What Do Those Weather Terms Mean?

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Have you ever thought of the differences between the terms “partly cloudy” versus “mostly sunny”?  Does one sound more optimistic than the other?  Well in this second installment of a two-part series, I will offer a perspective on the rhetoric used in general weather forecasts.  The first part of this two-part blog series centered on the rhetoric used in severe weather watches and/or warnings.  While it is certainly important to understand what the various terms mean when dangerous weather warnings are issued, having an understanding of basic weather terminology can facilitate the understanding of basic weather messages, including the local forecast.

Sky Condition

When you hear the forecast for your area, you will hear if it will be cloudy, sunny, etc. This describes the predominant sky condition based on the percentage of opaque (not transparent) cloud cover.  Although there is some subjectively in what someone perceives as the character of the sky, the National Weather Service has some general guidelines in describing sky condition.

Cloudy / Overcast  88-100%

Mostly Cloudy / Considerable Cloudiness  70-87%

Partly Sunny / Mostly Cloudy  51-69%

Mostly Sunny / Partly Cloudy  26-50%

Sunny / Mostly Clear  6-25%

Sunny / Clear  0-5%

So as you can see the terms “partly sunny” and “mostly cloudy” are interchangeable.  As are “mostly sunny” and “partly cloudy”.  The choice of words is really dependent on the mood of the weather forecaster, sort of as a seeing the glass as half full thing.  For more enlightenment on this issue, click here for a brief discussion on this topic from another meteorologist.

Precipitation Chances

According to the National Weather Service, the Probability of Precipitation (POP) is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is equal to or greater than 0.01”.   Terms such as occasional, intermittent, or periods of are used to describe a precipitation event that has a high probability of occurrence (80% +) but is expected to be of an “on and off” nature.

When precipitation is in the forecast, often you will hear a percentage assigned to it. These percentages represent the probability assigned for the type of precipitation specified.  The rhetoric used is slightly different when stratiform (non-convective) rain and/or snow is expected versus convective precipitation events.  Below are the percentages typically assigned to a precipitation forecast and the rhetoric used with these percentages.

20 percent = slight chance of rain or snow or isolated thunderstorms

30, 40, and 50 percent = chance of rain or snow or scattered thunderstorms

60 and 70 percent = likely rain or snow or numerous thunderstorms

Temperature

Believe it or not, there is a lot of ambiguity in a forecast temperature.  One meteorologist may think a temperature in the lower 40s can include temperatures up to 45 degrees, and another meteorologist may think the lower 40s only includes 41-43 degrees.  Then there is the issue when the terminology “around” is used.  How about the adjectives used to describe the character of the temperature?  Again this is highly subjective, and is also location dependent.  If you are from Chicago and a high temperature of 42 degrees in January sounds like a nice, mild winter day.  However if you are from a warmer climate, New Orleans for example, a high of 42 sounds pretty chilly.  What it boils down to is the perception of “hot”, “cold”, “chilly”, and etc. are location dependent.  Being from the Chicago area, I associate temperatures 90 degrees and above as “hot” (I also think the mid to upper 80s are hot but 90, in my opinion, is the lower threshold for the “hot” adjective to be used.  Not so in Texas.  When I lived in Amarillo, my colleagues thought I was nuts when I expressed that I considered 90 degrees as hot.  They did have a point as I did live there during the hottest summer I ever experienced; 30 days of high temperatures of at least 100 degrees!  Now that is downright, furnace-like hot!

Winds

The last weather terminology I would like to provide some insight into are winds. Although a bit more straightforward than sky cover, precipitation chances, and temperatures, National Weather Service like to append adjectives to wind forecasts. Below are the commonly used adjectives for winds.

0-5 mph Calm / Light / Light & variable

5-15 mph / 10-20 mph (None used)

15-25 mph Breezy (mild weather) Brisk or Blustery (cold weather)

20-30 mph / 25-35 mph Windy

30-40 mph Very windy

40 mph or greater Strong, dangerous, high, damaging (High Wind Warning criteria)

Concluding remarks

Hopefully this second installment of this series of blog posts have provided you with enough information so you can make better sense of the daily weather forecast.  In addition, you may now have a better understanding of why certain adjectives are used in forecasts.  So if someone asks you if you know the difference between “partly sunny” and “mostly cloudy”, you will be able to explain to them what these weather terms mean.

