The weather is always a topic of conversation in countries like England where the weather is so changeable and often seems unpredictable. A weather diary is interesting from a scientific point of view and you might wish to get technical -note the wind direction, learn the names of cloud formations, note the temperature.
You may wish to note down how the weather makes things look: the colours of the sky, frost, sunshine on leaves.
You could mention what you have been doing and how the weather affects you – how the path was very muddy; how it was too cold to swim, how good it was to splash in the puddles.