Common Weather Misconceptions

Weather Misconceptions
Common Weather Misconceptions

Probability of Precipitation (Chance of Rain)
One of the challenges of forecasting weather in Alabama during the summer-time is the inability to determine the exact location where afternoon storms will develop.  In the summer months, as the afternoon heat builds from sunshine, the atmosphere destabilizes.  Air rises forming clouds, which eventually build to thunderstorms.  They form at rather random locations, but sometimes form on micro-scale boundaries, much like "mini cool fronts".  But since the conditions that determine where storms will develop occur on the micro-scale and the computer models we use to make forecasts are on a larger scale, the "synoptic-scale", they do a poor job of forecasting these features very far in advance.

As a result of these issues, on most summer days you will see a 10-30% chance of "widely scattered and afternoon showers and thunderstorms".  This literally means that about only 10 to 30% of locations in North Alabama will see rainfall that day.  These storms are very sporadic and typically do not move very much.  So in many cases one place may see a heavy downpour for up to an hour, while someone just a few miles down the road could see nothing but sunshine. This is something to keep in mind when you look at a weather forecast and automatically assume the weather forecast is wrong just because it rained on a day with a 10% chance of rain forecasted for that day.

There are ways to determine the areas that have the best chance of seeing the pop-up storms, if it is possible to detect the boundaries they are developing along.  Also by looking at visible satellite images you can detect building cumulus which could later form thunderstorms.  But for the most part pinpointing exactly where these "pop-up" (or "air mass") thunderstorms will develop during the afternoon is very difficult, if not impossible. 

The typical pop-up thunderstorms that occur during the afternoon hours in the summer-time are not associated with any larger weather system (another reason it is nearly impossible to predict exactly where they will develop).  As a result, they typically are not severe, but instead "pulse" up quickly and may briefly produce strong winds and small hail - before quickly dissipating.  However, they sometimes can produce a damaging wind gust as the storm collapses.

The key point to remember is that on a summer day in Alabama, a 20% chance of rain does NOT mean there is 20% chance that all of North Alabama will get rain, it means that about 20% of North Alabama will get rain. Hopefully this information will help you to better understand the weather forecasts you see each day during the summer-time here in Alabama.

HEAT LIGHTNING
Lightning is ALWAYS followed by thunder as the air particles explode from the extreme heat. “Heat lightning” is just lightning in a thunderstorm that is so far away you can’t hear the thunder. Lightning is ALWAYS associated with clouds and a thunderstorm. Lightning DOES NOT occur do to excessive heat, but from differences in the charge of the clouds and ground, or different clouds.

Doppler Radar Data
Base reflectivity on radar is the standard view you see when you want to know where it is raining. The radar sends out a signal and the signal is reflected back to the radar and the radar displays a certain reflectivity value depending on the size of the object its signal was reflected off. So the heavier the rainfall, the higher the DBZ (measure of radar reflectivity) value and the brighter colors you will see on the radar display. A high DBZ level (or bright colors) DOES NOT mean that the storm is severe, it is just indicative of heavy rainfall. So just because you see RED or PURPLE does NOT necessarily mean the storm is severe. Also, tornadoes rarely occur in the area of the thunderstorm where you see the reds, purples and other bright colors, they often form on the southwestern end of the storm. Weather radars broadcast microwaves to detect precipitation, and will not be incorrect - if a radar is not working properly it can easily be detected.

Lightning
Rubber shoes or tires on a car DO NOT protect you from lightning strikes. Cars are safe from lightning because of their metal frame. As long as you do not touch the metal, a car is a pretty safe place to be in a thunderstorm. Lightning can enter your home through phone lines, plumbing, and cable. Don’t touch these during a thunderstorm.

Tornadoes
Many people today say that tornado warnings are not as reliable as they were 30 years ago. That is because 30 years ago a tornado warning was not issued until the tornado was on the ground doing damage and killing people. Today we have technology to try to give early warning on a tornado threat. But we can’t be 100% accurate in doing so. But its better to have too many warnings than to have not enough.

Tornado Warnings
A tornado warning indicates that either a tornado has been spotted by a storm spotter or law enforcement officials, or there is strong indication of a tornado on Doppler radar. This means that people in these areas should take cover immediately. Get in the lowest level of the building in an interior room with no windows, covering your head with your hands. Use mattresses or pillows to help protect yourself as well.

Tornado Watch
A tornado watch is NOT issued for a specific storm, but instead it is issued for a very large area and indicates that the conditions in place are sufficient for the development of severe thunderstorms and tornadoes. Severe weather may not be imminent, but it means you should keep an eye on the weather just in case.

Hail Formation
Hail is formed when water droplets are pushed high into the storm by the updraft where they freeze. They are continually pushed up into the storm until they become to heavy to be held up by the updraft and fall to the ground. The stronger the updraft, the larger the hail stones will be.

Temperature Inversion
Due to t he temperature inversion on many nights in Alabama, the temperature on the ridge tops will be as much as ten to fifteen degrees WARMER than valley locations. The cooler, more dense air sinks into these valleys. So low temperatures will vary a great deal on these nights. Temperature inversions can cause a ten degree difference in temperature in just a few hundred yards distance.

Freezing Rain and Sleet
These precipitation types are confused by people all the time. All precipitation starts off as ice, in the case of freezing rain in melts in mid air, and then freezes on contact when the surface temperature is below freezing. In the case of sleet it falls from the cloud, melts to liquid but then refreezes in mid-air and falls to the ground as ice pellets. Sleet forms in a completely different way than hail. (see the hail formation info) Freezing rain and sleet are two completely different forms of precipitation, and freezing rain is probably the most dangerous because it coats everything with layers of ice. Sleet generally does not cause much of a problem.

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