Glossary
Cap :
A term used to describe a layer in the atmosphere where temperature increases with height. A cap acts as a lid which inhibits the development of thunderstorms. A strong cap will suppress clouds altogether; a weak cap allows stronger updrafts to develop, possibly into thunderstorms.
Dewpoint :
The temperature to which the air must cool in order to be 100 percent saturated. The higher the dewpoint the more moisture present in the atmosphere. Dewpoints 50 degrees and above are usually enough for severe thunderstorm development. When the dewpoint is within 5 degrees of the temperature, fog is a possibility.
Funnel cloud :
A rotating column of air not in contact with the ground, extending from a towering cumulus or cumulonimbus base. Funnel clouds are found at the rear of the storm, usually from a wall cloud.
Gust front :
The leading edge of the thunderstorm downdraft. The gust front is most prominent beneath the rain-free base and on the leading edge of an approaching thunderstorm. It often precedes the thunderstorm precipitation by several minutes. Shelf and roll clouds sometimes accompany gust fronts, especially when the gust front precedes a line of thunderstorms.
Mammatus :
Hanging rounded protuberances or pouches seen on the underside of the thunderstorm anvil. These clouds do not produce severe weather.
Relative humidity :
The percent of moisture present in the atmosphere in relation to the amount of moisture the atmosphere can hold at the present temperature.
Roll cloud :
A low-level, horizontal, tube shaped accessory cloud completely detached from the thunderstorm base. It is located along the gust front and most frequently on the leading edge of a line of thunderstorms. Roll clouds are not and do not produce tornadoes.
Scud cloud :
Low ragged and wind-torn appearing cloud fragments, usually not attached to the thunderstorm base, often seen in association with, and behind gust fronts. Scud clouds DO NOT produce severe weather. Scud clouds are often mistaken for wall clouds and tornadoes, especially when attached to the thunderstorm base. A way to differentiate scud clouds from wall clouds is to watch their relative position with respect to the rain area: scud clouds move away from the rain area while wall clouds maintain the same relative distance.
Severe Thunderstorm :
Thunderstorms which contain one or more of the following features: winds over 50 knots (58 miles per hour), 3/4 inch or larger hail, funnel clouds, or tornadoes.
Shelf cloud :
A low-level horizontal wedge-shaped accessory cloud, usually attached to the thunderstorm base, that forms along the gust front. The leading edge of the shelf cloud is often smooth and at times layered or terraced while the underside is concave upward and appears turbulent, boiling, or wind-torn. Tornadoes rarely occur with shelf clouds. Remember that shelf clouds are usually found on the leading edge of an approaching thunderstorm.
Stability :
A measure of the atmosphere's ability to develop and sustain rising currents of air. A "Stability Index" usually compares the temperature and moisture in the lower atmosphere with that of the upper atmosphere. Warm, moist air in the lower atmosphere, and cold air in the upper atmosphere, all contribute to more unstable conditions.
Tornado :
A violently rotating column of air, in contact with the ground, usually found in the southwest quadrant of the storm. Tornadoes are usually pendant from wall clouds or directly from the thunderstorm base, within a few miles to the southwest of the precipitation shaft. Tornadoes are still called tornadoes even after they lift off the ground. The MAJORITY of tornadoes are found at the REAR of the storm.
Virga :
Wisps or streaks of rain falling out of a cloud but not reaching the ground. When seen from a distance these streaks can be mistaken for funnels or tornadoes.
Wall cloud :
A local and often abrupt lowering of a rain-free cumulonimbus base, either rotating or non-rotating, from 1 to 4 miles in diameter, and usually situated in the southwest portion of the storm. Wall clouds are found in the rear of the storm; NEVER on the leading edge.
Warning :
A warning is issued by the local Weather Service Office when severe weather has developed in the area. They are statements of imminent danger and are for relatively small areas. Warnings are issued for severe thunderstorms, tornadoes, or flash floods. The MSP Weather Service also issues "Very Severe Thunderstorm Warnings" when they expect winds in excess of 75 miles per hour (hurricane force winds).
