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This page is meant to serve as an explainer for terms that appear on the city forecast pages. It will offer both a regular definition of a term and a "plain English" explanation of how that term might just affect your life.
For a more comprehensive reference guide to the weather, see washingtonpost.com's Weather Glossary. |
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 If you see this image next to a definition, click on it to get an interactive calculator for computing that weather factor. |
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| Barometer |
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- Definition
- A barometer is used to measure atmospheric pressure. A typical barometer employs a mercury-filled glass tube, sealed at one end with a mercury reservoir at the other end. The mercury then adjusts between the tube and the reservoir to reflect atmospheric pressure. Low pressure will cause the mercury to drop in the tube. High pressure forces the mercury to rise in the column. Another type of barometer is an aneroid barometer. Aneroid barometers use a tightly sealed box with a partial vacuum inside. Small changes in external air pressure cause the cell to expand or contract, which reflects outside atmospheric pressure.
- So What Does This Mean for Me?
- Okay, so it's not the actual barometer you're interested in, but barometric pressure. Barometric pressure is the amount of pressure the air is exerting. It is measured in inches (though scientists prefer millibars), which indicates the height of mercury in barometers. Barometric pressure is an indicator of incoming weather systems. Low pressure systems often bring with them clouds and precipitation, because air rises in low pressure systems. The higher the altitude, the cooler the air gets, causing condensation, clouds and precipitation. High pressure systems usually result in mostly clear days. High pressure systems push air closer to the ground, preventing the formation of clouds. If you see the barometer rising on our current conditions forecast, it is an indicator of clear weather on the horizon. If it is falling, it can be read as a prelude to cloudy or rainy weather. If it's steady, then expect more of the same of whatever is outside of your window.
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| Dew Point |
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- Definition
- A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).
- So What Does This Mean for Me?
- Dew point can be used to gauge the likelihood of fog. The difference between the dew point and current temperature can also gauge how much moisture is present in the air. The smaller the difference between current temperature and the dew point, the more moisture is present in the air. High dew points can also lead to unstable weather. Dew points also are used as an indicator for determining the likelihood of thunderstorms in the summer.
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| Heat Index |
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- [A heat index chart is available]
- Definition
- Heat index is used to describe the apparent temperature outside. The index combines the temperature and humidity in a formula.
- So What Does This Mean for Me?
- In the summer,
or in warmer climates, the heat index will tell you how it actually feels outside (also described as the apparent temperature). When both the temperature and humidity are high, the heat index could be as high as 30 degrees above the actual temperature. For example, if it's 90 degrees and the humidity is 90 percent, it will feel like it's 122 degrees outside.
| Heat Index |
Impact |
| 80-90 | Fatigue is possible with prolonged exposure |
| 90-105 | Sunstroke and heat exhaustion are possible |
| 105-139 | Sunstroke and heat exhaustion are likely. |
| 130+ | Sunstroke and heat exhaustion are highly likely. |
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| Humidity |
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- Definition
- The amount of water vapor present in the air. The higher the temperature, the greater the number of
water molecules the air can hold. Humidity is also commonly used to describe the relative humidity.
- So What Does This Mean for Me?
- Humidity
has its greatest impact when it's warmer outside, because the air will hold more moisture. High humidity produces a muggy, stuffy feeling that makes it feel hotter than it actually is outside. The body's primary cooling mechanism is the evaporation of sweat. When the humidity is higher, sweat will not evaporate as fast because the air is already moist. Humidity and temperature are used in warm weather to produce the heat index, which measures how hot it actually feels outside.
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| NEXRAD |
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- Definition
- NEXt-Generation Weather RADar. Technologically-advanced weather radar being developed to replace the current
WSR-57 and
WSR-74 units. NEXRAD is a high-resolution
Doppler radar with increased emphasis on automation, including use of algorithms and automated volume scans. NEXRAD units are known as WSR-88D.
- So What Does This Mean for Me?
- You mean you didn't understand the scientific gobbledygook? (**Sigh**) Okay. NEXRAD was developed in 1988 (thus the -88) and uses Doppler radar (thus the D). The radar is used to measure precipitation and can also measure both the direction and speed of objects being moved by the wind (such as rain, hail, snow, etc). NEXRAD shows where current precipitation is occurring, where it's going and how fast it's going there. The units are ground-based radar stations. The National Weather Service is the process of installing NEXRAD at all its weather stations.
