Final answer:
Thick masses of snow accumulate and can lead to avalanches because they are b) compressed over time. The transfer of heat necessary for snow melting is a gradual process, which is influenced by air temperature. Insights into phase changes of water help to understand why snow can remain even in temperatures above freezing due to insufficient heat exchange.
Step-by-step explanation:
The thick masses of snow that can create big avalanches accumulate primarily because b) they are compressed over time. Snow forms from ice crystals and is therefore a solid phase of water. Accumulation and compression occur because enormous heat is necessary for phase changes, and it takes time for heat to be transferred from the air. Even if the air temperature is above freezing, this heat exchange does not happen instantaneously. The speed at which snow melts is dependent on air temperature - the warmer the air, the quicker the heat exchange and the faster the snow melts. However, snow can remain on mountain slopes because it reflects most of the incoming energy from the sun, absorbing only a minimal amount which is often insufficient to cause significant melting. Additionally, phase changes regarding water and snow involve significant energy exchange. This aspect is critical in various environmental conditions. For example, because ice is less dense than water, lakes freeze from the top down, allowing aquatic life to survive underneath the insulating ice layer during winter. The same principle of heat required for phase change ensures that snow accumulates over time on mountain slopes, leading to the potential for avalanches. In contrast, substances like ice mixed with salts used on roads lower the melting point of the snow, hastening its phase change and making the roads sloppy and messy in winter.