Heating Degree Day (HDD)- useful in determining heating
energy requirements. Defined as (65 - MT)= HDD,where MT is Mean
Temperature (F). If MT is greater than 65 degrees F, HDD=0. MT= Mean
(average) Temperature of the day.
MT=(High Temperature+Low Temperature)/2
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) favour 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,
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,
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 favourable 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 - Generally, a measure of the water vapour content of
the air. Popularly, it is used synonymously with relative humidity.