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A quick look back at May weather, then clouds

The mean monthly temperature for May 2017 was much cooler than in May 2018

Believe it or not, I got into a disagreement with someone about how nice/bad May’s weather was/has been. The person in question was positive that last May was nicer than this May. This brings me back to the whole notion of the weather brain, or weather memory. Just like most things in life, we tend to remember the most significant events with good clarity, but in between, our brains are remarkably good at filling in the blanks — sometimes correctly, sometimes not so much.

Looking back at May, the average temperature across our three main reporting stations was about 14 C, which is a good 3 C above the long-term average. But this number, as we know, isn’t the best number to quantify how the temperatures across a month play out. Take April as an example: the first half was way below average, with the second half being above average. Overall, the month turned out to be below average, but not nearly to the extent that the first half of the month was.

We can also look at the last day of frost across the region, but again, this can be skewed by one short cold snap late in the month. Looking back at the data, Winnipeg’s last frost was on May 19, which was earlier than the long-term average. Brandon’s last spring frost also occurred on May 19. Dauphin was the warm spot, with its last spring frost occurring on May 10. Even this number can be viewed in another way; while Dauphin had the earliest final spring frost, all three locations only had between five and six days of frost during the entire month. Finally, on the opposite side of the scale, we have the number of warm days. Looking back at May, all three locations saw between four and five days with daytime highs above 30 C. This compares to the long-term average of about one day.

This still doesn’t answer the question of whether this May saw nicer or worse weather compared to last May. Well, looking back, last May saw a mean monthly temperature that was much cooler than this May. The last day of frost last May was around May 19, which was bang on the same time as last year. Looking at the number of hot days, this May has recorded between four and five days above 30 C, which is way more than we saw last year, and more than the long-term average. In fact, with another +30 C day recorded this June, we are already almost tied with the total number of +30 C days we saw during all of last year!

Collision-coalescence

OK, now on to this week’s weather topic: clouds and how they form. For clouds to form we need to cool the air so condensation will occur. Condensation is the process by which the tiny molecules of water in the air come together to form larger droplets or ice crystals. Without going into topics such as vapour pressure, mixing ratios and absolute and specific humidity, condensation occurs because, as the air cools, the ratio of evaporation to condensation tilts in favour of condensation.

Evaporation and condensation of water vapour are continuous and ongoing processes in the atmosphere. Usually, more evaporation is taking place than condensation, so, as the microscopic water droplets or ice crystals collide and grow, they quickly evaporate back to their original microscopic size. When we cool a parcel of air enough, the amount of evaporation that takes place decreases, because evaporation takes energy that comes from heat. Now, as the microscopic water droplets or ice crystals collide and grow, they do not evaporate as quickly. As more and more collisions occur they eventually grow big enough to become visible; they have grown large enough to be called a cloud droplet.

When talking about the size of these cloud droplets we are talking about drops of water that range in size from around five microns up to around 50 microns (one micron is one-millionth of a metre, so these are some small drops of water!). Another way to compare the size of cloud droplets is that it takes about one million of them to make a raindrop. Therefore, clouds (or the water in them) don’t fall out of the sky. The cloud droplets are so small that they only fall at a rate of about one cm a second or about 0.04 km/h, so it doesn’t take much of an updraft to keep these tiny droplets airborne.

There are two main processes that allow for clouds, rain and snow to form: the collision-coalescence process and the Bergeron process. The first of these processes is self-explanatory. The microscopic water droplets collide and combine (coalesce). With each collision the droplets get larger, eventually becoming visible and possibly getting large enough to even form a raindrop.

The second process, the Bergeron process, happens with ice crystals. Here, ice crystals grow because they absorb water vapour more readily than water droplets. That is, in a cold cloud, water droplets can be supercooled below the freezing point and still remain liquid; if an ice crystal is near one of these supercooled water droplets, the ice crystal will absorb the water from the water droplet by causing the water droplet to evaporate while the ice crystal uses this newly evaporated water to grow.

Ok, so now we know that clouds form when atmospheric moisture condenses into droplets (or ice crystals) that are large enough to be visible to the naked eye. We also know it requires that the air be cooled so that condensation occurs more rapidly than evaporation. Now I feel we are ready to move on to our look at the different cloud types — but that will have to wait until next time.

About the author

Co-operator contributor

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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