In the Rocky Mountains, downslope winds are referred to as Chinook winds. Also known as the “snow eaters”, Chinook winds impact the region's weather, climate, and environment in various ways. Chinook winds influence snowmelt, flooding, wildfire activity, vegetation, wildlife, health, infrastructure, and transportation. These warm and dry winds, named after the Chinook Tribe who first noticed them, have been observed and documented for over one hundred years. The various impacts of Chinook winds will be explored below.

What is the mechanism of Chinook winds?

The setup for Chinook Winds involves a low-pressure system to the east of the Rockies with high pressure to the west, though these features are often oriented from southwest to northeast and can occur from Alberta down through New Mexico.

Image: Atmospheric pattern for a Chinook wind event in Colorado and Wyoming. A high pressure was positioned west of the Rockies in Colorado, along the border of New Mexico, while a low pressure was in the plains to the northeast. 

Image: Location of Chinook winds in the western US and western Canada. Courtesy of traveltalesoflife.com.

The pressure difference between the high-pressure system to the west and the low-pressure to the east creates a strong gradient. The pressure gradient drives air from the high-pressure area in the west towards the low-pressure area in the east, resulting in winds that flow from west of the Rocky Mountains to the eastern side of the range.

As warm, moist air moves inland from the coast along the pressure gradient, it encounters a barrier, the Rocky Mountains. Forced to ascend over the mountains, the air cools as it rises. During this ascent, the air mass releases much of its moisture as precipitation. Once the now dry air mass reaches the Continental Divide, it begins to descend the eastern slopes. As it descends, it warms rapidly due to compression. This process is enhanced because dry air warms more quickly than moist air. Consequently, the descending air becomes significantly warmer and drier than the original air mass that arrived from the Pacific Ocean. 

Chinook winds along the Rockies cause dramatic temperature increases, decreased humidity, and strong winds. They most commonly develop in the autumn and winter months but can occur at any time of the year. 

Image: Schematic of Chinook winds in the western US. Courtesy of WGN-TV Chicago.

Wildfires

Chinooks play an important role in wildfire activity east of the Rockies in the United States and Canada. The combination of strong winds and dry air can exacerbate wildfires. The descending air on the leeward side of the range warms quickly and loses much of its moisture, leading to low humidity levels. This can cause vegetation to dry out rapidly, turning grasses, shrubs, and trees into highly combustible materials. 

Chinook winds eliminate moisture in the air near the ground, which typically keeps the ignition temperature of vegetation higher. The decreased moisture increases the speed at which wildfires can spread. Chinooks prevent temperatures and relative humidity from recovering overnight when firefighters can get a better handle on fires.

These winds result in the rapid spread of wildfires as well, transporting embers long distances that ignite new fires far from the source. The rapid spread and intense burning that results from Chinook winds present additional challenges to firefighters.

Several large and destructive wildfires along the Rocky Mountain front have been worsened by Chinooks. The Yellowstone Fires (1988), First Old Stage Fire (1990), Second Old Stage Fire (2009), Fort McMurray Fire (2016), and the Marshall Fire (2021) are some of the worst Chinook wind-driven fires and are summarized below.

Yellowstone Fires (1988)

• Location: Yellowstone National Park, WY
• Acreage Burned: Over 793,000 acres (36% of the park)
• Structures Destroyed: Infrastructure within the park
• Fatalities: 15 people
• Details: A series of wildfires began in the summer of 1988, driven by a combination of dry conditions and strong Chinook winds. Despite extensive firefighting efforts, the fires spread rapidly due to abundant dry vegetation. The fires subsided with cooler weather and snow in September. This event led to significant changes in wildfire management policies in national parks.

The First Old Stage Fire (1990)

• Location: Colorado Springs, CO
• Acreage Burned: Approximately 3000 acres
• Structures Destroyed: 15 
• Fatalities: 1
• Details: The 1990 Old Stage fire was driven by strong winds, rapidly spreading through dry vegetation and threatening residential areas. The fire resulted in significant property loss and one fatality.

