Question

Snow Why does a pile of snow slowly shrink even on days when the temperature never rises above the freezing point of water?

Short answer

A snow pile shrinks even on days when the temperature remains below the freezing point of water due to the process of sublimation. Sublimation occurs when snow (solid-state) changes directly into water vapor (gaseous state) without melting into liquid water. This is caused by various factors including low relative humidity, wind, and solar radiation. The combination of these factors accelerates the transition of snow particles into water vapor, thus reducing the size of the snow pile over time.

Step-by-step solution

Introducing the states of water

Water exists in three states: solid (ice or snow), liquid (water), and gaseous (water vapor) forms. For a snow pile to shrink, some of the solid-state water (i.e., the snow) is transforming into the gaseous state (i.e., water vapor).

Understanding sublimation

Sublimation is the process where a solid changes directly into a gas without passing through the liquid state. In the case of the snow pile, sublimation occurs when the snow (solid-state) changes directly into water vapor (gaseous state) without melting into liquid water.

Factors contributing to sublimation

Several factors contribute to the sublimation of snow, even when temperatures are below the freezing point of water. 1. Relative humidity: Lower relative humidity in the air increases the rate of sublimation because the air has a greater capacity to absorb the water vapor produced by the snow pile. 2. Wind: Wind helps to remove water vapor from the surface of the snow pile, accelerating the sublimation process. 3. Solar radiation: Sunlight heats the surface of the snow, supplying energy for sublimation, even if the air temperature remains below freezing.

Result of the snow pile shrinking

The combination of these factors causes the snow pile to shrink through sublimation, even on days when the temperature never rises above the freezing point of water. As snow particles become water vapor, the size of the snow pile decreases over time.

Key concepts

States of Matter

Matter can exist in different _states_, primarily solid, liquid, and gas. These states are defined based on the arrangement and energy of particles. In a **solid**, particles are tightly packed and vibrate in place, providing a fixed shape and volume. A **liquid** has particles that are more loosely arranged, allowing the substance to take the shape of its container while maintaining a consistent volume. In a **gas**, particles have the highest energy and are spread out, allowing the gas to fill the volume of its container completely.
Water, which most of us are familiar with, is a great example of something that can exist in all three states: ice (solid), water (liquid), and steam (gaseous water vapor). Each state has properties influenced by temperature and pressure. When temperatures hover around the freezing point, water can fluctuate between a solid and gas, demonstrating the fascinating dynamic nature of matter.

Solid to Gas Transition

The process where a solid changes directly into a gas without first becoming a liquid is called **sublimation**. This might sound complex, but it's a fascinating concept. Imagine, instead of ice melting into water and then evaporating into steam, the ice goes straight to gas. It skips the liquid stage!
This transition is what happens with snow shrinking on cold, dry winter days. Despite temperatures staying below freezing, the snow doesn't need to melt to disappear. The solid snow particles gain enough energy to break free directly into the gas phase. You might see this when a mound of snow slowly gets smaller even when it's cold outside. The snow turns to water vapor and seems to vanish!
Common examples of sublimation apart from snow include dry ice (solid carbon dioxide) and certain air fresheners.

Factors Affecting Sublimation

Sublimation isn't just a result of a change in state; it's influenced by various **factors**. Understanding these will help make sense of why snow vanishes even without melting. Here are the key players:

• **Relative Humidity:** When the air is dry or has low relative humidity, there's less water vapor in it. This dryness draws water vapor from the snow, speeding up sublimation.
• **Wind:** A breezy day contributes to sublimation by moving the air around, whisking away the water vapor from the snow's surface. This removal encourages more snow to turn to vapor.
• **Solar Radiation:** The sun plays a crucial role. Even when the air is cold, sunlight can heat the top layer of snow. This extra heat provides the energy needed for sublimation to occur, much like how the ice cubes disappear from your freezer over time.

These factors combined ensure that snow sublimates effectively, which is why you may see mounds of snow shrinking over time, even if melting temperatures are not achieved.