Temperature, Pressure, and Density
Aviation Routine Weather Report (METAR)
Terminal Aerodrome Forecast (TAF)
Graphical Forecasts for Aviation (GFA)
Winds and Temperatures Aloft Forecast (FB)
In-Flight Weather Advisories (WA, WS, WST)
Constant Pressure Analysis Charts
The earth’s atmosphere is a mixture of gases made up primarily of nitrogen and oxygen. The atmosphere is in layers, with each layer having its own characteristics:
Troposphere—The layer extending from the surface up to about 7 miles. All of earth’s weather occurs in the tropospere because it contains water vapor. Temperature decreases steadily with altitude in the troposphere.
Tropopause—The boundary between the troposphere and the stratosphere. The tropopause slopes from about 20,000 feet over the poles to about 65,000 feet over the equator, and it is higher in summer than in winter.
Stratosphere—The layer above the troposphere in which there is relatively little change of temperature with altitude, except for a warming trend near the top.
Energy received from the sun in the form of solar radiation is the primary driving force of the weather on the earth. The earth’s surface and its atmosphere reflect about 55 percent of the radiation and absorb the remaining 45 percent, converting it to heat. The earth in turn radiates energy, and this outgoing radiation is called terrestrial radiation.
The standard temperature of the atmosphere at mean sea level is 15°C and 59°F. The standard pressure at mean sea level is 29.92 inches of mercury, 1013.2 millibars, and 14.69 pounds per square inch.
Almost all weather is caused by heat transferred to the earth by the sun through solar radiation. Most of this energy is reradiated, but the rest is converted into heat.
The temperature of the air in the troposphere decreases with altitude at a rate of 2°C per 1,000 feet. This is called the average lapse rate. There are two other lapse rates that are of interest to pilots: dry adiabatic lapse rate and moist adiabatic lapse rate. The dry adiabatic lapse rate is the change in temperature with altitude for unsaturated air; it is 3°C per 1,000 feet. The moist adiabatic lapse rate is the change in temperature with altitude for saturated air. Because of the condensation of moisture from this air, the moist adiabatic lapse rate is less than the dry adiabatic lapse rate. The actual rate depends upon the dew point of the air. When we know the temperature at any given level and the lapse rate, we can find the freezing level:
A temperature inversion is a change in temperature in which the air gets warmer as the altitude increases. A surface inversion occurs when terrestrial radiation on a clear night cools the surface of the land and lowers the temperature of the air immediately above the surface. The air temperature increases with altitude for a few hundred feet. An inversion aloft occurs when a current of warm air aloft overruns cold air near the surface. A low-level temperature inversion with high relative humidity will trap fog, smoke, low clouds, and other restrictions to visibility. The air will normally be smooth in an inversion.
Pressure altitude is the altitude measured above the standard pressure level at sea level of 29.92" of mercury (Hg), or 1013.2 millibars (mb). In the lower levels of the troposphere, the atmospheric pressure decreases approximately 1 "Hg for each 1,000-foot increase in altitude. We can find the pressure altitude by setting the barometric scale of the altimeter to 29.92 "Hg, or 1013.2 mb, and reading the altimeter indication. We can also compute the approximate pressure altitude by using this standard lapse rate of 1 "Hg per 1,000 feet. If the altimeter indicates 1,850 feet when the barometric scale is set to 30.18 inches of mercury, it would indicate 260 feet lower if it were set to the standard sea level pressure of 29.92 "Hg.
The density of the air is affected by its temperature, pressure, and moisture content. It is the density of the air that determines the performance of an aircraft engine and the aerodynamic forces that are produced by an airfoil. Density altitude is the altitude in standard air where the density is the same as that of the existing air. It is found by correcting pressure altitude for nonstandard temperature. As the density of the air decreases because of an increase in temperature or water vapor, or a decrease in pressure, the density altitude increases. An airspeed indicator is a differential pressure indicator which measures the dynamic pressure of the air. When the density of the air decreases, the static pressure will decrease and the true airspeed will increase.
