Gallery of Weather Phenomena
The goal of this page is to describe phenomena
in terms that hopefully most people will understand, and more importantly,
describe the abilities and limitations in forecasting these phenomena,
so the public can best use the information and forecasts that we meteorologists
provide. I have divided the phenomena into Thunderstorm-Related
Phenomena , Winter
Weather Phenomena , Tropical
Systems and Non-Precipitation
Phenomena - Characteristics
of thunderstorms can change from season to season, but their effects are
always the same. Each phenomenon will be loosely classified into typical
cold season and warm season characteristics.
Heavy rain and flash flooding - Terrain is an
important factor in the frequency and severity of flash flooding. Hilly
or mountainous terrain can enhance flash flood potential since upslope
wind flow can provide a persistent mechanism for continued thunderstorm
development, and rainfall runs downhill. Urban areas, where many paved
surfaces exist, can aggravate flooding as well, since rainfall cannot soak
into the soil. It must exit through storm drains, which may not support
large flows of water.
Hail - Severe hail by definition is 3/4"
(dime-sized) or larger. Smaller hail can still cause problems, though.
Large accumulations of small hail can cause crop damage and hazardous driving.
3/4" or larger hail can chip paint off cars and produce significant
crop damage. Denting of cars and serious crop damage usually occurs with
hail of 1" or larger.
- Warm season - loosely defined as April through
- Storm tops generally higher than during the cool
season, allowing moisture to rise to higher levels above the ground, and
more available moisture to be converted into rainfall
- Wind flow from the ground surface to higher levels
is generally weaker than during the cool season due to the northern position
of the polar upper-level jet stream during the warm season
- Weak wind flow can sometimes result in nearly
stationary thunderstorms, resulting in persistent heavy rains over a given
location, and resultant flash flooding
- Frequency of thunder and lightning is not an
effective indicator of severity of a thunderstorm, in terms of potential
heavy or flooding rains. Some thunderstorms may have little if any thunder
and lightning, yet still produce flooding rains.
- Cold season - loosely defined as October through
- Storm tops generally lower than during the warm
season, so moisture isn't as deep, and rainfall amounts are generally lighter
than during the warm season
- Wind flow from near the ground surface to higher
levels is generally stronger than during the warm season, and if strong
low-level winds bring large amounts of moisture to an area, even low-topped
thunderstorms can produce large amounts of rainfall due to a continuous
source of moisture over a relatively longer period of time, when associated
with a slow-moving large-scale system
- Frequency of thunder and lightning is not an
effective indicator of the potential for heavy rainfall since any thunder
and lightning is rare during the cold season.
Damaging non-tornadic winds - These are also
known as "straight-line" winds. Thunderstorm downdrafts hit the
ground and can spread out, sometimes violently, producing winds that can
topple trees, telephone poles, and damage buildings.
- How it forms - Thunderstorms can produce hail
any time of the year. Formation of hail in any thunderstorm is largely
dependant on whether the rising air within the thunderstorm, or updraft,
can push through the freezing level, which is the height above ground that
is as 32F. If an updraft extends far above the freezing level, then raindrops
and water vapor can freeze. Frozen water droplets can blow around to different
parts of a thunderstorm, and accumulate more water and re-freeze many times
before it is heavy enough to fall to the ground.
- Hail size - The largest hailstones occur in thunderstorms
that have the strongest updrafts, the most moisture, and the lowest freezing
level. If the temperature at the ground surface is warm, then hail can
melt significantly as it falls, reaching the ground much smaller than when
it began falling, or even end up being just a large rain drop. Since the
process of hail growth is dependant on very small-scale circulations within
thunderstorms, and cannot be monitored by any meteorological instruments,
forecasting hail size in any thunderstorm is nearly impossible until reports
are received, and we can associate hail sizes with what we see on NWS Doppler
radar. Often, there is a variety of hail sizes that fall in a thunderstorm.
Even thunderstorms that produce large hail often have non-severe hail mixed
- Hail shape - Most hailstones are generally round,
but just about any shape is possible, and in fact, a huge variety of shapes
have been reported. So, how do you determine if the hail is severe? It's
always safest to just take the maximum diameter of irregularly-shaped hailstones,
and if it is 3/4" or larger, then it is severe.
Tornadoes - All details about tornadoes can be
found in Dr. Doswell's "Definition of a tornado" essay, but there
is some important basic information that I hopefully can summarize.
- Why do some thunderstorm downdrafts produce damaging
- The strength of a thunderstorm downdraft depends
on many different factors, but can be generally divided into three categories.
- Intensity of precipitation - The more rain or
hail, the heavier the precipitation, and the faster the precipitation will
fall to the ground. Also, if the precipitation core originates at a higher
level above ground, then it can build up more momentum than if originating
closer to the ground.
- Cooling within the downdraft - Air that is cooler
than its environment has a tendency to accelerate downward, or build up
speed as it sinks. If precipitation within a thunderstorm downdraft evaporates
(dries up) and cools, then the sinking air within the downdraft will fall
faster with time until it hits the ground. Precipitation will evaporate
when dry air is mixed into the thunderstorm.
