Just when you think nothing is going on…

2012-11-20 17:19:12.000 – Brian Fitzgerald,  Weather Observer/Education Specialist

A distinct layer of haze looking west this PM.

Even though this past week’s weather pattern has been extremely quiet, that doesn’t mean interesting things aren’t happening in our atmosphere. A few days ago I spoke about sun dogs that were a sign of very high level moisture (more specifically ice crystals that help form high-level cirrus clouds) in our atmosphere, which was pretty much the only moisture to be found, given how low our relative humidity on the rockpile has been lately (on average < 30%). Co-currently, there has been a small amount of surface-level moisture that has been manifesting itself as early morning fog in the valleys. For the past three days now low-level haze has been engulfing valleys across the state with a smoky, milky-white obscuration during the afternoons which has been happening beneath 4,000 feet our so. Why is this happening, you may be asking? Well, two major factors come into play: a thermal inversion (in this case a slight warmer air mass sitting on top of a cooler air mass) and air pollution. With a warmer air mass sitting on top of a cooler one (which is the opposite of how surface air masses typically interact) the base of the warmer air mass acts as an invisible ceiling trapping particles close to the surface instead of being carried off. Haze is being formed when pollutants, like fine particles (from automobiles, factories, coal-fired power plants, etc) are released into the air and combine with moisture droplets in the air and are then exposed to sunlight- which in turn refracts sunlight, causing reduced visibility.

When haze is being formed in the air, it’s likely ozone is as well- which spells trouble for sensitive groups who suffer from asthma or other lung-related ailments. Today’s air quality forecast across the state has most counties with a ‘moderate’ air quality rating (meaning unhealthy for very sensitive groups), which is not very typical in or near the winter months except when thermal inversions occur. With many families and businesses emitting smoke from wood stoves and people driving cars, it doesn’t take long for particles to build up and get trapped. Along with slow moving high pressure very little air is moving around, and with all of these factors coming together we have ourselves low-level haze.

For more information about haze and air quality forecast for the state of New Hampshire visit NH Department of Environmental Services’ website. Be sure to look our for our air quality forecasts in MWO’s Regional Forecast.

Brian Fitzgerald,  Weather Observer/Education Specialist