Differences in temperature create differences in pressure, and these pressure differences cause winds to blow. We can tell a lot about wind by studying weather maps that show lines of equal barometric pressure, called isobars. When isobars are close together on a surface weather map, the pressure gradient is steep: there is a large amount of pressure change in a small distance, and the wind velocities are strong. Wind blows from an area of high pressure into an area of low pressure, but it does not cross the isobars at right angles.
The Coriolis force, caused by the rotation of the earth, acts at right angles to the wind, and in the Northern Hemisphere it deflects the wind to the right until it blows parallel to the isobars.
Friction between wind and the surface is a third force that acts to change wind direction. Friction slows wind. The rougher the terrain and the stronger the wind speed, the greater the effect of friction. As the friction slows windspeed, the Coriolis force decreases, but friction does not affect the pressure gradient force, which causes an imbalance in the pressure gradient and Coriolis forces. This stronger pressure gradient force turns the wind at an angle across the isobars toward the low pressure area.
Winds at altitude follow isobars, but because of friction, surface winds flow at an angle across isobars. In the Northern Hemisphere, wind flows around low pressure areas in a counterclockwise direction. This is called cyclonic flow. When planning a long east to west flight, you can get an advantage from wind by flying to the north of a low pressure area and to the south of a high pressure area. See Figure 8-1. Wind circulation in the Northern Hemisphere is clockwise out of a high and counterclockwise into a low. When flying from a high-pressure area into a low-pressure area, wind will blow from the left.

Figure 8-1. Wind circulation in the Northern hemisphere
Wind velocities are generally greater in a low-pressure area than in a high-pressure area, so when flying from a high pressure into a low, the wind velocities will be increasing. Air flowing counterclockwise into a low-pressure area cannot flow outward against the pressure gradient, nor can it go downward into the ground; it must go upward. Therefore, a low pressure area, or trough, is an area of rising air. Air moving out of a high, or ridge, flows in a clockwise direction and depletes the quantity of air. Highs and ridges, therefore, are areas of descending air.
Convective circulation patterns associated with sea breezes occur because land surfaces warm and cool more rapidly than water surfaces. The land is warmer than the sea during the day, and wind blows from the cool water to warm land. At night the wind reverses and blows from the cool land to warmer water.
Relative humidity is the ratio of the amount of water vapor in the air to the amount the air can hold at its present temperature. Dew point is the temperature at which a mass of air can no longer hold water in its vapor state. It is the temperature to which the air must be cooled for it to become saturated. The difference between the temperature and the dew point (the spread) decreases as the relative humidity of the air increases.
Precipitation is a term that includes all types of atmospheric particles that grow in size and weight until they can no longer remain suspended and they fall. Some rain evaporates before it reaches the ground. This is called virga, which appears as dark streamers hanging below the cloud.
When cold air moves over a warm lake, the warm water adds heat and water vapor to the air. This additional water in the air condenses out as the air is cooled on the lee side of the lake and causes showers. If the air is warmer than the water in the lake, the air may become saturated by evaporation from the water while also becoming cooler in the low levels by contact with the cool water. This causes fog which often becomes extensive and dense to the lee of a lake.
Rain falling through colder air may become supercooled, freezing on impact as freezing rain; or it may freeze during its descent, falling as ice pellets. Ice pellets always indicate freezing rain at a higher altitude.
Sublimation is the process in which ice forms on a surface directly from water vapor on a cold, clear night. The water does not pass through the liquid state as it changes from water vapor into ice.
Stable air resists any upward or downward displacement. Unstable air allows an upward or downward disturbance to grow into a vertical or convective current. Atmospheric stability can be determined by the ambient or existing lapse rate which is the actual decrease in temperature with altitude.
