You have two issues:
- When mounted, many M42 lenses leave a gap exposing the body contacts. This creates a fairly direct route for water to get into the body. There is some risk with non-sealed K-mount lenses, but much more with many M42.
- The lens itself might be damaged from exposure to the elements
Of the two, I would consider the first to be the more serious. In regards to the second, it has been my experience after using that class of gear for the last 44 years that most vintage primes (and many zooms) are surprisingly good at resisting moisture and dust incursion due to the nature of how they are made. So, what to do?
- Take reasonable care when conditions are harsh. If the grit is in your eyes and teeth, it is best to leave the vintage gear in the bag. Similarly, it is best to keep the camera/lens out of direct rain/spray. Loose-fitting rain gear and/or umbrellas (with an assistant) are useful.
- The rare snowflake is not a cause for concern
- Use a lens hood to keep snow/rain off the front element
- Wipe the lens barrel clear of moisture/dust after the shoot before it gets a chance to get inside
In regards to cold, the main issues are the same as for modern gear: thickened lubricants and condensation. Keep in mind that your lenses (modern or vintage) were not designed to be used under extremes of cold or heat. A good rule of thumb is that if it is cold enough to kill you, it is probably too cold for your gear. Condensation is a more difficult thing. Consider these two cases:
- Camera and lens have been in the nice warm ski lodge where humidity is fairly high due to moisture from drying clothes and tracked in snow. This moisture can and will enter both camera and lens.* Now take the camera out into the -10 C cold. As the camera cools, the moisture deposits on the inside and rear elements of the lens as well as the camera's innards. Not good.
- Camera and lens have been in the very cold and dry -10 C outside and have just been carried into the warmer, but very humid ski lodge. The result is instant fog on all outer surfaces and as the moisture intrudes, many inner surfaces as well.
There are multiple similar scenarios involving moisture and warm vs. cold temperatures. So, what to do? I have a few friends who are working pros that specialize in wildlife photography. The have made it their practice to bag their gear in zip-lock bags before going inside and to avoid opening the bags in a humid room.
Steve
* This is the case even with WR gear. Eventually the inside environment will come to be similar to the outside environment. Remember, we are dealing with gaseous water.