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 http://meteorologynews.com/offbeat/… . 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