A frequency distribution for the hour of most frequent occurrence of drizzle for North America reveals a sharp increase from 9 UTC to 12 UTC followed by a steady decline from 13 UTC to 23 UTC. East coast stations of the United States tend to report drizzle earlier in the day (~10-11 UTC) than West coast stations (~14-15 UTC). This result suggests that drizzle is affected by the diurnal cycle because the increase in mixing of the boundary layer at sunrise produces instability, resulting in turbulence that increases opportunities for collision-coalescence to produce drizzle.
A monthly distribution of maximum drizzle occurrence reveals that 40% of the stations in North America have a drizzle maximum from November to January. These stations cover much of North America, except for central Canada where drizzle occurs most frequently in September. Due to either local effects or a low number of drizzle observations over the 15-year period, a small percentage (13%) of stations dispersed around the continent report drizzle maxima from June to August.
The combination of low visibility and supercooled drizzle droplets presents a weather hazard, especially for aviation. An analysis of the surface conditions associated with drizzle indicates that 68% of the drizzle observations occurred at visibilities less than or equal to 6400m (4 mi). Many stations in Alaska, Canada, and the northern United States have reports of drizzle at these low visibilities with subfreezing temperatures. An understanding of the hourly and monthly distributions, surface conditions, and upper-air conditions associated with drizzle in this region will help forecasters better anticipate these potentially hazardous events.