Lightning bolts through the ominous clouds above Mount Lemmon. One … Two … Three … Four … Five … Boom. Thunder rattles the sky. The storm sits roughly one mile away from the mountain based on the “five seconds to one mile” theory.

The difference between the speed of light and the speed of sound allows this theory to estimate the distance of lightning from a person’s location. Near sea level, the sound of thunder travels approximately one mile in five seconds, while lightning flashes and disappears in an instant. This can protect hikers from possible danger during monsoon season.

The Southwest monsoon season begins in late May as the summer sun evaporates water from the Pacific Ocean and builds up humidity in the atmosphere. The dry spell in the Southwest continues throughout late June. 

In early July, a southeasterly shift in the wind direction pulls the moisture from the tropical Mexican air into the Southwest. This stream of moisture is the final puzzle piece in the heavy rainfall associated with monsoons.

Simply put, the monsoon is a seasonal shift in wind direction that pulls moisture into the Southwest region and allows the development of thunderstorms. Monsoon season provides the arid desert ecosystem with needed rainfall and lower temperatures while also inflicting powerful flood waters and numerous lightning strikes.

All of the factors of the monsoon vary, from the timing of the thunderstorms to the amount of rainfall. The variability of monsoons makes these thunderstorms dangerous to the ill-informed.

“It is difficult to predict where [monsoon] storms will exactly be at a specific time,” said Dr. Thomas Galarneau, an assistant professor of atmospheric sciences at the UA.

A Surge of life

The Southwest monsoon season replenishes life in the dry Sonoran Desert. It produces more than half of Arizona’s annual rainfall in a matter of months while suppressing the hot summer temperatures, according to associate professor Michael A. Crimmins in the UA Department of Soil, Water and Environmental Science.

Arizona sits in the “sweet spot” of this complex weather pattern, Galarneau said. It is important to the state’s water supply, agricultural production and the life cycle of plant and animal species, which remain dormant until these wet months.

Drowning in consequences

The city of Tucson averages about 6 inches of rain per year during monsoon season, according to Crimmins. However, the benefits of heavy rainfall come with some negative ramifications.

Lightning is one of the main ignition sources for wildfires. Wildfire season stretches from May until July, a time in which dry plants increase the risk of fire until the rains return. A late monsoon season can be devastating to property and the environment by extending the period of increased likelihood for wildfires.

Flash flooding occurs in a matter of minutes in dry stream beds and narrow canyons. These powerful floods have the capability to move thousands of pounds of water. 

A small car can be washed away in just 12 inches of water with the right force. The National Weather Service created the “Turn Around Don’t Drown” slogan to promote safety practices around flood water.

Arizonians need to “respect the monsoon,” Crimmins said. Education and planning are the best courses of action to minimize property damage and prevent deaths.

As the month of September continues, the thunderstorms and lightning will start to dwindle as the wind shifts back to its original southwest direction. Mount Lemmon and the surrounding wilderness areas become dry and safe once again.

Until next time, monsoon season.

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