Air Quality in the Winter

Earlier this month, Delmarva experienced days where the sky looked a bit hazy and even an air quality alert was issued. These are common things to expect during the hot summer months but this event took place in the winter.

The common reason for an air quality alert to be issued during the winter is inversion. An inversion is when temperatures increases while the height increases. Normally in the atmosphere, the temperatures decreases as you go in height, especially in the lowest layer of the atmosphere which is the troposphere.

In a typical atmosphere, warm air rises and cold air sinks but when an inversion takes place, this concept gets complicated and creates for a noticeable dip in air quality. The air that rises from the surface contains pollutants from cars and industries; but when the inversion takes place that air becomes trapped in that lower layer. A calm or light wind will help set up that poor air quality, by not allowing the mixing of the air in the atmosphere, thus keeping that polluted air stagnant near the surface.

The setup for a strong inversion usually takes place at night when calm winds, clear skies and cold temperatures are present. The combination of those 3 elements will lead to air on the surface, especially in the winter, become cooler than the air aloft. At sunrise, this inversion will be at its maximum as the surface will have its lowest temperature of the day, but usually it will not last. The rising sun will weaken the inversion despite a low sun angle, as the ground warms. A strong high pressure like the one observed earlier this month; however can offset this scenario and keep the poor air quality around during the daytime hours and even for a few days.

The Salisbury’s skew-T are computer models for the 4th (left) and 5th  (right) of February, indicating an inversion taking place at 12Z (7AM) for both days. The temperatures at the surface are at the lowest for the day since it’s around sunrise at around 0°C (32°F)to 7°C (45°F), with a warming occurring around 10°C (50°F) at 925 mb or about 2,500 ft. The 3 days with the air quality alert issued; highs in Salisbury climbed from a mild 57° on the 3rd to a warm 69° on the 5th, while lows on all 3 days were in the chilly mid to upper 20s. The air quality has improved over the last week, thanks to a couple of systems bringing mainly rain over the peninsula and this unsettled pattern will continue over the next 7 days.

A code orange air quality alert was issued during those 3 days indicating unhealthy for sensitive groups. This means that individuals with cardiac and respiratory problems along with young children and the elderly should limit time outdoors. The air quality index is a daily report on the air quality which affects your breathing. The main pollutants measured for the index are ozone, particle pollution, carbon monoxide and sulfur dioxide. The air quality index is measured in 6 levels with each level increasing the health concern, as the air quality index values increase. For more information on air quality, check out



Categories: Weather Blog