Glossary of common weather terms

 

Barometric Pressure:

The weight of air that makes up our atmosphere exerts a pressure on the surface of the earth.  This pressure is known as atmospheric pressure.  Generally, the more air above an area, the higher the atmospheric pressure, this, in turn, means that atmospheric pressure changes with altitude.  For example, atmospheric pressure is greater at sea-level than on a mountain top.  To compensate for this difference and facilitate comparison between locations with different altitudes, atmospheric pressure is generally adjusted to the equivalent of sea-level pressure.  This adjusted pressure is known as barometric pressure.

Barometric pressure also  changes with local weather conditions, making barometric pressure an extremely important and useful weather forecasting tool.  High pressure zones are generally associated with fair weather while low pressure zones are generally associated with poor weather.  For forecasting purposes, however, the absolute barometric pressure value is generally less important than the change in barometric pressure.  Rising pressure tends to indicate improving weather while falling pressure indicates deteriorating weather conditions.

 

Dew Point:

Dew point is the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur.  Since atmospheric pressure varies only slightly at the earth's surface, the dew point is a good indicator of the air's actual water vapor content.  High dew points indicate high water vapor content; low dew points, low water vapor content.  Addition of water vapor to the air increases the dew point; removing water vapor lowers it.  When the air temperature and dew point are equal, the air is saturated and the relative humidity is 100%.  The dew point is an important measurement used to predict the formation of dew, frost and fog.  If dew point and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night.  A high dew point indicates a better chance of rain, severe thunderstorms and tornados.  You can also use dew point to predict the minimum overnight temperature.  Provided no new fronts are expected overnight and the afternoon Relatively Humidity is greater than or equal to 50%, the afternoon's dew point gives you an idea of what minimum temperature to expect overnight, since the air can never get colder than the dew point.

 

Evapotranspiration (ET)

ET is a measurement of the amount of water vapor returned to the air in a given area.  It combines the amount of water vapor returned through evaporation (from wet vegetation surfaces and the stoma of leaves) with the amount of water vapor returned through transpiration (exhaling of moisture through plant skin) to arrive at a total.  Effectively, ET is opposite of rainfall, and it is expressed in the same units of measure.

 

Fog:

Like any cloud, fog usually forms in one of two ways:  (1) by cooling - air is cooled below its saturation point (dew point); (2) by evaporation and mixing - water vapor is added to the air by evaporation, and the moist air mixes with relatively dry air.  Once fog forms it is maintained by new fog droplets, which constantly form on available nuclei.  In other words, the air must maintain its degree of saturation either by continual cooling or by evaporation and mixing of vapor into the air.

Radiation Fog forms best on cool nights when a shallow layer of moist air is overlain by dryer air.  Since the moist layer is shallow, it does not absorb much of the earth's outgoing infrared radiation.  The ground, therefore, cools rapidly and so does the air directly above it, and a surface inversion forms.  The longer the night, the longer the time of cooling and the greater the likelihood of fog.  So, radiation fogs are most common over land in late fall and winter.

Advection Fog forms in the following manner.  Cooling surface air to its saturation point may be accomplished by warm moist air moving over a cold surface.  The surface must be sufficiently cooler than the air above so that  the transfer of heat from air to surface will cool the air to its dew point and produce fog.

Evaporation (Mixing) Fog occurs when two unsaturated bodies of air mix together to produce fog.  An example of this is when you see your breath on a cold day.  When moist air from your mouth meets the cold air and mixes with it, the air becomes saturated, and a tiny cloud forms with each exhaled breath.  A warm rain falling through a layer of cold moist air can produce fog.  When a warm raindrop falls into a cold layer of air, the saturation vapor pressure over the raindrop is greater than that of the air.  This vapor pressure difference causes water to evaporate from the raindrop into the air.  This process may saturate the air and, if mixing occurs, fog forms.  Fog of this type is often associated with warm air riding up and over a mass of colder surface air.  The fog usually develops in the shallow layer of cold air just ahead of an approaching warm front or behind a cold front.

 

Heat Index:

The Heat Index uses temperature and relative humidity to determine how hot the air actually feels.  When humidity is low, the apparent temperature will be lower than the air temperature, since perspiration evaporates to cool the body.  However, when humidity is high (the air is more saturated with water vapor) the apparent temperature feels higher than the actual air temperature, because perspiration evaporates more slowly.

