. The climate and weather of Baltimore . iform precipitation throughout the year. This is a condi-tion which prevails with limited exceptions between latitudes 35° and60°. Within the tropics, and over limited areas elsewhere, as in theupper Missouri Valley, there are large areas in which most of the annualfall of rain occurs in the summer months, with light rain in winter andspring. In other regions, as in the Pacific coast states, nearly all of theprecipitation occurs in the winter months with little or no rain in sum- MARYLAND WEATHER SERVICE 165 mer. Lastly there are the arid regions of the
. The climate and weather of Baltimore . iform precipitation throughout the year. This is a condi-tion which prevails with limited exceptions between latitudes 35° and60°. Within the tropics, and over limited areas elsewhere, as in theupper Missouri Valley, there are large areas in which most of the annualfall of rain occurs in the summer months, with light rain in winter andspring. In other regions, as in the Pacific coast states, nearly all of theprecipitation occurs in the winter months with little or no rain in sum- MARYLAND WEATHER SERVICE 165 mer. Lastly there are the arid regions of the world which are nearlyfree from rain throughout the year. Hourly Amount of Rainfall. A diurnal period in the relative amounts and frequency of rainfall ismost distinctly revealed in tropical countries, but is still clearly shownin the summer months of the middle latitudes. The precipitation whichoccurs in connection with the movement ot a general barometric depres-sion has a fairly uniform distribution throughout the day; that occur-. FiG. 44.—Average Hourly Precipitation. The average amount of rainfall or snowfall during each hour of the day, for everymonth of the year, is shown by the heavy black lines and the shaded areas. The lightshades show the time of day and year when the precipitation is usually lightest. Thefigures attached to the curved lines show the amount of the precipitation in hundredthsof an inch. The values are based on the ten years record of a tipping-bucket rain-gage and on eye observations. Only days with an appreciable amount of precipitationwere considered in determining the average amount of precipitation for the day. ring in connection with thunderstorms is restricted mostly to the after-noon hours, and is intimately associated with the diurnal variation intemperature and pressure. The most conspicuous feature of the diagram representing the hourlyquantity of rainfall (see Fig. 44) is the uniform distribution of the pre-cipitation throughout the d
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