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Forecasts and Observations Queries FAQs

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Understanding The Weather Network’s Local Forecast
There are many parts to a Weather Network forecast; understanding the parts can help in understanding the whole forecast.

The first distinction to be made with a forecast is that it usually contains two parts: observations and a look ahead.

The observations are a report of current weather conditions at a particular time and place. Most observations are recorded at airports across the country by meteorological technicians or weather observers. The observations are taken at the top of each hour in Canada and around the world. Temperature, precipitation, opacity (sky coverage), humidity, pressure, dewpoint, visibility and ceiling are recorded.

A forecast is a look ahead at the expected conditions. The Weather Network’s Local Forecast contains a short and long term forecast and a 14 day outlook.

A short term forecast is a look at the next 24 hours broken into 6 hour time blocks. The long term forecast is a look at the next 5-7 days. The 14 day outlook is a trend of the expected maximum temperatures over a 14 day period and is compared to the average normal daily maximum. There are two information lines on the 14-day outlook. The yellow line represents the output of a numerical model (numerical models are just one of the tools used by meteorologists to create their forecasts) and indicates the forecasted high temperatures. The white line indicates the average daily maximum for the period. It is not unusual for a long term forecast to change daily.

During the Local Forecast every ten minutes on the ten’s, The Weather Network provides both a high temperature and minimum temperature. The high and minimum temperatures are the extremes for the time period. Usually the high temperature for a particular day will be achieved during the daylight hours because of the effect of daytime heating, however, occasionally the high temperature may come during the overnight hours as a warm front moves through the region. A temperature forecast is considered to be correct within +/- 2 degrees Celsius.

Probability of precipitation (P.O.P) is one of the most misunderstood elements of any forecast. It does not predict when, where or how much precipitation will occur and encompasses space, time and confidence. It is a statistical analysis of the chances that any random place in a forecast area, such as your home, could receive measurable precipitation. Measurable precipitation means at least 0.2 mm of rain or the water equivalent of snow. So what does it mean if the forecast calls for P.O.P of 30%? It means that there is a 3 in 10 chance that there will be measurable precipitation in the forecast area. With a POP of 100% the forecaster has a very high confidence that rain will fall over most or all of the forecast area. These high P.O.P. values are associated with very organized weather systems covering large areas of the country. Forecast areas are large. If you don’t see rain where you are located, it likely means that the system has changed track somewhat but it may be raining elsewhere in the forecast area.

A wind forecast includes direction and speed. A wind forecast is based on a number of conditions. The strength and direction of the wind is based on the track the pressure system takes, the distance between the isobars (lines of equal pressure) and the proximity of high and low pressure systems. If the distance between the isobars or the proximity of the low and high pressure systems is very close then the winds will be stronger than if they were further apart. Although some local effects are taken into consideration such as topography, elevation, lakes and valleys when generating a wind forecast, winds can be affected by urban structures such as buildings. Urban structures are not considered when formulating a wind forecast for a particular city. The forecasted wind conditions are an average for the area.

The forecast also includes an icon and a brief description of the expected conditions. Click here for a list of icons and their description.

The humidex is a factor during the summer months for many areas of Canada. Humans begin to feel the effects of the humidex when it reaches 29 degrees Celsius.

The wind chill factor during the winter months is very important for many people as it represents the cooling effect of our bodies. The wind chill is a calculation which begins to appear on The Weather Network when the wind chill factor reaches 0 degrees Celsius.

The normal maximum or minimum temperature is an average daily high or low temperature for the time period and is based on 30 years of data. When the daily normal or mean temperature is referred to, it is the average of the daily maximum and minimum temperatures for a particular day, month or year based on a thirty-year database.

Where can I find historical weather records and climate statistics?
Whether you're looking for historical weather records and climate statistics for a school project, business pursuits or just for curiosity, theweathernetwork.com can help you. To find historical weather records it's as easy as going to your local forecast page (ex: Toronto). In the Current Conditions section on the page you will see a Historical Data search box, which will provide you with historical weather data since 2001.

