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In 2010 Swackett (sweater, jacket or coat) redefined the weather report for millions of people in 67 countries worldwide. We convert complex weather data into easily-understood visual weather reports — designed for people of every age. An instant hit, Swackett saw 30,000 new users on launch day — 100,000 users after 10 days — and more than 1 million unique user accounts during it’s. 5-day weather forecasts for the United States in a nice, easy to read format, AND the 1- to 3-day out accuracy of other weather forecasters (Accuweather, The Weather Channel, MyForecast.com, Intellicast, and the National Weather Service) including links to their forecasts.
< High School Earth Science
Weather forecasts are better than they ever have been. According to the World Meteorological Organization (WMO), a 5-day weather forecast today is as reliable as a 2-day forecast was 20 years ago! This is because forecasters now use advanced technologies to gather weather data, along with the world’s most powerful computers. Together, the data and computers produce complex models that more accurately represent the conditions of the atmosphere. These models can be programmed to predict how the atmosphere and the weather will change. Despite these advances, weather forecasts are still often incorrect. Weather is extremely difficult to predict, because it is a very complex and chaotic system.
Lesson Objectives[edit]
Collecting Weather Data[edit]
Figure 16.38: Barometers are Mercury columns used to measure air pressure.
To make a weather forecast, the conditions of the atmosphere must be known for that location and for the surrounding area. Temperature, air pressure, and other characteristics of the atmosphere must be measured and the data collected. Thermometers measure temperature. One way to do this is to use a temperature-sensitive material, like mercury, placed in a long,very narrow tube with a bulb. When the temperature is warm, the mercury expands, causing it to rise up the tube. Cool temperatures cause the mercury to contract, bringing the level of the mercury lower in the tube. A scale on the outside of the thermometer matches up with the air temperature.
Because mercury is toxic, most meteorological thermometers no longer use mercury in a bulb. There are many ways to measure temperature. Some digital thermometers use a coiled strip composed of two kinds of metal, each of which conducts heat differently. As the temperature rises and falls, the coil unfolds or curls up tighter. Other modern thermometers measure infrared radiation or electrical resistance. Modern thermometers usually produce digital data that can be fed directly into a computer.
Meteorologists use barometers to measure air pressure (Figure 16.38). A barometer may contain water, air, or mercury. Like thermometers, barometers are now mostly digital. Air pressure measurements are corrected so that the numbers are given as though the barometer were at sea level. This means that only the air pressure is measured instead of also measuring the effect of altitude on air pressure.
A change in barometric pressure indicates that a change in weather is coming. If air pressure rises, a high pressure cell is on the way and clear skies can be expected. If pressure falls, a low pressure is coming and will likely bring storm clouds. Barometric pressure data over a larger area can be used to identify pressure systems, fronts and other weather systems.
Other instruments measure different characteristics of the atmosphere. Below is a list of a few of these instruments, along with what they measures:
Figure 16.39: A land-based weather station. Since some of the instruments must be protected from precipitation and direct heat, they are held behind a screen.
These instruments are placed in various locations so that they can check the atmospheric characteristics of that location. Weather stations are located on land, the surface of the sea, and in orbit all around the world (Figure 16.39). According to the WMO, weather information is collected from 15 satellites, 100 stationary buoys, 600 drifting buoys, 3,000 aircraft, 7,300 ships and some 10,000 land-based stations.
Instruments are also sent into the atmosphere in weather balloons filled with helium or hydrogen. As the balloon ascends into the upper atmosphere, the gas in the balloon expands until the balloon bursts. The specific altitude at which the balloon bursts depends on its diameter and thickness, but is ordinarily about 40 km (25 miles) in altitude. The length of the flight is ordinarily about 90 minutes. Weather balloons are intended to be used only once, and the equipment they carry is usually not recovered.
Figure 16.40: A weather balloon with a radiosonde beneath it. The radiosonde is the bottom piece and the parachute that will bring it to the ground, is above it.
Weather balloons contain radiosondes that measure atmospheric characteristics, such as temperature, pressure and humidity (Figure 16.40). Radiosondes in flight can be tracked to obtain wind speed and direction. Radiosondes use a radio to communicate the data they collect to a computer.
Radiosondes are launched from around 800 sites around the globe twice daily (at 0000 and 1200 UTC; UTC is Coordinated Universal Time; it is the same as Greenwich Mean Time—the time in the city of Greenwich, England) at the same time to provide a profile through the atmosphere. Special launches are done when needed for special projects. Radiosondes can be dropped from a balloon or airplane to make measurements as they fall. This is done to monitor storms, for example, since they are dangerous places for airplanes to fly.
Weather information can also come from remote sensing, particularly radar and satellites (Figure 16.41). Radar stands for Radio Detection and Ranging. In radar, a transmitter sends out radio waves. The radio waves bounce off the nearest object and then return to a receiver. Weather radar can sense many characteristics of precipitation: its location, motion, intensity, and the likelihood of future precipitation. Most weather radar is Doppler radar, which can also track how fast the precipitation falls. Radar can outline the structure of a storm and in doing so estimate the possibility that it will produce severe weather.
