File:Smoke over the upper Midwest (MODIS 2017-09-17).jpg

English: In mid-September, 2017, smoke from fires burning in the Pacific Northwest, the western United States, and Canada, draped across much of the United States.

The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Aqua satellite acquired this true-color image on September 13. Gray smoke moves beneath white clouds across the Midwest, rising and mixing with the clouds in some areas. In some locations red hotspots mark other actively burning fires, which contribute to the smoky scene.

The heavy smoke has been entrained in the jet stream, which is sort of like an atmospheric “conveyor belt”, which carries air, smoke, or smog from the west towards the east in the United States. Smoke from a West Coast fire can reach the East Coast in just a matter of days. Such smoke may also journey across the Atlantic Ocean to darken the skies of Ireland, the United Kingdom, and France. Typically such far-traveling smoke flows high in the atmosphere, so it is not as much of a concern for human respiratory health as smoke that stays closer to the ground.