Unsaturated air moving upward cools at about 3.0°C (5.4°F) per 1,000 feet. Moving downward, it warms at the same rate. If a body of unsaturated air has a lapse rate greater than that of the air surrounding it, it is colder than the surrounding air and is stable, causing it to sink. But if its lapse rate is lower than that of the surrounding air, it is unstable. It is warmer than the surrounding air, and it accelerates upward, causing a convective current.
Unstable air is characterized by cumuliform clouds, showery precipitation, rough air (turbulence), and good visibility except in blowing obstructions. Stable air, on the other hand, is characterized by restricted visibility, usually caused by haze and smoke.
Clouds may be divided into four distinct families:
When a body of unsaturated air is lifted, it cools until it reaches its dewpoint temperature. At this temperature, the water vapor becomes water droplets and clouds form. Clouds formed in stable air are predominantly stratiform, or layer-like clouds, while clouds formed in unstable air are cumuliform, or billowing clouds. Cumulonimbus clouds, or thunderstorms, are formed when moist unstable air is lifted. The base of cumuliform clouds form at the altitude where the temperature and dew point become the same. Dew point decreases at approximately 1°F (0.5°C) per 1,000 feet, and unsaturated air in a convective current cools at 5.4°F (3°C) per 1,000 feet. The temperature and dew point come together at a rate of 4.4°F (2.5°C) per 1,000 feet. To find the height of the base of a convective cloud, in thousands of feet, divide this rate into the temperature/dew point spread.
When a cold, moist air mass moves over a warm surface, the air is warmed from below and has an unstable lapse rate. The air is turbulent with strong updrafts and there are cumuliform clouds and good visibility. Often cumulonimbus clouds form, and any precipitation is showery in nature. If the surface temperature is high, strong updrafts and cumulonimbus clouds can be expected.
If warm, moist air moves over a cold surface, the air is cooled and has a stable lapse rate. The air is smooth, and there are stratiform clouds and fog and poor visibility. Any precipitation is steady or continuous.
Fronts are the zones between two different air masses. Across this zone the temperature, humidity, and wind direction and velocity change, often changing rapidly over a short distance. There are three basic types of fronts:
Frontal waves and cyclones (areas of low pressure) usually form on slow-moving cold fronts or stationary fronts. A wave forms along the front, and as it increases in size, cyclonic (counterclockwise) circulation develops. One section begins to move as a warm front while the section next to it moves as a cold front. When the cold front catches up with the warm front, the two close together, or occlude, and form an occluded front. When a wave forms on a stationary front running east and west across the United States, the cold air circulates counterclockwise and forms a cold front to the west of the wave and a warm front to the east.
In a cold-front occlusion, a mass of cold air forces its way under a mass of warm air, and the coldest air is under the cold front. When the cold front overtakes the warm front, it lifts the warm front aloft, and the air ahead of the warm front is warmer than the air behind the overtaking cold front.
Convective currents are one cause of turbulence at low altitudes. These currents are localized and have vertical ascending and descending air in the same general area. For every rising current, there is a corresponding descending current. Convective currents in moist air are easily identified by the presence of towering cumulus clouds. These clouds form when moist air is lifted until its temperature and dew point are the same.
When stable air crosses a mountain barrier, the air flowing up the windward side is relatively smooth, and the wind crossing the barrier tends to flow in layers. The air dips sharply downward immediately to the lee side of a ridge before rising and falling in a wave motion for a considerable distance downstream. The waves remain nearly stationary while the wind blows rapidly through them. Wave crests extend well above the highest mountains, sometimes into the lower stratosphere.
Under each wave crest is a rotary circulation, or rotor, which forms below the elevation of the mountain peaks. Turbulence can be violent in the rotor, and it is most severe in and below the standing rotors just beneath the wave crests at or below mountain-top levels. If the air is humid and the wave is of large amplitude, crests of the standing waves may be marked by stationary lens-shaped clouds called standing lenticular clouds. Flight over mountains may be hazardous in high wind conditions because of the violent downdrafts in these waves on the lee side, especially when flying into the wind.
