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Questions for Claire

Archives: July

With our recent fires, I have heard a great deal about a weather condition called "crossover", when the humidity is lower than the temp. Can you explain what this means, and what it does during fire season?

Friday, July 31, 2009 | 09:44 AM PT

Question submitted by Lisa Nagy
(Kelowna, BC)

Lisa! This is the first time I have ever been truly stumped by a question!! Congratulations.

I went to the BC Wildfire Management Branch, Ministry of Forests and Range, and got some help from a couple of great people there.

Here's what they had to say: "The formal definition of Crossover as listed by CFFC (Canadian Forest
Fire Centre) is "the point at which the numerical value of the ambient relative humidity is less than, or equal to, the numerical value of the ambient air temperature". This may be used as a rule of thumb indicator of extreme burning conditions. It is a rule of thumb indicator primarily used by fire suppression staff but there are some limitations to it being used as a definitve law."

So there you go! And thanks for the challenging question.. I now know more about fire weather forecasting too!

As the earth spins one rotation per day, does it spin the weather patterns? For example does England eventually get our "weather" and do we get China's?

Wednesday, July 29, 2009 | 12:00 PM PT

Question submitted by David Mallory
(Vancouver, BC)

Fantastic question! But I have to go back to basics to explain this:
Weather is created due to the day by unequal heating of the atmosphere, and the motion of the earth spin, drags the atmosphere around with it, with the overall result being a combination of global patterns of wind which follow a regular yearly cycle.

Wind and weather are created by the action of the Sun, heating up different parts of the land and sea by varying amounts. For example, the sun is overhead in the tropics, therefore it has less atmosphere to pass through so the effects of heat are greater. Warm air rises and cooler air flows alongside, replacing the displaced air resulting in large scale air movement or wind as we know it.

The rotation of the earth means that it is a spinning globe where at one point of the equator is traveling at 1100 km/hour, whilst a point at the poles is not affected by rotation. This results in what is known as Coriolis forces which act to cause an apparent deflection of a motion. This can be explained in terms of an imaginary force that can be translated into terms of gravitational pressures.

Due to the effects of the Sun and the effect of varying degree of heat on certain areas such as the equator and the Poles, resulting in surface winds or air moving towards the equator at the surface and towards the Poles at altitude. "At mid-latitudes we can see that air moves towards the Poles via the surface and moves to the Equator at altitude".

The effects of wind force and direction dictates to some extent to the type of weather we are going to experience, and how that weather will affect people "along the way".

What exactly is humidex? How come sometimes you mention it, and sometimes you don't?

Tuesday, July 28, 2009 | 01:57 PM PT

Question submitted by Phyllis Gee
(Burnaby, BC)

The humidex is an index (a computed value as opposed to something measured) devised to describe how hot or humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature. It takes into account these two important factors that affect summer comfort. It is therefore a better measure of how stifling the air feels than either temperature or humidity alone. A humidex of 40 with, for example, a temperature of 30 degrees means that the sensation of heat when it is 30 degrees and the air is humid is more or less the same as when it is 40 degrees and the air is dry. We must be careful not to depend on this interpretation alone: it is a mere indication of physiological reactions, not an absolute measure.

The humidex is particularly significant when its value is greater than 30. We only display humidex values of 25 or higher for a location which reports a dew point temperature above zero (0°C) and an air temperature of 20°C or more. Below this value, the humidex is too close to the air temperature to be considered significant.

It is suddenly hot and humid here in Vancouver, how unusual is this? Will records fall?

Tuesday, July 28, 2009 | 01:51 PM PT

Question submitted by Tim Allen
(Vancouver, BC)

Hi there Tim,
Vancouver does not normally get this hot and humid! As I write this (12 noon on July 28th) we're 28 deg C, with a humidex of 34. Abbotsford is 32 deg C, with a humidex of 40.
Our all time record hot temperature was set back in August of 1960 at 33.3 deg C. Victoria has already set a record for this date at 31.6 deg C, beating it's old record of 29.4 deg C set back in 1958.

Footnote added July 31st: Since originally answering this question, we have had two unprecedented days of heat: on July 29th we (YVR) broke our all time temperature record when we hit 34.0 deg C, then on the 31st to my surprise we beat THAT all time record, when we hit 34.4 deg C! I am sometimes humbled by what Mother Nature can throw at us.

How is humidex calculated?

Tuesday, July 28, 2009 | 01:33 PM PT

Question submitted by Bonnie Almso
(Vancouver, BC)

The Humidex formula is based on the work of J.M. Masterton and F.A. Richardson at the Atmospheric Environment Service (now MSC) of Environment Canada in 1979. It is a standard for Canada, but variations are used around the world. The dew point temperature should be given in kelvins (temperature in K = temperature in °C + 273.1) for the formula to work. The magic number 5417.7530 is a rounded constant; it's based on the molecular weight of water, latent heat of evaporation, and the universal gas constant.

e = vapour pressure in hPa (mbar), given by:
e = 6.11 * exp [5417.7530 * ( (1/273.16) - (1/dewpoint) ) ]

h = (0.5555)*(e - 10.0);
humidex = (air temperature) + h