Figure 16.41: Radar view of a line of thunderstorms.
Weather satellites have been increasingly important sources of weather data since the first one was launched in 1952. Weather satellites are the best way to monitor large scale systems, like storms. Satellites can also monitor the spread of ash from a volcanic eruption, smoke from fires, and pollution. They are able to record long-term changes, such as the amount of ice cover over the Arctic Ocean in September each year.
Weather satellites may observe all energy from all wavelengths in the electromagnetic spectrum. Most important are the visible light and infrared (heat) frequencies. Visible light images record images the way we would see them, including storms, clouds, fires, and smog. Infrared images measure heat. These images can record clouds, water and land temperatures, and features of the ocean, such as ocean currents. Weather patterns like the El Niño are monitored in infrared images of the equatorial Pacific Ocean.
Two types of weather satellites are geostationary and polar orbiting (Figure 16.42). Geostationary satellites orbit the Earth at the same rate that the Earth rotates; therefore, they remain fixed in a single location above the equator at an altitude of about 36,000 km (22,000 miles). This allows them to constantly monitor the hemisphere where they are located. A geostationary satellite positioned to monitor the United States will have a constant view of the mainland, plus the Pacific and Atlantic Oceans, as it looks for hurricanes and other potential hazards.
Figure 16.42: One of the geostationary satellites that monitors conditions over the United States.
Polar orbiting satellites orbit much lower in the atmosphere, at about 850 km (530 miles) in altitude. They are not stationary but continuously orbit making loops around the poles, passing over the same point at around the same time twice each day. Since these satellites are lower, they get a more detailed view of the planet.
Forecasting Methods[edit]
There are many ways to create a forecast, some simple and some complex. Some use only current, local observations, while others deal with enormous amounts of data from many locations at different times. Some forecasting methods are discussed below.
Perhaps the easiest way to forecast weather is with the 'persistence' method. In this method, we assume that the weather tomorrow will be like the weather today. The persistence method works well if a region is under a stationary air mass or if the weather is consistent from day to day. For example, Southern California is nearly always warm and sunny on summer days, and so that is a fairly safe prediction to make. The persistence method can also be used for long-term forecasts in locations where a warm, dry month is likely to lead to another warm dry month, as in a Southern California summer.
The 'climatology' method assumes that the weather will be the same on a given date as it was on that date in past years. This is often not very accurate. It may be snowing in Yosemite one New Year's Day and sunny and relatively warm on the next. Using the 'trend' method, forecasters look at the weather upwind of their location. If a cold front is moving in their direction at a regular speed, they calculate when the cold front will arrive. Of course, the front could slow down, speed up, or shift directions, so that it arrives late, early, in a strengthened or weakened state, or never arrives at all. Forecasters use the 'analog' method when they identify a pattern. Just like an analogy compares two similar things, if last week a certain pattern of atmospheric circulation led to a certain type of weather, the forecaster assumes that the same pattern this week will lead to the same weather. There are lots of possible variations in patterns and changes often occur, so this method is also not entirelyaccurate.
Numerical Weather Prediction[edit]
The most accurate weather forecasts are made by advanced computers, with analysis and interpretation added by experienced meteorologists. These computers have up-to-date mathematical models that can use much more data and make many more calculations than would ever be possible by scientists working with just maps and calculators. Meteorologists can use these results to give much more accurate weather forecasts and climate predictions.
In Numerical Weather Prediction (NWP), atmospheric data from many sources are plugged into supercomputers running complex mathematical models. The models then calculate what will happen over time at various altitudes for a grid of evenly spaced locations. The grid points are usually between 10 and 200 kilometers apart. Using the results calculated by the model, the program projects that weather further into the future. It then uses these results to project the weather still further into the future and so on, as far as the meteorologists want to go. The final forecast is called a prognostic chart or prog.
Certain types of progs are better at particular types of forecasts and experienced meteorologists know which to use to predict different types of weather. In addition to the prog, scientists use the other forecasting methods mentioned above. With so much data available, meteorologists use a computerized system for processing, storage, display and telecommunications. Once a forecast is made, it is broadcast by satellites to more than 1,000 sites around the world.
NWP produces the most accurate weather forecasts, but as anyone knows, even the best forecasts are not always right. Some of the reasons for this are listed below:
Weather Maps[edit]
Weather maps simply and graphically depict meteorological conditions in the atmosphere. Weather maps may display only one feature of the atmosphere or multiple features. They can depict information from computer models or from human observations. Weather maps are found in newspapers, on television, and on the Internet.
On a weather map, each weather station will have important meteorological conditions plotted. These conditions may include temperature, current weather, dew point, the amount of cloud cover, sea level air pressure, and the wind speed and direction. On a weather map, meteorologists use many different symbols. These symbols give them a quick and easy way to put information onto the map. Figure 16.43 shows some of these symbols and Figure 16.44 explains what they mean.
