The Whys of Weather

How is a Weather Forecast Made?

The National Weather Service issues many types of forecasts. Aviation forecasts anticipate sky conditions, wind speed and direction for the next 24 hours. National Oceanic and Atmospheric Administration (NOAA) Weather Radio and television stations broadcast the local forecasts. These forecasts predict sky condition, high and low temperatures, restrictions to visibility and precipitation for the next 48 hours. Extended forecasts issued for periods up to five days show the general weather conditions beyond the range of the local forecast. All these forecasts, updated at least every six hours, compensate for changing conditions.

Besides routine forecasts, the National Weather Service issues watches to alert people of potential severe weather and warnings to warn of impending or existing weather hazards. The National Weather Service also produces weekly, monthly and seasonal outlooks.

Doppler radar, high resolution satellites and powerful computers are just a few of the tools in the modernized National Weather Service. The combination of the most advanced technology and the skills of the forecaster yields more accurate forecasts.

The first step in the forecasting process is describing the present state of the atmosphere. To accomplish this, weather data are collected in hourly observations at over 300 locations across the United States. Surface weather maps produced using the data gathered at these locations creates charts every three hours. Additionally, observations of the upper atmosphere occur twice daily through a worldwide network of simultaneously launched weather balloons carrying radiosondes. A radiosonde is an instrument that measures the temperature, pressure and humidity of the air. Analyzing data from the radiosonde also yields wind speed and direction information.

Data is also collected from other sources. Examination of satellite images and return signals from radar provide information on the location and type of clouds and precipitation. In some situations, special networks of weather spotters and cooperative observers feed important information into this process.

The super computers at the National Center for Environmental Prediction in Washington, D.C. collect, process and analyze observational data. Surface and upper air charts, based on the radiosonde data, provide forecasters with a three-dimensional view of the atmosphere.

Using current observational data, computers calculate future atmospheric conditions based on the principles of modern physics. It requires a half billion calculations to the United States with a model atmosphere. Scientists obtain information about future conditions by moving this ideal atmosphere over an imaginary earth. Results reported in the forms of printed forecasts, charts and numerical data are important guidelines for forecasters. At National Weather Service Forecast Offices, forecasters apply their skill and experience to produce the finished forecast.

As weather forecasts become increasingly accurate, the benefits continue to grow. Fields such as transportation, agriculture and construction gain from advanced knowledge of weather patterns.

Frostbite and Hypothermia

When exposed to cold temperatures, a person's body tissues begin to freeze. This condition, called frostbite, is divided into three categories based on severity. Symptoms of the first degree (called frostnip) cause a loss of feeling and a white or pale appearance in fingers, toes, ear lobes and the tip of the nose.  Continued exposure will lead to the second degree, also called superficial frostbite. When this happens, the outer layer of skin will feel hard and frozen and blistering of the skin is likely. Deep frostbite (third degree) causes skin to become blotchy and blue and the skin and underlying tissues are hard and very cold and the skin will blister. Frostbite is a serious medical condition and attention is necessary; however if you must wait for help, slowly warm affected areas.  Do not rub frostbitten areas…it will cause further tissue damage.  Warm affected areas by tucking hands and feet next to warm skin or by immersion in warm water. If you also note symptoms of hypothermia, warm the body core before the extremities.

Hypothermia occurs when the core body temperature drops below 98.6 degrees Fahrenheit.  Some common warning signs are uncontrollable shivering, disorientation, incoherent or slurred speech and cold pale skin.  If the condition goes untreated, drowsiness, extreme confusion and slowed breathing will occur. When the body’s core temperature drops below 95 degrees, shivering may stop, but he or she is suffering from hypothermia and may lose consciousness or even die. Seek medical care immediately!

Cold-related problems can be even worse when the wind blows.  As wind speed increases, heat is carried away from the body at an accelerated rate, driving down the body temperature. Wind chill is the term used to describe and quantify the rate of heat loss from exposed skin. It combines the effects of wind and cold temperatures.

Although hypothermia is more commonly recognized as an outdoor hazard, it can occur indoors as well. Elderly or infirm persons are at risk even indoors and should take precautions to keep thermostats above 65 degrees and dress warmly.

Frostbite and hypothermia are serious conditions, so listen to the local forecast and wear appropriate clothing to reduce your risk.  Dress in layers of warm clothing so the air trapped between the layers will act as insulation. To prevent the body from losing heat, wear a hat.  If possible, wear mittens instead of gloves.  Eat well because the body turns food into heat, but don’t drink alcohol (it makes the body lose heat faster).  By following these common sense precautions, you can avoid cold weather hazards.


Lightning is the visible effect of a thunderstorm. It develops when rising and descending air, ice pellets and rain droplets within the thunderstorm separate into positive and negative charges. Interactions of these charged particles produce an electrical field within the cloud with the frozen upper levels of the cloud being positively charged and the lower portions negatively charged.

