Chapter 2: Weather
What does "dew point" mean?
Dew point is the temperature to which a sample of air must be cooled to attain the state of saturation.
Where can you find a listing of AFSS/FSS and weather information numbers?
Number for these services can be found in the Airport/Facility Directory under the "FAA and NWS Telephone Numbers" section. They are also listed in the US Government section of the local telephone directory.
Name the main types of icing an aircraft may encounter in-flight.
Structural, induction system, and instrument icing.
Why do surface winds generally flow across the isobars at an angle?
Surface friction.
When temperature and dew point are close together (within 5 degrees), what type of weather is likely?
Visible moisture in the form of clouds, dew, or fog. Also, these are ideal conditions for carburetor icing.
Why is wind shear an operational concern to pilots?
Wind shear is an operational concern because unexpected changes in wind speed and direction can be potentially very hazardous to aircraft operations at low altitudes on approach to and departing airports.
Define the term "wind shear", and state the areas in which it is likely to occur.
Wind shear is defined as the rate of change of wind velocity per unit distance; conventionally expressed as vertical or horizontal wind shear. It may occur at any level in the atmosphere but three areas are if special concern: A. Wind shear with low-level temperature inversion. B. Wind shear in a frontal zone or thunderstorm. C. Clear air turbulence (CAT) at high levels associated with a jet stream or strong circulation.
At what rate does atmospheric pressure decrease with an increase in altitude?
1" Hg per 1,000 feet
What are the standard temperature and pressure values for sea level?
15 C and 29.92" Hg
What are Center Weather Advisories (CWA)?
A Center Weather Advisory (CWA) is an aviation warning for use by aircrews to anticipate and avoid adverse weather conditions in the en route and terminal environments. The CWA is not a flight planning product; instead is reflects current conditions expected at the time of issuance and.or is a short-range forecast for conditions expected within 2 hours of issuance. CWAs are valid for a maximum of 2 hours. If conditions are expected to continue beyond the 2-hour valid period, a statement will be included in the CWA.
What is METAR and what are the type types?
A METAR is an hourly surface observation of conditions observed at an airport. There are two types of METAR reports - a routine METAR report that is transmitted every hour and an aviation selected special weather report (SPECI). This is a special report that can be given at any time to update the METAR for rapidly changing weather conditions, aircraft mishaps, or other critical information.
Describe the basic elements of a METAR.
A METAR report contains the following elements in order as presented: A. Type of reports - the METAR, and the SPECI B. ICAO station identifier - 4 letter station identifiers; in the conterminous US, the 3 letter identifier is prefixed with K. C. Date and time of report - a 6 digit date/time group appended with Z (UTC). First two digits are the date, then two for the hour, and two for minutes D. Modifier - if used, the modifier AUTO identifies the report as an automated weather report with no human intervention. If AUTO is shown in the body of the report, AO1 or AO2 will be encoded in the remarks section to indicate the type of precipitation sensor used at the station. E. Wind - 5 digit group; first three digits are wind direction; next two digits are the average wind speed in knots F. Visibility - surface visibility in statute miles, space, fractions of statute miles, and the letters SM. G. Runway visual range (RVR) - as required H. Weather phenomena - broken into two categories; qualifiers, and weather phenomena. I. Sky conditions - amount/height/type or indefinite ceiling/height. Heights are recorded in feet AGL. J. Temperature/dew point ground - 2 digit format in whole degrees Celsius, separated by a solidus (/). Temperatures below zero are prefixed with M. J. Altimeter - 4 digit format presented in tens, units, tenths, and hundredths of inches of mercury prefixed with A. The decimal point is not reported or stated. K. Remarks (RMK), as required - operational significant weather phenomena, location of phenomena, beginning and ended times, direction of movement. Example: METAR KLAX 140651Z AUTO 00000KT 1SM R35L/4500V6000FT -RA BR BKN030 10/10 A2990 RMK AO2
What is a SIGMET (WS)?
A SIGMET (WS) advised of non-convective weather that is potentially hazards to all aircraft. SIGMETs are issued for the six areas corresponding to the FA areas. The maximum forecast period is four hours. IN the conterminous US, SIGMETs are issued when the following phenomena occur or are expected to occur: A. Severe icing not associated with a thunderstorm; B. Severe or extreme turbulence or clear air turbulence (CAT) not associated with a thunderstorm; C. Dust storms or sandstorms lowering surface or inflight visibilities to below 3 miles; D. Volcanic ash.
What are Radar Weather Reports (SD)?
