Tuesday, September 16, 2014
   
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Climate and Weather

Bow River Basin

The Canadian Prairies generally have a continental climate, characterized by warm or hot summers, cold winters, and usually low precipitation. Air and the associated weather patterns commonly travel from west to east across southern Canada. The westerly winds must pass over the Rocky Mountains, forcing the air to drop a large amount of precipitation (orographic precipitation). The lack of large water bodies across the prairies causes temperatures to vary greatly between summer and winter.

Temperature

Two main air masses typically affect Alberta - a continental polar air mass coming from the interior of Canada and a maritime polar air mass coming from the north Pacific. In the summer, a maritime tropical air mass occasionally enters Alberta from the Gulf of Mexico or the Pacific. This sometimes results in frontal precipitation due to the added moisture. Air from the Arctic is dominant in the winter.

The Palliser Triangle area experiences the greatest temperature ranges in Canada. The record high temperature for Medicine Hat was 42°C, recorded on July 12, 1886. The record low temperature was -46°C, recorded on January 22, 1886.

Southern Alberta often experiences a Chinook wind – a warm, dry wind that descends from the Rockies as it travels from west to east. This air has dropped much of its moisture after travelling over several mountain ranges in British Columbia. The Chinook can warm temperatures by more than 20˚C in several hours. The farther from the Rockies the land is located, the smaller effect the Chinook has on temperature.

Air temperatures above 5°C are considered “growing temperature”. One measure for temperature over a period of time is growing degree-days (GDDs); this statistic can be useful for agricultural purposes. Growing degree-days are the number of days with temperatures higher than 5°C multiplied by the number of degrees above 5°C that occur over a period of time. For example, if the temperature stays at 7°C for three days, this counts as six GDDs. Relative to 5°C, most locations around Medicine Hat have values of about 1,500 to 1,900 GDDs annually. In some parts of the Cypress Hills the average value is as low as 1,000 GDDs annually.

Degree-days are also sometimes calculated for temperatures above 0°C. The number of frost-free days in a year also determines how agriculture fares because crops can be damaged by frost. In a year, southeast Alberta can have more than 125 days above 0°C – more than other parts of Alberta but less than many of the maritime regions of Canada. Most of the South Saskatchewan River Basin experiences more than 115 days annually above 0°C.

The Cypress Hills region is generally a few degrees cooler than the surrounding land, and tends to receive more clouds and thunderstorms.

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about mean annual temperature conditions in the region.

Precipitation

The SSRB is located within the Palliser Triangle, a large area of the Prairies characterized by dry conditions. The Triangle is the Canadian portion of an area with fairly uniform climate. The west boundary and the north/east boundary of the Triangle are defined based on climate. The west side runs along the edge of the Rocky Mountains. The north/east side runs along the edge of parkland fringe in Alberta, Saskatchewan and Manitoba, toward the southeast. A similar climate extends south into the USA, although the Palliser Triangle only includes Canadian land.

Most of the Canadian Prairies receive little precipitation. The land loses more water through evaporation (from lakes, ground, plants, etc.) than it receives in rain or snow; this is known as a moisture deficit. Precipitation is especially low in southeast Alberta. Among Canadian cities, Medicine Hat has the most days per year without measurable precipitation (271 on average). Southeast Alberta, including much of the SEAWA area, has low precipitation levels. On average, Medicine Hat receives 322 millimeters of precipitation annually.

By most definitions, Medicine Hat’s climate is dry, but not that of a true desert. According to the Meigs classification system, places with 250 to 500 millimeters of precipitation annually are semi-arid lands. In contrast, Toronto receives about 800 millimeters of precipitation while Phoenix, Arizona only receives about 190 millimeters.

Across the region, precipitation is even lower in the winter than the summer; snow only makes up about 30% of annual precipitation.

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about precipitation and annual potential evapotranspiration conditions in the region.

Drought and Floods

Floods occur occasionally in southern Alberta: recent examples of this occurred at Medicine Hat in 1995, 2005 and 2010. In the June 2005 flood, several other communities in southern Alberta were affected. In fact, the flooding did not occur immediately after the heavy rains (in early June), but happened later that month, when saturated soil and increased water runoff led to significantly greater river flows.  More recently, during the June 2010 flood, high levels of rainfall in the Cypress Hills led to flood conditions throughout the watershed including major flooding in the town of Irvine and the closure of the Transcanada Highway for several days.

A significant amount of the summer rainfall is convective precipitation. Convective storms can drop a large amount of rain in a short time. In Medicine Hat, the record rainfall for one day occurred on August 14, 1927, when 122 mm of rain was dropped. Convection is the transport of heat in the air through vertical motion. First the Sun heats the ground, which heats the air immediately above it. The resulting movement of air upward causes clouds to form. Convection is associated with storms, which can produce short, intense rainfalls. Dry soil has less moisture to hold earth together, so a sudden heavy rainfall can cause significant erosion. Dry soil also increases the risk of wind erosion, and wind erosion risk tends to be higher in southeast Alberta. A lack of vegetation also contributes to erosion from water and wind.

An ecological region, or ecoregion, is an area that has fairly uniform climate, vegetation, soil, topography, animals, water patterns, and land use throughout. Most of the South Saskatchewan River Sub-Basin is in the “Mixed Grassland” ecoregion. In this ecoregion, precipitation averages about 250 to 350 millimeters/year and average temperatures are higher than in other parts of Alberta. The Mixed Grassland ecoregion has Chernozem soils – a dark soil which is high in organic matter. The Cypress Hills region is called either the “Cypress Upland” ecoregion or a “Fescue Grassland” ecoregion similar to the Foothills area of Alberta. Fescue prairie is wetter than mixed prairie and has a greater abundance and variety of wildlife.

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about Climate Change and its Effects.

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