Chapter 7: Climate

Weather: The state of atmosphere at a given time and place.

Climate: The typical weather of a region, commonly defined by averaging weather records over a 30-year period. Climate data gives insight to the range and extremes of expected weather.

Climate Classification

Areas are divided into climate classes based on mean monthly and annual values of precipitation and temperature. The Koppen Classification System is what’s used today to describe these classes. The main factors influencing climate of an area include the following.

Latitude. The most significant variable to climate is the amount of solar radiation it receives. Polar regions receive the least sun, and are extremely cold. Regions near the equator are hot.

Altitude. Altitude influences air temperature, precipitation, atmospheric moisture, winds, and solar radiation.

Ocean Currents. Poleward moving ocean currents will cause air temperatures to be warmer than expected. The opposite is true for areas influenced by ocean currents moving toward the equator.

Bodies of Water. Variations in air temperature are much greater over land than water. This distance from large bodies of water create three classes:

• Maritime: Close to major bodies of water, mild and moist.
• Continental: Inland, more extreme, and with a much wider range of conditions.
• Transitional: Somewhere in-between Maritime and Continental, exhibiting properties of both.

Prevailing Winds. The maritime influence will be much greater on the windward side of a continent than the leeward. Prevailing winds will carry the maritime air mass much farther inland on the windward side, whereas on the leeward side – winds blown land to ocean – will have more continental characteristics.

Mountain Ranges. Most ranges in North America run North to South. The mountains act as a barrier to prevailing winds from the West, causing orographic precipitation on the windward side, and a rain shadow on the leeward side. This results in rapid transition from maritime to continental climate.

Snow Avalanche Climates

The character of snow avalanches differs between maritime and continental climates, and is therefore useful to classify the snow climate of a mountain range.

Maritime Snow Climates.

• Heavy precipitation and mild temperatures
• Snowpacks tend to be deep (15m-25m annually)
• Rain may fall at high elevations at any time during winter. May be affected by cold arctic air several times per winter
• Persistent weak layers are not as common due to warm temperatures and deep snowpack, but ice layers and crusts are common

Continental Snow Climate.

• Low precipitation and cold temperatures
• Low snow density
• Snowpacks tend to be shallow (8m annually)
• Persistent weak layers, including a weak foundation of depth hoar are common due to the shallow snowpack and lower temperatures.

Transitional Snow Climate.

• Displays characteristics of both maritime and continental snow climates.
• Snow packs can be deep, but exhibit the highest prevalence of persistent weak layers, particularly surface hoar.

Application. Understanding snow climates is useful in discussing snow and avalanche character in general terms. Avalanches are caused by the sequence of weather event, and not the average weather of an area, so it does not have a role in describing or forecasting immediate conditions. Having background knowledge however of the snow climate can be very helpful in determining which observations to focus on.

Climate Change. Climate change may result in a higher frequency of storms, and greater variability. This will have notable effect on avalanche hazards and forecasting.