While thunderstorms are often associated with the heat of the day, a common query is whether the icy precipitation known as hail can also fall after the sun goes down. The short answer is yes, hail can and does occur at night, although it is a less frequent event than during the late afternoon. Understanding the conditions that create hail helps explain why it is primarily a daytime phenomenon and what allows it to form under the cover of darkness. The mechanism for creating hailstones remains the same, requiring a specific set of atmospheric ingredients to come together.
The Atmospheric Recipe for Hail
Hail formation begins inside a towering cumulonimbus cloud, which is a thunderstorm cloud characterized by strong, sustained currents of rising air called updrafts. These updrafts are responsible for lifting water droplets far above the freezing level into extremely cold regions of the atmosphere. Once a droplet is carried into an area where the temperature is below freezing, it becomes supercooled liquid water, which remains unfrozen even at temperatures below 0°C.
A small particle, like a frozen raindrop or a piece of graupel, acts as a nucleus, or hail embryo, around which the hailstone will grow. As the embryo cycles through the cloud, it collides with the abundant supercooled water droplets, which freeze onto its surface in a process called accretion. The intensity of the updraft is a direct factor in the size of the final hailstone, as a stronger vertical wind can suspend the growing ice mass longer. Once the hailstone’s weight overcomes the force of the updraft, it falls to the ground.
How Daytime Solar Energy Affects Storms
The sun plays a significant role in fueling the atmospheric instability required for powerful hailstorms. Solar radiation warms the Earth’s surface throughout the day, which in turn heats the air directly above it. This surface heating creates buoyant air parcels that rise freely, a process that builds up atmospheric energy called Convective Available Potential Energy (CAPE).
High CAPE values indicate a large amount of energy available to drive deep convection and produce the powerful updrafts necessary for large hail growth. This mechanism explains why the frequency of severe thunderstorms, including those that produce hail, typically peaks in the late afternoon or early evening hours. When surface heating diminishes after sunset, the source of this instability is removed, which leads to a decrease in the intensity of convection.
Frequency and Location of Nighttime Hail Events
While the surface-based instability fueled by the sun fades at dusk, hail can still occur at night because other atmospheric lifting mechanisms remain active. One common context for nocturnal hail is within large, organized storm complexes known as Mesoscale Convective Systems (MCSs). These expansive systems often form in the late afternoon and can persist, growing in size and power throughout the night.
MCSs are frequently sustained by a feature called the low-level jet stream, a ribbon of fast-moving, moist air that develops several hundred meters above the ground. This jet provides a continuous influx of warm, moist air to the storm system, maintaining the required instability and strong updrafts aloft, even as the air near the surface cools and stabilizes. Consequently, the central United States Great Plains, which frequently experience these nocturnal MCSs, see a notable, though still secondary, peak in hail events during the overnight hours. Hail events are statistically less frequent at night compared to the peak daytime hours, but the continued presence of elevated instability means the possibility is real.