Precipitation types the preceding adjective "freezing", becoming known as freezing rain or freezing drizzle. Frozen forms of precipitation include snow, ice needles, sleet, hail, and graupel. Their respective intensities are classified either by rate of fall, or by visibility restriction. Precipitation falls in many forms, or phases. They can be subdivided into: The parenthesized letters are the METAR codes for each phenomenon. Precipitation occurs when local air becomes saturated with water vapor, and can no longer maintain the level of water vapor in gaseous form. This occurs when less dense moist air cools, usually when an airmass rises through the atmosphere. However,
February 9–10, 2010 North American blizzard and out of Bradley International Airport, and Metro North operated trains on a reduced schedule on February 10. Around the local Philadelphia area, snow began during the early evening on the 9th. As warmer air moved in aloft, the snow changed to sleet and freezing rain between 3 a.m. and 6 a.m. EST on the 10th. Surface temperatures responded slower and the sleet and freezing rain changed over to rain during the morning of the 10th. Some sleet mixed in from time to time. As the low pressure system moved northeast of the region, the rain changed back to snow