Wind shear is a serious type of turbulence associated with either a wind shift or a wind speed gradient that occurs within a very short distance, and can occur at any level of the atmosphere. There are three basic types of wind shear that are of interest:
A temperature inversion can form near the surface on a clear night with calm or light surface wind. If the wind above the inversion is relatively strong, a wind shear zone may develop between the calm and the stronger winds above.
Eddies in the shear zone cause airspeed fluctuations as an aircraft climbs or descends through the inversion. When passing through the inversion, an aircraft is most likely either climbing from takeoff or approaching to land; therefore, airspeed is slow—only a few knots above the stall speed. The fluctuation in airspeed can induce a stall precariously close to the ground.
Microbursts are small-scale intense downdrafts which, upon reaching the surface, spread outward in all directions from the downdraft center. This causes the presence of both vertical and horizontal wind shears that can be extremely hazardous to all types and categories of aircraft, especially at low altitudes. Due to their small size, short life span, and the fact that they can occur over areas without surface precipitation, microbursts are not easily detectable using conventional weather radar or wind shear alert systems.
An individual microburst will seldom last longer than 15 minutes from the time it strikes the ground until dissipation. The horizontal winds continue to increase during the first 5 minutes, with the maximum intensity winds lasting 2 to 4 minutes. The downdrafts in a microburst can be as strong as 6,000 feet per minute. Horizontal winds near the surface can be as strong as 45 knots resulting in a 90-knot shear (head wind to tail wind change for a traversing aircraft) across the microburst. These strong horizontal winds occur within a few hundred feet of the ground.
Two conditions are necessary for the formation of structural ice on an aircraft in flight:
Aerodynamic cooling can lower the temperature of an airfoil to 0°C, even though the ambient temperature is a few degrees warmer. The most rapid accumulation of clear ice on an aircraft may occur when flying through cumuliform clouds when the temperature is between 0°C and -15°C. Freezing rain is most generally caused by rain falling from air which has a temperature of more than 0°C into air having a temperature of 0°C or less.
A thunderstorm is a violent, localized storm produced by a cumulonimbus cloud. For a thunderstorm to form these conditions must be met:
The life cycle of a thunderstorm consists of three stages:
A squall line is a nonfrontal, narrow band of active thunderstorms. Often it develops ahead of a cold front in moist, unstable air, but it may develop in unstable air far removed from any front. Squall lines often contain severe steady-state thunderstorms and present the most intense weather hazards to aircraft, which include destructive winds, heavy hail, and tornadoes. They usually form rapidly, generally reaching their maximum intensity during the late afternoon and the first few hours of darkness.
Hail is possible in any thunderstorm, especially beneath the anvil of a large cumulonimbus. It forms in the mature stage, when supercooled drops of water begin to freeze. Once a drop freezes, other drops latch on and freeze so that the hailstone grows as it is carried up and down inside the cloud by the vertical wind currents. Hail falls out of the bottom of the cloud, and some is thrown from the top into clear air several miles ahead of the cloud movement. Large hail is most commonly found in thunderstorms which have strong updrafts and large liquid water content. It is produced during the mature stage of a cumulonimbus cloud, or thunderstorm, in which there are strong updrafts and a large amount of liquid water. Water is carried above the freezing level where it freezes. It escapes from the updraft and drops down where it collects more water and is again carried upward and freezes. Hailstones can grow to the size of golf balls and cause serious damage to persons and property on the ground. Hailstones may be thrown outward from a storm cloud and have been observed in clear air several miles from the parent thunderstorm.