Click here for climate statistics where you will find data for nearly 250 cities across Canada and for hundreds more cities around the world.

The Weather Network's Farmzone web site also provides information on historical weather records, including maximum, minimum and mean temperatures, for many locations in Canada.

Partly cloudy and partly sunny?
Like so many viewers, Patricia Thorpe of Elliot Lake Ontario wants to know the difference between partly sunny and partly cloudy. These two terms are among the most poorly understood of all weather terms. Forecasters have been hassling over the definition of "partly" for years; that's why it is not widely used. In forecasting sky conditions, meteorologists describe what percent of the sky, from horizon to horizon, rounded to the nearest 10, is expected to be covered by clouds. A sunny day is one when there are no clouds or only 10% of the sky is expected to have cloud. With 20% or 30% cloud, the forecast calls for mostly or mainly sunny, or sunny with cloudy periods. When more cloud than sun is forecast, say 70 to 80% cloud, it's mostly or mainly cloudy or cloudy with clear periods. When 90% or more of the full sky is covered by cloud then it's a cloudy or overcast sky. When it's about half cloud and half blue sky, the wording is mix of sun and clouds or variable cloudiness. The Weather Network doesn't use "partly" in its forecast, only to describe current weather conditions; in this case, partly means mainly or mostly.

How exactly do you determine the P.O.P.? What's the error on it usually as a percentage?
The exact definition of the probability of precipitation (P.O.P.) is the chance of receiving 0.2mm of precipitation (snow or rain) over a specific spot. P.O.P. forecasts apply to precipitation that will fall over a rain gauge, usually located at airports. It does not cover a large city or region and are for specific times, say 7am.

The definition is very site specific and time dependent. A 40% P.O.P. for London means that there is a 4 out of 10 chance that the rain gauge in Windsor will receive 0.2mm of rain in that forecast period. As for the error factor for each P.O.P. value, it is rally hard to pin down. P.O.P. are expressions of degrees of confidence so, in general the higher the forecasts P.O.P. value, the better the track record of being of correct.

How often the weather observations are taken?
Environment Canada holds agreements with the World Meteorological Organization (the worlds standardizing body for meteorology) to take observations once an hour in fair or quiet weather. When more severe weather, such as strong gusts, hail, or freezing rain comes in, the observations are taken as often as necessary to alert the public of sudden, dramatic weather changes.

In the U.S. all the automatic weather stations report every 15 minutes regardless of the type of weather that is out there. Perhaps, one day automatic Canadian weather stations might also report every 15 minutes!

What is the purpose of having a dew point in the winter? If the temperature reached the dew point and it was still below what happens?
The purpose of having a dew point in the winter is the same as it is in the summer. We want to see how much moisture is in the air even in the winter. If the spread between the dew point and the temperature is small then the air is moist, if the spread is large, the air is dry. The more moisture the air has, the greater the risk of fog, rain or snow. If the dew point is below zero and the air temperature reaches the dew point then the precipitation will be in the form of snow.

Why do we always tap a barometer before reading it?
Most barometers should be tapped slightly with the fingers before reading. Mercury barometers were used almost exclusively until the beginning of this century when aneroid mechanisms evolved. Mercury does not usually wet glass. However, there is always a slight tendency for the mercury to cling to the side of the glass, causing the upper curved surface of the mercury called the meniscus, in reference to its shape, to change its shape when the pressure is rising, steady or falling. Tapping the glass near the top of the column and in the cistern will break down the frictional forces and allow the meniscus to assume its correct shape and height. Even these non-liquid aneroid barometers should be tapped on the dial glass because sometimes the levers, weights and bearings get stuck. The metal membrane or cell, with an inside vacuum, will move with even the slightest change in pressure, but this may not be transferred to the needle or indicator because of dirt and dust or the build up of static friction around the mechanical workings of the barometer. Finger tapping will unstick the barometer and move the dial or indicator accordingly. The only barometers you normally don't have to tap are electrical or electronic ones.