The Rhetoric of Weather Products – Severe Weather

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The skies are darkening, lightning flashes across the sky, the loud rumble of thunder seems to shake you to the core.  These are the familiar signs of thunderstorms – one of the most fascinating atmospheric phenomena to occur.  While you may be familiar with the terms “tornado watch” and “severe thunderstorm warning”, do you know why the National Weather Service uses the language in these products to get us to better understand what they mean? Perhaps you have read other weather blogs and/or watch The Weather Channel on a regular basis and have come across some strange weather terms.

The original intent of this site is to help discern the way language is used to describe the weather or how language affects our understanding of the weather.  This objective is fairly broad so in order to not bombard you with a ton of weather language in one post, I will break it up into smaller chunks, grouped by topic.  This first installment will cover the various weather terminology used during instances of severe weather.

Single Cell Storms

To start, it is important (and interesting) to note that there exists a storm spectrum, which attempts to classify the different varieties of storms that exist.  Sure all thunderstorms have the same basic structure (an updraft and a downdraft) but storms can vary by duration and certainly by size. In fact the first type of storm I’ll mention are relatively small in scale and have a relatively short lifespan. They are called “single cell storms” . They rarely produce any type of severe weather, but heavy rainfall and small hail can accompany these storms.

Multicell Cluster Storms

Multicell cluster storms occur when thunderstorms form in a cluster, with numerous storms in various stages of development, or thunderstorm life cycle merging together. There is a higher probability of severe weather associated with multicell cluster storms and flash flooding can be a possibility if additional storms developing upwind of the main cluster of storms moves over the same location.  This is referred to as “training”.

Multicell Line Storms

Sometimes thunderstorms can form a line that will extend laterally for hundreds of miles. These multicell line storms, or the more common term “squall line” can pack quite a punch. New storms develop along the leading edge of the system, with heavy rain and hail following. Are you familiar with the feeling of a sudden temperature drop and winds picking up as you see storms heading your way?  Those sensations you are feeling are from the thunderstorm downdraft.  Strong winds and large hail are the most common severe weather elements to occur with these storms.  Long-lived squall lines, which can travel many hundreds of miles and produce considerable damage are called derechoes.

Supercell Storms

The final and most important type of storm are supercell storms.  These storms are the most organized, persist for many hours, and are responsible for the majority of injuries, fatalities, and property damage than all other types of thunderstorms.  These storms are unique because they consist of a special ingredient the other storms do not have, wind sheer.  Wind sheer is when winds are veering, or turning clockwise with height.  When winds change in speed and direction with height, it creates storm scale rotation which can lead to the development of tornadoes and large hail.  In fact supercell storms are responsible for nearly all of the significant tornadoes in the United States and instances of large hail (for example baseball size hail).  Flash flooding can also accompany supercell storms.

How to Interpret Storm Watches and Warnings

Most of us are familiar with the term “severe thunderstorm watch” or “tornado watch” and likewise with warnings.  At times you may hear of a tornado watch being termed a “PDS watch”. This stands for Particularly Dangerous Situation.  These types of watches are issued occasionally and meteorologists at the Storm Prediction Center will issue a PDS tornado watch when a major tornado outbreak is possible, and there is a potential for EF 4 or EF 5 tornadoes.

Over recent years, the National Weather Service has warned the public of an approaching violent tornado in an alarming but effective way.  Instead of issuing a standard tornado warning which warns those in the path that a tornado has been sighted or detected by Doppler Radar, meteorologists have added the rhetoric “tornado emergency”.  Given the heightened urgency in the use of “emergency”, this is reserved for when a violent tornado is expected to impact a densely populated area.  Remember the EF 5 tornado that destroyed most of Greensburg, KS, the EF 4 tornado that hit Tuscaloosa, AL, or the EF 5 tornadoes that tore through Oklahoma City, including the town of Moore?  All of those were preceded by a tornado emergency.

If you are interested in additional information on the thunderstorm spectrum along with some nice photos of storms, click here.  This compilation of information was gathered from the National Weather Service storm spotting presentations.  I urge you to attend a storm spotting class near you when applicable.  They are held during the late winter/early spring in different locations across the country.  Chances are there is a class near you. (Just click on your part of the map to access your local National Weather Service office and they should have information on available storm spotter classes in the late winter/early spring.) I have taught numerous storm spotting classes while I worked for the National Weather Service and these classes are designed to teach you the basics of thunderstorm development, how to tell if a storm is severe or not, and what to report to the National Weather Service if you witness any severe weather.  Next I’ll offer a rhetorical perspective on basic weather terminology.