It’s the most anticipated part of the forecast… will it rain or snow? Sometimes it’ll do both. That’s quite often the million-dollar equation.
All around the globe, we have several thousand showers happening at once. It could be rain, snow, sleet, a wintry mix. While most will fall as rain, when this moisture reaches colder air it can be one of several types of precipitation or a mixture. This causes some hazards locally which is why it’s important to get it right. And while all precipitation falls from clouds, it doesn’t all fall as the same type.
Here are the most common types of precipitation we see here during the changing of the seasons (graphic below). When we have a column of air that is all colder than 32°. This supports all snow. Snow that’s made in the clouds can freely fall to the ground as a snowflake. When we introduce a layer of warm air in the upper atmosphere… this melts the snowflakes and once it falls through another layer of cold air, it won’t turn back into a snowflake – that process only happens in the clouds. It then turns into an ice pellet. This is sleet.
Then if we introduce a larger layer of warmer air but we’re below freezing at the surface, this allows liquid raindrops to freeze on contact. This is freezing rain. And of course, if the entire column of air is above freezing, this will support all rain.
Some of the more hazardous precipitation types cause what’s known as “black ice’. This can come from freezing rain, or the melting and refreezing of ice or snow… and it’s invisible. We will see this in North Dakota this season for sure. So when the temperatures are below freezing and the roads look wet, although they may be treated, it’s best to assume they could be slippery.