Background knowledge in meteorology is crucial to understand how weather affects the mountain snowpack and avalanche formation. This chapter introduces key concepts used in weather forecasting.
Atmospheric Composition. The atmosphere is a gaseous layer that surrounds the Earth’s surface. It protects us from the sun, and it is where energy exchange and weather occurs.
The atmosphere contains 78% Nitrogen, 21% Oxygen, 1% other, and 0.04% CO2. Carbon Dioxide is important because it is an effective energy absorber, thus influencing the heating of the atmosphere.
Other components that are important in weather and climate include water vapour, dust particles, and ozone. Water vapour is the source for cloud formation and precipitation. It exists in all three states, and when it changes states it absorbs or releases heat. This heat transfer is an important energy sources in driving weather. Dust particles are important because they can act as a surface on which water vapour will condense, and they will absorb or reflect incoming solar radiation. The ozone is important because it protects the Earth’s surface from UV radiation.
Structure of the Atmosphere. The majority of atmospheric gasses are contained in the lower 30 kilometres of the atmosphere. The atmosphere is divided into four different layers based on temperature. They are the:
Between the four different layers are the:
The Troposphere is where weather occurs. This is where we focus our attention.
Troposphere. The troposphere is the bottom layer of the atmosphere, thicker around the tropics, thinner around the poles. Within the troposphere, air temperature decreases with altitude. This is referred to as the lapse rate. The normal lapse rate is 6.5oC per 1000m, however it can vary from 4.5oC to 9oC depending on the moisture content of the atmosphere. It is possible for shallow layers to form where temperatures increase with altitude, and this is called a temperature inversion. Temperature inversions can have a significant impact on the snowpack and avalanche conditions.
Temperature is the measure of how warm an object is. Heat is the transfer of energy from one object to another because of the temperature gradient. Heat flows from higher to lower temperatures, and stops at equilibrium. Heat may occur in three ways:
These three mechanisms work together to create weather and atmospheric circulations.
Solar Radiation. Solar radiated energy is either absorbed, transmitted, or reflected. ‘Albedo’ refers to the percentage amount that is reflected, as opposed to that which is absorbed. Clouds and snow have a high albedo, whereas earth, trees, rocks, and old snow have a low albedo. The angle of the sun (time of day, latitude, slope angle, and slope aspect), age and density of snow, and cloud coverage all affect how much radiation is absorbed or reflected. Radiation heating or cooling is a significant factor in snow stability, as it is often responsible for surface hoar, crust formation, and rapid changes in stability through freeze/thawing near the surface.
Temperature. Temperatures vary over the Earth’s surface primarily due to latitude. This is due to the angle of the sun and the length of day light. Other factors influencing temperature include altitude (6.5oC per 1km), ocean currents (transporting warm water from the tropics, or cold water from the poles), and differential heating of the land and water (water changes temperature slower than land).
Used in weather maps, an isotherm is a line that connects two points on the map with the same temperature. The amount of temperature change over a given distance is the temperature gradient, indicated by the spacing of isotherms on the map. Closer isotherms equal higher temperature gradients.
Humidity is described as the amount of water vapour content (WVC) in the air. There are several ways to express humidity, and this course focuses on two:
Relative Humidity. Relative humidity is the ratio of WVC to WVC at dew/frost point. The relative humidity of a parcel of air increases as air temperature decreases. During the day, air is warmer and relative humidity is lower, while at night air cools and relative humidity increases.
Dew Point. Dew point is the temperature the air needs to be cooled to in order to become saturated. Cooling below the dew point causes water vapour to condense and form clouds. If there is a large difference between air temperature and dew point, the air is dry. The difference between air temperature and dew point is the dew point spread.
Surface Hoar Formation. If saturation occurs at temperatures of 0oC or below, frost will form. This is the frost point. Water changes from a gas to solid without entering liquid state. Frost is referred to as surface hoar when it forms on the snowpack on clear, calm, and humid nights. Surface hoar requires the following conditions:
Once buried, surface hoar can become a significant persistent weak layer within the snowpack. It is often very thin and may have a complex distribution pattern. Surface hoar has a low shear strength, but it is resistant to settlement and strength-gain by loading.
Air pressure is defined as the pressure exerted by the weight of air above it. It is closely tied to temperature, moisture, and wind, and the average air pressure at sea level is 1013.25hPA (or 101.325kPa).
Vertical Pressure Changes. Air pressure decreases non-linearly with altitude, at a rate fastest at sea-level. Air pressure is roughly around 50% at 5.5km above sea level, and around 25% at 10km above sea level.
Horizontal Pressure Changes. Horizontal pressure differences from place to place can be small, however these differences can be enough to cause significant weather. Local pressure variations arise from differences in temperature, water vapour, and air flow.
Air that converges horizontally (aloft) in an area causes a downward air flow and increase pressure at the surface. As air descends, there is divergence (air flowing out) at the surface. Surface air pressure will fall as divergence outpaces convergence. The opposite is true when convergence outpaces divergence. The amount of pressure change over a certain distance is the pressure gradient.
Wind. Horizontal differences in temperature result in horizontal differences in pressure. These changes in pressure cause the wind to blow as the atmosphere attempts to equalise pressure difference by moving air from an area of high pressure to an area of low pressure. The larger the pressure difference, the stronger the wind.
The Coriolis Force: The Earth’s rotation and surface friction prevent direct air-flow from lows to highs. All free-moving objects (like wind) are deflected to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. Also, because of the Earth’s off-axis, it will deflect winds moving east-west. The amount of deflection increases towards greater latitudes, and there is no deflection along the equator. In the Northern Hemisphere, air in Highs spiral outward and downward (divergence and subsidence) in a clockwise direction, whereas Lows spiral inward and are forced to rise (convergence and lifting. The spiralling circulation around a Low is cyclonic, whereas a High is referred to as anti-cyclonic.
