I don't think you are reasonable to have to got cross with them, not least because you didn't explicitly tell them to keep her in if it was bad weather.
However, I don't think comparing other children's experiences really helps sometimes. My husband's country is very cold and snowy in the winter, and children do play outside, however, they would never ever go out without a waterproof coat/leggings/hat/gloves, all kitted out etc. Children there are bundled up like you wouldn't believe. Secondly, it's very sunny in winter, so children tend to go out in the day in the sunshine and play in the snow, but as soon as the sun goes in, or their mittens get wet or they get cold, they would come in again. So, they wouldn't go out in gale force weather, as it is today.
Finally, yes, a cold or pneumonia isn't caused (as in directly) by cold weather, but there is some research that shows that putting the body under stress (psychological or physical) makes people more vulnerable to infection. So, extremes of anything, whilst not wrapped up properly, having had three bouts of hospitalization doesn't sound that great to me.
But, clearly you shouldn't have shouted, that's your emotionality over your dd's illness coming out and you need to apologise tomorrow.