Why does newly fallen snow squeak when you walk on it?Joe Fenton, Hemel Hempstead, Hertfordshire, UK

On cold mornings I notice that train windows mist up on the inside. Once the outside temperature reaches about 6 or 7 °C the windows normally clear. If it is raining, however, the windows mist up at these temperatures or even higher. Why does rain on the outside of the train, even at higher temperatures, cause the inside of the windows to mist up?Klaus Forroby, North Harrow, Middlesex, UKThe following answers were selected and edited by New Scientist staff. You can add your replies in the comments section below.The windows of the train cool through contact with the cold air outside. Heat conducted through the glass (assuming it is single-glazed) will keep the inside face quite close to the outside temperature.The inside face of the glass will steam up when its temperature is lower than the "dew point" of the inside air - that is the temperature to which this air needs to be cooled to become saturated, so that moisture begins to condense out of it. In the case described, the dew point of the air in the train could be just above 7 °C.On a rainy day the passengers' clothing will be wet and their umbrellas will be dripping onto the floor and dampening the upholstery. The heat inside the train will cause the rainwater to evaporate, markedly raising the moisture content of the air. The dew point inside the carriage will therefore be higher. Consequently, more moisture will condense on the window on a rainy day than on a dry one. This effect will be exaggerated if windows are shut, trapping the moist air inside the carriage.Martin Young, Honiton, Devon, UKOne factor not to be dismissed is that rain wets the outside of the window and, as it evaporates, it cools the glass. If the train is moving, wind increases the evaporation and adds to the cooling. So the window glass will be significantly colder than on a dry day of the same temperature. Now, I wonder if I can use this to cool a bottle of beer?Spencer Weart, Director, Center for History of Physics, American Institute of Physics, College Park, Maryland, US

How do clouds like this form (see Photo, left)?B. J. Baxter, Ilford, Essex, UK

Living in an older home with single-glazed windows, I have grown used to seeing intricate patterns of frost on the panes each winter. However, I was truly impressed by a twining vine-like pattern that appeared this January (see Photo), and I would love to know how it came about. The vine-like shapes formed in a 20-by-30-centimetre section of the window and were surrounded by standard snowflake-shaped frost. The "vines" were 1 centimetre wide with small dots running up the centre, and they twisted about each other with leaf-like shapes sprouting from the sides. The photo shows a section measuring about 6 by 10 centimetres. It had been a particularly cold day (-20 °C) and the sun was shining on the window. My wife suggested that the sunlight shining through the branches of a tree 2 metres away had caused this, but there were no distinct shadows visible on the window at the time.Ken Zwick, Neenah, Wisconsin, USNo clear answer to this one, but some hypotheses. This phenomenon has been seen by many people - Ed