Flagstaff, AZ – Earth Notes: The Summer Monsoon
In the arid Southwest, the idea of a rainy season comes as a surprise to many newcomers. But the region does enjoy a distinct monsoon period that usually begins in early July and lasts into the middle of September.
Colorado Plateau residents eagerly anticipate the season, keeping close watch as cumulus clouds tower into the sky. When the rains finally come, it's cause for rejoicing.
Navajos call them niltsa bika', or "male rains." For the Hopi, the annual ceremonial cycle is closely tied to the arrival of the summer's precious moisture.
Plants and animals are also closely attuned to the monsoon regime. Ponderosa pines require warm summer rains for seed germination. Spadefoot toads hunker underground through the year, emerging only at the sound of raindrops drumming hard on the dirt. Some songbirds wait to nest until the summer moisture arrives, causing insects to hatch.
While the toads wait, meteorologists watch changing global circulation patterns. They look for a shift in the wind, from prevailing westerlies to winds coming out of the south or southeast. As a subtropical high-pressure system builds, moisture is pumped northward into the region from the Gulf of California and the Gulf of Mexico.
Summer storms can deliver nearly half the region's yearly precipitation. When the monsoon season is in full force, normally dry streambeds fill with ferocious flashfloods, and cyclonic dust devils swirl up off the land.
The dramatic storms nourish plants, animals, and people and underline the infinite wonder of water in a dry land.