UPdate noon Sunday: Air quality Unhealthy for Sensitive Individuals (orange) and Unhealthy (for everyone, red) in Pennsylvania, especially the Lehigh Valley. See Airnow.gov for updates. Forget the groundhog. Air quality forecasting continues to improve as weather forecasts gets better. But as emissions of pollution have decreased, it can be more challenging to predict when air quality will cross the line into unhealthy concentrations. The state and local air quality offices have issued an advisory for people who are sensitive to particulate pollution. Some areas of southeastern Pennsylvania had code orange concentrations already on Saturday. Particle pollution is a combination of smoke from combustion and sulfates and nitrates formed in the air. Everything that burns fuel – cars, trucks, power plants, open fires – makes smoke. And they make sulfate particles from sulfur impurities in our fuels. Taking sulfur out of our oil and gasoline reduces the amount of particles that get into our air and hurt our lungs, especially the developing lungs of children. This weekend, as temperatures increase aloft and some areas have snow on the ground keeping the surface cool, we have conditions that keep pollution trapped near the ground. The cold air is denser than the warm air, so the warm air aloft keeps the cold air down. After the last week of bitter cold air, we’ve all had the experience of warm air rising and the cold air sitting on the floor near our freezing feet. So it is outdoors. The Naval Research Center works to understand how smoke and dust form and move around the globe. They don’t like the idea of our naval forces not being able to see folks sneaking up on our ships in fog, smoke, dust or haze. So, they forecast the formation and transport of these particles. Here’s the forecast from Saturday through mid-next week: