We are able to observe the sunrise about two minutes before the sun gets above the horizon because of the phenomenon of refraction. When the Sun is slightly below the horizon, the light coming from the Sun travels from a less denser air to a denser layer of air due to which the light gets refracted in downward direction. So, due to the atmospheric refraction, the sunrise is observed two minutes before the Sun gets above the horizon.

It can be shown with the help of following diagram: