SCIN 137 AMU week 5 lesson WEATHER FORECASTING & THUNDERSTORMS AND TORNADOES Introduction to Meteorology American Military university
- Lesson Overview
This week we cover weather forecasting and severe events of thunderstorms and tornadoes. I often say forecasting is more of an “art” than a “science” as the atmosphere is never static. Forecast models involve complex mathematics and physics, but ultimately it is the forecaster who must interpret that data. I hear people complain that forecasts are “always wrong”, but in fact, they are often correct. For example, I have had students comment how there was a forecast for 30% chance of rain, but it didn’t rain 30% of the day/30% of the area, etc. The fact is that the forecast is that there is a 1 in 3 chance it will rain that day for that forecast area – if it doesn’t rain, the forecast is “correct” as it also means there’s a 70% chance it will not rain! Remember to learn what the forecast is really about in this week.
Students will be able to:LO-33. Discuss how weather observations are obtained and disseminated. LO-34. Explain how weather data are analyzed manually and with computer models. LO-35. Identify and explain the different kinds of forecasts for extreme weather conditions. LO-36. Discuss thunderstorms and what causes ordinary thunderstorms to become severe. LO-37. Understand atmospheric electricity and the causes for lightning. LO-38. Explain the causes and effects of tornadoes.
The following activities and assessments need to be completed this week:
- Read Barry and Chorley: Chapters 8 and 9
- Research Project Outline
- Week 5 Lesson
- Week 5 Forum
- Week 5 Lab
- Week 5 Quiz
- COMET Modules (Optional):
- Weather Radar Fundamentals
- Principles of Convection I: Buoyancy and CAPE
- Principles of Convection III: Shear and Convective Storms
- This lesson introduces the most common types of weather forecasting, and how we use satellites to gather data. The lesson also studies severe weather phenomena, such as thunderstorms, lightning, thunder, and tornadoes, which represent a hazard to human life and property. Topics to be covered include:
- Weather forecasting and its history
- Numerical weather prediction
- Shortcomings of numerical weather prediction
- Weather data collection
Weather forecasts help us prepare for the day, knowing to carry an umbrella on a rainy day or buy milk before a snowstorm. Even more important is knowing to prepare for severe weather – evacuating before a hurricane or seeking shelter during a tornado can save lives.
Collecting Data for Weather Forecasts
A weather forecast is the prediction of the change of the state of the atmosphere at some geographic location. How is a weather forecast made? The simplest method is by visual observation of the weather. Automated stations are also located around the world, on land and sea, monitoring the weather, collecting many kinds of weather data. Weather stations report at least four times a day, or in some cases every fifteen minutes. The U.S. Geological Survey has an online map where you can click on a map to see the rainfall and river level information reported by their automated stations. Besides the earth-based weather stations, satellites and airplanes also monitor changes in the weather.
Processing Data for Weather Forecasts
Weather information from all parts of the world is gathered by the World Meteorological Organization (WMO), which shares information with 175 nations. The U.S. member of this group is the National Center for Environmental Prediction (NCEP), a branch of the National Weather Service (NWS). The meteorologists at the NCEP use a near constant stream of information to provide weather forecasts to public agencies and private firms, as well as making information available on the Internet. They also ingest this data into numerical weather prediction models which create maps and charts that are used for weather prediction at the local weather forecast offices. If conditions favor hazardous weather but timing is imprecise, the NWS issues a weather watch or, if the severe weather is imminent or in progress, a weather warning.
Thunderstorms are storms with thunder and lightning. They can have heavy rain, violent wind, and hail. As they are the result of rising warm air, they are convective storms, finding their beginnings as moist, warm air rises in a conditionally unstable atmosphere. The warming air could range from balloon-size to larger than a city block. As you have learned, when air is warmer than its surroundings, a buoyant force acts on it, causing it to rise. The conditions that make this happen could be any of these triggers:
Keep in mind throughout this lesson that warm does not mean summer temperatures. It is the temperature difference that matters, that the air is warmer than its surroundings.
The vast majority of thunderstorms are ordinary cell thunderstorms (often referred to simply as ordinary thunderstorms), short-lived scattered storms growing from rising moist, warm air which is not associated with significant weather fronts. Their predictable life cycle often spans less than an hour. Severe thunderstorms are capable of producing large hail, strong surface winds, flash floods and even tornadoes, and have a more complex origin.
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- CUMULUSAlso known as the growth stage, the beginning of a thunderstorm is often triggered by heat from the sun. In the cumulus stage, warm humid air rises, cools due to lower temperatures at high altitudes, and condenses into water droplets that form one or more cumulus clouds. The condensation of water releases a large amount of latent heat, so the cloud becomes even warmer than the surroundings and continues to rise. There is no rain during this stage as upward movement keeps water droplets suspended. The air reaches a point where the surrounding air is below freezing and some ice forms. The particles in the cloud become more dense in this colder environment and the particles grow larger as they collide with each other. Towards the end of this stage, they become too heavy to remain suspended and start to fall. This entire stage may take place in just a few minutes.t the same time as the cumulus cloud forms, drier, cooler air is drawn into the cloud during what we call entrainment. The drier air causes some evaporation of the cloud droplets, cooling the air. This colder air is now heavier and descends as a downdraft. As it descends, ice crystals melt which further chills the air.
Lightning and Thunder
Lightning is an electrical discharge from a thunderstorm cloud, created by friction from moving air similar to the shock you get when you scuff your feet on carpet in the winter and touch a metal object like a lamp. Lighting can go from cloud to cloud (most common), from cloud to the ground, or from the cloud to surrounding air. The cause is similar to what is happening with you and the carpet. When you walk across the carpet, your socks rub electrons off the carpet, giving your socks a positive charge. Electrons repel each other, so they spread out over your body. When your hand gets near a metal object, which has freely moving electrons, the excess electrons on your hand jump off to the lamp, heating up the air in between, which you appears like a tiny bolt of lightning. Similarly, Ahrens (2011) proposes that when hailstones collide with colder ice crystals, electrons are transferred giving the upper part of the cloud a negative charge and the lower part a positive charge. When a sufficient charge builds up, lightning strikes, heating its path to a temperature of around 18,000 °F. This rapid, extreme heating causes the air to expand explosively, creating the pressure wave we know as thunder.
If you are close enough to hear the thunder, you are close enough to be struck by the lightning (NOAA, n.d.). If you have heard the saying that every second between the lightning and thunder means it is that many miles away, you heard a lie. The distance from you to the storm is one mile for every five seconds between seeing the lightning and hearing the thunder. So if you count three seconds, the storm is little more than half a mile away.