A quick way to find you leak is to pressurize the cooling system with a pump (see images below or find a shop that knows X5's quirks).

I think you will find that it the cold to hot cycling that is making it appear only in the colder weather.

Attached Images

 

Attached Images

X5 3.0 cooling system image.pdf (147.5 KB, 568 views)

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2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green