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.

Thunderstorm-Related 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.

• Warm season - loosely defined as April through September
• 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 March
• 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.

• 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 in.
• 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.

• Why do some thunderstorm downdrafts produce damaging winds?
• 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 damaging winds?
• 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 winds.
• 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.

• 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 surface.
• 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 ground surface.

• Formation of lightning
• Positive and negative charges within a thunderstorm separate.
• 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 lightning.
• 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 for lightning.
• Thunderstorms whose tops do not extend much above the freezing level will have fewer frozen particels, less charge separation, and less lightning.

Winter 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.

• 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

• Thundersnow

Tropical Systems - 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 tornadoes.

Non-Precipitation Phenomena - Includes fog, frost and other phenomena.

• Global Warming



• Global Cooling