 

Moon Phase:
 
New Moon - The Moon's unilluminated side is facing the Earth. The Moon is not visible (except during a solar eclipse).
Waxing Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
First Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
Waxing Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
Full Moon - The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight.
Waning Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
Last Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
Waning Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
Following waning crescent is New Moon, beginning a repetition of the complete phase cycle of 29.5 days average duration. The time in days counted from the time of New Moon is called the Moon's "age". Each complete cycle of phases is called a "lunation".

Because the cycle of the phases is shorter than most calendar months, the phase of the Moon at the very beginning of the month usually repeats at the very end of the month. When there are two Full Moons in a month (which occurs, on average, every 2.7 years), the second one is called a "Blue Moon".

Although Full Moon occurs each month at a specific date and time, the Moon's disk may appear to be full for several nights in a row if it is clear. This is because the percentage of the Moon's disk that appears illuminated changes very slowly around the time of Full Moon (also around New Moon, but the Moon is not visible at all then). The Moon may appear 100% illuminated only on the night closest to the time of exact Full Moon, but on the night before and night after will appear 97-99% illuminated; most people would not notice the difference. Even two days from Full Moon the Moon's disk is 93-97% illuminated.

New Moon, First Quarter, Full Moon, and Last Quarter phases are considered to be primary phases and their dates and times are published in almanacs and on calendars.  The two crescent and two gibbous phases are intermediate phases, each of which lasts for about a week between the primary phases, during which time the exact fraction of the Moon's disk that is illuminated gradually changes.

The phases of the Moon are related to (actually, caused by) the relative positions of the Moon and Sun in the sky. For example, New Moon occurs when the Sun and Moon are quite close together in the sky. Full Moon occurs when the Sun and Moon are at nearly opposite positions in the sky - which is why a Full Moon rises about the time of sunset, and sets about the time of sunrise, for most places on Earth. First and Last Quarters occur when the Sun and Moon are about 90 degrees apart in the sky. In fact, the two "half Moon" phases are called First Quarter and Last Quarter because they occur when the Moon is, respectively, one- and three-quarters of the way around the sky (i.e., along its orbit) from New Moon.

The relationship of the Moon's phase to its angular distance in the sky from the Sun allows us to establish very exact definitions of when the primary phases occur, independent of how they appear. Technically, the phases New Moon, First Quarter, Full Moon, and Last Quarter are defined to occur when the excess of the apparent ecliptic (celestial) longitude of the Moon over that of the Sun is 0, 90, 180, and 270 degrees, respectively. These definitions are used when the dates and times of the phases are computed for almanacs, calendars, etc. Because the difference between the ecliptic longitudes of the Moon and Sun is a monotonically and rapidly increasing quantity, the dates and times of the phases of the Moon computed this way are instantaneous and well defined.

The percent of the Moon's surface illuminated is a more refined, quantitative description of the Moon's appearance than is the phase. Considering the Moon as a circular disk, the ratio of the area illuminated by direct sunlight to its total area is the fraction of the Moon's surface illuminated; multiplied by 100, it is the percent illuminated. At New Moon the percent illuminated is 0; at First and Last Quarters it is 50%; and at Full Moon it is 100%. During the crescent phases the percent illuminated is between 0 and 50% and during gibbous phases it is between 50% and 100%.

For practical purposes, phases of the Moon and the percent of the Moon illuminated are independent of the location on the Earth from where the Moon is observed. That is, all the phases occur at the same time regardless of the observer's position.

 

NEXRAD

NEXRAD or Nexrad (Next-Generation Radar) is a network of 158 high-resolution Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration (NOAA) within the United States Department of Commerce. Its technical name is WSR-88D, which stands for Weather Surveillance Radar, 1988, Doppler. NEXRAD detects precipitation and atmospheric movement or wind. It returns data which when processed can be displayed in a mosaic map which shows patterns of precipitation and its movement. The radar system operates in two basic modes, selectable by the operator: a slow-scanning clear-air mode for analyzing air movements when there is little or no activity in the area, and a precipitation mode with a faster scan time for tracking active weather. NEXRAD has an increased emphasis on automation, including the use of algorithms and automated volume scans.

Base Reflectivity is a measure of the intensity of precipitation occurring, and is reported in units of DBZ. The WSR-88D emits pulses of energy into the atmosphere at regular intervals. When this energy impacts something (i.e. a raindrop, a snowflake, a mountain, etc.), some of the energy is scattered back to the radar dish. The amount of energy which is received back at the radar dish is measured in units of DBZ (decibels). The higher the DBZ value the larger the object. Large raindrops and hail, for example, produce high DBZ values. In general, DBZ values greater than 15 indicate areas where precipitation is reaching the ground; DBZ values less than 15 usually are an indication of very light precipitation which in most cases is evaporating in the atmosphere before it reaches the ground.


