• When the reader clicks on the link ‘Forecast Next night’ he/she accesses to the forecasts of the next night i.e. the night that is coming. When he/she clicks on the link ‘Forecast Previous nights’ he/she accesses to the forecasts of the three last previous nights. In both options (‘Next nights’ and ‘Previous nights’) the reader can find a summary of the most important key figures of merit condensed in a ‘Forecast in a nutshell’. By clicking on the different curtains visible on the top of the page, he/she accesses to a complete and detailed description of the characteristics of the individual night. The access to ‘Forecast Next night’ is protected by a password. It is accessible only to the LBT community i.e. astronomers in charge to carry out observations, instrument scientists, people in charge to schedule observations and instrumentation to be used during the night.


  • Model forecasts cover the whole night of observations, each night being identified by the date expressed in Mountain Standard Time (MST) when the night is starting (sun set). MST is the local time at Mt.Graham.

    In Fig. 1 are shown the UT and MST time axes. Different days are identified following the UT time.
    Considering that in principle people could connect to the web site from different locations in the world (Tucson, Italy, Germany, US), in the following lines we report instructions to interpret correctly the model outputs with precise temporal references in order to avoid misunderstandings.

    The web site permits the vision of the forecast of the night that is coming (link ‘Forecast Next Night’) let’s call that DAY J (where J is the day in Universal Time (UT)) and the forecasts of the previous night (link ‘Forecast Previous Nights’) let’s call that DAY (J-1).
    The simulation of the night DAY J starts at 23:00MST of the DAY (J-1) (symbol ‘diamond’ on the MST axis).
    At 21:00UT/14:00MST of DAY (J-1) (symbol ‘star’ on the MST axis) the forecast of the DAY J are displayed on the website through the link ‘Forecast Next Night’. At the link ‘Forecast Previous Nights’ are visible the forecasts of the DAY (J-1).
    The simulations of DAY J done by the model cover the period starting from 00:00UT of DAY J to 15:00UT of DAY J (or 17:00MST of DAY (J-1) to 8:00MST of DAY DAY J).

    The displayed ‘Forecasts Next Night’ covers the time frame between the sunset and the sunrise of the DAY J, which are automatically computed for the LBT site (considering position, altitude and refraction index of the atmosphere). Where possible also the times of the astronomical dawn and dusk are shown. The displayed ‘Forecast Previous Nights’ cover the time frame between the sunset and the sunrise of the DAY (J-3),DAY (J-2),DAY (J-1).

    At 16:00UT/9:00MST of DAY (J-1) (symbol ‘circle’ on the MST axis) the website is updated and the forecasts related to the night that have just finished are moved to forecast of previous night (see link ‘Forecast Previous Nights’). Just after 16:00UT/9:00MST of DAY (J-1) the website in the link ‘Forecast Next Night’ will show a warning stating that the forecasts of the DAY J will be displayed at 21:00UT/14:00MST of the DAY (J-1).

    Initialisation data (i.e. forecast from the ECMWF) are calculated at 00:00UT of the DAY (J-1).
Forecast timeline

Fig. 1: Forecast timeline with all the relevant events. Click on the figure to magnify the image.


  • The whole year is defined into two macroscopic seasons:
    "SUMMER": [April – September]
    "WINTER": [October - March]
    When computing time averages, the whole night time frame is divided into three parts, identifying the first, central and last part of the night. The external thresholds are the dusk and dawn. The internal time thresholds depend on the current season as defined previously (see Fig. 2).
    FIRST PART OF THE NIGHT: The thresholds are 06:10UT/23:10MST (summer) or 05:30UT/22:30MST (winter).
    CENTRAL PART OF THE NIGHT: The thresholds are 06:10UT/23:10MST and 8:50UT/01:50MST (summer) or 05:30UT/22:30MST to 09:30UT/02:30MST (winter).
    LAST PART OF THE NIGHT: The thresholds are 08:50UT/01:50MST (summer) or 09:30UT/02:30MST (winter).
Night partition

Fig. 2: Night partition. Click on the figure to magnify the image.


Each day is divided in phases by the following delimiters (see Fig.3):
  • SUNRISE: The sun disk rises crossing the horizon (which is relative to the observer position) and daytime starts officially.
  • SUNSET: The sun disk falls crossing the horizon. Daytime ends and twilight period starts. During the twilight, ground illumination is not direct but is instead coming from the sunlight scattering in the upper atmosphere.
  • DUSK: The sun disk falls below 18° from the horizon. At this point there is almost no illumination coming from the sun on the lower Earth's atmosphere.
  • DAWN: The sun disk rises above 18° from the horizon. The lower atmosphere starts to be illuminated by scattered sunlight moving into the twilight phase.
Day/Night delimiters

Fig.3: Graphic representation of the twilight,
sunset, sunrise, dawn and dusk.