Convergence and divergence is stronger nearer the earth’s surface, and winds flow at an angle to the isobars due to the friction imparted by the surface. In addition, air flow is affected by terrain features and local heating/cooling. As a result, surface winds bear little resemblance to winds at higher elevations.
Areas of equal pressure are connected by isobars. The spacing of isobars indicate pressure gradient. Close isobars represent a steep pressure gradient and therefore strong winds. Wide isobars represent a weak pressure gradient and therefore light winds.
When a section of troposphere hundreds of kilometres across has a fairly uniform temperature and moisture, the air takes on the characteristics of this surface and is referred to as an air mass. Air masses are created at source regions – either polar, desert, or ocean. Winter weather in Western Canada is generally determined by interactions of the following air masses:
Maritime Polar (mP): Cool, damp air.
Maritime Tropical (mT): Warm, moist air.
Continental Polar (cP): Cold, dry air.
Continental Arctic (cA): Very cold, dry air.
Air masses modify the air over the area they are traversing, and also the surface they are moving over. cP and cA typically modify once they reach Southern BC. Periodic outbreaks of arctic air to the South and West occur across southern Alberta and BC. These bring clear skies, very cold temperatures, and northerly winds. Commonly strong outflow winds also flow out of coastal inlets and valleys that run from the continent to the coast.
Fronts. When air masses move out of their source regions, they displace other air masses. The transition zone between two different air masses is the front. Moisture content, temperature, pressure, and wind change rapidly in the front. A cold front is a cold air mass (High) advancing on a warm air mass (Low). A warm front is a warm air mass (Low) advancing on a cold air mass (High). A quasi-stationary front is when neither air mass is advancing.
Frontal waves form at the Earth’s surface at a point along a stationary front. When wind flows in an opposite direction along the front, a bend forms where cold air displaces warm air, forcing the warm air to lift. Under the right conditions, this displacement develops into a wave-like kink that moves along the frontal boundary.
TROWALS. In a developing wave, the cold front of a trailing section of the wave moves faster than the leading warm front. As the heavier, cold air catches up, it pushes under the lighter, warm air. This lifts the warmer air aloft. Once the warmer air is completely off the ground, it is referred to as an occluded front. This air mass of lifted warm air is referred to as a TROWAL when it separated from its air mass and out of contact from the ground (Trough of warm air aloft).
Wave Cyclones. Unstable waves form over the ocean when a trailing upper low promotes convergence and instability. Strong, moisture-laden and unstable, wave cyclones can create powerful storms and are the major snow-producing weather systems in the mountains of western Canada.
Air masses cool and expand as they lift. As they cool, their ability to carry moisture decreases. Expansion occurs due to the decrease in pressure with elevation. There are four ways in which lift becomes significant enough to produce weather:
Orographic Lift – 50-70% of Mountain Precip in Winter. Mountains act as barriers to the flow of air, where the air mass slows and is forced to ascend. The lifting results in adiabatic cooling, which is temperature change due to decreasing pressure. Relative humidity increases eventually to cloud formation and precipitation. Slowing often stalls weather systems on the windward side, producing longer lasting precipitation. Steeper slopes and more perpendicular winds will result in greater precip amounts.
The leeward size of a mountain is often drier than the windward size. Much of the air’s moisture has been removed through precip on the windward side, and as the air descends, it is warmed adiabatically. Relative humidity is reduced, making precip less likely. The leeward descending is referred to as ‘lee subsidence’.
Frontal Lift. At a cold front, heavier cold air advances and pushes under ad lifts the warmer and lighter air. Cold fronts are fast-moving and have steep slopes, causing strong lift and resulting in cloud formation and precip. Precip is usually intense and short. At a warm front, lighter warm air advances and is lifted up as it rides over heavier, cold air. Warm fronts are slow-moving and have shallow slopes, but the lifting occurs over a much larger area. If sufficiently moist clouds form, the precip is less intense but longer lasting. A TROWAL can result in clouds and heavy precip if sufficient moisture exists. Fronts are at their strongest when a TROWAL is forming. When the TROWAL is fully formed, the front is already dissipating.
Orographic Convergence. If air converges at or near the surface, some of it is lifted, such as a lower pressure centre, or when the horizontal wind flow is forced together by terrain. Air from a high-pressure centre does the opposite, diverging and subsiding, becoming warmer and drier.
Convection. Air heated near the Earth’s surface is forced to rise because of its low density relative to surrounding air. Convection cells are very local, so precip is short-lasting, but may be very intense. However, convective cells following cold marine air over warm ocean water can contribute to significant precip in coastal mountains.
Stability is the tendency of a parcel of air to rise or fall of its own accord after a very small vertical displacement.
Unstable. Warm air surrounded by cold air has a buoyancy and tends to rise. Unstable air produces convective type clouds, with showery or intense precip of short duration. The passage of a cold front, which produces cold air aloft, can lead to unstable air and associated weather. Warm parcels of air near the surface become unstable as they rise, resulting in convective cloud build-up.
Stable. Colder air surrounded by warmer air is denser and tends to sink back to the level it was lifted from. Stable air or neutral air produce layers of clouds with widespread and steady precip. If forced lift continues, this saturated air can become unstable.
Air cools and expands at a rate dependent on its moisture content (moist, saturated air cools slower). If it cools faster than surrounding air, it will tend to sink as lift stops. If it cools slower, it will continue to rise.
Neutral Stability. If a lifted parcel of air cools at the same rate as surrounding air, it will remain in place as lift stops.