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About the Weather ...

About the Weather - Air Pressure

About the Weather - Fronts

About the Weather - Hurricanes

About the Weather - In Depth

About the Weather - Polar Vortex

About the Weather - Temperature

About the Weather - Winds

About the Weather - Video:  a Year

Alaska Weather

Canadian Weather

Doppler Radar

Hawaii Weather

Infrared Satellite

Jetstream

NOAA Weather Advisory Charts

Relative Humidity

Sky Cover

Surface Winds

Snow Cover

Temperatures

US Weather

Visible Satellite

Water Vapor Satellite

Wind Chill

World Weather Overview

24 Hour Precipitation Forecast

30-Day U.S. Outlook


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Current and Forecast Weather Summary

For weather forecasts, see our Short-term Forecast, our Seven-Day Forecast, and our Ten-Day Temperature Forecast.

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For the seven-day U.S. weather forcast, click here.

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Current Temperatures

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For the ten-day U.S. temperatures forcast, click here.

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Infrared Satellite

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Doppler Radar ( in motion )

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Sky Coverage

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For the Sky Coverage Current and Forecast, click here.

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Surface Winds

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For the Winds Surface and Aloft Current and Forecast, click here.

For a detailed description of Wind Barbs, click here.

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Wind Chill

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Current Jetstream

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For the Jetstream Current and Forecast, click here.

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Water Vapor

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Relative Humidity

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    Dew Point

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Visible Satellite (not reliable at night)

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Current Snow Cover

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NOAA Weather Advisory Charts

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Key:
    Air Quality Alert
    Beach Hazards Statement
    Blizzard Warning
    Brisk Wind Advisory
    Coastal Flood Advisory
    Coastal Flood Statement
    Coastal Flood Warning
    Coastal Flood Watch
    Dense Fog Advisory
    Excessive Heat Warning
    Fire Weather Watch
    Flash Flood Warning
    Flood Advisory
    Flood Warning
    Flood Watch
    Freeze Warning
    Frost Advisory
    Gale Warning
    Gale Watch
    Hard Freeze Warning
    Hard Freeze Watch
    Hazardous Seas Warning
    Heat Advisory
    Heavy Freezing Spray Warning
    High Surf Advisory
    High Wind Warning
    Hurricane Force Wind Warning
    Hydrologic Outlook
    Lake Wind Advisory
    Lakeshore Flood Warning
    Low Water Advisory
    Marine Weather Statement
    Red Flag Warning
    Rip Current Statement
    Severe Thunderstorm Warning
    Severe Weather Statement
    Small Craft Advisory
    Special Marine Warning
    Storm Warning
    Special Weather Statement
    Tornado Warning
    Tornado Watch
    Tropical Storm Warning
    Wind Advisory
    Wind Chill Advisory
    Winter Storm Warning
    Winter Storm Watch
    Winter Weather Advisory

For our NOAA Weather Advisory Charts, click here.

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National Weather Service 24-Hour Precipitation Forecast and Categorical Outlook

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    For more, see our Two-Day Forecasts, Seven-Day Forecasts, Ten-Day Temperature Forecasts, and our Ten-Day Local Forecasts.

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30-day Outlooks - Temperature and Precipitation

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Weather and climate forecasting is not a science.

Predicting the weather is a statistically-based guess (It has been noted that looking back at the weather for the same date in the previous year is often the most reliable forecast).

Anything beyond about three days:  a month, three months, a year, in 2030, or in 2100 is a shot in the dark.  The longer-term the forecast, the increasingly less accurate it will be.

Be aware, too, that the parameters used in any computer modeling can be set up to predict any outcome desired.  Just as problematic, seemingly-insignificant changes to any model's parameters can result in a radically-different forecast.  Computer forecast models of the weather, like those of the stock-market, have no validity.

See these various issues affecting the accuracy of weather forecasting.

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Alaska Doppler Radar

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Hawaii Doppler Radar

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Current Canadian Weather

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World Weather Overview

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About the Weather ...

Weather and climate forecasting is not a science.

Any weather forecast is a statistically-based guess.  Anything beyond about three days:  a month, three months, a year, in 2030, or in 2100 is a shot in the dark.  Call it weather or climate, the math is all the same.

The longer-term the forecast, the increasingly less accurate it will be.

Prove it yourself:  On any seven-day forecast, save the forecast for seven days out.  Compare it to the actual weather when that day comes.

Note these weather and climate forecasting issues:

(1) The parameters used in any computer modeling can be set up to predict any outcome desired (i.e., due to personal bias or that of the organization funding the forecast).

(2) Seemingly-insignificant changes to any model's parameters can result in a radically-different forecast.  Computer forecast models of the weather have no validity; they are only accurate to the degree that the future is just like the past, which is never true.

See these various issues affecting the accuracy of weather forecasting.



Air pressure

Air pressure is shown in millibars or inches.  The average (standard) air pressure at sea level is 1013 millibars (mb), or 29.92 inches.

