Your reasons for choosing to go out after barbel in summer time are simple, there is a good chance of bites and the countryside is wonderful at this time. When low, warm and clear the river is conducive to feeding fish. If it is overcast or raining, even better really, they are even less suspicious.
Then comes winter and the rise and fall of temperature and pressure, catch the rivers right and you can be in for busy sessions. I recommend keeping a close eye for rises in temp and drops in pressure………..”A Time To Fish”