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Formulae and Measurement FAQs

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Formulae and Measurement FAQs

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Measuring the water content of snow
Tommy Daye of Kingston Ontario has heard that 10 cm of snow is equal to 1 cm of water. But surely, writes Tom, wet snow yields more water than the same amount of dry snow. You are right! As a very general rule, though, the depth of freshly fallen snow is converted to a water equivalent by dividing that depth by 10. However, the water content of snow varies greatly from storm to storm, depending on the temperature, wind speed and moisture content of the air through which the snow falls. Very wet plastery snow falling in air near freezing may have a water equivalent of 6:1. On the other hand, in dry, powdery snow, the ratio may be as high as 30:1. In these two examples, this amount of wet sloppy snow would melt down to 10 cm of water, whereas that same amount of dry snow would yield only 2 cm of water. The only sure way to measure the water content of fresh snow is to melt it. Whereas, at most locations, we use the convenient 10 to 1 ratio, at major stations, the standard instrument to measure water equivalent is the Nipher snow gauge. The snow in this receptacle is taken it inside, where it is left to melt... the contents are measured to get the true water content of snow since the last observation.

How is snow measured?
Rod Murray of Oakville, Ontario writes to ask "what is the precise method used to measure snowfall amounts"? Snowfall measurements are the easiest and most commonplace of all weather observations but just may be the least precise. In most countries snowfall measurements are no more complicated than simply going out and sticking an ordinary ruler into the snow in several places and taking an average depth. In practice though it is rarely that easy. Finding a representative site can be a problem. The best place to take measurements is in an undisturbed, flat, open area away from overhangs. The standard snow ruler is 1 metre long and marked in centimetres and fifths. Anything less than 0.2 cm is noted as a trace. So many things happen to snow as it sits there waiting to be measured:

— if it's warm, snow melts as it hits the ground
— if it's windy, snow drifts or blows away
— if it accumulates over 10 to 15 cm, snow settles and compacts under its own weight and evaporates

Because of these fates, snow depth on the ground sometimes decreases even as snow continues to fall.

Why kilopascals and not millibars?
David Hembling of Kamloops wrote to ask why did Canada adopt the kilopascal as the unit of pressure. What was wrong with the millibar? In 1971 Parliament adopted the modern form of the metric system known as the International System of Units or SI. That same year the international body responsible for maintaining standards of measurement decided to replace the "bar" as in millibars, with the Pascal as the SI unit of pressure or stress, i.e., force applied to an area. Millibar is metric but not SI metric. The SI metric system is founded on 7 well-defined base units including metre for length, kilogram for mass, second for time. A second group contain units that are derived directly from the base units. For example, pascals derive from kilograms, area in square metres and seconds. The derived unit of pressure, pascal (Pa) is named in honour of Blaise Pascal, a 17th century French physicist and writer. Most home barometers measure air pressure in inches or millimetres of mercury or in units called millibars. All official pressure readings, though, are expressed in kPa. If your barometer has units in inches, to convert to kilopascals multiply your barometer's readings by 3.386. For example, if your barometer is reading 29.91 inches, that's 101.31 kPa.

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.

How do you set a barometer?
A barometer is usually corrected to sea level pressure. This is done by knowing that, on average, atmospheric pressure decreases by about 1 kilopascal (kPa) for every 100 meters increase in elevation. To convert from station pressure to sea level pressure, add 1 kPa for every 100 m that the station is located above sea level. For example, if the station pressure at Ottawa International Airport (elevation 114 m above sea level) is 100.05 kPa, add 1.14 kPa to find the sea level pressure. The sea level pressure would be 101.19 kPa.

Where should I place my barometer in my house?
Wherever it looks good to you! Weather lore may claim that you should "never hang a barometer on an outside wall." In actual fact, the air pressure is the same on any wall in your house -- inside or outside.

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.

Where should I place a thermometer? Indoor and Out.
Where you place your thermometer all depends on its type:

— Outside thermometers should be placed away from any heat source. For example, heat transfer or radiation may affect a thermometer that is attached to the side of a house. If possible, place your thermometer away from any heat sources (buildings, houses, parking areas and streets) and away from direct sunlight.
— The same basic rules apply to an indoor thermometer. Keep it away from direct sunlight and away from heat sources such as heaters or household appliances. If the thermometer is dual purpose, the sensors for indoor and outdoor use should follow the same rule of thumb as mentioned earlier.

