NLCs (noctilucent clouds) are clouds that sometimes occur in summer in the mesosphere. As the sun is close to the horizon at night in summer, the NLCs are then still illuminated by sunlight while it is dark below. This diagram allows computing for a certain location and time how high above the horizon NLCs are still illuminated by the sun (provided they are present at all!). This maximum NLC elevation angle is computed with respect to the azimuth of the sun. E.g. if the sun is located at 315° (NW) the NLCs can be seen up to the maximum elevation angle above that point. Elevation angles greater than 90° indicate that the visible portion of the NLCs reaches beyond the zenith to the opposite part of the sky.
Clouds in the troposphere far from the observing location can intercept sunlight so that the NLCs are plunged into darkness earlier than compared to a clear atmosphere. This fact can be adjusted by using the parameter Tropospheric clouds at x km. Additionally, you can adjust the altitude of the NLCs themselves. However, these usually occur in a narrow region between 80 and 86 km.
Longitude is in degrees East, i.e. negative for degrees West. All other parameters should be self-explanatory.
The dark blue line in the diagram shows the maximum NLC elevation angle above that point of the horizon where the sun is located. The respective azimuth values of the sun are shown just above the plot area. Additionally, the direction is given as well.
The solid yellow line in the diagram represents the elevation of the sun during the daytime and its depression at night. The dashed yellow lines show important twilight thresholds, sunset, sunrise, midnight and midday. If NLCs are present, they can usually be seen after and before civil twilight, i.e. when the sun is at least 6° below the horizon (depression of +6°).