What are downslope winds?

Downslope winds are a meteorological phenomenon characterized by warm, dry air descending the leeward side of a mountain range. The term "leeward" refers to the side of a geographic feature, such as a mountain or an island, that is opposite from the prevailing wind. These winds are known by different names depending on the region. They can cause significant and rapid changes in weather patterns, affecting local climates and various human activities including winter sports. 

Image: After moving over a ridgeline, the air descends and accelerates, resulting in strong winds and rising temperatures. (Image courtesy of the NWS).

Why are they Important?

Downslope winds occur in various regions around the world, with major impacts on weather and climate, human health, agriculture, and the environment. This phenomenon can lead to rapid snowmelt and flooding, increased avalanche risk, stress to crops and livestock, health impacts, infrastructure damage, hazardous driving conditions, and increased wildfire risk.

Downslope winds have different names depending on the region:

• Alps (Europe) - France, Italy, Switzerland, Austria, Germany: Referred to as "Föehn."
• Rocky Mountains (North America) - Alberta, Montana, Wyoming, Colorado, New Mexico: Known as "Chinook winds."
• Andes (South America) - Argentina: Called "Zonda winds" in Argentina.
• San Gabriel and San Bernadino Mountain ranges (United States) - California: Known as "Santa Ana winds."

Each of these examples of downslope winds has profound impacts in their respective regions. 

Avalanche Risk and Snowmelt

For skiers and snowboarders, downslope winds present several issues in the mountains. The sudden warming associated with downsloping winds can lead to accelerated snowmelt. These winds are referred to as "snoweaters". More importantly, the impact the warmth and winds have on snow stability and avalanche risk poses problems for those of us enjoying activities in the mountains. 

Downslope winds cause a sudden rise in temperatures, which can decrease the stability of the snowpack by weakening layers of the snow. Further, strong downslope winds can transport snow from the windward slope and deposit it on the leeward slope, resulting in wind loading. This decreases the stability of the snowpack and can lead to increased avalanche activity. 

Wildfires

Wildfires are another major consequence of downslope winds, especially in southern California. Downslope winds are typically very dry, with low humidity levels. This dries out vegetation, making it more flammable and increasing the overall fuel load for potential fires. 

Downslope winds can be very strong, with gusts sometimes exceeding 100 mph. These winds can rapidly spread existing fires by pushing flames across large distances and carrying embers ahead of the fire front, igniting new fires. The strong winds can increase the intensity of wildfires by providing more oxygen to the flames, making them burn hotter and faster.

While the most significant impacts to wildfires occur in southern California, downslope winds affect fires in Europe and the Rocky Mountains as well. In all of these regions, downslope winds play a crucial role in exacerbating wildfire risks by drying out vegetation, increasing fire intensity, and promoting rapid fire spread.

Flooding

The sudden and significant rise in temperature caused by downslope winds can accelerate snowmelt, leading to a large volume of runoff over a short period. This can overwhelm rivers and streams, leading to flash floods as well as river flooding.

Agriculture and Livestock

Downslope winds can have a variety of significant impacts on agriculture and livestock. The rapid warming can lead to heat stress in crops, damaging plant tissues, reducing photosynthesis, and hindering plant growth. The sudden temperature rise can cause premature bud break in regions where fruit trees and other perennials are cultivated. This makes the plants vulnerable to subsequent frosts, potentially damaging the buds and reducing yields. 

Increased evapotranspiration associated with downslope winds can lead to greater water loss from plants, stressing crops and necessitating increased irrigation. The dry air can desiccate plant tissues, particularly the leaves, which can lead to wilting, leaf burn, and reduced photosynthetic efficiency. This is especially detrimental to young or tender plants.

Strong downslope winds can physically damage crops by breaking stems, dislodging fruits, and causing abrasion to leaves and stems from blowing dust and debris. High winds can lead to soil erosion, particularly in dry conditions, removing the topsoil that is essential for nutrient availability and root stability. This erosion can reduce soil fertility and long-term agricultural productivity.

The combination of warm temperatures and low humidity can rapidly dry out the soil, decreasing soil moisture levels. This can lead to water stress for crops, especially if irrigation is not available or insufficient. Farmers may need to significantly increase irrigation to compensate for the moisture lost due to the drying effects of downslope winds. This can strain water resources, especially in regions already facing water scarcity.

Health

Downslope winds have a complex and multifaceted impact on human health, influencing respiratory, cardiovascular, neurological, and dermatological conditions. These winds often pick up dust, pollen, and other allergens, increasing their concentration in the air. This can exacerbate respiratory conditions like asthma, bronchitis, and other chronic obstructive pulmonary diseases. The smoke from wildfires caused or exacerbated by downslope winds can significantly worsen respiratory conditions and overall air quality as well.

In Europe, the downslope winds are associated with a condition known as "Foehn sickness," characterized by symptoms such as headaches, irritability, anxiety, depression, and sleep disturbances. These symptoms are believed to be triggered by the rapid changes in atmospheric pressure and ionization that accompany downslope winds.

Infrastructure

Downslope winds can cause extensive damage to infrastructure, including buildings, power lines, transportation systems, and water supply networks. These strong winds can lead to structural damage, such as broken windows, dislodged roof shingles, and even more severe damage in poorly constructed buildings. The high winds can knock down power lines and poles, leading to widespread power outages. Hazardous driving conditions, especially for high-profile vehicles like trucks and buses, can result from downslope winds. Blowing debris and reduced visibility due to dust can also increase the risk of accidents.

What is the mechanism of Downslope winds?

Downslope winds result from specific atmospheric processes involving air movement, pressure changes, and temperature variations. The development of downslope winds begins when moist air masses are directed toward a mountain range and are forced to ascend. 

Image: Comparison of upslope and downslope winds. (Image courtesy of KOAA NEWS5).

As the air rises, it cools, causing condensation and precipitation. This process releases latent heat, which then warms the air. Much of the moisture is lost on the windward side due to precipitation. The air that reaches the mountain ridge is considerably drier. 

Once this now drier air descends the leeward side of the mountain range, it continues to warm further. The descending air can warm significantly, often resulting in temperatures much higher than those on the windward side at the same elevation. The descending air is now warmer and drier, and oftentimes moves at high speeds resulting in strong winds.

Image: Example of downslope winds in Salt Lake City, Utah. (Image courtesy of the NWS).

Downslope winds are a significant meteorological phenomenon that can dramatically influence local climates and environments. They are driven by complex interactions between atmospheric pressure, temperature, moisture, and topography. Downslope winds occur in various regions around the world with a wide range of significant impacts.

Luke Stone
Forecaster, OpenSnow