H

Hail - The worst hailstorm happened in Denver July 11, 1990. Insurance claims mounted to the hundreds of millions of dollars. The hail ranged from pea to golf ball size.
Scientists can learn all about a storm's structure, duration and strength by cutting a hailstone in half. Resembling a cross section of an onion or a tree, the rings allow scientists to determine approximately how many trips the stone made up and down through the storm. Water droplets and ice crystals rise and fall in strong updrafts and downdrafts within a storm. The pellets eventually become so large the updraft can't support them, and they fall to the ground as hailstones. The size of the hail determines how fast it falls. For instance, pea size hail drops at about 12 miles per hour - walnut size at a little faster (25 mph) and a golf ball size stone at nearly 60 miles per hour.

Halo - The term halo is generic for all rings, arcs and spots produced by the reflection and refraction of light by ice crystals in the atmosphere. There are two major types of ice crystal, the hexagonal (six sided) plate and the hexagonal column - these two types account for almost all observed halos. They can create a variety of shapes - rings, arcs, etc, because the tiny crystals are swirled around in different ways by the motions of the air.

Reflection is a familiar concept to everyone as we see our reflection in a mirror or on the surface of a calm lake. When light hits a flat, smooth surface, it bounces off that surface at the same angle as it approached. Hexagonal plate type ice crystals behave like millions of tiny flat mirrors which reflect the image of the sun or moon. This can create a false sun or moon that can be seen to the side of the actual object. If the ice crystal are shaped into six sided columns instead of flat plates, they reflect the light differently as they tumble through the atmosphere and may create a column of light - we call this a "sun pillar". Sometimes these pillars can be seen in the winter during extremely cold weather as tiny ice crystals float above street lights and create a similar effect.

Refraction is a phenomena that occurs when light penetrates the surface of a flat water or ice surface and the light rays bend slightly. This is due to the fact that the light travels at a slightly different speed in air compared to liquid water or ice. Because the amount of the bending that occurs depends upon the wavelength of the light (blue is bent more strongly than red light), white light can be separated into it's component colors. A prism is a familiar example of this and the hexagonal shape of ice crystals can act as tiny prisms. When light is refracted by ice crystals, the light is bent from it's original direction and fans out away from the light source to create an arc of light around the sun or the moon. Depending upon the density, size and the type of ice crystal (plates or columns), the arc may be a complete circle, a partial circle or even two circles, one close to the sun and another farther outside.

Heat Lightning - A distant thunderstorm in which you can see lightning, but you cannot hear the thunder. Simply put, someone else's thunderstorm.

Helicity - A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2.

High Risk (of severe thunderstorms) - Severe weather is expected to affect more than 10 percent of the area. A high risk is rare, and implies an unusually dangerous situation and usually the possibility of a major severe weather outbreak. (See slight risk, moderate risk, convective outlook.)

Hodograph - A plot representing the vertical distribution of horizontal winds, using polar coordinates. A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height. Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc.).

Hook (or Hook Echo) - A radar reflectivity pattern characterized by a hook-shaped extension of a thunderstorm echo, usually in the right-rear part of the storm (relative to its direction of motion). A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development.

HP Storm or HP Supercell - High-Precipitation storm (or High-Precipitation supercell). A supercell thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the mesocyclone (Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes difficult and very dangerous. Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e., usually in the eastern portion). HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large damaging hail events.
Mobile storm spotters are strongly advised to maintain a safe distance from any storm that has been identified as an HP storm; close observations (e.g., core punching) can be extremely dangerous. See bear's cage.

Humidity - A measure of the amount of water vapor in the air. Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.

Hurricane - A tropical cyclone whose maximum sustained winds are equal to a greater than 74 mph.