At sunrise and sunset the sun is very low in the sky, which means that the sunlight we see has travelled through a much thicker amount of atmosphere. Because blue light is scattered more strongly by the atmosphere, it tends to be scattered several times and deflected away in other directions before it gets to us. This means that there is relatively more yellow and red light left for us to see.
The diagram above shows a simplified illustration of these effects. A person standing at position A would see a blue daytime sky, as there is plenty of blue light being scattered in all directions.
At position B, it is evening and a person standing here would see a familiar orange sunset because the blue light has mostly been scattered away in other directions, leaving the reds and yellows.
So, this begs the question 'why is the sky blue?'
To understand why the sky is blue, we first need to understand a little bit about light. Although light from the sun looks white it is really made up of a spectrum of different colours, as we can see when they are spread out in a rainbow.
We can think of light as being a wave of energy, and different colours all have a different wavelength.
When the sun's light reaches the Earth's atmosphere it is scattered, or deflected, by the tiny molecules of gas (mostly nitrogen and oxygen) in the air.
Shorter wavelengths (violet and blue) are scattered the most strongly, so more of the blue light is scattered towards our eyes than the other colours. You might wonder why the sky doesn't actually look purple, since violet light is scattered even more strongly than blue. This is because there isn't as much violet in sunlight to start with, and our eyes are much more sensitive to blue. I'm afraid there's not much sunshine (or blue sky) around today (Wednesday) and I'm afraid there won't be much tomorrow either but at least when the clouds do break, you'll know why the sky is blue!