Answered by Shaykh Ilyas Patel
The Definition between Subh Kadhib (False Dawn), Subh Sadiq (True Dawn)& Shuruq (Sunrise)
Dawn is that moment when the reflected and refracted rays from the sun begin to reach the earth. Dawn happens much in advance of the actual sunrise.
In the very beginning, a vertical white streak of light appears above the horizon [Termed Subh Khadib] and as the sun approaches, the rays of light change their direction and begin to spread over the horizon. [Termed Subh Sadiq]
Then with further approach of the sun, the refracted rays also appear with the result that the intensity of the light increase and the colour changes from white to pinkish and then to yellowish. The sunrise follows this moment. [Termed Shuruq]
[from: Prayer Times for the United Kingdom & Ireland (with a detailed discussion of the Twilight Phenomenon and the solutions related to the Absence of true Night in higher latitudes) ]
Compiled by Moulana Yakub Qasmi and Tariq Muneer B Eng, Msc. Phd, MI Mech E
It is mentioned in Nurul Idah:
The time for morning is from the rising of the true dawn until before the sunrise
(Imam Shuranbulali, Nurul Idha pg 55)
The explanation to this text by Imam Izaz Ali in his marginal commentary is as follows:
The second dawn is called True because it is truthful and clear. And the first is called False because there is illumination on the horizon but it gets dark and the light fades away and darkness appears. Thus it is termed False.
And Allah Knows Best
[Shaykh Ilyas Patel of Keighley]