December 2001 Agdex 070-1

Weather Forecasting and TerminologyThe Atmospheric Environment Service of

Environment Canada is responsible for generalweather services in Canada. The Alberta Weather Centre,one of eight regional centres, provides services for Albertaand northeastern British Columbia from its office inEdmonton.

The Alberta Weather Centre operates 24 hours a day, yearround. Its primary responsibility is to issue weatherwarnings and advisories for severe or hazardous weatherconditions, which could cause serious inconveniences orendanger life or property. In addition, forecasterscontinually issue weather information to serve the generalpublic, the media, and industries such as agriculture.

Figure 1. Satellite photo showing band of typicalsummer thunderstorms over central Alberta,causing isolated showers.

Further information, consultations, and localinterpretations can be obtained from weather offices inLethbridge, Calgary, Edmonton, and Grande Prairie.

Forecast productionThe latest in modern technology - satellites, conventionaland doppler radars, computers, communications systems,lightning detectors, and other remote sensing devices - areused in conjunction with surface observations to gatherinformation about the atmosphere. The information issorted and analysed at major computing centres inToronto and Montreal. While technology is used toidentify and forecast medium and large scale weathersystems, knowledge of how local topography influencesthe weather is necessary to determine how weather mightaffect a particular area.

Forecast limitations andinterpretationTopography and water bodies will cause the weather todiffer over relatively short distances. Forecasters mustgeneralize to keep the forecasts a reasonable length, butusers can often modify the forecast to account for theirown geographic situation.

A typical summer forecast may be read as follows:

“Today..a few clouds. Scattered late afternoon showers orthunderstorms. Probability of precipitation is percent 40 today.”

This forecast can be understood to mean that for most ofthe day there will be more blue sky than cloud, althoughcloud may cover half of the sky for short periods. Someshowers or thunderstorms can be expected in the region,but they should not be widespread. There is a 40 per centchance of having showers at any point in the region.

Most of the area in the forecast will get no measurablerainfall. Some areas will get brief periods of rain ranging

from a few sprinkles to a few millimetres, anda few small areas will receive 10 to 20

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millimetres if a thunderstorm passes directly overhead.With these small scale, short-lived weather systems, it isimpossible to forecast exactly the location or the timing ofindividual showers. Any forecast of precipitation amountsunder these conditions attempts only to give a range ofpossible amounts over a given area. Even over one farm, itis possible rainfall could vary from a few millimetres overone section of land to none over another.

Forecasts are issued at 5 and 11 a.m. and at 4 and 9:30p.m. In addition, forecasts are updated whenever asignificant change in the weather occurs or is forecast tooccur. A weather outlook for three, four and five daysahead is issued early every morning and late everyafternoon.

Forecast contentForecasts regularly include: sky condition, temperatures,wind direction and speed, and probabilities ofprecipitation. During the growing and harvest seasons thefollowing information of interest to agricultural users isadded: maximum and minimum relative humidities,drying indices, and more detailed forecasts of winddirection and speed as well as expected rainfall amounts.Forecasts include wind chill values above 1600 in thewinter.

Forecast disseminationThe Atmospheric Environment Service depends onnewspapers, radio and television stations to distributeforecast information. Each media outlet decides when andhow much of the information will be used.

Individuals or groups living within 60 kilometres ofEdmonton, Calgary, Red Deer, Lethbridge, or GrandePrairie can obtain weather forecasts directly from theAtmospheric Environment Service. Weatheradio Canada,owned by Environment Canada, broadcasts 24 hours aday on the VHF-FM band at 162.400, 162.475 or162.550 MHz from these sites. These signals cannot bereceived on regular AM-FM radios. Weatheradio receiverscan be purchased at many radio shops, some with anautomatic alert feature that is activated when a weatherwarning is issued.

Weather warningsThe weather warning program is part of the publicweather service. Warnings are issued whenever current orexpected weather conditions might endanger lives orproperty. The bulletins will be issued, when possible, sixhours in advance of the forecast weather conditions butnormally no more than twelve hours before. The bulletins

will give the regions affected, the time of occurrence, theduration and severity.

