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Dirty downpour

Hail, wind and thunderclaps sent people scrambling for shelter at the U’s campus on Tuesday, April 18. The storm left sullied souvenirs in its wake, coating cars and clothes with muddy raindrop residue.

So, what happened? Dust, said McKenzie Skiles, assistant professor of geography who researches dust and its impacts on snowpack. Skiles spoke with @theU to explain how dust from Utah’s West Desert caused the dirty deluge.

What was going on with that dirty rain?

Before the rain, there were high wind speeds, known as prefrontal wind, that started transporting dust from the West Desert. As the storm approached, there was still dust in the air, which mixed with the precipitation in the atmosphere. As the rain started to fall, it brought the dust with it—hence, dirty rain. For dust, this is known as wet deposition.  

How does it work?

The storm approached from the west and kicked up a lot of dust—satellite imagery picked up large dust plumes coming from the Salt Flats/West Desert area. Even when storms don’t produce these dramatic dust plumes, high prefrontal wind speeds often carry dust from smaller or more diffuse sources that still contribute to dust, reduced visibility, and “dirty rain.”

Are these events common?

Dust events are not uncommon in the spring, as dust-source regions dry out and high prefrontal winds precede our frequent spring storms. These events typically bring dust from the south or west of Salt Lake Valley—it’s less common to see dust from the north, although events like that have been observed. These events don’t always mix with precipitation, but it will lower visibility and deposit dust on surfaces as it settles out of the atmosphere, known as “dry deposition.”

Wet deposition events like we saw this week are less frequent, but we typically get a few every spring. They are less noticeable when the dust mixes with snow, and we had a relatively large event like this a few weeks ago that is currently visible as a dust layer in the snowpack (picture below). It’s more noticeable when it mixes with rain because it is immediately visible on surfaces like cars and windows. 

Is dusty rain a health hazard?

Not necessarily. Dust can lower air quality, and dust events that lower visibility should be treated like other low air quality periods—avoid outdoor activity, stay inside if possible. During wet deposition events, the dust is actively being scavenged out of the atmosphere via the rain or snow, so breathing in dust particles is less of a concern. There is other pollution in the atmosphere that gets scavenged by rain, though, so I wouldn’t recommend collecting the dirty rain and drinking it!

Will more happen in the future?

A woman stands at the bottom of a square pit in the snow. You can see bands of dirt deposited around the outside that shows where dust deposited.

PHOTO CREDIT: McKenzie Skiles

Undergraduate researcher Ava Lessing in a snowpit at Atwater Study Plot (Alta, UT). The prominent thick dust layer visible near the surface was a wet deposition event.

Download Full-Res Image

When dust sources regions are dry, and wind speeds are high enough to pick up and transport dust, the likelihood of dust events (wet or dry) is high. These conditions are typically met every spring. It is challenging to say how storms themselves might change in the future as the climate warms, but we do know that land use change, surface disturbance, and greater exposure of playa, like the dry lakebed of the Great Salt Lake, do increase dust levels by making surface soils more available for transport. Although dust is often considered a natural aerosol, modern levels of dust that we see in the Western U.S. are elevated due to human activity. Humans tend to disturb stable surfaces like natural salt and biological crusts, or remove vegetation that limits dust emission by reducing wind speeds at the surface.

Besides us, does the dirty rain impact anything else? 

Besides needing a car wash or cleaning windows, the direct impact of dirty rain on our daily lives in relatively minimal. But more broadly dust emission, transport and deposition are complex processes that play a really important role in the world around us. Dust can lower air quality and visibility, transport nutrients, scatter light in the atmosphere and accelerate snowmelt by darkening the normally bright surface.