Sonoran desert weather

Leo Martin
Wed, 24 Aug 2016 23:31:47 PDT
Our spring and fall have very low relative humidity, and it almost never
rains. Days are warm to hot, and nights cool down. Summers are tripartite:
Hot and arid; monsoon, with thunderstorms; hot and arid. Winters are cool,
with occasional Pacific winter storms arriving from the northwest,
featuring long spells of gentle, cool rain.

The start of spring is usually recognized in retrospect. The potential for
cool winter rain stopped, and nights still cool down. Spring may last until
early March, or until mid June.

Winter-growing plants continue growing in the spring only if watered
regularly, because rain has stopped. They begin going dormant slowly if
watering stops, but rapidly after only a few days of hot weather. Often a
hot spell will be followed by a long period of cool weather, but the
dormancy message has been received.

The first part of summer has increasingly hot days, and relative humidity
is extremely low. This is when the temperature records are set, in the last
week before the monsoon arrives. We hit 118 F / 47.8C a couple of times
this June. First-summer relative humidity is under 15% almost all the time,
day and night, and usually around 5%. Day and night temperatures steadily
increase, but nights are cooler than days, though still hot. This has
changed from the past due to the heat island effect in metro Phoenix. 30
years ago, early summer nights cooled down quite a bit.

The second part of summer is the monsoon. Warm, moist air from the Sea of
Cortez advances farther north each day, and recedes at night. The
possibility of thunderstorms arises. Daytime temperatures on monsoon days
are not near the records set at the end of first summer. Nights are
uncomfortably warm, especially when there is cloud cover. This is the time
when night high temperature records may be set, and plants with
crassulacean acid metabolism struggle mightily. Echeverias are winter
annuals here, as are many Aizoaceae and other Crassulaceae. The jade plant
cannot be grown here successfully because of hot summer nights. The monsoon
reaches Phoenix around the second week of July, and Tucson 2-3 weeks before
this. More than half of our annual precipitation falls during the monsoon.
Because Tucson has a longer monsoon, and is closer to the Sea of Cortez, it
gets more monsoon rain than does Phoenix. Tucson is 1,000 feet / 267 meters
higher in elevation than Phoenix, so it is very slightly cooler, as well.
It is much smaller than Phoenix, so it is not so severe a heat island.

The monsoon has been defined as beginning when the average dew point is
above 55 degrees F / 12.8C for three consecutive days. Dew point is the
temperature at which water will condense from the air spontaneously; the
dew point rises as air becomes more humid. Evaporative coolers don't
function well at, nor above, this dew point, so it is a practical measure.
During our first summer, evaporative cooling works at least as well as air
conditioning, and many people find the cool, humid air more pleasing than
cold, dry air.

This definition was too difficult for low-intelligence people to
understand, or most television and newspaper weather reporters to explain,
so it has been discarded. Now the monsoon is defined as beginning June 15.
As defined under the old system, the monsoon never once during the history
of weather recordkeeping began as early as June 15 in Phoenix.

In the monsoon there are substantial periods of higher relative humidity,
when it may be 20%-40% for several days, to over a week. Daytime
temperatures are in the range 100 -110 F / 38-43C. Clouds form near
mountains almost every afternoon, and thunderstorms may form at any time.
This is interspersed with spells of hot, dry and low-humidity weather, when
relative humidity falls to 10% or below and there is no chance of rain.
Temperatures during these monsoon interruptions are not as hot as the last
week of June, but regularly over 110 F / 43.3C.

Late afternoon and evening thunderstorms are common during our monsoon.
These tend to be very localized, and are often not more than a mile in
diameter. The sun heats the ground, and the air above rises; by late
afternoon the ground is very hot, and the air rises very fast and very
high, high enough for the water vapor to condense. Intense winds arise,
blowing towards the center of the storm, as ground-level air rushes to
replace that carried aloft by updrafts. Regional air currents can blow such
storms along their path, dropping an inch of rain in 10-20 minutes, then
passing to the next part of the city. Sometimes it rains on only one side
of a street. This kind of thunderstorm only rarely occurs in metro Phoenix
any more, compared to 30 years ago, due to the vastly increased concrete
and pavement coverage as the city expands. There is no humid air over
pavement. Almost every monsoon afternoon, a wall of thunderstorms can be
seen ringing Phoenix, but a good 20-30 miles away from the city center. 30
years ago I could expect 5-7 late afternoon monsoon storms at my house, but
more recently, I get 1-3.