Any area with a lower value is an area of low pressure.  Any area with a higher value is an area of high pressure.  The further the pressure is from 1013, the greater the speed of the associated winds.

Weather charts will also label an area as a low, even if it is above 1013 but it is surrounded by higher air pressures.

High pressure tends to bring sunny weather.

Low pressure tends to bring clouds and precipitation (rain, sleet, or snow, depending on the temperature).

Air pressure declines as the altitude rises above sea level, which means you get less oxygen with each breath.  At 11,000 feet, the air pressure is only two-thirds (670 mb), which means you are only getting two-thirds as much oxygen in each breath as normal.  At 18,000 feet, the air pressure is only half (506 mb).  These are standard values; the actual air pressures will vary, as they do at ground level.

Peoples' tolerance for getting less oxygen varies; some people can go no higher than 5,000 feet (843 mb); others can go as high as 14,000 feet (595 mb) with no problem.  The bodies of people who live at high altitudes adapt, and those people can go higher.  Vision can also be affected by high altitude, particularly at night.

The lowest pressure ever recorded (at sea level) was 867.93 millibars, in the eye of typhoon Tip over the Pacific Ocean on 12 October 1979.

The highest air pressure ever recorded was 1085.68 millibars at Tosontsengel, Mongolia, on 19 December 2001.

Fronts

A front is a meeting of two air masses, usually with differing winds, temperature, and humidity.

Low pressure areas tend to be part of a front.

High pressure areas tend to be the central part of an air mass.

Air masses, highs, and lows vary in intensities, and when the differences are minimal, they are not always clearly defined.

In the northern hemisphere, winds circulate clockwise around a high, and counter-clockwise around a low.  In the southern hemisphere, circulation is the opposite.

Warm fronts tend to lie more or less east-west, and divide two fairly static air masses.  Warm fronts are more common in the summer, resulting in the humid, stagnant air with afternoon thunderstorms so familiar in the South.

However, warm fronts can occur even in the colder months, as these two charts from mid-December show.

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A warm front has built up along the Tennessee - South Carolina borders.

It is being held in place by a Jet Stream which is stationary just north of the front, over Kentucky and Virginia, blocking the passage of cooler northern air masses southward as well as the normal movement of cold fronts from west to east.

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Under these conditions, a constant presense of low clouds, high humidity, minimal winds, and light rain can persist for a number of weeks in the South.

Cold fronts tend to run more or less north-south and move eastward, with (in the northern hemisphere) the south end trailing, and can stretch for a thousand miles or more.

Cold fronts are usually the leading edge of a cooler air mass moving east, overtaking or pushing out the existing air mass.

Being an area of low pressure, winds will generally circulate counter-clockwise around a cold front.  That means that a cold front will generally be preceded by winds bringing warmer southern air northward, and followed by cooler northern air moving southward.

Cold fronts often include precipitation, due to moisture carried north from the Gulf of Mexico.  As that moisture moves northward, it cools and so condenses (because cooler air can hold less water than warmer air), resulting in precipitation.

The stronger the winds, the greater the temperature swings.  Warmer southern air reaches further north before cooling, and cooler northern air reaches further south before warming.

Examples are most obvious in the winter, when unusually warm temperatures extend up into Iowa, preceding rain or snow on the leading edge of the cold front, or a cold snap reaches down into the Carolinas, after the passage of a cold front.

Note the big swath of high temperatures for the mid-country shown in the chart below, taken at 5:30am in late October, vs. the more-typical pre-dawn temperatures in the eastern U.S., and the colder temperatures further west.

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That pattern is a fairly reliable predictor of the next 2-3 days' weather:  if it is unusually warm, expect precipitation.  The higher the temperature and winds, the more intense the precipitation and the less time it will take the cold front to pass through.

The quicker and more intense the precipitation, the colder the temperatures and the higher the winds will be when the cold front has passed.

The passage of the front will be followed by a gradual return to more seasonal temperatures, until above average temperatures and southern winds again signal the approach of another cold front.

Temperatures

Temperatures rise during the day as the Sun's infrared radiation warms the Earth's surface.  Temperatures fall at night as the captured warmth radiates away into space.  Clouds insulate, reducing the change in temperature when present.

Typically, temperatures vary about 25 degrees between the high and low temperatures each day.  If the temperature falls less at night, either cloud cover prevents heat from radiating away, or warm air is moving north from the south.

A greater drop in temperature at night or a lesser increase during the day indicates colder air moving in from the northwest:  e.g., an "Alberta clipper".

Air temperature declines about 3℉ / 2℃ per thousand feet rise in altitude, which is why it is cooler in the mountains.

The graph at right shows the full spectrum of Solar radiation, which includes light and heat.  It also shows the amounts of each part of that spectrum which is absorbed by various components of the Earth's atmosphere.

Hurricanes

Hurricanes (cyclones) are a counter-clockwise (in the northern hemisphere) swirling of high-speed winds around a low-pressure center.

