what change happens in the nature when 'fajr ul sadiq starts

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Indeed there are two kinds of Fajr.

1-     The true Fajr which starts when dawn breaks horizontally from the east direction without any darkness following it. Allaah Says (what means): {…and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), …}[Quran 2:187].

2-     The false Fajr: it starts before the true Fajr showing its light vertically in the sky and it is followed by darkness. However, the true and false Fajr might not appear to the people who live in cities and they might not be able to distinguish between them as there are many lights in the city. So, the city-dwellers rely on the approved calendars, and these calendars differ from place to place, and each country has its own rulings and they rely mostly on natural signs upon which Allaah conditioned the rulings of the time of the prayer.

Allaah Knows best.