge cumulonimbus clouds.  Severe air currents inside the clouds cause water droplets and ice crystals to crash into each other continually, and the friction between these particles creates static electricity in the cloud.  Over time, opposite charges build up between the top and bottom of the cloud, and the bottom of the cloud and the earth.  When these opposing charges become intense, a gigantic spark occurs (lightning) which jumps the gap between the top and bottom of the cloud or the cloud and the earth.  The thunder accompanying lightning is noise produced by the discharge.  Thunderstorms are most likely in the spring and summer months, during the monsoon, and during the afternoon and evening hours. 

Thunderstorms often produce heavy rains and strong winds.  Many hazardous weather events are associated with thunderstorms:  Under the right conditions, rainfall from thunderstorms can cause flash flooding, killing more people each year than lightning, tornadoes, or hurricanes.  Lightning is responsible for many fires around the world each year, and in 2019 killed 20 people in the U.S.  Hail, up to the size of softballs, damages cars and windows, and kills livestock caught out in the open.  Strong (up to 120 miles per hour) straight-line winds knock down trees, power lines, and mobile homes. 

Some interesting facts about lightning:

a.       Lightning tends to strike tall objects on the ground because of the shorter distance from the cloud.

b.       Energy from lightning heats the surrounding air from 18,000 - 60,000 degrees Fahrenheit.

c.    Since 1989, over the contiguous U.S. 48 states, an average of 20,000 cloud-to-ground lightning flashes have been detected every year by the Lightning Detection Network.  About half of these flashes have more than one strike point on the ground.

d.      Dry lightning can occur when thunderstorm rain evaporates before reaching the ground and is a major cause of wild fires.

e.        The odds of an individual in the U.S. being struck by lightning in a given year are one in 1.2 million.

f.         Lightning can hit the same spot more than once.

Some interesting facts about thunder:

a.       Thunder starts out as a shock wave (for the first 10 yards), after which it becomes an ordinary sound wave.

b.       Thunder can be heard up to 25 miles away from the lightning discharge.

c.      Since you see a lightning flash almost instantaneously (speed of light), and sound travels much   slower at about one fifth of mile per second, you can count the seconds from the time you see a   lightning flash until you hear the thunder and divide by five to estimate the number of miles to the   lightning flash.

Thunderstorms are common in Tucson, particularly during the monsoon season.  We have all experienced heavy rains, and flash flooding of washes, low-lying road sections, or bridge underpasses.  And when it rains long enough, we see the rebirth of the Rillito River! 

Thunderstorm over Tucson, Arizona.  (Courtesy of Brian Snider)

Tucson also gets its share of lightning.  In 2018 there were 56,000 detected cloud-to-ground lightning flashes in Pima County.  As I write this paragraph, on June 10, 2020, there are two lightning-caused fires in the Santa Catalina and Tortilla Mountains north of downtown Tucson.  No city has a better setup to view and photograph lightning.  Tucson is surrounded by mountain ranges, giving photographers many high viewpoints for spectacular photographs. 

Spectacular lightning flashes over Tucson, with the moon in the background, 2013.  (Courtesy of Keith Kent)

A lightning-caused fire burns on Pusch Ridge in Tucson's Santa Catalina Mountains, June 10, 2020.  (Courtesy of Pat Wood)

Tornadoes.  Tornadoes are violently rotating columns of air that extend down from the base of a thunderstorm to the ground. These swirling winds spin at phenomenal speeds up to 400 miles per hour.  In the U.S., tornadoes are around 500 feet across on average and travel on the ground for five miles.  However, there is a wide range of tornado sizes.  Weak tornadoes, or strong, yet dissipating tornadoes, can be exceedingly narrow, sometimes only a few feet across.  Tornadoes focus their destructive power and can destroy all but the strongest man-made structures.  Tornadoes are difficult to predict and often strike without warning.  On average, about 1,000 tornadoes are reported nationwide each year, according to the National Oceanic and Atmospheric Administration.

Funnel clouds are spotted frequently in Tucson but very few of them become tornadoes (touch the ground).  Based on records dating back to 1950, 21 tornadoes have touched down within a 30-mile radius of Tucson International Airport.  Two of these tornadoes caused deaths and injuries, three others caused one or two injuries, and 16 tornadoes caused no deaths or injuries.  Two tornadoes in one day occurred on June 23, 1974 and July 17, 1984.  Of the 21 tornadoes, 17 occurred in the monsoon season.

 The two tornadoes that caused deaths were Tucson’s strongest tornadoes, with winds between 113-157 miles per hour:

a.       August 27, 1964:  Tucson’s deadliest tornado, touching down near San Xavier Mission.  It caused two deaths and eight injured persons.

b.       June 23, 1974:  The tornado went through a trailer court at Los Reales and Cardinal in southwest Tucson.  It caused 1 death and 40 injured persons.

A tornado escapes from a thunderstorm in south Tucson on August 27, 1964.

Rescue workers tend to an injured person after a tornado went through a Tucson trailer court at Los Reales and Cardinal on June 23, 1974.

Hurricanes.  Hurricanes, known as tropical cyclones, are low-pressure systems with associated thunderstorm activity that form over tropical or subtropical waters.  They derive their energy through the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation.  As storm systems strengthen into hurricanes, the surface winds move continuously in a circular motion.  These rotating winds lead to the development of the characteristic “eye” of the hurricane, the calm, clear center of the storm.    Hurricanes highest wind speeds are 20 to 30 miles from the center.  Major hurricane winds can reach speeds over 180 miles per hour with gusts of 200 miles per hour.  A typical hurricane is 400 miles in diameter and has an average forward speed of 15 miles per hour.  The average life span of a hurricane is nine days.  

