Moisture-laden air spreading across the Southwest in recent days has brought much-needed rain to drought-ridden areas, while also unleashing thunderstorms, dust storms, strong winds and flooding. This seasonal weather pattern, known as the monsoon, has led to some spectacular — albeit disruptive and even dangerous — weather.

In the Nevada desert, winds toppled tents and briefly shut down the entry gates to the Burning Man festival this weekend. In Phoenix on Monday, a wall of dust moved through the city and grounded flights at the airport. By Wednesday, waterfalls at Yosemite National Park that are typically dry at this time of year had started flowing.

The monsoon affects the West every summer, though its duration and strength vary annually. It occurs when the prevailing winds shift from the west to the south, transporting moisture into the Southwest from the Sierra Madre ranges in Mexico, the Gulf of Mexico and the Pacific Ocean.

Its effects are usually strongest in Arizona and New Mexico, where heavy thunderstorms can bring cooler weather on hot days, but the moisture can spread across the region, as far north as Colorado and Northern California.

The onset of the monsoon varies every year, but it generally begins to affect the Southwest in July and runs into September. This season it came late — “about as late as it can get,” said Mike Kaplan, professor emeritus at the Desert Research Institute in Reno, Nev.

The delay meant that Arizona and New Mexico had been unusually dry so far this year. The Dragon Bravo fire exploded in Grand Canyon National Park in July, in part, because the landscape was more parched and vegetation more flammable from a lack of monsoon rains.