CBC British Columbia
Questions for Claire

Archives: January

What does a "30% chance of rain this morning" mean? Thirty percent of what?!

Friday, January 30, 2009 | 10:02 AM PT

Question submitted by Frank Cosco
(Vancouver, BC)

Oh, this is such a common question. I've taken my answer straight from the Enviroment Canada web site:
The probability of precipitation (POP) is the chance that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any point of the forecast region during the forecast period. For example, a 30% probability of precipitation means that the chance of you getting rained over (or snowed over in winter) is 3 in 10. In other words, there is a 30% chance that rain or snow will fall on you, and, therefore, a 70% chance that it won't. It must also be noted that a low POP does not mean a sunny day: it only means a day where the chance of rain or snow is low.

What is the difference between wind speed & gusts, and how you meausure the gusts of the wind?

Tuesday, January 27, 2009 | 10:23 AM PT

Question submitted by Cody Simmons
(Fruitvale, BC)

Cody, wind measurements are taken with anemometers located 10 meters above ground level (about 33 feet), at official Environment Canada weather observing sites. The latest anemometers are changing from a rotating cup variety of the past, to a new sonic version, with no moving parts, in a replacement program that began about a year and a half ago.

The "wind speed" reported in each observation is an average speed for the most recent two-minute period prior to the observation time. This is also considered the "sustained wind" for routine surface observations (just to confuse matters a bit, in hurricane forecasts, the sustained wind is a one-minute average). This two minute average is calculated from a series of 24 five-second average values.

A wind "gust" is also reported when the peak "instantaneous" wind during the most recent ten-minutes prior to the observation is more than 10 knots greater than the lowest "lull" in the wind during that time. If that is the case, the highest instantaneous wind during that ten minute window is reported as the gust value. The wind speed is recorded by a pen mark on piece of paper held on a rotating drum, attached to the anemometer output. Hence the gusts is the spike in the chart that rolls out with tme.

A squall is a sudden increase in wind speed which is typically associated with active weather, such as rain showers, thunderstorms, or heavy snow, but which lasts much longer than a gust. Squalls refer to an increase in the non-sustained winds over an extended time interval, as there may be lower gusts during a squall event.

How is "dew point" determined, and why does it matter?

Tuesday, January 20, 2009 | 01:54 PM PT

Question submitted by John Birk
(Kamloops, BC)

John, sometimes the seemingly simplest of questions are the best!

The dew point is the temperature at which the air would become saturated if it were cooled sufficiently. If the air is cooled below this point, spontaneous condensaton of the water vapour in the air will occur. The dew point is actually calculated from two readings on two different thermometers - a dry bulb and a wet bulb thermometer.

The temperature/dew point spread measures the same thing as relative humidity,
but relative humidity is given as a percent. As the relative humidity goes up, the temperature/dew point spread decreases.

So, answering the second part of your question first (!), the measurement of the dew point of the air, essentially gives us the relative humidity of the air.Dew Point values provide useful insights into the characteristics of an air mass at the surface. Generally, higher Dew-Point Temperatures during periods of warm or hot weather will indicate a greater likelihood of discomfort because the air is nearer saturation and therefore less capable of absorbing moisture from the surface of one’s skin. Likewise, low Dew-Point Temperatures will indicate greater comfort during warm weather because moisture on exposed skin evaporates more rapidly, therefore increasing heat loss from the body, making us feel cooler.

Dew Point values can also give insights into the amount of energy available at the surface for the development of thunderstorms. Higher Dew-Point values can indicate a greater supply of heat and moisture at the surface. Greater potential energy in the form of heat and moisture provides better "fuel" for the growth of thunderstorm updrafts.

So the dew point is a crucial piece of the puzzle when creating a forecast.

Can you tell me why at winter solstice the sunrise actually gets later after Dec 21 and then stays at this same time until the middle of Jan?

Tuesday, January 13, 2009 | 10:22 AM PT

Question submitted by Janice Rosang
(Abbotsford, BC)

Actually it doesn't! What you're probably noticing is a "rounding" error in the publication of the sunrise time. However the sunrise time does remain virtually unchanged for a matter of about a week or so.
And to explain what is going on I'm going to include a discussion about equinoxes as well - simply because you are the second person to ask this type of question. Basically astronomical equinoxes and solstices' are hard and fast explicit moments in time, but they do not mark exact lengths of days etc at our latitude. Here's why:

The seasonal significance of the winter solstice is in the reversal of the gradually lengthening nights and shortening days. How cultures define this is varied, since it is sometimes said to astronomically mark either the beginning or middle of a hemisphere's winter. "Winter" itself is a subjective term, so there is no scientifically established beginning or middle of winter but the winter solstice itself is clearly calculated to within a second. Though the winter solstice lasts an instant, the term is also colloquially - however quite inaccurately - used to refer to the full 24-hour period.

As humans we generally measure the "day" as the time between sunrise and sunset. Sunrise is defined as when the TOP EDGE of the sun breaks the horizon and sunset is defined as when the TOP EDGE disappears below the horizon. Equinox is measured based on the CENTER of the sun's disk, not at the edge. This difference means that at the equinox moment, the day is actually slightly LONGER than 12 hours.

Not only that, but the earth's atmosphere refracts (bends) sunlight coming over the horizon so that the sun appears to have risen even before it really has. This adds 7-8 more minutes to the length of the day at the equinoxes.

Both of these effects combine so that the true 12-hour day for high northern latitudes can occur several days before the Vernal Equinox and several days after the Autumnal equinox. The effect is lessened for latitudes closer to the equator. And the opposite holds true for those latitudes in the southern hemisphere.

In addition, since the earth is not rotating at the actual moment of the astronomical equinox (can't rotate if there is no time change) then only certain points on earth even see a sunset or sunrise at the ACTUAL moment of the equinox. And then the other end of the day for those locations would be before or after the equinox anyway. So no one location on earth can possibly see a sunrise and sunset at the equinox moment.

Still, this third "effect" does not contribute to the lengthening the day as much as the other two.

Hope this answers it!

How do you calculate the wind chill factor?

Thursday, January 8, 2009 | 09:48 AM PT

Question submitted by Jackie Ward
(Surrey, BC)

Environment Canada has a web site dedicated to the subject of wind chill:

But basically the calculation is as follows:
Tair = air temperature, V = wind (km/h):
WC = 13.12 + 0.6215 × Tair - 11.37 × V (exp 0.16) + 0.3965 × Tair × V ( exp 0.16)