- Low-level jet reaching the ground - This occurs
mostly in low-topped, cool-season thunderstorms. When strong warm or cold
fronts are approaching, the winds from the ground surface can strengthen
dramatically with height (for reasons to complex to explain here). Heavy
precipitation within a thunderstorm can drag some of the strong or damaging
winds from 1,000 to 5,000 feet, down to the ground surface.
- Why don't all thunderstorms produce strong or
- Most thunderstorms don't have precipitation that
is intense enough to produce damaging winds.
- The precipitation cores of most thunderstorms
don't originate from high enough above the ground surface to produce damaging
- Most thunderstorms have only limited dry air
mixed in, so there usually isn't enough evaporational cooling within the
downdraft to produce damaging winds.
- Sometimes a surface wind off the ocean, or if
a warm front is approaching your region, surface temperatures can cool
enough so that the temperature rises with height within a couple of thousand
feet of the ground surface. The cooler air at the ground surface is denser
than the warmer air above the ground, so a thunderstorm downdraft may not
be able to affect the cooler surface layer of air, preventing most, if
not all the wind from reaching the ground.
Lightning - This is one of the least forecastable
weather elemements, yet it kills more people each year than any other type
of weather. Lightning is nothing more than electricity, a static discharge,
much like touching a doorknob during a winter day.
- Tornadoes form when a circulation at the ground
surface links to a circulation at the base of a thunderstorm. The ground
circulation shrinks or tightens through any one of many processes, causing
the circulation to spin faster, until it can cause damage on the ground
- A tornadic circulation may extend beyond what
is visible, so damage may very well occur beyond its visible funnel. The
tornado is defined as the part that contains the damaging winds (which
may be only on one side of a relatively weak, fast-moving tornado) at the
- Formation of lightning
- Positive and negative charges within a thunderstorm
- Frozen particles at the top of the thunderstorm
are mainly positively charged.
- Liquid water droplets at the base of a thunderstorm
are mainly negatively charged.
- The negatively charged base of the thunderstorm
repels negative charges and attracts positive charges into the ground surface,
sort of like a magnet.
- The more frozen particles at the top of the thunderstorm,
the more positive charge at the top of the storm, and the more negative
charge at the base of the storm.
- The larger the charge difference from the top
of the thunderstorm (positive), to the base of the thunderstorm (negative),
to the ground surface (positive), the better the chance for the static
discharge, or lightning "spark".
- A stream of negative charge forms from the cloud
to the ground surface, which is not visible.
- Once the stream touches the surface, a stream
of positive charge flows from the ground into the cloud, this is what we
see as the "spark".
- Due to the mix of frozen and liquid particles
within the many circulations in a thunderstorm, lightning can jump from
cloud to cloud, or cloud to air, without touching the ground. The "spark"
will occur wherever the positive and negative charge separation supports
- Characteristics of thunderstorms that support
and reduce lightning formation
- The more frozen particles at the top of the storm,
the more charge separation will likley exist, and the better the chance
- Thunderstorms whose tops do not extend much above
the freezing level will have fewer frozen particels, less charge separation,
and less lightning.
Weather Phenomena - In my opinion,
one of the most neglected fields of research, and also the most poorly
predicted phenomena. Frozen and freezing precipitation can occur as a result
of many different atmospheric processes. Many "surprise" events
are often more forecastable than some people think and the following information
will hopefully illustrate my point.
Snow - Snowstorms are a result of many weather
features from large scale to mesoscale, working together to produce the
snow. Traditional methods of tracking snow-producing storms aren't always
valid anymore, such as following the surface low pressure center, or assuming
that certain temperatures and wind directions are too warm to allow accumulating
snow to fall at some future time.
- Classic Nor'easter
- Classic Nor'easters are the easiest to predict
since they usually have produced snow someplace else, which gives the forecaster
a pretty good idea of what could potentially occur in his/her forecast
area. Even if they haven't formed yet, the current computer forecast models
handle the large-scale systems well, and provide somewhat reliable guidance.
The word guidance is important, because continuosly updated satellite,
surface and upper air data is vital in modifying the computer model guidance.
- Storms such as the Superstorm of 1993 and the
Blizzard of 1996 are 2 examples of classic Nor'easters that were reasonably
well forecasted over a large area. Of course the exact track of any Nor'easter
will determine what locations will receive all snow, a mix, ice, or rain.Usually,
these large storms affect a large area, and extend very high into the atmosphere,
so local effects such as upslope winds (in hilly or mountainous areas)
and ocean effects can result in a huge difference in snow amounts in a
very short distance, even within one city or county.
- "Clipper" snows
- More tropical systems affect the
eastern U.S. than one would imagine. After a hurricane, tropical storm
or tropical depression makes a land fall, the remnants can often track
for hundreds of additional miles, producing devastating flooding and occasionally
Heavy Rainfall and Associated Flooding
Phenomena - Includes fog, frost
and other phenomena.
- Global Warming
- Global Cooling