The Second Old Stage Fire (2009)

• Location: Colorado Springs, CO
• Acreage Burned: Approximately 3000 acres
• Structures Destroyed: 2 homes
• Fatalities: 0
• Details: Similar to the 1990 fire, the 2009 Old Stage fire was fueled by strong Chinook winds, causing rapid spread and challenging firefighting efforts. The fire underscored the recurring risk of wildfires in this area.

Fort McMurray Fire (2016)

• Location: Fort McMurray, AB
• Acreage Burned: About 1.5 million acres
• Structures Destroyed: Over 2400
• Fatalities: 2 indirect
• Details: The Fort McMurray fire, also known as "The Beast," was exacerbated by dry conditions and strong winds. It led to the evacuation of nearly 90,000 residents and caused extensive damage to the city and surrounding areas.

The Marshall Fire (2012/2022)

• Location: Boulder County, CO
• Acreage Burned: Approximately 6000 acres
• Structures Destroyed: 1084
• Fatalities: 2
• Details: The Marshall Fire, which began on December 30, 2021, is considered one of the most destructive wildfires in Colorado's history. The fire rapidly spread across Boulder County, fueled by strong Chinook winds and dry conditions.

Snowmelt and Flooding

Chinook winds, or “snow eaters”, are notorious for the rapid disappearance of snow. The sudden increase in temperatures that results from Chinooks causes snow to melt quickly. Some of the most dramatic and rapid temperature changes ever recorded were caused by Chinook winds.  

Here are a few notable examples:

1. Spearfish, South Dakota (1943)

• Date: January 22, 1943
• Temperature Change: The temperature rose from -4°F to 45°F (-20°C to 7°C) in just two minutes. This sudden increase of 49°F (27°C) is one of the fastest temperature changes ever recorded.
• Details: As the Chinook winds ceased, the temperature plunged back to -4°F (-20°C) within 27 minutes. This rapid fluctuation caused significant disruptions and highlighted the powerful impact of Chinook winds on local weather.

2. Great Falls, Montana (1980)

• Event: January 1980.
• Temperature Change: The temperature climbed from -32°F to 15°F (-35.5°C to -9.4°C) in just seven minutes, a rise of 47°F (26.1°C).
• Details: This rapid warming significantly affected the local environment, causing snow to melt fast and creating icy conditions as the melted snow refroze when temperatures dropped again​.

3. Lethbridge, Alberta (1962)

• Event: January 1962.
• Temperature Change: The temperature increased from -2°F to 72°F (-22°C to 19°C) in a short period, marking a dramatic 74°F (41°C) rise.
• Details: The rapid melting of snow led to hazardous road conditions. Initially, the melting snow created slushy and muddy roads, and subsequent refreezing led to icy patches, complicating travel and increasing the risk of accidents.

4. Loma, Montana (1972)

• Event: January 14, 1972.
• Temperature Change: The temperature climbed from -54°F at 9 am to 49°F (-35.5°C to -9.4°C) the next day. The 103°F (57.2°C) rise is the greatest change in temperature ever officially measured on earth within a 24-hour period.
• Details: This warming caused rapid snowmelt, creating icy conditions as the melted snow refroze when temperatures dropped again​.

Snow can rapidly disappear through a process called sublimation as well. Snow sublimation is the process by which snow and ice transition directly from a solid state to water vapor without first becoming liquid water. The three atmospheric impacts from Chinook winds are warm temperatures, low humidity, and strong winds, which are the three conditions that favor snow sublimation. The strong, warm, and dry winds vaporize the snow, diminishing the snowpack and removing water from it as well. The rapid snowmelt from the Chinooks can cause or worsen flooding events. 

Images: Three-part schematic showing how Chinook winds result in warmer air on the leeward side of the Rockies that rapidly melts snow. Courtesy of Fox 21 Colorado.

