The Cold Conveyor Belt - The Wizard Controlling the Precipitation Type

#0418 "Sheena's Sky" Phil was about to learn
some meteorology from this one trick pony...

I am a firm believer in making mistakes. Ask anyone. I feel that you need to keep pushing the knowledge envelope to become skilled. Strive to learn. If you are not making mistakes, you are not learning. Nature is the best teacher and it is best to be always paying attention. One never knows when the next natural lesson will be presented.

Conveyor Belt Conceptual Model of a Mid-Latitude Storm
The humble Blue Arrow is the Cold Conveyor Belt but ignore
this Wizard of the Weather at your own risk.

I recall one Day Shift at the Ontario Storm Prediction Centre in the early 1990’s. The Cold Conveyor Belt taught me a lot. I made a tremendous number of mistakes – but I never made those mistakes again. A large and severe winter storm with a good feed of Gulf of Mexico moisture was developing with a huge warm conveyor belt. The equally impressive dry conveyor belt was carving a path through the dry slot in the comma cloud pattern. I was all over these dramatic patterns in the severe storm and excited to provide an accurate warning service.  I was in charge. Or was I? Then it happened.

Rain started to fall in the eastern forecast regions. This was a winter storm! No problem, we will fix the heavy snowfall warnings. The rain changed to freezing rain. Big problem! We had to really adjust the warnings. The freezing rain changed to ice pellets. What next? The ice pellets changed to heavy snow. Wonderful! We finally caught up to the weather that we were originally warning for but we had spent the morning back pedaling and confused. I learned a lot. I made even more mistakes. That day was burnt into my memory more than any of the forecast successes. The humble Cold Conveyor Belt had ruled the day and the weather. The Cold Conveyor Belt Wizard was pulling the strings behind the curtain of the three dimensional deformation zone.

Rain Sounding

The cold conveyor belt originates from low levels in the atmosphere poleward and east of the low pressure center. The cold and dry air is drawn toward the low. The operative word is actually “dry” instead of cold. In this case the temperature started above freezing but the dew-point was well below freezing.

The snow flakes were in their free-fall at 1 metre per second. Those flakes melted in the last kilometre of their descent. That explained the winter storm starting as rain.

Freezing Rain

The energy required to melt the snow flakes along with the saturation of that lowest kilometer of air created a profile with an above freezing layer of air aloft and below freezing near the ground. That explained the rain changing to super-cooled water droplets. These super-cooled water droplets freeze on contact producing a treacherous ice glaze on exposed surface - freezing rain.

Ice Pellets Type A

The continued cooling and saturation of the air mass increased the amount of cold air at the surface. The super-cooled water droplets were freezing within the deeper cold layer at the ground thus converting the freezing rain to less dangerous ice pellets.

Snow

The continued loss of energy required to melt the snow flakes and saturate the air mass in the lowest kilometre cooled the entire profile to the wet-bulb temperature and eliminated the above freezing layer aloft. The falling snow would fall that lowest kilometer and remain unchanged as large dendritic flakes. These large flakes would accumulate quickly and verify our original warning. Before that happened though the cold conveyor belt had been exceedingly dry and taught me a lot of lessons.

I quietly slid the term wet-bulb temperature in the previous paragraph. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature). In dry air, the wet-bulb temperature is achieved by evapourating water into the air parcels and taking the latent heat required to do that evapouration from the air parcel itself. The wet-bulb temperature is a conservative property of air as it moves in the conveyor belts of a storm. The wet-bulb temperature is the lowest temperature that can be reached under current ambient conditions by the evaporation of water only.

The cold conveyor belt taught lessons about precipitation typing and the wet-bulb temperature that day. It also taught about the perils of being distracted by large and energetic patterns while neglecting the subtle features of the wizard behind the deformation zone curtain. This weather is now called a "wintry mix" by the media thus side-stepping the potential challenges and science. I feel it is best to understand and work with nature - and never leave home during a freezing rain event.

Warmest regards and keep your paddle in the water,

Phil the Forecaster Chadwick

Next is how the Cold Conveyor Belt can make you a better weather observer.