The top two maps on this plate show the mean mixing ratios for January and July, respectively. Mixing ratio is defined as the ratio of the mass of the water vapour to the mass of dry air in a unit volume. The two mixing ratio maps on this plate are based on observations taken at midnight, 6 a.m., noon, and 6 p.m. Greenwich Mean Time over the ten year period from 1941 to 1950 inclusive. Humidity data were obtained from the regular reading of artificially-ventilated dry and wet bulb thermometers housed in standard thermometer shelters. Knowing the dry bulb temperature and the difference in temperature between the dry and wet bulbs, it is possible to calculate the different measures of humidity. The remaining maps show the mean number of days with fog for each of winter, spring, summer and fall. Winter is considered as the months of December, January and February; spring as March, April and May; summer as June, July and August; and fall as September, October and November. Fog is defined as an obscurity in the surface layers of the atmosphere caused by water droplets in suspension in the air which reduce visibility. A day with fog is considered to be a day on which the visibility was lowered to less than five-eighths of a mile (approximately 1 km) by fog for some part of the day.