A radar weather report (SD/ROB) contains information about precipitation observed by weather radar. It is a textual product derived from the WSR-88D NEXRAD radar without human intervention. Reports are transmitted hourly and contain the following; location ID, time, configuration (CELL, LN, and AREA), coverage, precipitation type and intensity, location, maximum tops, cell movement, and remarks. The resolution of an SD/ROB is very coarse, up to 80 minutes old, and should only be used if no other radar information is available.
What is a "ridge"?
A ridge is an elongated area of relatively high atmospheric pressure. Air moving out of a high or ridge depletes the quantity of air; therefore, these are areas of descending air. Descending air favors dissipation of cloudiness; hence the association of high pressure and good weather.
What are Terminal Aerodrome Forecasts (TAFs)?
A terminal aerodrome forecast (TAF) is a concise statement of the expected meteorological conditions significant to aviation for a specified time period within five statue miles (SM) of the center of the airport's runway complex (terminal). The TAFs use the same weather codes found in METAR weather reports, in the following format: A. Type of reports - a routine forecast (TAF), an amended forecast (TAF AMD), or a corrected forecast (TAF COR). B. ICAO station identifier - 4 digit station identifier. C. Date and time of origin - the date and UTC for when the forecast was actually prepared, displayed in ICAO format. Valid time, TEMPEs and PROBS are presented ddhh/ddhh, FROM groups are presented ddhhmm. D. Valid period date and time - valid forecast period is a 2-digit date followed by the 2-digit beginning and 2-digit ending hours in UTC. Routine TAFs are valid for 24 hours and are issued four times daily at 0000Z, 0600Z, 1200Z, and 1800Z. E. Forecasts - wind, visibility, significant and vicinity weather, cloud and vertical obscuration, non-convective low level wind shear, forecast change indicators (FM, TEMPO and PROB).
What is a "trough"?
A trough is a elongated area of relatively low atmospheric pressure. At the surface when air converges into a low, it cannot go outward against the pressure gradient, nor can it go downward into the ground; it must go upward. Therefore, a low or trough is an area of rising air. Rising air is conductive to cloudiness and precipitation; hence the general association of low pressure and bad weather.
What pertinent information should a weather briefing include?
A. Adverse Conditions B. VFR Flight Not Recommended C. Synopsis D. Current Conditions E. Enroute Forecast F. Destination Forecast G. Winds Aloft H. NOTAMs I. ATC Delay J. Pilots may obtain the following from FSS briefers upon request; information on special use airspace (SUA) and SUA-related airspace, included alert areas, MOAs, MTRs (IFR, VFR, CR, and SR training routes), warning areas and ATC assigned airspace (ATCAA); a review of the printed NOTAM publication; approximate density altitude data; information on air traffic services and rules; customs/immigration procedures; ADIZ rules; search and rescue; LORAN-C and military NOTAMs; runway friction measurement value NOTAMs, GPS RAIM availability; and other assistance as required.
State two basic ways that fog may form.
A. Cooling air to the dew point. B. Adding moisture to the air near the ground.
During the preflight planning, what type of meteorological information should you be aware of with respect to icing?
A. Location of fronts - A front's location, type, speed, and direction of movement. B. Cloud layers - The location of cloud bases and tops, which is valuable when determining if you will be able to climb above icing layers or descend beneath those layers into warmer air, reference PIREPs and area forecasts. C. Freezing level(s) - Important when determining how to avoid icing and how to exit icing conditions if accidentally encountered. D. Air temperature and pressure - Icing tends to be found in low-pressure areas and at temperatures at or around freezing.
Describe several types of weather observing programs available.
A. Manual observations - reports made from airport locations staffed by FAA or NWS personnel. B. AWOS - Automated Weather Observing Systems; consists of various sensors, a processor, a computer-generated voice subsystem, and a transmitter to broadcast local, minute-by-minute weather data directly to the pilot. Observations will include prefix AUTO in data. C. ASOS/AWSS - Automated Surface Observing System/Automated Weather Sensor Systems; the primary US surface weather observing systems. AWSS is a follow-on program that provides identical data as ASOS. Both systems provide continuous minute-by-minute observations that generate METARs and other aviation weather information. Transmitted over a discrete VHF radio frequency or the voice portion of a local NAVAID, and are receivable to a maximum of 25 NM from the station and a maximum altitude of 10,000 feet AGL. Observations made without human intervention will include the modifier "AUTO" in the report data.
What valuable information can be determined from Winds and Temperatures Aloft Forecasts (FB)?
