Smoke covering parts of Sydney this morning was a timely reminder that the bushfire season is just around the corner for NSW. Smoke from hazard reduction burns drifted over the Sydney Basin last night and became trapped near the ground underneath a temperature inversion. An inversion forms when cool air becomes trapped under a layer of warmer air, creating an invisible boundary that prevents air - and smoke - from rising into the atmosphere. Once trapped, smoke will linger until the temperature inversion is broken by warming at the surface or strengthening winds. Smoke can also make the previously invisible inversion visible. Hazard reduction burns have been carried out by the NSW RFS to the north, west and south of Sydney in recent days. These controlled burns are a regular occurrence at this time of year and aim to reduce bushfire fuel loads before we enter the warmer months of spring. While hazard reduction burns are normal at this time of year, the start of the upcoming bushfire season has the potential be more active than usual. Vegetation is currently drier than usual across much of NSW due to a lack of winter rainfall. The state as a whole registered its driest June-July period in 15 years and some areas have received less than half of their average winter rainfall so far this season. As of 9am today, Katoomba's 127mm of rainfall during winter to date is well below the season's average of 279mm. If the recent trend of dry and warm weather continues into spring, it could increase the risk of early-season bushfires in NSW.