Single-site NEXRAD base reflectivity data are updated every 5, 6, or 10 minutes, depending on whether the radar is in normal precipitation mode, storm precipitation mode, or clear air mode. The same NEXRAD base reflectivity information (excluding clear air mode data) is incorporated into the WSI NOWrad national and regional mosaics, except that the NOWrad images go through rigorous quality control procedures that remove most false echoes caused by ground clutter and anomalous propagation (AP). Caution must be exercised in attempting to interpret the single site NEXRAD images because they often contain 'artifacts' that are not actual precipitation echoes.

The Radial Velocity maps allow you to view single site Nexrad data for many sectors. Colors indicated the direction of atmospheric particles (raindrop, snowflakes, smoke, dust etc) from the radar site. Green indicates movement towards the radar site and red indicates movement away from the radar site. This is particularly useful in showing rotation within thunderstorms, which may lead to tornadic development.


Relative Humidity:

While relative humidity is the most common way of describing atmospheric moisture, it is also the most misunderstood.  The concept of relative humidity may at first seem confusing because it does not indicate the actual amount of water vapor in the air.  Instead, it tells us how close the air is to being saturated.  The relative humidity (RH) is the ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at that particular temperature (and pressure).  It is the ratio of the air's water vapor content to its capacity.

It is important to realize that relative humidity changes with temperature, pressure and water vapor content.  A parcel of air with a capacity for 10 g of water vapor which contains 4 g of water vapor, the relative humidity would be 40%.  Adding 2 g more water vapor (for a total of 6 g) would change the humidity to 60%.  If that same parcel of air is then warmed so that it has a capacity of 20 g of water vapor, the relative humidity drops to 30% even though the water vapor does not change.

Relative humidity is an important factor in determining the amount of evaporation from plants and wet surfaces since warm air with low humidity has a large capacity to absorb extra water vapor.

 

Solar Radiation:

Technically known as Global Solar Radiation.  This is a measure of the intensity of the sun's radiation reaching a horizontal surface.  This irradiance includes both the direct component from the sun and the reflected component from the rest of the sky.  The solar radiation reading gives a measure of the amount of solar radiation hitting the solar radiation sensor at any given time, expressed in watts/sq. meter (W/m2).

 

Temperature/Humidity/Sun/Wind (THSW) Index

The THSW Index uses humidity and temperature like the Heat Index, but also includes the heating effects of sunshine and the cooling effects of wind (like wind chill) to calculate an apparent temperature of what it feels like out in the sun.

 

UV (Ultra Violet) Radiation

Energy from the sun reaches the earth as visible, infrared and ultraviolet (UV) rays.  Exposure to UV rays can cause numerous health problems, such as sunburn, skin cancer, skin aging, cataracts, and can suppress the immune system.  UV readings are displayed in two scales:  MEDs and UV Index.

MED (Minimum Erythemal Dose) is defined as the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after sun exposure.  In other words, exposure to 1 MED will result in a reddening for the skin.  Because different skin types burn at different rates, 1 MED for persons with very dark skin is different from 1 MED for persons with very light skin.

EPA SKIN PHOTOTYPES

Skin Phototype

Skin Color

Tanning & Sunburn history

1  Never tans, always burns Pale or milky white; alabaster Develops red sunburn; painful swelling; skin peels
2  Sometimes tans, usually burns Very light brown; sometimes freckles Usually burns, pinkish or red coloring appears; can gradually develop light brown tan
3  Usually tans, sometimes burns Light tan; brown or olive; distinctly pigmented Rarely burns; shows moderately rapid tanning response
4  Always tans, rarely burns Brown, dark brown or black Rarely burns; shows very rapid tanning response

 

UV Index assigns a number between 0 and 16 to the current UV intensity. 

Index Values

Exposure Category

0-2

Low

3-4

Moderate

5-6

High

7-9

Very High

10+

Extreme

 

Wind Chill

Wind chill takes into account how the speed of the wind affects our perception of the air temperature.  Our bodies warm the surrounding air molecules by transferring heat from the skin.  If there's no air movement, this insulating layer of warm air molecules stays next to the body and offers some protection from cooler air molecules.  The wind sweeps that comfy warm air surrounding the body away.  The faster the wind blows, the faster heat is carried away and the colder you feel.