High pressure controls New England this week, bringing lovely weather. In a high pressure system, air descends from above; since the cold air high in the atmosphere cannot hold very much water vapor, this descending air is dry and causes clear, sunny conditions. Due to the lack of clouds, the surface radiatively cools very efficiently at night, leading to cool mornings and a large diurnal temperature gradient. Low pressure systems have the opposite effect; moist surface air convects upwards, condensing and forming clouds at the altitude where the temperature is too low to hold excess water vapor. At least for this week, we shouldn’t see those sorts of clouds!