What is meant by "POP" in the local forecast?
These two letters, one from Ian Kellogg of Toronto and the other from Robert Brawn of Ayr, Ontario are typical of the many inquiries from viewers of The Weather Network, asking what POP refers to when displayed on the local weather forecast segment. As many of you know, POP refers to probabilities of precipitation. A POP of 60% doesn't mean that it will rain 60% of the day, or that rain will fall on 60% of your area. It also says nothing about how much rain or snow will occur. A POP of 60% means that the forecasters have calculated that in a 100 similar weather situations, rain has fallen 60 times in the forecast area. POP is for any point in your forecast area, not the whole area. So, for instance, a POP of 90% for rain means that 9 times out of 10 when this is predicted, you ought to get rain somewhere in the forecast area, eg., at your home, playground or at the airport. In the past, meteorologists used descriptive terms such as slight chance, chance of, occasional, or possibility of. However, the public much prefer numerical statements like 20%, 80% to describe the likelihood of precipitation.

When meteorologists report that an inch of rain has fallen, it always seems like more. I have also heard conflicting reports on how rain is measured. Is it calculated over a specific area such as an acre of land, or just by putting a pot out to catch the rain?
Yes, rainfall is calculated over a specific area. At most weather stations, rainfall is measured with two types of gauges, both having different surface areas. One has a surface area the size of a coffee tin or around a 30 square centimetre opening. This is the world wide standard rain gauge. The other type is a little larger with a 120 square centimetre opening, and it's called a tipping rain gauge.

You're right, when an inch of rain in these types of rain gauges it is not as impressive as over a larger area, let's say a square kilometre, where 1 inch of rain equals many 1000's of ton's of water.

I recently looked up the weather report for Charlottetown that updated at 1500 AST which I know is 3:00 pm. What is the 1900 GMT that follows?
GMT means Greenwich Mean Time which is the time of the Greenwich Astronomic Observatory. The observatory is located in London, England where the longitude is 0 degrees.

To calculate your local time for Charlottetown from GMT time, subtract 4 hours. GMT does not take into account daylight savings time so when applicable, it must be factored in. GMT is also known as Zulu (Z) time and Coordinated Universal Time (UTC).

I would like to know how the ceiling is measured? For example, if the ceiling is 2,000 feet, how was that determined.
Now a days, cloud ceilings are measured with an instrument called a Laser Ceilometer. This large oil drum shaped instrument has a laser inside that fires a laser beam up into the sky. As the laser hits the cloud ceiling, it bounces back to the instrument. The time difference between the laser firing and the laser light bouncing back is what we use to calculate the height of the cloud ceiling.

In the old days, we used small weather balloons. The weather balloon would be released and we would watch it until it disappeared into the clouds while timing how long it took for the balloon to disappear. Knowing the ascent rate of the balloon, we calculated the height of the ceiling. Believe it or not, we still use the balloon method at some stations that are not equipped with a laser ceilometer!

How is the Dew Point temperature measured?
The dew-point temperature can be measured with a device called a wet-dry bulb thermometer. A wet-dry bulb thermometer is basically two thermometers placed side by side. The bulb of one is covered by a wick that is always kept wet (wet bulb) and the other is left alone (dry bulb).

Just like water evaporating form a person's body right after they come out of the shower, the thermometer with the wet wick will cool as the water evaporates from the wick. The less water vapour in the air or the drier the air around us is, the more water will evaporate from the wet wick. The more water evaporates the cooler the reading on the wet bulb thermometer. The dry bulb or regular thermometer is used as a reference to give us the current air temperature. The temperature difference between the two thermometers tells us how much moisture is in the air. The greater the difference the dryer the air.

For example, a dry bulb temperature of 22 degrees Celsius and a wet bulb temperature of 10 degrees Celsius results in a difference of 12 C. In this case, the air would be considered dry. A dry bulb temperature of 22 C and a wet bulb temperature of 20 C Celsius results in a difference of only 2 degrees. In this case the air would be considered moist.