Fog is a surface-based cloud composed of either water droplets or ice crystals. Fog forms when the temperature/dew point spread is small and the relative humidity is high. It may be formed by cooling the air to its dew point, or by adding moisture to air near the ground. Some of the most common types of fog are:
Radiation fog—forms near the surface when terrestrial radiation cools the ground, and the ground in turn cools the adjacent air. When the air is cooled to its dew point, fog forms. Radiation fog is most likely to occur when there is high humidity during the early evening, on a cool cloudless night with light winds and favorable topography. Radiation fog is restricted to land because water surfaces cool little from nighttime radiation.
Advection fog—forms when moist air moves over colder ground or water. Advection fog deepens as wind speed increases up to about 15 knots. Wind much stronger than 15 knots lifts the fog into a layer of low stratus or stratocumulus. Advection fog is usually more extensive and much more persistent than radiation fog, and it can move in rapidly regardless of the time of day or night.
Upslope fog—forms as a result of moist, stable air being cooled adiabatically as it moves up sloping terrain. Once the upslope wind ceases, the fog dissipates.
Precipitation-induced fog—forms when relatively warm rain or drizzle falls through cool air, as happens in a warm front. Evaporation from the precipitation saturates the cool air and forms fog.
Ice fog—occurs in cold weather when the temperature is much below freezing and water vapor sublimates directly as ice crystals.
If a high-pressure air mass stagnates over an industrial area, it will concentrate smoke and haze and cause poor surface visibility with little chance of improvement.
Clear air turbulence (CAT) is most likely to be encountered in areas where vertical wind shear exceeds six knots per 1,000 feet or horizontal wind shear exceeds 40 knots per 150 miles.
Convective circulation patterns associated with sea breezes are caused by the land absorbing and radiating heat faster than the water. The land is heated on warm, sunny days, and sea breezes usually begin during the early forenoon, reach a maximum during the afternoon, and subside around dusk after the land has cooled. The leading edge of the cool sea breeze forces the warmer air inland to rise. Rising air from over the land returns seaward at a higher altitude to complete the convective cell.
Cool air must sink to force the warm air upward in thermals. Therefore, in small-scale convection, thermals and downdrafts coexist side by side. The net upward displacement of the air must equal the net downward displacement. Fast rising thermals generally cover a small percentage of a convective area, while slower downdrafts predominate over the remaining greater portion.
Thermals are strongest under smooth cumulus clouds that have concave bases and sharp upper outlines. The most favorable type of thermals for cross-country soaring are found along “thermal streets,” which are generally parallel to the wind. A pilot can soar under a cloud street maintaining generally continuous flight and seldom, if ever, have to circle. In the central and eastern United States, the most favorable weather for cross-country soaring occurs behind a cold front. Most thermal cross-country flying in these areas is done after a cold front passes and ahead of the following high-pressure center.
The movement of surface dust and smoke can be used as an indication of a thermal. The rising air in a thermal will cause the streamers of dust or smoke to converge in the low-pressure area caused by the rising air. You must be careful when soaring in a dust devil to avoid the eye of the vortex. A wall of turbulence surrounds the core, or the eye.
A thermal index (TI) is a forecast value based on the temperature difference between sinking and rising air at a given altitude. The greater the temperature difference, the stronger the thermals. A TI of -10 predicts very good lift and a long soaring day. A TI of -5 indicates good lift, and a TI of +5 shows no hope of thermals at the reported altitude. For thermals to exist, the existing lapse rate must be equal to or greater than the dry adiabatic rate of cooling.
Soaring under stable-air conditions can be done in the lift that is found on the upwind side of hills or ridges when moderate winds are blowing. A mountain wave, in a manner similar to that in a thermal, means turbulence to powered aircraft; but to a slowly moving sailcraft, it produces lift and sink above the level of the mountain crest.
