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THE NORTH AMERICAN MONSOON

The hot, dry weather of May and June abruptly changes in July as a mass of moist tropical air typically moves in from the southeast. Skies, which have been clear for weeks, start to fill with puffy cumulus clouds by midmorning, and the afternoon often brings towering thunderstorms with strong gusty winds, heavy rain, hail, and lightning. Arizona lies at the northern limit of the North American Monsoon, which as the name implies is much more active south of the international border. The stormy weather follows a surge and break pattern, with active periods of several days or a week often followed by lulls when the moist air is pushed south of the state by minor weather disturbances. Still, take precautions during the monsoon. To avoid lightning, plan to be off high ridges and peaks by midday. Because of the potential for floods, never camp or park a vehicle in dry washes or other drainages. Heavy rain may fall in the headwaters, miles from your location, and cause streams and dry washes to rise suddenly and without warning. If thunderstorms are building during the afternoon, plan to camp in a sheltered area, if possible. Thunderstorm gust fronts can produce winds of 60 miles per hour or more. Although most hailstorms are extremely local in nature, and seldom produce hailstones larger than 0.25 inch, larger hailstones do occur and can be dangerous to hikers caught in the open. That said, the summer rains are a delight. Monsoon mornings dawn clear and sweet, with mountain meadows misty with dew, and the afternoon clouds and rain bring welcome relief from the midday heat.