Why does smoke disperse but clouds seem to stick together?
The particles that make up smoke and clouds have important differences, which explains how they appear
This week's question comes from Gary Schajer in Vancouver. He asks:
During the wildfire season, the smoke came and spread evenly across the sky. This is much different from clouds. Why don't clouds spread out like smoke?
Nadine Borduas-Dedekind, an assistant professor in the Department of Chemistry at the University of British Columbia says it is important to understand the difference between the two.
Smoke is the result of visible amounts of tiny particles in the air. The smoke can spread both vertically and horizontally, carried by their own heat and wind.
Clouds are made when tiny particles, including the ones that can come from forest fires, experience high humidity so that water vapour condenses on them, forming visible droplets.
High in the atmosphere, clouds can appear to stand still if there's no wind, but clouds can spread as well depending again on temperature and wind. However, unlike smoke, when a cloud spreads, it can disappear if the relative humidity drops.
When that occurs, the water from the droplet evaporates, and only small, effectively invisible, particles are left behind. This does not happen for a smoke plume because the visible particles don't disappear. That explains why it can appear from an observer on the ground that clouds don't spread like smoke.
Produced and written by Mark Crawley