As air spills over the crest like a waterfall, it causes strong downdrafts. The violent overturning forms a series of rotors in the wind shadow of the mountain, which are hazardous even to a sailplane. Clouds resembling long bands of stratocumulus sometimes mark the area of overturning air. These rotor clouds appear to remain stationary, parallel to the range, and stand a few miles leeward of the mountains. Turbulence is most frequent and most severe in the standing rotors just beneath the wave crests at or below mountain-top levels. The greatest potential danger from vertical and rotor-type currents in the vicinity of mountain ranges is usually encountered beneath the wave crests on the leeward side at or below mountain-top levels. This turbulence will generally be encountered as you are flying into the wind, attempting to climb over the area of lift. A strong mountain wave requires:
An international weather reporting code is used for weather reports (METAR) and forecasts (TAFs) worldwide. The reports follow the format shown in Figure 8-2.

Figure 8-2. TAF/METAR weather card
For aviation purposes, the ceiling is the lowest broken or overcast layer, or vertical visibility into an obscuration.
Aircraft in flight are the only means of directly observing cloud tops, icing, and turbulence; therefore, no observation is more timely than one made from the flight deck. While the FAA encourages pilots to report inflight weather, a report of any unforecast weather is required by regulation. A PIREP (UA) is usually transmitted in a prescribed format. See Figure 8-3.
Pilots seeking weather avoidance assistance should keep in mind that ATC radar limitations and frequency congestion may limit the controller’s capability to provide this service.

Figure 8-3. Pilot report form
A Terminal Aerodrome Forecast (TAF) is a concise statement of the expected meteorological conditions at an airport during a specified period (usually 24 hours). TAFs use the same code used in the METAR weather reports (see Figure 8-2).
TAFs are issued in the following format:
TYPE / LOCATION / ISSUANCE TIME / VALID TIME / FORECAST
Note: the “/” above are for separation purposes and do not appear in the actual TAFs.
The GFA at the Aviation Weather Center (AWC) website is an interactive display providing continuously updated observed and forecast weather information over the continental United States (CONUS). It is intended to give users a complete picture of weather critical to aviation safety. The GFA display shows user-selected weather categories, each containing multiple fields of interest at altitudes from the surface up to FL480. Depending on the field of interest chosen, weather information is available from -6 in the past (observed) to +15 hours in the future (forecast).
The GFA is not considered a weather product but an aggregate of several existing weather products. The information and data from the various weather products are overlaid on a high-resolution basemap of the United States: aviationweather.gov/gfa. The user selects flight levels and current time period for either observed or forecast weather information. Mouse-clicking or hovering over the map provides additional information in textual format, such as current METAR or TAF for a selected airport. The GFA replaces the textual area forecast (FA) for the CONUS and Hawaii with a more modern digital solution for obtaining weather information.
The winds and temperatures aloft forecast is displayed in a 6-digit format (DDffTT). It shows wind direction (DD), wind velocity (ff), and the temperature (TT) that is forecast to exist at specified levels. For example, “234502” decodes as: winds from 230 degrees true north, at 45 knots, temperatures 02°C.
When the wind speed (ff) is between 100 and 199 knots, the wind direction (DD) portion of the code will be greater than 50. In cases such as this, you will need to subtract 50 from the coded wind direction, and add 100 to the coded wind speed in order to decipher the code. For example, “734502” decodes as: winds from 230 degrees true north at 145 knots, temperature 02°C.
Temperatures with a negative symbol in front of them (DDff-37) are negative. For flight levels above 24,000, temperatures are always negative and will not have a negative symbol. Light and variable winds or wind speeds below 5 knots are indicated by 9900, followed by the forecast temperature. For example, the coded winds aloft forecast for flight level FL270 (flight level 27,000) is “990017” and decodes as: winds are light and variable, temperature negative 17.
The observed winds aloft chart shows temperature, wind direction, and speed at selected stations. Arrows with pennants and barbs indicate wind direction and speed. Each pennant is 50 knots, each barb is 10 knots, and each half barb is 5 knots. Wind direction is shown by an arrow drawn to the nearest 10 degrees, with the second digit of the coded direction entered at the outer end of the arrow. Thus, a wind in the northwest quadrant with the digit 3 indicates 330 degrees, and a wind in the southwest quadrant with the digit 3 indicates 230 degrees.