Generally the earth has a negative charge compared to the atmosphere. As a thunderstorm passes over the ground, the negatively charged cloud base produces a positive charge on the ground below. The charge on the ground follows the storm like a shadow. The attraction between positive and negative charges makes the positive ground current flow up buildings, trees and other tall objects trying to establish a flow of current. On average, lightning produces 15 million volts of electricity, although up to 100 million volts can be generated. However, air is a poor conductor of electricity and acts to insulate the cloud and ground charges, preventing the flow of current until enormous electrical charges have built up.

Lightning occurs when the difference between the positive and negative charges becomes great enough to overcome the resistance of the insulating air. This forces a conductive path to flow between the two opposite charges. Lightning strikes travel from cloud to cloud, from cloud to ground and where there are tall structures, from ground to cloud! Lightning does not strike in a single "bolt". Instead, a stream of charge particles called a leader, moves toward the earth in a series of steps. These steps are approximately 150 feet long with a pause between steps of about 50 microseconds. When the leader reaches the earth, the charged (called ionized) particles, surge back toward the cloud. This occurs so quickly, it produces the optical illusion of lightning flashing downward from a cloud.

The air surrounding a lightning stroke is heated to nearly 50,000 degrees Fahrenheit. This is hotter than the surface of the sun! Extreme heating of the air near the lightning stroke causes a shock wave that results in thunder. Lightning has often been described in terms of its appearance--streak, fork or a sheet. Streak lightning is the most common, taking on the appearance of a jagged line. Fork lightning describes several lightning strokes occurring in the same area simultaneously, taking on the look of a fork in the road. Cloudy skies diffuse the visual intensity of lightning the phenomena of sheet lightning happens.

Myths about lightning are common, including the idea of heat lightning, the color of lightning and how often it occurs in the same location. First, there is no such thing as heat lightning. "Heat lightning" is simply lightning associated with a distant storm. The belief that lightning occurs in various colors is also incorrect because lightning is generally whitish in color. However, when the air is very moist, lightning may appear red. Also, lightning observed near fluorescent lights imparts a bluish hue. What about the old wives' tale that lightning cannot strike the same place twice? Definitely not true! Lightning can and often does strike the same area twice! Lightning is dangerous, so you should always follow the safety rules to avoid injury!

Visual Phenomena

Why is the sky blue?  As visible light from the sun passes through the atmosphere, particles of dust and droplets of moisture either absorb or scatter the light.  The wavelength of light scattered most by the atmosphere is blue and so the sky appears that color.

But if the light from the sun usually makes the sky appear blue, what happens at sunrise and sunset to make the sky look red?  Due to the angle of the sun, light must pass through more of the atmosphere and more particles than if the sun was higher in the sky. The colors of the visible spectrum that have a shorter wavelength such as blue and purple do not predominate and the sky appears various shades or red.

Red sky in the morning, sailors take warning; Red skies at night, sailors' delight. Like other weather folklore, this has basis in truth.   A reddish hue to the sky shows light from the sun filtered through more water vapor than if the sky appeared blue.  The more water vapor in the atmosphere, the more clouds and a better chance for rain and possibly storms.  If the red sky appeared to the mariner at night, the bad weather was behind him and it was smooth sailing before him.  That’s probably why rainbows are associated with good luck. To see the luminous bands of color, an observer's eyes must be aligned with a source of light and the atmosphere must be moist.  Although rainbows are usually seen in the aftermath of a rain shower, they may be seen in foggy areas and those prone to sea spray.

A ring of light is sometimes visible around the sun or moon.  When this happens, it’s because light is refracted through ice crystals in the atmosphere.  Another solar phenomenon is the sundog or mock sun.  It also produced by refraction of light through ice crystals.  When the sun is near the horizon, a sundog would appear as brightly colored spots on either side of the sun.

Several phenomena are due to diffraction, or light being bent around objects.  A bright ring of light may appear to rest on top of the moon when it is viewed through thin, high clouds.  This is known as a corona (or crown).  A similar effect is possible with the sun, but because the sun is much brighter, it is more difficult to discern.   Iridescence is another diffraction phenomena.  It occurs when droplets of different size are present within a cloud.  The clouds, especially the ones near to the sun, can appear pink, blue or green!

When light passes through layers of the atmosphere that have different densities, objects may appear far from their true position, and upside-down. These conditions can lead to a number of optical phenomenon, including looming (when something below the distant horizon is made visible), mirages (inverted images either above or below the true image), or the Fata Morgana (a type of multiple mirage).

A common type of 'inferior' mirage can be seen on hot days when road surfaces absorb a great deal of incoming solar radiation. The air just above the road is relatively cooler and when the light from the sky above the road is refracted into our eyes, the pavement appears wet (the 'sky' appears on the road!). When warm air rests over a cold surface, the Badlands are sometimes “visible” in Rapid City through the phenomena termed 'looming'. Occasionally, inverted images of the badlands appear above the visible badlands in a 'superior' mirage. Multiple superior mirages sometimes form under just the right conditions, when several erect and inverted images appear above one another...this phenomenon
is known as the Fata Morgana.

So now you not only know why the sky is blue, but why some of these stunning visual phenomenon occur.

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