As is true for any area of low-pressure, the lower the pressure at the center, the stronger will be the storm's winds.

Hurricanes form in summer or early fall, feeding and growing on the radiated warmth of ocean water and the heat of the sun, gathering energy.  Hurricanes lose energy and diminish as they travel to the cooler north and over land.

Hurricanes typically form in the Atlantic Ocean off the coast of northern Africa (sometimes originating as a storm over the Sahara that flows westward off the coast), or in the Gulf of Mexico.

Those forming in the Atlantic move west and then eventually northwest.  If a hurricane turns north early enough, it will stay out to sea, traveling north and then northeast up through the Atlantic Ocean without making landfall.  If a hurricane goes a bit further west, it will travel up the U.S. Atlantic seaaboard.

If it travels far enough west to go into the Caribbean Sea, it will typically go ashore into Central America or Mexico's Yucatan Peninsula, or turn north and travel into the Gulf of Mexico.

On extremely rare occasions, a hurricane will cross the Panama isthmus and reform in the Pacific Ocean, where it will continue traveling west towards Southeast Asia and southern China.

Hurricanes in the Gulf of Mexico move west and northwest.  If they are far enough west, they will make landfall along the Mexico-Texas coast.

If they turn north further east, they will make landfall on the southern U.S. coast, or possibly even travel eastward and cross the northernmost part of the Florida peninsula, and then travel up the Atlantic coast.

Once a hurricane reaches as far north as the southern U.S. shoreline and northern Florida, it will move generally northeast.

What is so magic about the U.S. shoreline of the Gulf of Mexico?  That latitude is approximately the northernmost limit of the equatorial trade winds, which generally flow east to west; the actual limits vary over time, like any weather feature.  North of that latitude is the northern hemisphere's temperate zone, where the prevailing winds move west to east.  A hurricane follows the existing prevailing winds of the latitude where it resides.

Gulf hurricanes that come ashore west of Panama City, Florida, will usually go up the Mississippi River Valley.  Hurricanes making landfall east of Panama City will typically pass over central Georgia and the Carolinas, usually diminishing into windy rainstorms.  On rare occasions, a hurricane will cross northern Florida, and move into the Atlantic.  From that point, its movement follows the same pattern as any Atlantic hurricane i.e., usually up the Atlantic coast of the U.S.

Hurricanes almost never straddle the Appalachians.  The mountains disrupt and interfere with air flow, dissipating energy, so the part over flatlands remains stronger, and that focus of energy dominates, pulling the hurricane away from the mountains as it moves northward.

Those travelling up the Mississippi River Valley will generally continue on into the Ohio River Valley, crossing the Appalachian mountains in Pennsylvania.  In some cases they will go further north into northern Ohio and Quebec, usually due to circumventing a strong high-pressure area parked off the Virgina-Carolinas coast (e.g., the classic Bermuda High).  In either case, by that time they will have lost a lot of energy and been reduced to heavy rainstorms.  However, heavy flooding is possible.

For hurricanes traveling up the Atlantic seaboard, the part over land quickly loses strength, while the part remaining over the ocean weakens less quickly, feeding on the energy radiated by the warm ocean water.  It will also push lots of rain-filled airflow landward along its north side, as a result of its windy counter-clockwise circulation.  The greater energy on the ocean side dominates, causing the hurricane to hug the coastline, and not go inland.

Hurricanes traveling up the Atlantic coast will typically meet up with a cold front as the front moves eastwards across the U.S., and the two will merge sometime during the passage of both through the mid-Atlantic states.  That combination will often result in a "nor'easter" off the shore of the New England states, as the storm finally heads back out to sea, off Maine and Nova Scotia.

Once out to sea in the northern Atlantic, the storm will head for Europe.  There it will often make landfall with enough energy to be a fairly-heavy rainstorm, which may include some flooding.  The storm will diminish as it moves eastward across Europe and dissipate as it continues on into western Asia.

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Winds

Regarding winds and the movement of air masses, the Earth can be divided into three general areas: the polar regions, the temperate zones, and the equatorial regions.  For more about prevailing winds, click here or on the diagram at right.



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Polar Vortex

What is a polar vortex?  Basically, it is when a very cold polar air mass travels down into the temperate zone.

The animated diagram at left shows airflows during a polar vortex (in purple), looking down from above North America.


A Year's Weather Unfolds:  Watch 2013's Weather Across the Planet


In-Depth:  More about the Weather

For a better understanding of the weather, learn about Skew-T Log-P Charts.

For more information about the weather and flying conditions, see our Aviation Weather webpage, which includes links to METARs and TAFs for many airports.

For a better understanding of climate change, see our Climate Information & News.

Bothered by all those scary climate predictions about CO2 levels?  See the answers to your questions about CO2, and The Earth is Getting Greener!.

See also our list of External Sources for Weather Information and Education

Weather and climate forecasting is not a science.  See these various issues regarding the accuracy of weather forecasting.

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