Many   calculations   in   meteorology   and oceanography  use  the  Kelvin,  or  Absolute,  temperature scale. A kelvin is exactly equal to a Celsius degree in scale, but the starting point of measurement on the Kelvin scale (0 kelvin) is absolute zero, or -273.16°C. That is the temperature at which, theoretically, all molecular  motion  would  stop.  Water  freezes  on  the Kelvin  scale  at  273.16  K  and  boils  at  373.16  K. Conversions may be easily made between the Kelvin and the Celsius scales by addition or subtraction using the  following  formulas: C = K – 273.16 or K = C + 273.16, where K is kelvin and C is Celsius degrees. TIME STANDARDS Though the hour, minute, and second convention is universally used in keeping time, various time zones are also used. In North America, eastern standard time (EST), central standard time (CST), mountain standard time (MST), and Pacific standard time (PST) are used. Standard time zones generally cover strips of the globe, extending north and south parallel to the longitude lines. Each time zone covers about 15° longitude centered on 0° longitude, with all longitudes evenly devisable by 15. Time zone boundaries that cross land masses have been adjusted by local agreement, and often zig-zag. Standard time zones for the world are provided in Appendix III. Throughout most of the world, standard time is 1 hour earlier for each time zone to the west and 1 hour later for each time zone to the east. A list of countries,  provinces,  and  states,  with  their  local standard time departure from the 0° longitude standard zone is provided in the Nautical Almanac, published each  year  by  the  U.S.  Naval  Observatory.  When standard time is used, it is referred to as local standard time (LST) or by a standard zone designation, such as Eastern Standard Time (EST) or Ppacific Sstandard Time (PST). Daylight  savings  time  or  summer  time  is  the convention  adopted  by  most  regions  in  North  America. On the first Sunday in April at 0200, the clocks are set ahead 1 hour. On the last Sunday in October at 0200, they are set back 1 hour. During the summer, time in these regions is called daylight time; for example, Eastern  Daylight  Time  (EDT)  or  Pacific  Daylight  Time (PDT). Other regions of the world have also adopted this  practice. Another measurement of time sometimes used is local mean time (LMT). Local mean time is time measured in 24-hour days relative to the movement of the sun. When the sun is highest in the sky, local mean time is 1200 noon. Within a time zone, local mean time may be off standard time by up to several hours. Local mean time changes by 4 minutes for every degree of longitude. To prevent confusion between the different zones and  types  of  time,  meteorological  and  oceanographic records, charts, and reports use Coordinated Universal Time (UTC). UTC time is kept by using the 24-hour clock.  UTC  is  the  local  mean  time  at  the  Royal Greenwich Observatory in East Sussex, England, at 0° longitude. This time is the same all over the world, regardless of local time conventions. All times in UTC are suffixed with a Z for identification. Because of this, UTC time is sometimes referred to as "Zulu Time." The term Coordinated Universal Time and the abbreviation UTC,  by international agreement, have replaced the older  term  Greenwich  mean  time  and  the  older abbreviation  GMT. ORDER OF OBSERVATION Surface weather observations are completed and transmitted every hour. The various weather elements are actually observed from 5 to 15 minutes before the hour, for routine observations. The time to begin monitoring the elements should be adjusted as the observer gains speed and experience. As a general rule, first observe the elements from outside the weather office, and then from the equipment inside the office. Pressure elements and those elements changing quickly should be observed last. Even when automatic  observation  equipment  is  used,  these  general rules apply. Most of the necessary observation data will come from equipment located within the office spaces, but outside measurements, such as cloud type and visibility,  should  be  done  first. Although many observation records and reporting codes are in use, the formats have many elements in common.   For   example,   all   surface   weather observations  include  data  for  state-of-the-sky, visibility, and temperature. Some observation formats require additional data, such as sea condition, seawater temperature, and sea ice conditions. The following sections cover the various data types and the methods used to obtain the data. These data types are not arranged in any particular code format, but are generally arranged in observation order. 1-2