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| Precipitation Radar |
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- Definition
- Precipitation radar is a made-up term for our use on washingtonpost.com. Essentially, it is the same as NEXRAD, but who knows what NEXRAD is? Washingtonpost.com offers two types of precipitation radar images: D.C. metro area and regional. The D.C. metro precipitation radar images is taken from a ground station in Baltimore. The regional radars are composites from individual NEXRAD stations throughout an entire region.
- So What Does This Mean for Me?
- You can see cool images and check out where and how hard it's raining.
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| Temperature |
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- Definition
- A measure of the energy in the air. The more heat energy in the air, the higher the temperature. As the surface warms in reaction to the sun's heat, it also warms the air above it.
- So What Does This Mean for Me?
- Temperature
alone is not an accurate measure of how warm or cold it feels outside. In the summertime or in warmer climates, temperature and humidity are combined to produce the heat index, which is the measure of how warm it actually feels outside. In the winter or in colder climates, temperate and wind speed are combined to produce the wind chill, which is a measure of how cold it feels outside.
- Why are these separate? When it's cold, the humidity in the air has less of an impact on the temperature, because the air holds less moisture when it's cold. However, a brisk wind on a cold day can make it feel 10 or more degrees cooler than it actually is outside. When it's warm, the air holds more moisture, producing a muggy feeling, particularly when the air isn't moving. That muggy feeling makes it feel hotter than it actually is. A breeze is nice in the summertime, but a hot wind doesn't do much to make it feel much cooler outside.
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| UV Index |
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- Definition
- The UV (Ultraviolet) Index indicates the day's exposure to ultraviolet rays. The UV index is measured around noon for a one-hour period and rated on a scale of 0-15.
- So What Does This Mean for Me?
- Long-term exposure to UV rays has been proven to be harmful.
| Index Number |
Exposure Level |
| 0-2 | Minimal |
| 3-4 | Low |
| 5-6 | Moderate |
| 7-9 | High |
| 10+ | Very High |
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| On clear days during the summer, exposure to UV rays can be particularly high. Clouds, fog, mist and smog all cut down the body's exposure to UV rays. To avoid sunburns, sunblock and hats are recommended on high UV days.
- The ultraviolet index is a forecast of the ultraviolet intensity at the Earth's surface, taken over a one-hour period around noon. It is measured once a day. Clouds will affect the intensity, measured on a scale of 0-15. (10+ is considered very high). People need to have skin block and dress properly to avoid sunburn. Hats are a good idea on high-UV days. The heat index and temperature can affect the UV Index, but not directly.
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| Visibility |
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- Definition
- Visibility is the distance one can see at ground level. On clear days, visibility can stretch for dozens of miles. But haze, smog, fog and clouds all reduce visibility at ground level.
- So What Does This Mean for Me?
- Visibility is particularly important for pilots, drivers and boaters, because decreased visibility makes it more difficult to spot potential obstacles, giving the operator far less reaction time. Visibility can tend to be low in the early morning hours, as morning fog is common in many areas.
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| Wind Chill |
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- [A wind chill chart is available.]
- Definition
- The perceptive difference between actual temperature and the way it feels when factoring in the wind speed. Wind reduces the amount of body heat retained, so the effects of outdoor temperatures on the body must factor in the current wind chill.
- So What Does This Mean for Me?
- Wind
speed and temperature are factored together to compute the wind chill, which measures how cold it feels outside (the apparent temperature). The colder the temperature, the greater the impact of wind speed on the apparent temperature. For example, if it's 10 degrees and the wind speed is 20 mph, it will feel like it is 24 degrees below zero.
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| Wind Speed |
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- Definition
- The speed at which the wind is moving in relation to the ground.
- So What Does This Mean for Me?
- Wind speed mainly affects the wind chill, which in turn impacts how cold it actually feels outside.
