In 3.70 beta3 the weather data is not saved in a file named WEATHER.DAT, but now it is in GLOBWEATHER.DAT.
Ukhata, a screenshot would be a helpful, to determine what is "in the middle of summer" in your game. Snow and ice in the early June doesn't sound like a bug to me, just one of those years when summer starts late. But snow and ice in mid-July sounds a lot more like a bug. So, to clarify this, we'd need to know what you mean by "in the middle of summer".