June is typically the wettest month of the year in much of the interior of BC, but not in 2017. The month that is typified by unstable air, thunderstorms, and rain produced a rainless month for much of the Chilcotin and very little elsewhere in southern British Columbia.
2017 was beyond normal, especially on June 26th when a freak meteorological event pushed the relative humidity below 16% over a wide area several hundred kilometers across. One forestry station at Lone Butte even measured 1%!
This would be world record territory… but, did it really happen?
Before answering that, understand that most weather records never actually happened, not necessarily at individual stations, but at the level of states and provinces. For example, Ontario’s all-time record cold never happened nor did Newfoundland and Labrador’s all-time high temperature (I will explain why at a future date).
And those are temperature records, which are some of the more reliable variables we measure. Precipitation records, snow records, wind records, sunshine records, and others are all subject to error.
Humidity is especially problematic because automated weather stations do no have sensors that can reliably measure the relative humidity below 10%.
That’s not usually a problem since relative humidity readings below 16% are quite rare. If it does happen, it’s typically one or two stations, but on the 26th no less than 17 stations fellow below 16%, and many of those below 10%.
The air was so dry, it was literally off the charts. The humidity map below was generated from BC Forest Service weather stations which snapshots the data at 1pm PDT. Places like Alexis Creek recorded 7% for the day, but the lowest of the lot was Lone Butte which bottomed out at 1%.
What is relative humidity?
Relative humidity is basically a measure of the difference between the air temperature and the dew point. Dew point is an expression of the actual (not relative) moisture content of the air. Because air can hold more moisture as it warms, relative humidity drops even as the amount of moisture in the air stays the same. That’s why the relative humidity can be 100% in the morning and 30% in the afternoon without the amount of moisture in the air changing. Similarly, 30% humidity in the summer contains much more moisture in the air than 30% humidity in the winter. Actually, cold winter air is very dry even if it’s at 100%. In other words, when someone tries to tell you that cold moist air along the ocean “feels” colder than interior dry air at the same temperature, they’re mistaken. There’s almost no moisture in the air in either case, and certainly not enough to be able to physically detect. The reason it might “feel” colder on the coast is that there’s more wind.
Environment Canada has a number of stations around the red zone above, but instead of recording the relative humidity, stations will report an error when the temperature difference between the air and dew point is above a certain level. This is because Environment Canada is more concerned with accuracy than Forestry, and since extremely low humidity falls outside of the instrument’s specifications, the number is considered invalid. Given past and current measurements, it seems that the cut-off is 7% humidity.
The above map reveals that drawing a line from Lillooet to Clearwater cuts through an area entirely below 10%, but there are a number of reasons to question any measurement as reliable in that range. Chief among them is that hygrometers are glitchy below 10%.
Skipping Lillooet and Clearwater, I will just focus on Ashcroft and the two Clinton stations. Ashcroft is the most arid place in Canada with very low relative humidity in the summer afternoons, and the nearby village of Clinton has two stations, one of which measured 7% without breaking the hygrometer.
On a normal day the dew point remains stable throughout the day because the moisture in the air remains constant, but on June 26th, the dew point dropped 10 degrees in a single hour at Clinton. The other stations stopped working during the heat of the day because the air was too dry, but it stands to reason their dew points were similar.
Back on May 13th, 2012, Ashcroft measured 7%. That was with a dew point of -10°C and an air temperature of 29°C. On June 26th, 2017, the measurement could not be made, but if we assume the dew point matched Clinton’s -13°C figure, and given the measured air temperature of 35°C, the relative humidity can be calculated to be 4%!
This is definitely possible, and would perhaps be a Canadian record, but how about the 1% reading at Lone Butte? Unlike Ashcroft, the plateau on which Lone Butte sits is not arid or semi-arid, so it’s almost certain that this value is fake. It’s more reasonable to peg the number at 7% because the elevation is similar to the Clinton airport. Even still, that’s almost unbelievable at that elevation and latitude.
As we enter into July, we are extremely dry under the scorching sun with high and extreme fire danger across the province, and yet, not a single weather station is recording relative humidity under 16%. That goes to show you how rare and unusual the June 26th event happened to be.