Hail can occur in the summer, despite the idea of ice falling from the sky during a hot season seeming contradictory. Hail is a form of precipitation composed of solid, irregular lumps of ice that only form within a specific type of storm cloud. Although surface temperatures peak during the summer, the atmospheric conditions high above the ground are frequently optimal for hail production. Hail events are most common in the late spring and early summer in many mid-latitude regions.
Essential Atmospheric Requirements
Hail formation requires three atmospheric components, all characteristic of powerful summer storms. A strong thunderstorm, specifically the towering cumulonimbus cloud, can extend tens of thousands of feet into the atmosphere. This cloud serves as the factory where hailstones are created.
The second requirement is an abundant supply of moisture, readily available in warm, humid summer air, which provides the water droplets and vapor necessary for ice to form. Finally, there must be a freezing layer of air aloft, where temperatures are at or below 0°C (32°F), even if the ground is hot. This freezing level is located well above the ground, often between 10,000 and 20,000 feet, providing the necessary sub-zero environment.
The Mechanics of Hailstone Growth
Hail formation begins when water droplets are carried upward into the frigid upper reaches of the cumulonimbus cloud by powerful, rising air currents called updrafts. Once these droplets rise above the freezing level, they transition into tiny ice particles or collide with existing particles to form a hail embryo. The environment high in the cloud contains supercooled water droplets, which are liquid water existing below the normal freezing point.
As the hail embryo is suspended by the updraft, it collides with these supercooled droplets, which instantly freeze onto its surface in a process known as accretion. Updraft strength determines how long the hailstone remains suspended and how large it can grow. A cross-section of a large hailstone often shows an onion-like structure of alternating clear and opaque layers, representing cycles through different regions of the cloud.
The hailstone continues to grow until it becomes too massive for the updraft to support its weight. At this point, the stone begins its descent, falling through the cloud and eventually reaching the ground. Larger hailstones fall at higher speeds, sometimes exceeding 90 miles per hour, which allows them to survive the descent through the warmer air near the surface without completely melting.
How Summer Heat Fuels Hailstorms
Paradoxically, high summer surface temperatures drive the most severe hailstorms. Intense solar heating causes the air near the surface to warm rapidly, creating extreme atmospheric instability. This instability means that a pocket of warm, buoyant air will rise freely and rapidly when given a slight upward push.
This rapid ascent creates the necessary violent, towering updrafts that are essential for hail formation. The stronger the updraft, the longer a hailstone can be held aloft in the freezing zone, allowing it to collect more supercooled water and grow to a larger size. Summer storms generate the most powerful updrafts because of the significant temperature contrast between the hot, moist surface air and the cold air aloft.
While summer months have a higher freezing level in the atmosphere, the intensity of the convective energy overcomes this. The extreme buoyancy drives moisture high enough to sustain the growth process for a longer duration, resulting in the large, damaging hailstones commonly associated with summer severe weather. The combination of heat, moisture, and instability makes summer the peak season for these powerful storms.
Clarifying Ice: Hail, Sleet, and Graupel
Hail is often confused with other forms of frozen precipitation, but its formation process gives it unique characteristics. Hail consists of hard, solid balls or irregular lumps of ice that form exclusively inside the violent updrafts of a thunderstorm. The size of hailstones typically starts at 5 millimeters in diameter, with some growing to the size of a grapefruit.
Sleet, by contrast, is not associated with thunderstorms and forms through a different atmospheric temperature profile. It consists of small, translucent ice pellets that form when snowflakes or raindrops fall through a warm layer, partially melt, and then refreeze completely in a deep, sub-freezing layer near the ground. This process is common in winter, not summer.
Graupel, sometimes called soft hail or snow pellets, is composed of soft, opaque pellets formed by riming. Graupel develops when a snow crystal falls through a layer of supercooled water droplets, which freeze onto the crystal’s surface. Unlike the hard, dense structure of hail, graupel is fragile and easily crushed between the fingers.