I would like to know the relationship between the various terms used in measuring atmospheric pressure: inches of mercury, kilopascals, millibars and inches. Why can't they be correlated in the reporting of weather conditions? It is very confusing.
All the above relationships are just a different way of measuring a height of mercury column in a Mercury Barometer. The reasons why there are 4 or 5 different measurements is because they are equivalents in different measurement systems: Imperial (inches of mercury, pounds per square foot), metric (millimetres of mercury) and SI-International Scientific Units (millibars, kilopascal).

Yes they all could be correlated.
1012 millibars = 101.2 Kilopascals =29.92 inches of mercury
For most media outlets, say your 15-second weather spot on the radio or TV there just isn't time to list all the conventions that are available.

Can you explain what the dewpoint is and how it is relevant in the forecasts?
The dew point is the temperature at which water vapour condenses into liquid water. If the liquid water condenses onto objects (grass, autos, windows, an airplane wing) at a temperature above freezing, it is called dew. If it condenses onto objects at a temperature at or below freezing, it is called frost.

It is important to know the dew point because it is the temperature at which clouds form. When air rises from the ground, it cools until it reaches the dew point and then a cloud forms in the atmosphere.

How are the meteorologists able to predict the amount of snow or rain that will fall?
The numerical forecast models used by meteorologists contain what is called QPF (quantitative precipitation forecast) this water equivalent can be converted to snowfall by use of certain mathematical formulas.

What is the famous Saxby Gale?
Florence Terriss of Halifax wanted me to tell the story of the Great Saxby Gale.

In 1868, a Lieutenant Saxby of the Royal Navy sent a letter to several British newspapers predicting an intense storm and exceptional tides that he said would occur almost a year later, at 7:00 on the morning of October 5, 1869.

Late in the afternoon of October 4, 1869, scarcely half a day away from Saxby's deadline, a storm of hurricane proportions struck the Bay of Fundy. At Saint John, heavy waves pounded ships against the wharves. Buildings were unroofed or blown down, cattle drown, and almost every community around the Bay of Fundy was flooded.

This amazing prediction has been called a miracle of meteorology. Because the great storm coincided so closely with Saxby's prediction, it has been known ever since as the Saxby Gale. The accuracy of Saxby's forecast, however, owed more to luck than to science. Although the alignment of the sun and moon explains the unusually high tides, it does not account for the storm. You see, Saxby was indefinite as to the place where the storm would occur. He just said somewhere in the world. That it occurred in New Brunswick makes it a Canadian legend.

When did the Farmer's Almanac first appear?
I get dozens of letters from Weather Network viewers from Courtenay, BC to Corner Brook, NF asking me about the Farmer's Almanac, especially how reliable their forecasts are.

This pocket-sized, yellow-covered annual has been around for 204 years. But it was by no means the first.

In fact, the first almanac dates back 5000 years ago to Egypt. Throughout the centuries almanacs have been a source for the movements of planets and stars, the phases of the moon, dates of eclipses, and precise times of sunrise and sunset and high tides for every day of the year. Our own Canadian version of an almanac first appeared in the 1850s.

Of course, almanacs are most noted for their weather predictions, whose accuracy, by the way, is not any more accurate than pure chance. On occasion, of course, the almanac weather prediction has turned out to be right on target. A classic example was the forecast for July 13, 1816. when it accurately predicted snow in New England.

According to almanac lore, the manuscript was about to go to press when someone realized that no prediction was made for July 13. the editor was home ill, so a copy boy was sent to get a weather prediction for the missing date. Put in anything you want, he was told, so the young man inserted, rain, hail and snow. Sure enough, parts of New England had all three on that July 13th. While the editor denied making such a ridiculous forecast for the summer ahead, when it really did snow on that day, he changed his tune and took full credit, saying told you so.