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| Heat Index Chart |
| Heat Index ChartHumidity |
| 100% | 72° | 80° | 91° | 108° | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 95% | 71° | 79° | 89° | 105° | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 90% | 71° | 79° | 88° | 102° | 122° | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 85% | 71° | 78° | 87° | 99° | 117° | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 80% | 71° | 78° | 86° | 97° | 113° | 136° | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 75% | 70° | 77° | 86° | 95° | 109° | 130° | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 70% | 70° | 77° | 85° | 93° | 106° | 124° | 144° | --- | --- | --- | --- | --- | --- | --- | --- |
| 65% | 70° | 76° | 83° | 91° | 102° | 119° | 138° | --- | --- | --- | --- | --- | --- | --- | --- |
| 60% | 70° | 76° | 82° | 90° | 100° | 114° | 132° | 149° | --- | --- | --- | --- | --- | --- | --- |
| 55% | 69° | 75° | 81° | 89° | 98° | 110° | 126° | 142° | --- | --- | --- | --- | --- | --- | --- |
| 50% | 69° | 75° | 81° | 88° | 96° | 107° | 120° | 135° | 150° | --- | --- | --- | --- | --- | --- |
| 45% | 68° | 74° | 80° | 87° | 95° | 104° | 115° | 129° | 143° | --- | --- | --- | --- | --- | --- |
| 40% | 68° | 74° | 79° | 86° | 93° | 101° | 110° | 123° | 137° | 151° | --- | --- | --- | --- | --- |
| 35% | 67° | 73° | 79° | 85° | 91° | 98° | 107° | 118° | 130° | 143° | --- | --- | --- | --- | --- |
| 30% | 67° | 73° | 78° | 84° | 90° | 96° | 104° | 113° | 123° | 135° | 148° | --- | --- | --- | --- |
| 25% | 66° | 72° | 77° | 83° | 88° | 94° | 101° | 109° | 117° | 127° | 139° | --- | --- | --- | --- |
| 20% | 66° | 72° | 77° | 82° | 87° | 93° | 99° | 105° | 112° | 120° | 130° | 141° | --- | --- | --- |
| 15% | 65° | 71° | 76° | 81° | 86° | 91° | 97° | 102° | 108° | 115° | 123° | 131° | --- | --- | --- |
| 10% | 65° | 70° | 75° | 80° | 85° | 90° | 95° | 100° | 105° | 111° | 116° | 122° | 128° | --- | --- |
| 5% | 64° | 69° | 74° | 79° | 84° | 88° | 93° | 97° | 102° | 107° | 111° | 116° | 122° | 128° | --- |
| 0% | 64° | 69° | 73° | 78° | 83° | 87° | 91° | 95° | 99° | 103° | 107° | 111° | 117° | 120° | 125° |
| 70° | 75° | 80° | 85° | 90° | 95° | 100° | 105° | 110° | 115° | 120° | 125° | 130° | 135° | 140° |
| Air Temperature (°F)
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| Wind Chill Chart |
Wind Chill Chart
Wind Speed
(mph) |
| 0 | 35° | 30° | 25° | 20° | 15° | 10° | 5° | 0° | -5° | -10° | -15° | -20° | -25° | -30° |
| 5 | 32° | 27° | 22° | 16° | 11° | 6° | 0° | -5° | -10° | -15° | -21° | -26° | -31° | -36° |
| 10 | 22° | 16° | 10° | 3° | -3° | -9° | -15° | -22° | -27° | -34° | -40° | -46° | -52° | -58° |
| 15 | 16° | 9° | 2° | -5° | -11° | -18° | -25° | -31° | -38° | -45° | -51° | -58° | -65° | -72° |
| 20 | 12° | 4° | -3° | -10° | -17° | -24° | -31° | -39° | -46° | -53° | -60° | -67° | -74° | -81° |
| 25 | 8° | 1° | -7° | -15° | -22° | -29° | -36° | -44° | -51° | -59° | -66° | -74° | -81° | -88° |
| 30 | 6° | -2° | -10° | -18° | -25° | -33° | -41° | -49° | -56° | -64° | -71° | -79° | -86° | -93° |
| 35 | 4° | -4° | -12° | -20° | -27° | -35° | -43° | -52° | -58° | -67° | -74° | -82° | -89° | -97° |
| 40 | 3° | -5° | -13° | -21° | -29° | -37° | -45° | -53° | -60° | -69° | -76° | -84° | -92° | -100° |
| 35° | 30° | 25° | 20° | 15° | 10° | 5° | 0° | -5° | -10° | -15° | -20° | -25° | -30° |
| Air Temperature (°F) |
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| Source List |
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- TAMMI (Weather Glossary), National Weather Service
- Joan Von Ahn, customer service meteorologist, National Weather Service
- Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign
- National Weather Service, Louisville, Ky.
- Jack Williams "USA Today Weather Almamac", Vintage Books, 1994
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© Copyright 2001 The Washington Post Company
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