Question

Describe weather conditions during which hailstones form and the process by which they form.

Short answer

Hailstones form in thunderstorms with strong updrafts, as ice pellets accumulate layers through repeated cycles in the storm's updraft and downdraft, growing until they fall.

Step-by-step solution

Understanding Hailstones

Hailstones are solid ice precipitation that forms within strong thunderstorm updrafts. They are typically spherical or irregularly shaped lumps of ice.

Necessary Weather Conditions

Hailstones form during thunderstorms with strong updrafts, which are upward movements of air in the storm. These storms often form in environments with high instability in the atmosphere, usually during warm months when surface temperatures are high, creating strong convection currents.

Initial Formation

As storm updrafts carry raindrops upward into extremely cold areas of the atmosphere, these raindrops freeze into tiny ice pellets. This initial process requires temperatures below freezing in the upper atmosphere.

Hail Growth Through Updrafts

The small ice pellets are carried by the updrafts above the freezing level of the storm. As they collide with other raindrops and ice particles, they pick up additional layers of ice, growing larger each time they are lifted and fall back down, only to be swept up again.

Falling and Final Size Determination

When the hailstones become too heavy for the updrafts to support, they fall to the ground as precipitation. The size of the hailstones depends on the strength of the updrafts; stronger updrafts can support larger, more layered stones.

Key concepts

Thunderstorm Dynamics

Thunderstorm dynamics encompass the intricate processes within a storm that contribute to the development of hailstones. Thunderstorms occur due to a complex interaction between moisture, instability, and lift in the atmosphere. The dynamics of a thunderstorm are largely driven by the formation of strong updrafts and downdrafts.

Updrafts are powerful upward currents of air that occur within thunderstorms. They are responsible for lifting water droplets high into the atmosphere, where temperatures are cooler. When these updrafts are particularly intense, they transport these droplets into regions with temperatures below freezing, setting the stage for hail formation.

Downdrafts, on the other hand, are downward movements of air that eventually carry precipitation, including hailstones, back toward the ground. The balance and intensity between updrafts and downdrafts define how hail grows and falls with the storm.

Atmospheric Instability

Atmospheric instability is a critical factor in hail formation as it provides the energy needed to power thunderstorms. Instability occurs when air parcels at lower levels of the atmosphere are warmer and less dense than the air above them. This difference causes the warmer air to rise vigorously.

During warm months, when surface temperatures are high, the sun quickly heats the Earth's surface. This warming generates unstable atmospheric conditions, making thunderstorms more likely. As these surface-level air parcels rise into cooler regions of the atmosphere, they create convection currents. These currents further increase the instability by enhancing the upward movement of air, which is essential for hail formation.

In short, without atmospheric instability, the necessary vertical movements within a thunderstorm, such as updrafts, would be much weaker, limiting the potential for hail development.

Convection Currents

Convection currents play a pivotal role in the formation of hailstones by facilitating the upward and downward air movements essential for thunderstorm development. When the sun heats the ground, it warms the adjacent air, causing it to rise. This rising air cools and can condense into cloud droplets as it ascends.

High convection currents result when these droplets are carried higher into the atmosphere where temperatures are below freezing. As droplets freeze and become ice pellets, these currents continue to lift and sustain their journey within the storm.

The cyclical pattern of convection currents ensures that ice pellets are repeatedly circulated within the storm, allowing them to collide with more moisture and freeze layer by layer. This layering process directly influences the size of the hailstones that eventually reach the ground.

Ice Pellet Formation

Ice pellet formation is the initial stage in the creation of hailstones, occurring in the upper regions of a thunderstorm. When updrafts carry liquid raindrops into extremely cold air, these drops freeze into small ice particles. The freezing process begins at temperatures below 0°C (32°F), which are commonly found at higher altitudes within a thunderstorm.

As these ice pellets accumulate moisture and freeze, they increase in size. Each ascent and descent within the storm allows the ice pellet to gain additional layers of ice. The stronger the updraft, the larger the pellets can become, as they are held aloft for longer periods of freezing and growth.

The final size of hailstones depends on how long they remain within the storm, continuing to circulate and collect ice. Eventually, the mass of these ice pellets exceeds the capacity of the updrafts to support them, causing them to fall to the ground as hail.