If the air is not humid enough to sustain thunderstorms, a dust storm may
ensue, as air rushes to replace updrafts. See…
. We have even more Valley fever cases (coccidioidomycosis) than does
California's Central Valley, after which it was named. This is a fungal
infection whose spores are spread during dust storms.

Another kind of monsoon thunderstorm forms on the prevailing wind side of
mountains almost every afternoon. Air masses collide with the mountains,
and rise; the air cools as it rises, and clouds form. This is seen best in
the Santa Catalina Mountains, at the base of whose southern slope Tucson
reposes. If you want to see thunderstorms and lightning, stay at the
Ventana Canyon Resort in Tucson some week in mid July, and ask for
a mountain view room. Summer rates are quite reasonable. These
mountain-induced storms are not affected by the heat island effect, since
the mountains haven't been built over. Yet.

Region-wide monsoon storms happen mostly at night, related to large air
masses moving over the land. There are 0-3 of these most years.

My Gethyllis sp, if it is to bloom during a given year, seems to bloom
shortly after a good thunderstorm in July.This year we didn't have one, so
it didn't.

Amaryllis belladonna here would bloom during the mid monsoon. Mine have
bloomed twice since they were well-established. It had been so long since
I'd seen them that once I didn't recognize them, and I had to look at my
records to see what I'd planted. Lycoris radiata may bloom later during the
monsoon, right about now, about once every 3-4 years. It hasn't this year.
Lycoris aurea grows here, though I don't have it. Other Lycoris don't
survive here. Neither surviving species blooms every year.

The monsoon ends when the moist air recedes to the south, replaced by hot,
dry air. This happens sometime from the end of August to the end of
September. We have a third summer, similar to the first summer, but days
and nights steadily get less hot.

Fall begins when nights begin cooling to something comfortable. This may
happen from late September to late October, Days are often still over 100 F
/ 38C through the beginning of November. There is no fall rain. A few
winter bulbs sprout on their own when they feel like it. A lot of Albuca,
Oxalis and the red squill, Urginea or Drimia maritima, sprout
inflorescences in late August no matter the weather. There is no rain in
our fall, so bulbs that depend on fall rains to sprout will not do so
unless watered. It is tricky deciding when to water; it might suddenly turn
hot again tomorrow, and will the wet bulbs rot?

One day in November it is suddenly winter. Nights become chilly, and night
frosts are possible. The earliest frost at my house in the last 30 years
was November 20, and there have been years with no frost here. One winter
there was no frost until the first week of February. Days always rise well
above freezing when the sun is up. Only once has my pond frozen solid, in
the polar express of late December 1990. We did not have another extremely
cold spell until January 2007, which, while cold, was nowhere near as cold
as 1990. Pacific winter storms may arrive after dumping most of their rain
on California. It rains here a day after it rains at Disneyland in southern
California. Winter growing plants do extremely well here; when it is not
raining, relative humidity is very low, so there is no fungus trouble at
all. If there has been no rain, though, the gardener needs to water. Days
are cloudless and sunny unless a storm is present.

Tucson is farther south and east from Phoenix and the Pacific, so Tucson
gets much less winter rain than we do. Because of the heavier monsoon in
Tucson, it averages 14 inches / 355mm of rain per year, while Phoenix
averages 8" / 200mm.

My paperwhite narcissus begin blooming between the last week of December
and the second week of January. I have various winter bulbs in bloom all
winter outside.

The last recorded frost for Phoenix was on March 15, but there hasn't been
frost after mid February for many years. Spring begins between the end of
January and mid March. The chance for winter storms is gone, and there will
be no rain until the monsoon arrives. If hot weather comes early, spring
bulb flowering is pitiful. Spring has been over the first week of March,
and the second week of June. Winter growers much prefer like longer springs!

Leo Martin
Zone 9?
Phoenix Arizona USA

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