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.

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.

During the warmer months, the relative humidity is given together with the temperature displayed on television. When is the relative humidity used, and how is it calculated?
Relative humidity is always calculated at all times of the year but the humidex is usually given in the warmer months from May to September.

Relative humidity can be calculated using various methods and instruments. It is a calculation to find out how many grams of water vapour can be held at a given temperature. Usually, the warmer the air the higher it's capacity for holding water vapour. Each specific temperature has a holding limit for water and the actual amount of water held in the air at the time of measurement can be represented as a percentage.

For example, if a parcel of air at 20 C has a limit or capacity of 16 grams of water vapour and 16 grams of water vapour is measured, the air's holding capacity is at it's maximum, the relative humidity (RH) of the air is said to be 100%. Take the same air with 8 grams of water vapour and your RH is 50%. Relative humidity is a percentage of water vapour per area at a specific temperature.

My house is located at 3100'/950M AMSL and our closest weather station is Kamloops. What is the correction factor I should apply to set my barometer from the Kamloops reading? Secondly, what is the correction factor per 1000'/100M(?) for any elevation change?
You're right! You have to adjust your barometer to correct it to Sea Level using the Kamloops weather station reading and your station height. Pressure at any given time and place will vary with height. In your example, your "station pressure" will always be lower than Kamloops because you are higher up in elevation. So, you'll have to correct your pressure to Sea Level, like Kamloops.

To do this first you obtain the Kamloops Sea Level pressure, then (I've done the math for you) add 15.0 mb to the current Kamloops MSL (mean sea level) reading and you get your MSL pressure. Remember, also, that the most important use of a barometer is not the actual reading, but it's rate of change over a specific period of time.

What is meant by the weather term "Buys-Ballots Law"?
Buys-Ballot Law states that when a person stands with his/her back to the wind, high pressure is to the right and low pressure to the left.
Why does the weather change?
The weather changes because winds in the atmosphere carry pressure areas over the earth's surface. When the wind carries a high-pressure area over a region then that region usually has clear weather, whereas when the wind carries a low-pressure area over a region that region usually experiences cloudy weather with rain or snow.

How do meteorologists determine the number of lightning strikes during a thunderstorm?
Meteorologists can determine the number of lightning strikes during a thunderstorm from information from the "Canadian Lightning Detection Network". The network consists of 81 state of the art lightning detectors. These sensors can accurately detect lightning strikes to within 500 metres and are capable of detecting more than 90% of all lightning strikes. Depending on the type of detector the forecaster can identify cloud to cloud lightning, which suggests a developing thunderstorm, or cloud to ground lightning, which is important for personal safety, property damage and the ignition of forest fires, if certain conditions are met.

The system registers how many strikes occur and can be set for different time segments - for example it can be set for every 15 minutes or over a longer period of time, like a 24-hour period. There are other provincial agencies which have their own lightning detection networks such as the ministries of natural resources for Ontario, Saskatchewan, Alberta and I believe British Columbia. This system is utilized in the prediction of forest fires.

Does Canada use weather balloons?
Yes, Canada uses weather balloons, or as it's really called a "radiosonde" - the instrument attached to the balloon. There are 20 or so upper air stations in Canada and they launch the balloons (or radiosondes) twice a day, once in the early morning and then again in the evening.
Where can I find out about Wind Chill?
For everything you have ever wanted to know about wind chill in Canada, including the formula and cold weather ailments and safety tips, check out wind chill in TheWeatherNetwork.com's Glossary.
When is the first day of fall, winter, spring and summer?
To find out when the seasons start across Canada, see the Sun/Moon/Season Calculator on TheWeatherNetwork.com.

Why is the ceiling reported in feet, not metres?
Ceiling is a measurement primarily used by the aviation industry. The standard for the aviation industry is to report ceiling in feet.
When do we change the clocks?
To find out when to change your clocks and whether to turn them forward or back by one hour, see Daylight Saving Time on TheWeatherNetwork.com.

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Help: Formulae and Measurement

Help: Formulae and Measurement

Formulae and Measurement FAQs

Formulae and Measurement FAQs

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