Can contrails forecast the weather? Heather Kennedy of Bradford Ontario wants to know if the appearance and length of contrails tell you something about the coming weather.
Contrails or condensation trails are long white streaks of ice crystals left behind in the exhaust of flying jet aircraft. At 8 to 12 km above the ground the jet engine pulls in very cold, dry air and spews out hot, water-filled exhaust. The hot water vapour mixes with the colder surrounding air. In doing so, it expands and freezes in 1 or 2 seconds forming a trail of ice-crystal clouds.

Contrails do not cause weather, however, watching these contrails can provide a clue to upcoming weather. If a jet leaves no trail or a short trail or if the trail fades quickly, it is an indication that the air is relatively dry and sinking, which suggests that fair weather is likely to continue. On the other hand, if the exhaust trails linger for an hour or more or they spread across the entire sky, the surrounding air is probably moist and rising, foretelling the arrival of a storm in a day or two. Reading contrails gives you a glimpse as to the weather over the next 12 to 24 hours.

It seems like the long-term forecasts in the Farmer's Almanac are usually accurate. How do they make their predictions? What are the predictions for spring and summer in Winnipeg?
The track record of the Almanac is around 60%. Some month's it scores better than others. They have a tightly guarded secret method used in putting their forecasts together. All I know is that it involves a whole lot of factors such as, moon phases, solar cycles, nature's response to the upcoming season, etc. As for the Almanac's predictions for Winnipeg, I did not receive a personal copy for Christmas this year!
How do you show weather warnings on radar, satellite and pressure maps?
Warning and watch boxes can be drawn on any radar, satellite or weather map using a graphical software package which allows the user to select the region or hand draw the region where there is active weather.

Why are long range forecasts less reliable?
Long range forecasts are sometimes less reliable for a couple of reasons. Long range forecast issued by EC or TWN are based on computer generated numerical models. The models are based on a variety of weather elements, high level circulation patterns and moisture as well as precipitation patterns and pressure fields. In addition there is climatology ingested into the model outputs. This simply means the models look at conditions that occurred over the past 30 years and then project those conditions for that specific date into the future. The atmosphere is very dynamic and is in constant motion. The long-range models are issued twice daily, morning and late evening, and can change considerably from the previous day. When this computer generated guidance changes it results in the long-range forecast seeming to be less reliable.
What is a 'meteorological bomb'?
A meteorological bomb is an intense cyclone or low pressure system. The central pressure of the BOMB must fall 24 millibars or more in a 24-hour period. They are very strong with high winds and heavy snow or rain depending on the season.

Is the air pressure shown on the local forecasts a corrected figure or uncorrected? How do you convert one to the other?
The pressure on the local forecasts is corrected to reflect the sea level pressure. It's a little complicated. If you have a barometer at home and you look at it your barometer will actually read at a lower pressure than what you will see on the television screen.

Take Vancouver and Calgary for example. Vancouver is about 3 metres above sea Loveland Calgary is about 1000 metres above sea level. The pressure in Calgary is lower than it would be in Vancouver because the weight of the air above any given place in Calgary is less.
Here's how you convert it.

P (sea level) = P (observed) + {(1kPa/100m) x height above sea level}

For example, if observed pressure is 90kPa. In Calgary its pressure at sea level is as follows:

P (sea level) = 90 + (1000/100)
P (sea level) = 90 + 10 = 100 kPa.

Why would you do that?
We want to convert everyone to sea level so we can track the levels of lows and highs across the country.

How does The Weather Network develop its forecast?
The Weather Network takes pride in being able to provide Canadians with accurate, local forecasts for more than 3,000 cities across Canada as well as American and international destinations. The Weather Network is also proud to be Canada's largest private-sector employer of professional meteorologists.

Our team of meteorologists uses a variety of tools to formulate their forecasts including leading-edge proprietary technology developed by The Weather Network. A typical forecast involves the following steps:

— Information from several numerical weather models are loaded into the Pelmorex Forecast Engine (PFE) to formulate a forecast.
— The Weather Network's PFE allows our meteorologists to issue weather forecasts for the entire country down to an area 10 km by 10 km - making it one of the most advanced computer forecasting systems in the world.
— Once the data is loaded in the PFE, our meteorologists look at weather observations across Canada as well as satellite and radar data.
— This information is analysed to determine whether the weather system is increasing or decreasing in strength and the path the system is expected to follow. The amount of precipitation, cloud cover, winds and temperatures for the different forecast periods are also evaluated.
— Once these steps are completed, our meteorologists are ready to issue The Weather Network forecast!

