Areas of heavy vegetation generally have distinct climates, which may differ considerably from climates of  nearly  open  areas.  Falling  precipitation  caught  in trees  before  reaching  the  ground  may  be  evaporated, but precipitation, which reaches the ground, does not evaporate or run off readily. Heavily forested areas can absorb and store considerable quantities of water. Snow in forests can be protected from direct insolation by the trees  and  may  stay  on  the  ground  for  much  longer periods   than   snow   on   open,   exposed   surfaces.   In forests,  temperature  maximums  and  minimums  are higher than over open land at the same latitude. Relative humidity    is    also    higher    and    wind    speeds    are considerably lower. REVIEW QUESTIONS Q6-12.     Which climatic control has the biggest effect on climatic elements? Q6-13. A weather station on the western coast of the United States will receive the characteristics of  what  type  air  as  compared  to  a  weather station on the eastern coast? Q6-14. Generally,   how   do   ocean   currents   effect climate? CLIMATOLOGICAL DATA LEARNING OBJECTIVE: Describe the use of climatological data in meteorology and what references and services are available. Climatological records are based on the meteorological    observations    that    are    taken    at    a particular locality. This information may be presented in a number of ways. Temperature records generally include the following  temperature  values:  daily  maximums  and minimums   by   months;   the   extremes;   the   average temperature by year and month; the mean monthly and annual  temperature;  the  mean  monthly  maximum  and minimum  temperature;  and  (sometimes)  the  monthly and seasonal degree-days. Of great climatic significance is the range between the mean temperature of  the  warmest  month  and  the  coldest  month.  Other temperature   data   are   sometimes   given.   These   may include   the   number   of   days   with   the   following temperatures: maximum of 90°F and above; maximum of 32°F and below; minimum of 32°F and below; and minimum of 0°F and below. Precipitation records include the mean annual and monthly totals. The range between the highest and the lowest annual rainfall for a locality is the best indication of  the  dependability  of  the  precipitation.  The  records often show the absolute maximum rainfall and snowfall for   a   24-hour   period   by   months,   as   well   as   the maximum and minimum precipitation for each month. Climatic records usually show data on winds. Such information  indicates  the  mean  hourly  speed  and  the prevailing  direction  by  month.  Also  shown  are  the speed  and  direction  of  the  strongest  wind  for  the  12 months and the year in which it occurred. Data  on  cloudiness,  humidity,  thunderstorms,  and heavy fog are often included. Other helpful data would be   the   frequency   and   distribution   of   cyclones   and anticyclones;  passage  of  fronts;  proportion  of  rainfall and snowfall received from cyclonic storms and local, air  mass  thunderstorms;  and  climatological  data  on upper air conditions. METHODS OF PRESENTATION Climatological  information  is  presented  in  many different  ways.  Tables  are  frequently  used.  Maps  are particularly useful in presenting climatic information in cases  where  geography  is  an  important  factor.  Wind data can be given by means of a device called a wind rose, which presents information on the prevailing wind directions. (See fig. 6-4.) 6-13 4 15 30 15 10 6 10 10 N (NOTE :  NUMBERS REPRESENT THE  % OF  WINDS  FROM THAT  DIRECTION DURING A SPECIFIC TIME) AG5f 0604 Figure 6-4.—A wind rose.