Weather warnings are issued for the following conditions:

Blizzard – Visibility less than one km due to snow orblowing snow; a combination of wind and temperature toresult in wind chills 1600 or higher; conditions lastingfour hours or more.

Dust Storm – Visibility less than one km due to dust;wind speed greater than 40 km/h; and these conditionswill last six hours or more.

Freezing Rain – Freezing rain for one hour or more.

Frost – Temperature expected to fall below zero duringthe months of June, July and August and major damage isexpected. Not issued once the temperature of minus threeor colder has beenrecorded.

Cloudiness and contour of the land have a significanteffect on the incidence of frost. After sunset on a clearnight, the ground cools rapidly and its temperature soonfalls below that of the surface layer of air. The air incontact with the ground is cooled and becomes heavier.Over sloping ground the heavier cooled air tends to drainto lower levels and collect in hollows. This is why frost ismost likely to occur in low-lying area.

Heavy Rain – Rain accumulations of 50 mm or morewithin 24 hours.

Heavy Snow – Snow accumulations of 10 cm or morewithin 24 hours.

Wind – Wind speed 60 km/h and/or gusts to 100 km/h ormore; in Lethbridge region, wind speed 70 km/h and/orgusts to 120 km/h.

Wind Chill – Wind chill values of 2200 or greater whenno blizzard warning is in effect.

Weather AdvisoriesWeather advisories are issued when the actual or expectedweather conditions are not hazardous but may causeconcern or inconvenience.

Examples of problems that an advisory might address arefreezing drizzle, heavy drifting or blowing snow, orextensive thick fog.

Summer Severe WeatherSevere summer weather is dealt with in a special manner.Severe thunderstorms can produce any of the followingconditions:

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• a tornado• wind gusts to 90 km/h or more• hail - walnut size or larger• rainfall of 30 mm per hour or more

Regular analyses allow forecasters to determine a generalarea where potentially hazardous weather is likely and aSevere Thunderstorm Watch is issued to alert the publicof this possibility. When a severe storm is imminent orconfirmed in progress, a Severe Thunderstorm Warningis issued, and precautions should be taken. The warningareas are identified by county names. A Tornado Watch isissued when there is potential for significant tornados. ATornado Warning indicates that a tornado or funnelcloud is occurring within the warning area.

Forecast terminology anddefinitionsPrecipitationTerms such as rain, snow, showers, or flurries are used toindicate the occurrence and steadiness of the precipitation.Qualifying words and numbers provide details about theintensity, amount and frequency of the precipitation.

The verbal description applies to precipitation expectedover the region as a whole. The probability ofprecipitation represents the likelihood of measurableprecipitation occurring at any point in the region. Thus, aprobability of 30 per cent means that out of 100 similarsituations, precipitation should occur 30 times.

Rain, snow, periods of rain, or intermittent rain or snowwill normally appear with probabilities of 90 or 100 percent and indicate that a major weather system will affectthe region. The amount of precipitation may vary.

Freezing rain occurs when rain forms in warm air of theupper atmosphere and falls into air at temperatures belowfreezing near the ground.

The rain freezes on contact with cold surfaces and resultsin a coating of clear ice on exposed surfaces.

Showers or flurries affect areas of 10 to 50 km in diameterfor only a brief period (usually less than an hour andsometimes as short as a few minutes). Frequently, the skybrightens or the sun breaks out between the showers orflurries.

The term showers, flurries or occasional rain (or snow)implies that the precipitation will not be continuous andany point in the region is likely to get a measurableamount. These terms are normally combined withprobabilities in the 60 to 80 per cent range.

The word “scattered” is used to qualify the terms showersand flurries when only a portion of the region is expectedto get measurable precipitation. The probabilitiesassociated with “scattered showers” are in the 30 to 50 percent range.

When no measurable precipitation is expected, theprobability of precipitation will be 20 per cent or less, andno mention of precipitation will be made in a forecast. Ifthe probability of no precipitation is between 60 and 80per cent, precipitation amounts are occasionally includedin the forecast. If the probability is greater than 80 percent, precipitation amounts will be included.