Figure 16.43: Explanation of some symbols that may appear on a weather map.
Once conditions have been plotted, points of equal value can be connected. This is like the contour line on a topographic map, in which all points at a certain elevation are joined.
Weather maps can have many types of connecting lines. For example:
Surface weather analysis maps are weather maps that only show conditions on the ground (Figure 16.45). These maps show sea level mean pressure, temperature and amount of cloud cover. This information will reveal features such as high and low pressure cells.
Figure 16.45: Surface analysis map of the contiguous United States and southern Canada.
Weather maps can also depict conditions at higher altitudes. Aviation maps show conditions in the upper atmosphere, particularly those that are of interest to pilots. These include current weather, cloud cover, and regions where ice is likely to form.
Lesson Summary[edit]
Review Questions[edit]
Vocabulary[edit]![]()
Points to Consider[edit]
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While most scientists are revered for making sense of our complex universe (Einstein is practically a hero), meteorologists often face ridicule. How can we put a person on the moon or foretell planetary alignments years in advance, yet still fail to put together accurate weather forecasts?
First, to give credit where credit is due: Weather forecasters have improved their game significantly over the last 20 years. The three-day forecasts they deliver today are better than the one-day forecasts they delivered 20 years ago. They're also much better equipped to provide advanced warnings of severe weather, doubling the lead times for tornado warnings and giving people an extra 40 minutes to escape flash floods.
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Modern meteorologists wouldn't be nearly so accurate without numerical forecasting, which uses mathematical equations to predict the weather. Such forecasting requires powerful computers and lots of observational data collected from land, sea and air. A single weather station would never be able to collect so much information. Instead, thousands of stations across the globe are linked and their data pooled. Some of these stations -- ground-based wind gauges (what meteorologists call anemometers), rain collectors and temperature sensors -- resemble those used by amateur weather watchers. Others lie far out at sea, strapped to buoys. And still others travel on commercial airliners or shipping vessels, collecting weather data as passengers and goods are moved from point A to point B. Finally, weather satellites and balloons provide information from the upper regions of the atmosphere. Satellites photograph Earth's weather from their orbit in space, while balloons monitor upper-air data over a particular location.
Collectively, all of these sensors and gauges produce more than 1 million weather-related observations every day. A normal computer -- the kind you buy at your local electronics store -- would choke on all of this data. Luckily, meteorologists can rely on supercomputers, crazy-fast machines that perform millions of calculations per second. In the United States, these computers are housed at the National Centers for Environmental Prediction (NCEP), located in Camp Springs, Md. There, weather observations stream into a supercomputer's brain, which uses complex mathematical models to predict how, based on the incoming data, weather conditions might change over time. The computer's output form the basis of almost every forecast broadcast on radio and television channels across America.
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You might think that the National Centers for Environmental Prediction's supercomputers could never make mistakes, but even their abilities aren't up to the enormous challenge of weather forecasting. That's because they must take into account several large-scale phenomena, each of which is governed by multiple variables and factors. For example, they must consider how the sun will heat the Earth's surface, how air pressure differences will form winds and how water-changing phases (from ice to water or water to vapor) will affect the flow of energy. They even have to try to calculate the effects of the planet's rotation in space, which moves the Earth's surface beneath the atmosphere. Small changes in any one variable in any one of these complex calculations can profoundly affect future weather.
In the 1960s, an MIT meteorologist by the name of Edward Lorenz came up with an apt description of this problem. He called it the Butterfly Effect, referring to how a butterfly flapping its wings in Asia could drastically alter the weather in New York City. Today, Lorenz is known as the father of chaos theory, a set of scientific principles describing highly complex systems, such as weather systems, where small changes in initial conditions radically change the final results. Because of chaos, there is a limit to how accurate weather forecasts can be. Lorenz set this limit at two weeks.
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Modern meteorologists use state-of-the-art technology and techniques to tame chaos, such as the ensemble forecast, which consists of several forecasts, each one based on slightly different starting points. If each prediction in the ensemble looks the same, then the weather is likely to 'behave.' If any prediction looks radically different, then the weather is more likely to 'misbehave.'
Meteorologists also rely on Doppler radar to monitor weather conditions more effectively and improve forecasts. Doppler radar requires a transmitter to emit radio waves into the sky. The waves strike atmospheric objects and bounce back. Clouds moving away from the transmitter return different kinds of waves than clouds moving toward the transmitter. A computer in the radar converts data about the reflected radio waves into pictures showing cloud coverage and bands of precipitation, as well as wind speeds and direction.
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Because of this technology, meteorologists can now predict the weather better than ever, especially when they limit how far they look into the future. For example, up to 12 hours out, meteorologists offer fairly reliable forecasts of general conditions and trends. Unfortunately, thanks to chaos, they will never be able to predict the weather with absolute certainty, which is how surprise storms -- tornadoes and torrential, flooding rains -- continue to devastate communities with little warning. For this reason, it might be best to carry an umbrella, even on days forecasted to be bright and sunny.
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