Accretion (meteorology) Accretion (meteorology) Accretion is defined as the gradual collection of something over time. In meteorology or atmospheric science it is the process of accumulation of frozen water as precipitation over time as it descends through the atmosphere, in particular when an ice crystal or snowflake hits a supercooled liquid droplet, which then freeze together, increasing the size of the water particle. The collection of these particles eventually forms snow or hail in clouds and depending on lower atmosphere temperatures may become rain, sleet, or graupel. Accretion is the basis for cloud formation and can also be seen as water accumulates
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