Meteorological thermometers are often filled with either alcohol or mercury. By international agreement, the nations of the world have decided to measure temperature in a similar fashion. This standardization is important for the accurate generation of weather maps and forecasts, both of which depend on having data determined in a uniform way. Weather stations worldwide try to determine minimum and maximum temperatures for each and every day. By averaging these two values, daily mean temperatures are also calculated.
Many stations also take temperature readings on the hour.
Well ventilated instrument shelters are used to protect thermometers from precipitation, direct sun, and other physical elements. Construction standardization of these shelters, by international agreement, guarantees that measurements are comparable in any of the over 15,000 weather stations found worldwide. Thermometers found inside the instrument shelter are mounted approximate 1.5 meters above the ground surface. The top thermometer contains alcohol and is used to daily minimum temperatures. The lower thermometer uses mercury to determine the daily maximum temperature.