Definition and simple explanation:

The meteorological winter in the northern hemisphere extends over the months of December, January and February. Astronomically, the coldest season begins with the winter solstice, the shortest day of the year. This day usually falls on the 21st or 22nd of December.
At temperatures around freezing some animals keep hibernation, hibernation or even fall into a cold star. Some insects themselves defy snow and ice in their hiding places because their body fluid contains a natural antifreeze that prevents the formation of ice crystals. In contrast, migratory birds prefer to fly to the warmer south before the onset of the first cold snap.
When lakes and rivers gradually freeze, it pulls the fish to the bottom of their water. Water with a temperature of 4 ° C has the highest density and therefore always sinks down. Colder and warmer water has a lower density, causing it to settle over four degrees of cold water. In this context, one speaks of the density anomaly of the water. As long as the lake does not freeze to the bottom, fish can easily survive the winter until spring at a constant 4 ° C water temperature.
Over the winter months, deciduous trees do not leave any leaves to lose as little water as possible. For as long as the ground is frozen, a tree can not pull water out of the ground. Only conifers are still green in winter. Their leaves are especially protected against loss of water (see: Leaf and Needle Leaf).