In addition to the snowpack and water loss, Chinook winds can cause snow on roadways to melt fast. When the Chinooks subside, temperatures can fall rapidly, and cause the water from the melted snow to freeze on the roadway. This can lead to hazardous driving conditions. 

Infrastructure

Chinook winds, with their strong gusts and rapid temperature changes, have caused notable damage to infrastructure in many locations on the eastern side of the Rockies. They can cause structural harm to buildings and homes, including roof and window damage.

Image: Diagram of the Chinook setup in western Canada that brings the strong winds into Calgary. Courtesy of traveltalesoflife.com.

While these winds bring warmth, they often only melt the surface snow and ice, leaving a deep layer of frost underground. This frost, which can extend up to 10 feet deep, penetrates the area where most water mains are located, making pipes vulnerable to bursting from the extreme cold. During winter, the snow on the ground acts as insulation, protecting water mains and household pipes. However, when Chinook winds cause the snow to vaporize, this protective layer is lost, leaving pipes exposed to freezing conditions.

Chinooks can knock down power lines and utility poles, leading to widespread power outages. This can disrupt daily life and business operations, and repairs can be time-consuming and costly.

Transportation Hazards

Chinook winds significantly impact transportation systems in the US and Canada. These impacts include road, rail, and air travel challenges, posing risks to safety and infrastructure. High winds can blow debris such as branches and other objects onto roadways, creating obstacles for drivers. 

Chinooks can kick up dust and loose soil, reducing visibility and increasing the risk of accidents. Strong gusts can affect the stability of vehicles, particularly high-profile ones such as trucks, buses, and SUVs. This increases the likelihood of rollovers and accidents.

Chinook winds can create turbulence and wind shear, which pose hazards to aircraft. Pilots may need to adjust flight paths or delay flights to avoid these dangerous conditions. Airports in affected regions may face delays and cancellations during Chinook wind events. Similar to road transportation, the dust and debris stirred up by Chinook winds can reduce visibility around airports, complicating air traffic control and ground operations.

Health

Chinook winds pose several problems to the health of those in the affected regions. The dry air and potential for dust can exacerbate respiratory conditions such as asthma and bronchitis. Further, chinooks can cause headaches, irritability, anxiety, and dry skin.  

Vegetation and Wildlife

Vegetation and wildlife are significantly impacted by Chinook winds as well. These winds can alter the environment quickly, affecting plant health, soil conditions, and animal behavior. The warm, dry air of Chinook winds causes rapid evaporation of moisture from plant tissues. This can lead to dehydration, stress, and potential damage to vegetation, especially during winter when plants are not actively growing and are less capable of replenishing lost moisture.

The sudden warming can cause a premature thaw. If followed by a rapid return to cold temperatures, this can damage the roots and tissues of plants, leading to reduced vitality or death. The stress caused by Chinook winds can delay the growth and development of crops, impacting the overall productivity of agricultural systems.

The impact of Chinook winds on vegetation can reduce the availability of food sources for wildlife. Dehydrated and damaged plants can lead to a decline in forage quality and quantity for herbivorous animals. The rapid temperature changes associated with Chinook winds can cause thermal stress in wildlife. Animals adapted to cold winter temperatures may struggle to cope with sudden warmth, leading to physiological stress. 

Summary

Chinook winds are a defining feature of the Rocky Mountains, bringing both benefits and challenges to the region. Their warmth can provide much-needed relief from winter's chill and extend agricultural seasons, but they also pose risks such as increased fire danger, rapid snowmelt, and health impacts. Understanding and anticipating the effects of Chinook winds is essential for residents, farmers, and policymakers in the Rockies to mitigate potential hazards of this powerful natural phenomenon. Accurate weather forecasting and monitoring of temperature and wind patterns are crucial for predicting and managing the risks associated with Chinook winds.

Luke Stone
Forecaster, OpenSnow