...but it's a dry heat.

I know I've already complained about the heat, but let me tell you about monsoons.

In discussing desert heat my friends from the great white humid North insist that, sure, it's hotter here, but that it's a dry heat and therefore more tolerable than their hot and humid weather. I will grant that the humidity does add an element of "unbearability" to the summer, but have you been in Arizona from mid-July to mid-September? That's Monsoon season.

The Arizona Monsoon is a slightly different phenomenon from the famous monsoons heard about in India where it rains non-stop for weeks on end, flooding everything. Instead, it merely rains most afternoons. A good meteorological discussion of the Arizona Monsoon can be found here.

In short, this crazy thing happens where the winds shift to bring more moisture into the state from the Pacific Ocean and the Gulf of Mexico, and at the same time it gets crazy hot here. When both things happen in tandem and for prolonged periods of time (at least a couple of days), the moisture builds up in huge fluffy cumulus clouds. As we learn in science class, the hotter the air temperature, the more moisture can be held in it. So as Arizona heats up during the day, more and more moisture is held in the clouds. As Arizona begins to cool down a bit in the late afternoon and early evening, the air can no longer hold all that moisture and you have a downpour, or, monsoon.

If that was all that happened, we'd be alright with that. Desert monsoons are generally preceded by dust storms, called haboobs. Seriously, they usually describe these as lens-shaped dust walls that are generated by the thundercells. They can be up to 3000 feet tall, and move at upwards of 30 miles per hour (50 km/h). This part is really spectacular to watch, as long as you're not exposed, but can wreak havoc on trees.

Tree uprooted by monsoon on 17 July 05. This is on my street.

Early in the monsoon season, the storms are usually dry. By which I mean all I've just described happens, except it doesn't rain. It feels unbearably humid, but not enough moisture is yet present in our atmosphere to create the heavy rains characteristic of typical late-season monsoons.

So what I have described above is what happens in the Southwest during a typical monsoon season. Desert dwellers both rely on and look forward to monsoon seasons because a) we really need the water, and b) the temperature drops by at least a few degrees. Having said this, the monsoon is sort of an endangered species in Phoenix because of all Phoenecians have done to manipulate the environment of the valley.

Let's quickly discuss the many ways that Phoenix has increased its potential for heat retention:

• Paving surfaces: unpaved desert reflects a lot of heat, but once the sun sets cools down very quickly. Paved surfaces absorb heat and radiate it after the sun has set.
• Air conditioning: to get cool air inside you have to generate heat somewhere.
• Automobile exhaust: there is a very poor public transportation system and I haven't met anyone who has actually set foot on a bus. You can't get Phoenecians out of their cars. Couple this phenomenon with air conditioning.

The Phoenix area pushes so much heat skyward that storm cells approaching from the north east are pushed away, go around the city and reconvene at the south west corner. From the right vantage point you can actually watch it happening. Also, Phoenix doesn't cool down as quickly as most desert regions so monsoons don't occur until mid- to late-evening instead of late-afternoon/early-evening.

All this to say, dear friends in the North, that it's not a dry heat today.

Check current Phoenix weather here.