Thunderstorms are essentially overgrown showers thatproduce thunder and lightning. When isolatedthunderstorms are forecast, a probability of precipitationof 10 or 20 per cent will be used. Only a small part of theregion is likely to get rain but those that do, get intenseheavy rain for short periods. Thunderstorms may occurduring a continuous rain. Hail, strong winds, and eventornadoes can result from severe thunderstorms.

Snow that is lifted and moved around by the wind is calleddrifting snow. It may hinder traffic and occasionallyobscure the ground. Blowing snow is snow that is liftedabove eye-level by the wind. Visibility is lowered byblowing snow and can be reduced to zero.

Sky ConditionSky condition describes the average fraction of the skycovered by cloud.

Clear or sunny means that less than 20 per cent of the skyhas cloud.

Mainly sunny or a few clouds means that less than half thesky is covered by cloud on average, but there may be anhour or two when clouds cover more than half the sky.

Mainly cloudy or cloudy implies that more than half of thesky will be covered on average. Again, breaks of a coupleof hours or less may occur.

Overcast means the sky is completely clouded over, givingdull grey conditions.

Afternoon cloudiness implies that skies will be sunny ormainly sunny (clear or a few clouds) with the exception ofthe afternoon.

Clearing means that the cloud cover will change frommostly cloudy to mostly sunny (a few clouds) but notnecessarily to completely clear.

Clouding over or increasing cloudiness refers to anexpected increase in cloud cover from mostly sunnyconditions (a few clouds) to mostly cloudy conditions. Anovercast sky may or may not result.

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A forecast of precipitation implies a mainly cloudy sky, sothe sky condition may not be mentioned in the forecast.

TemperaturesThe afternoon maximum and the early morningminimum temperatures are usually specified. Iftemperatures are expected to rise or remain steady throughthe night or are expected to fall during the day, theabnormal trend will be identified. Early morning and lateafternoon reference temperatures are usually given.

WindsWinds of 20 km/h or less are described as light. Winddirection and speeds are given when the forecaster has ahigh confidence level. For wind speeds of 25 km/h orhigher, the specific expected wind direction and speed(including gusts) are always given.

Drying IndexThe drying index is a measure of the drying ability of theair and takes into account wind speed, sunshine,temperature, humidity, and precipitation. Descriptiveterms are used:

Poor – little drying with overcast skies, cool temperatures,high humidity, and possible rain.

Fair – some drying with mostly cloudy skies, moderatetemperatures, and light to moderate winds.

High – significant drying with mostly sunny skies, warmtemperatures, and moderate winds.

Very high – rapid drying under sunny, windy and hotconditions.

Wind ChillWind chill is a cooling rate not a temperature. Significantvalues range from 1200 to 2500 watts per square metre(W/m2). At 1600 W/m2, frost bite to exposed flesh ispossible. At 2000 W/m2, frostbite is possible in 1 to 3minutes. At 3000 W/m2, frostbite is possible within 30seconds.

Growing degree daysGrowing degree days is an energy term obtained bysubtracting the value five from the average dailytemperature. If the average temperature is below 5°C thegrowing degree day value for that day is zero.

The threshold value of 5°C is derived from the growthhabits of cereal and forage crops, which show little growthor development when average temperatures are below5°C. Daily growing degree day values are added togetherfrom the beginning of the season, providing an indicationof the energy available for plant growth. Growing degreeday totals are used for comparing the progress of agrowing season to the long-term average and are usefulfor estimating crop development stages and maturitydates.

Corn heat unitsCorn heat units is an energy term calculated for each dayand accumulated from planting to the harvest date.

Corn heat units are calculated daily using the dailymaximum and minimum temperature and constants asfollows.

CHU= [1.8 X (Tmin-4.4) + 3.33(Tmax-10.0) -0.084(Tmax-10.0)2] /2 where the values for Tmin andTmax are the minimum and maximum temperatures forthe day.

Environment Canada’s co-operation in producing thispublication is gratefully acknowledged.

For more information contact:Shane ChetnerAgricultural Weather Resource Specialist,Conservation and Development Branch,Alberta Agriculture, Food and Rural Development,Edmonton, Alberta.

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