Hurricanes form in the Atlantic Ocean and the northeastern Pacific Ocean.  Atlantic Ocean hurricanes often threaten the southeastern U.S., while Pacific Ocean hurricanes only occasionally threaten the southwestern and southcentral U.S. 

In the northeastern Pacific, hurricane season runs between May 15^th and November 30^th.  Hurricanes in Arizona are not common, since the predominant wind pattern steers most storms either parallel or away from the Pacific coast of northwestern Mexico.  Only six percent of all Pacific hurricanes enter U.S. territory.

Even so, Arizona has been affected by hurricanes on numerous occasions.  Usually, these storms make landfall in the Mexican states of Baja California or Sonora, and dissipate before crossing into the U.S.  In most cases, it is only the tropical cyclone’s remnant moisture that produces heavy rainfall - and on some occasions flooding - in portions of Arizona.  However, approximately every five years, a hurricane retains enough strength to the enter the state as a tropical storm or tropical depression.  Arizonans can expect indirect flash floods caused by remnants of tropical cyclones to occur about every two years.

Satellite view of a Pacific hurricane off the coast of northwestern Mexico.

Since 1950, Tucson has experienced five major hurricanes:

  

Major Hurricanes to Hit Tucson (within 150 miles).

Date	Name	Max Wind (mph)	Track
10/1958	unnamed	85	Northward over metro Tucson
9/1967	Katrina	85	Northeastward, northwest of Casa Grande
8/1968	Hyacinth	45	Northward to Benson
10/1989	Raymond	140	Northward, east of Benson
8/1992	Lester	85	Northward, just east of Tucson

Dust Storms.  Dust storms are meteorological phenomenon common in arid and semi-arid regions.  They arise when a gust front or other strong wind blows loose sand and dirt into the air from a dry surface.

In Arizona, dust storms can be some of the most dramatic weather events in the state, causing havoc for traveling automobiles and trucks suddenly caught in a storm.   As a dust storm builds, it can completely block out the sun, making it nearly impossible to see just a few feet ahead, causing multiple-vehicle accidents and pileups.

Dust storms are the third largest (behind extreme heat or cold, and flash flooding) cause of weather fatalities in Arizona over the last 50 years.  According to the National Weather Service, 157 people died and 1,324 were injured in 1,521 “dust incidents” on Arizona highways between 1955 and 2011.  Many of the deaths came during Spring winds and monsoon thunderstorms in the months of April and July.

Nearly half of the deaths occurred in the southern desert regions, which includes Phoenix and Tucson and heavily traveled interstate highways.  On Interstate 10, the accidents are concentrated in two agricultural belts - one near Picacho Peak, between Phoenix and Tucson, and the other between Benson and the New Mexico state line. 

The major culprit is abandoned farmland exacerbated by further disturbance of the land from all-terrain vehicle recreation or animals.  Active farmland can also produce dust in the time between tilling and planting.

Giant dust storms, called “haboobs” (Arabic for “blown”), are huge walls of dust created from high winds rushing out of a collapsing thunderstorm.  The wall of dust typically reaches heights between 1,500 and 3,000 feet, can stretch as far as 100 miles wide, and contains winds up to 50 miles per hour.  If you get caught outside during a haboob, you can be hurt by flying rocks and debris.  In Arizona, haboobs mostly occur in the Phoenix Valley.

On June 9^th of this year, state officials announced that a new dust storm warning system would be operational along a ten-mile stretch of Interstate 10, north of Tucson, in time for this summer’s monsoon season.  The system will be deployed between Eloy and Picacho Peak, and will employ multiple types of technology, including weather radar, visibility sensors, closed-circuit cameras, overhead message boards, and variable speed limit signs.

A dust storm on Interstate 10 near San Simon, Arizona, 2016.

 Dust Devils.  Dust devils are strong, well-formed, and short-lived whirlwinds, ranging from small (a foot and half wide and a few tens of feet tall) to large (more than 30 feet wide and more than 3,000 feet tall).  The primary vertical motion is upward.  Dust devils are usually harmless, but on rare occasions grow large enough to pose a threat to both people and property. 

Dust devils form over flat, barren terrain, with no or few clouds, and light or no wind. A pocket of hot air near the surface rises quickly through cooler air above it forming an updraft, and if conditions are right, the updraft may begin to rotate. Maximum winds within a dust devil range from 45-60 miles per hour.  Most dust devils dissipate less than a minute after forming.  Large, intense dust devils can last for upwards of 20 minutes before dissipating.

Typical dust devil near Tucson, Arizona

Rainbows.  Rainbows are a meteorological phenomenon that sometimes accompany rain.  Rainbows are optical illusions that are seen with the sun behind the observer.  They are caused by sunlight being refracted (like a prism) by the surface of a water droplet and divided into a spectrum of visible colors:  red, orange, yellow, green, blue, indigo, and violet.  Then the colored rays reflect off the back of the droplet and are further refracted as they exit to the viewer’s eye.  Rainbows take the form of a multi-colored circular arc, with the colors always in same order with red at top, and are centered on a line from the sun to the observer’s eye.

In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colors reversed, with red on the inner side of the arc.  This is caused by the light being reflected twice on the inside of the droplet before leaving it.  The second rainbow is also dimmer as a result of the two reflections.

Rainbows can occur even when it doesn’t rain, in dew and mists, fog, and clouds.  It happens when sunlight interacts with water droplets - of any kind.

This rainbow was observed while driving west on Tucson's Sunrise Road, alongside the Santa Catalina Mountains, March 27, 2010.  (Courtesy of Pat Wood)