That was exactly what happened. The rain came and about 5 umbrellas opened in all directions. He spooked and span round, and which point I quickly got off and calmed him down, but the umbrellas still came at him from all directions and he felt so tense (despite me shouting at everyone to put them down). I was then left in a difficult situation because there was absolutely nothing I could use to get back on him again, and to get back to my lorry to use my mounting block meant that I had to walk past the secretary's trailer where the whole queue of people were holding umbrellas!
I thought that I would start off by putting him in the field and then putting up an umbrella outside his field. I will then walk up the down the lane with the umbrella up and see what he does. Once he is happy with that, I will walk around the field putting the umbrella up and down. I thought that if he is in his own field he can do what he likes. Once he tolerates me walking round the field with an umbrella, I will let him free in the school and put the umbrella up there and once he gets used to that I will hold him and put one up directly next to him. When he is happy with that I will ask people to put umbrellas up and down when I am riding in the school. Hopefully this will mean that we won't have a repeat of yesterday!
Why do people put umbrellas up at horse shows anyway? Do people have no sense at all?