Watch :
An area in which the National Severe Storms Forecast Center in Kansas City feels conditions are favorable for the development of severe weather. They usually include an area 140 miles wide by 200 miles long and are issued for the potential of severe thunderstorms, tornadoes, or flash floods. A Watch means be aware of the potential for severe weather.
MISCELLANEOUS INFORMATION
1) Distance to thunderstorm
Count the time from the flash of lightning until you hear thunder. Take this time and divide by 5. This gives approximate distance to thunderstorm in miles.
2) Approximate height of convective clouds
To find the height of convective type clouds take the temperature (F) - dewpoint temperature (F). Multiply this number by 220. This will give approximate height in feet.
3) Approximate dewpoint from temperature and relative humidity
To find dewpoint take temperature x humidity + temperature. Take this result and divide by 2. This will give approximate dewpoint during a typical summer day.
Example: Temp = 80 (F) Relative Humidity = 60%
80 x .60 + 80 = 128. 128/2= 64. Dewpoint= 64
Stability Indices
1) SWEAT Index (Severe Weather Threat Index)
Thunderstorms unlikely
Slight risk, general thunderstorms
Moderate risk, approaching severe limits
Strong risk, a few severe t-storms/scattered tornadoes
High risk, severe t-storms/scattered tornadoes
High wind damage, not favorable for severe weather
2) Total totals
The Total Totals Index combines the effect of the atmospheric lapse rate, and low level moisture. It is computed by using the Cross Totals Index (the 850mb dewpoint minus the temperature at 500mb), and the Vertical Totals Index (the 850mb temperature minus the 500mb temperature). An index of 50 is a good starting point for thunderstorms. Scattered thunderstorms may be prevalent with values between 52-55, and anything above 56 will yield scattered severe thunderstorms with the possibility of scattered tornadoes.
There is a false sense of security when using this index. It is not meant to be used for the sole purpose of severe thunderstorm forecasting. High lapse rates and cold mid level temperatures will yield a high Total Totals number, but it does not take i nto consideration the low-level moisture that is needed for deep convection.
Values under 44..Limited potential for thunderstorms
Values 44-45.....Isolated light thunderstorms.
Values 46-47.....Scattered light/few moderate thunderstorms.
Values 48-49.....Scattered light/few moderate/isolated severe.
Values 50-51.....Scattered moderate/few severe/isolated tornado.
Values 52-55.....Numerous moderate/few severe/few tornado.
Values over 55...Numerous moderate/scattered severe w/tornadoes.
3) Lifted Index
The Lifted Index shows the stability of an air parcel. It is computed by lifting the parcel of air from the surface to the 500mb level then comparing the temperature of the air parcel to that of the environmental air temperature. Thr ough the forces of nature explained above, the temperature of the air parcel may be much higher than the surrounding air, causing it to be unstable in a sense that it wants to be displaced vertically. As the air parcel becomes saturated, its physical stat e is transformed from a gas to a liquid state, and in doing so (condensation) the energy absorbed is called latent heat. Once this "latent" heat is added to the air parcel, it now becomes lighter and warmer than its surrounding air. We look for negative n umbers with this index. The more negative the number, the more unstable the air is. Put another way, the more negative the number, the more potential there is for a stronger thunderstorm. Values of zero or below are a good indicator of general thunderstorms. Severe thunderstorms are possible when the values reach -4 or so. When MESO tracked the tornadic supercell near Coldwater, Kansas on Memorial Day 1999, Lifted Indexes were in the -12 to -14 range, indicating a very unstable atmosphere.
Values over +2.....Limited thunderstorm potential.
Values +2 to -2....Thunderstorm potential.
Values -2 to -6....Severe thunderstorm potential.
Values under -6....Tornado potential.