In-Flight Weather Advisories advise pilots en route of the possibility of encountering hazardous flying conditions that may not have been forecast at the time of the preflight weather briefing.
AIRMETs (WA) contain information on weather that may be hazardous to single engine, other light aircraft, and VFR pilots. The items covered are moderate icing or turbulence, sustained winds of 30 knots or more at the surface, widespread areas of IFR conditions, and extensive mountain obscurement.
SIGMETs (WS) advise of weather potentially hazardous to all aircraft. The items covered are severe icing, severe or extreme turbulence, and widespread sandstorms, dust storms or volcanic ash lowering visibility to less than 3 miles.
SIGMETs and AIRMETs are broadcast upon receipt and at 30-minute intervals (H + 15 and H + 45) during the first hour. If the advisory is still in effect after the first hour, an alert notice will be broadcast. Pilots may contact the nearest FSS to ascertain whether the advisory is pertinent to their flight.
Convective SIGMETs (WST) cover weather developments such as tornadoes, lines of thunderstorms, and embedded thunderstorms, hail greater than or equal to 3/4-inch diameter, and they also imply severe or greater turbulence, severe icing, and low-level wind shear. Convective SIGMET Bulletins are issued hourly at H + 55. Unscheduled Convective SIGMETs are broadcast upon receipt and at 15-minute intervals at the first hour (H + 15, H + 30, H + 45).
The Surface Analysis Chart, or Surface Weather chart, is a computer-prepared chart covering the contiguous 48 states and the adjacent areas. These charts are transmitted every 3 hours. The date-time group shows the valid time in GMT, and the chart shows the conditions that were existing at the time the observations were made. Isobars are solid lines that depict the sea level pressure pattern. They are usually spaced at 4-millibar (mb) intervals, and are identified with a two-digit number for the last two digits in the millibar pressure. For example, the isobar identified as 32 is the 1032.0 mb isobar. The isobar marked 92 is the 992.0 mb isobar. The closeness of the isobars shows the pressure gradient. The closer together the isobars, the steeper the pressure gradient, or the change in pressure, for a given horizontal distance.
Fronts are identified by the coded lines shown in Figure 8-4. The shape of the pips identifies the type of front, and which side of the line they are on shows which direction the front is moving. A stationary front has the warm-front pips on one side and the cold-front pips on the opposite side. A frontogenesis is a front that is building up, and a frontolysis is one that is dissipating.

Figure 8-4. Surface Analysis Chart symbols
A three-digit code marked along the front identifies the type of front, intensity, and characteristic. The weather existing at each station is shown by a station model such as the one in Figure 8-5.

Figure 8-5. Station model for Surface Analysis Charts
A Constant Pressure Analysis Chart is an upper air weather map where all the information depicted is at the specified pressure-level of the chart. Each of the charts (850 MB, 700 MB, 500 MB, 300 MB, 250 MB, and 200 MB) can provide observed temperature, temperature/dew point spread, wind, height of the pressure surface, and the height changes over the previous 12-hour period.
The Convective Outlook Chart (previously called Severe Weather Outlook Chart) is used primarily for advance planning. It provides an outlook for general and severe thunderstorms, tornadoes, and tornado-watch areas. Single-hatched areas will be annotated to indicate either slight, moderate, or high risk of possible severe thunderstorms. Crosshatched areas indicate a forecast risk of tornadoes.
An area labeled APCHG on a Convective Outlook Chart indicates probable general thunderstorm activity may approach severe intensity, which means winds greater than or equal to 35 knots, but less than 50 knots, and/or hail greater than or equal to 1/2 inch in diameter, but less than 3/4 inch in diameter (surface conditions).
Tornado watches are plotted only if a tornado watch is in effect at chart time. The watch area is cross-hatched, and no coverage is specified.
[10-2024]