Pray Fajr When There Is Light Pollution or Overlapping of Twilights
13-3-2019 | IslamWeb
Question:
In Germany around summer time we have the problem of "persistent twilight" which basically means that the end of evening twilight (Maghrib) "overlaps" with the morning twilight (Fajr), if we use 18° for Fajr. In cities we can not really observe it because of light pollution and hence the Muslims here have different prayer times. In Fatwa 268246 you recommended ISNA-Times for Northern Europe which means 15° for Fajr and Isha, which confuses me because why should the Fajr here look different than for example the one in the Arab lands (which usually use 18° to 19.5°)? Please tell me what website/app I could use to know the exact times, because the popular ones like IslamicFinder, Muslim Pro etc. show different times.And if it does not cause any inconvenience, could you tell me the prayer times for Cologne, Germany on 21/06/2019.
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
If you are confused about the beginning time of Fajr prayer because of the light pollution, or the overlapping of the two twilights -as you mentioned -then do not pray until you become certain that its time has started, or you predominantly believe so; this is very easy, praise be to Allah.
This is because the prayer, whether it is Fajr or other prayers, is not valid unless one is certain or predominantly believes that its time has started. The prayer is not valid while doubting that its time has started. Ibn Qudaamah 
said:
"If a person doubts that the time of the prayer has started, then he should not pray until he becomes certain that the time has started, or he predominantly believes so. An example for this is a person who has the habit of doing something estimated to last until the time of the prayer, or a reciter (of the Quran) who has the habit of reciting part of the Quran, (and he finishes when the time of the prayer becomes due), and other similar examples. Therefore, if one does so and predominantly believes that the time of the prayer has started, then it is permissible for him to pray. However, it is desirable for one to delay the prayer a little while longer after that, just for precaution, to increase the likelihood of the belief that the time actually started. Nonetheless, if one fears that the time of the prayer will be elapsed, then he should not wait this extra time." [End of quote]
We cannot tell you exactly the prayer times in Cologne or elsewhere. However, you can ask people and they may guide you to a useful site or application to use for this propose.
For more benefit, please refer to Fatwa 387526 and 326176.
Allah knows best.