Two pen arms operate to record both temperature (top) and relative humidity (bottom). Human hair actuates the lower pen (RH), while a bimetal strip actuates the top pen (temperature). Vertical red lines denote day/date/time. Horizontal red lines denote temperature in degrees Fahrenheit (top), and relative humidity in percentage (bottom). Here you can clearly see the effects of daylight on temperature and nighttime on humidity. Humidity goes up at night as temperatures cool to, or very near to (in most cases), dew point temperature. Temperatures rise during the day due to shortwave solar energy being absorbed at Earth’s surface and emitting from the surface at thermal longwave terrestrial radiation which warms the troposphere in which we all live.

~ Steve Woodruff and Devin Lussier