The other day, I gave a webinar to introduce some local growers to the new weather stations that we installed in 2022 in the Creston Valley of British Columbia. One of my intended take-home messages was explain why such a relatively small production area needs so many weather stations. There are a total of eight – seven new ones plus the existing Environment Canada station. Wouldn’t one or two strategically placed weather stations fulfill the needs of the region’s growers? I was looking for a couple of good examples of how temperature varies substantially over short distances – I found many.
For cherry producers, an untimely frost can mean the difference between an excellent crop or no salvageable crop whatsoever. Growers invest tens of thousands of dollars into wind machines, sprinklers, heaters, and helicopters, all to mitigate the effects of freezing temperatures on their crop. So being able to accurately detect frost is paramount.
The agricultural productions areas in the Creston Valley range in elevation from roughly 530 to 710 meters above sea level – a vertical difference of 180m. In a typical overnight period in the fall, when the wind is calm and the sky is clear, the colder dense air tends to “drain” from higher elevations to the lower terrain. This results in the lower areas experiencing the coldest temperatures. This was entirely noticeable in Creston where the two lowest elevation stations, located in the River Flats were 5°C to 7°C colder than the stations upslope. Clearly, overnight temperatures and potential frost damage varies significantly across the region.
Weather stations located at North Erickson and Creston Flats
The temperature trends observed at most of the stations were somewhat consistent. The temperature readings would steadily climb throughout the day to reach their maximum readings by the mid-afternoon. Then once the sun began to set behind the mountains, the air temperatures would rapidly drop and continue to decrease throughout the night. Lowest temperatures would occur in the early morning, just prior to sunrise.
But one location was very different. Instead of continuing to drop throughout the evening, the temperature at the North Erickson site, the location at the highest elevation would decrease through the early evening until about 8:00pm. Then the temperature would begin to increase abruptly, often by over 5°C in less than an hour. The temperature would then remain warmer than the surrounding stations for the entire night. The pattern continued nightly for several weeks.
Without more information, it was difficult to know exactly what was happening, so I started digging deeper into the data. What I found was at that while most stations recorded calm winds in the evening (we are using ultrasonic anemometers, which are very sensitive and accurate), the North Erickson station began to record average wind speeds between 10km/h and 15km/h and gusts up to 30km/h around that 8:00 pm time. By comparison, the wind speed at other stations remained below 5km/hr. In all cases, the stronger winds were coming from the North to Northeast. Given the location of the station, that explained everything.
While at the time, I couldn’t quite recall the phenomena, a quick review of one of my old meteorology textbooks jogged my memory – it’s the katabatic winds. The word ‘katabatic’ is derived from the Greek word katabasis, which means descending. As the air up the mountain cools, it becomes denser, heavier, and it travels downslope due to gravity. The air continues to descend into the surrounding dense air, becoming compressed. When parcel of air loses volume, it gets warmer. Since the air is quite dry, it warms at a predictable rate of 1°C per 100m of descent (dry adiabatic lapse rate). For air that is 5°C warmer than the surrounding area may have descended from 500m upslope (depending on its original temperature).
Map of weather stations in Creston Valley (elevation in background)
Air temperature and wind speed at North Erickson and Creston Flats
In the meteorology world, there are many known cases of katabatic winds. In Antarctica, air masses can flow hundreds of kilometers downslope at speeds as high as 70km/hr. Every fall, the Santa Ana winds originate over the Great Basin and upper Mojave Desert, bringing dry, hot air to Coastal Southern California. In December or 2011, these katabatic winds were recorded at sustained velocities of over 150 km/h and gusts over 250 km/h. Since the air is so dry (relative humidities as low as 10%), the Santa Ana winds often greatly increase the wildfire risk to the area.
Farmers in the Creston area are well aware of these sorts of local peculiarities – which parts of the valley get the earliest and latest frosts, where the rain most often occurs, or which areas will reliably support certain crops. They just might not know the actual physical forces that are causing these behaviours. As we continue to monitor local climates with more accuracy and granularity, we’ll gain a better understanding into what is occurring and how it might affect production. The more we measure, the more we’ll understand, the more we understand, the better we can adapt.