How can I find The Weather Network's forecasts?
You can access accurate, local forecasts from The Weather Network in a variety of ways to meet your planning and recreational needs including through:

— Our 24-hour television service available on basic cable and satellite services across Canada.
— Our web site: TheWeatherNetwork.com
— WeatherDirect and WeatherEye are customized to meet your weather needs. WeatherDirect delivers local forecasts for cities of your choice directly to your e-mail inbox. WeatherEye is The Weather Network's desktop weather service which conveniently and compactly displays current weather conditions for the city of your choice and the current temperature in Microsoft Windows system tray.
— The Weather Network also supplies weather information to many newspapers across the country.

What timeframe is meant when talking about morning, afternoon, evening and overnight forecasts?
The short-term forecasts created by The Weather Network are based on 6 hour segments. The breakdowns are:

— Morning: 6am - 12 noon
— Afternoon: 12 noon - 6pm
— Evening: 6pm - 12 midnight
— Overnight: 12 midnight - 6am

How are The Weather Network's seasonal forecasts created?
The Weather Network's seasonal forecasts are produced using a combination of:

— ensemble forecasting methods relying on approximately 11 model outputs from around the world
— proprietary methods and technologies
— the skills and weather expertise of The Weather Network's team of professional meteorologists

When are The Weather Network's seasonal forecasts issued?
The Weather Network's seasonal forecasts are issued on December 1 (Winter), March 1 (Spring), June 1 (Summer) and September 1 (Fall). Once issued, all outlooks are available on TheWeatherNetwork.com. Or tune into The Weather Network for further explanation of the outlooks by our weather specialists.

What is the difference between an advisory, watch and warning?
A Weather Watch alerts you that conditions are favourable for the development of severe weather. Watch the skies and listen for updated watches and possibly weather warnings.

A Weather Warning alerts you that severe weather is occurring or that hazardous weather is highly probable. Severe thunderstorm or tornado warnings may be issued less than one hour in advance. Other weather warnings may be issued six to twelve hours in advance.

A Weather Advisory means actual or expected weather conditions may cause general inconvenience or concern, but do not pose a serious enough threat to warrant a weather warning. An advisory may also be used when conditions show signs of becoming favourable for severe weather when the situation is not definite enough or too far in the future to justify a warning.

Source: Environment Canada

Why does the forecast change so often?
Weather is a favourite topic of conversation in Canada often because it changes so frequently! In many parts of Canada, the saying is that if you don't like the weather out your front door then look out your back door!

To reflect Mother's Nature's constantly changing "moods," The Weather Network's forecasts are regularly updated during the day to ensure that Canadians have access to the most accurate, local forecasts for their planning needs.

Using computer models, our team of professional meteorologists are always on the lookout to see if:

— weather systems are picking up pace or reducing speed
— if the moisture in the system is changing
— whether the system is gathering strength or losing its intensity or as meteorologists call it "filling"
— if the system is changing direction, becoming less moist or is changing in strength

When our meteorologists become aware of significant changes in the atmosphere, he or she will then issue an updated forecast based on these recent developments.

When is the best time to see the Aurora Borealis or Northern Lights?
The best viewing of the northern lights is away from the light pollution of urban areas. Northern lights appear all year round but they are less visible during the summer months because of the urban pollution trapped by heat. The northern lights are also more visible shortly after solar flares on the sun.

How can I find out more about The Weather Network's Extreme Weather Team?
To find out more about this exciting program visit our Extreme Weather Team section.

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Help: Forecasts and Observations

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Forecasts and Observations Queries FAQs

Forecasts and Observations Queries FAQs

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