Snow and hail are both forms of precipitation that fall as ice, but their creation processes and the atmospheric conditions required for them to form are fundamentally different. Snow develops in relatively calm, cold air masses, resulting in delicate, low-density flakes. Hail, conversely, is a product of violent, convective weather systems like intense thunderstorms, leading to the formation of hard, dense ice pellets.
The Formation of Snowflakes
Snow formation begins in clouds where temperatures are below freezing, typically initiated by deposition. This is the direct transformation of water vapor into ice without first becoming liquid water. Tiny airborne particles, such as dust or pollen, act as nuclei, providing a surface for the water vapor molecules to freeze onto and start the crystal’s growth.
The ice crystals grow into intricate, six-sided shapes based on how water molecules arrange themselves when they freeze. As the crystal falls, it absorbs more water vapor, which freezes onto its surface. The specific temperature and humidity levels encountered determine the crystal’s final shape, producing everything from simple hexagonal plates to complex stellar dendrites. These individual crystals often collide and stick together to form the larger, light, and fluffy aggregates we recognize as snowflakes.
The Turbulent Origin of Hail
Hailstones originate in towering cumulonimbus clouds, which are characteristic of severe thunderstorms. The process is driven by strong, sustained updrafts of warm, moist air rising rapidly from near the surface. These vertical winds lift water droplets into the very cold upper regions of the cloud, often well above the freezing level.
In these cold altitudes, water droplets remain liquid even at temperatures below \(0^\circ\text{C}\), a state known as supercooled water. Hail formation starts when a small ice particle, called a hail embryo, collides with these supercooled droplets. The droplets instantly freeze upon contact, a process called accretion, adding a layer of ice to the embryo. The powerful updrafts repeatedly suspend the growing hailstone, allowing it to cycle through the cloud, collecting more supercooled water and growing larger with each pass. This cyclical growth creates the distinct, concentric layers of ice often visible when a hailstone is cut open.
Distinguishing Factors: Storm Type and Structure
The primary difference between the two forms of precipitation lies in the required atmospheric stability and temperature profile. Snow typically requires cold temperatures throughout the entire column of the atmosphere, from the cloud base to the ground, and forms in stable, stratiform cloud systems. The ice crystals drift gently to the ground, resulting in the soft accumulation associated with winter weather.
Hail requires intense atmospheric instability to fuel the violent updrafts of a severe convective storm, such as a thunderstorm. These storms often occur in warmer months or during warm weather fronts, where surface temperatures are well above freezing. The physical structure is completely different: snowflakes are delicate, dendritic ice crystals with a low density, while hailstones are hard, dense, and often irregularly shaped pellets of layered ice. The size of a hailstone directly correlates with the strength and duration of the storm’s updrafts, as stronger winds can keep the ice suspended longer for greater growth.