A. Most favorable altitude - based on winds and direction of flight B. Areas of possible icing - by noting air temperatures of + 2C to -20C. C. Temperature inversions D. Turbulence - by observing abrupt changes in wind direction and speed at different altitudes.
Name several types of fog.
A. Radiation fog B. Advection fog C. Upslope fog D. Precipitation-induced fog E. Ice fog
What factors must be present for a thunderstorm to form?
A. Sufficient water vapor. B. An unstable lapse rate. C. An initial upward boost to start the storm process in motion.
Give some examples of current weather charts available at the FSS or NWSO used in flight planning.
A. Surface analysis chart B. Weather depiction chart C. Radar summary chart D. Short-range surface prognostic chart E. Significant weather prognostic chart F. Convective outlook chart G. Constant pressure analysis chart
What are some examples of other sources of weather information>?
A. Telephone Information Briefing Service (TIBS) (AFSS). B. Weather and aeronautical information available from numerous private industry sources. C. Direct User Terminal System (DUATS) can be accessed by pilots with a current medical certificate toll-free in the 48 contiguous states via personal computer. D. In Alaska, Transcribed Weather Broadcast (TWEB) and telephone access to the TWEB (TEL-TWEB).
What types of weather information will you examine to determine if wind shear conditions might affect your flight?
A. Terminal forecasts - any mention of low level win shear (LLWS) or the possibility of sever thunderstorms, heavy rain showers, hail, and wind gusts suggest potential for LLWS and microbursts. B. METARs - inspect for any indication of thunderstorm, rain showers, or blowing dust. Additional signs such as warming trends, gusty winds, cumulonimbus clouds, etc., should be noted. C. Severe weather watch reports. SIGMETs and convective SIGMETs - severe convective weather is a prime source of wind shear and microbursts. D. LLWAS (low level windshear alert system) - installed at 110 airpots in the US; designed to detect wind shifts between outlying stations and a reference centerfield station. E. PIREPs - reports of sudden airspeed changes on departure or approach and landing corridors provide a real-time indication of the presence of wind shear.
Define "aviation area forecast".
Abbreviated as "FA", this is a forecast of specified weather phenomena covering a flight information region or other area designated by the meteorological authority. Pilots should use the area forecast to determine forecasts en route weather and to interpolate conditions at airports that do not have a terminal aerodrome forecast (TAF). FAs are issued 3 times daily for each of the 6 areas in the contiguous 48 states. FAs are also issued for the Gulf of Mexico, the Caribbean, Hawaii, and Alaska.
What is advection fog, and where is it most likely to form?
Advection fog results from the transport of warm humid air over a cold surface. A pilot can expect advection fog to form primarily along coastal areas during the winter. Unlike radiation fog, it may occur with winds, cloudy skies, over a wide geographical area, and at any time of the day or night.
What is an AIRMET (WA)?
Advisories of significant weather phenomena that describe conditions at intensities lower than those which require issuance of SIGMETs, intended for use by all pilots in the preflight and en route phase of flight to enhance safety. AIRMET information is available in two formats: text bulletins (WA) and graphics (G-AIRMET). They are issued on a scheduled basis every 6 hours beginning at 0245 UTC. Unscheduled updates and corrections are issued as necessary. Each AIRMET Bulletin includes an outlook for conditions expected after the AIRMET valid period. AIRMETs contain details about IFR, extensive mountain obscuration, turbulence, strong surface winds, icing, and freezing levels.
What is a "temperature inversion"?
An inversion is an increase in temperature with height - a reversal of the normal decrease with height. An inversion aloft permits warm rain to fall through cold air below. Temperature in the cold air can be critical to icing. A ground-based inversion favors poor visibility by trapping fog, smoke, and other restrictions into low levels of the atmosphere. The air is stable, with little or no turbulence.
What are "isobars"?
An isobar is a line on a weather chart which connects areas of equal or constant barometric pressure.
What are constant pressure analysis charts?
Any surface of equal pressure in the atmosphere is a constant pressure surface. A constant pressure analysis chart is an upper air weather map where all information depicted is at the specified pressure of the chart. From these charts, a pilot can approximate the observed air temperature, wind, and temperature-dewpoint spread along a proposed route. They also depict highs, lows, troughs, and ridges aloft by the heigh contour patterns resembling isobars on a surface map. Twice daily, five constant pressure charts are issued from observed data obtained at 00Z and 12Z: 850 mb.......5,000 ft 700 mb.....10,000 ft 500 mb.....18,000 ft 300 mb.....30,000 ft 200 mb.....39,000 ft
What information is provided by an FA?
Area forecasts are issued for the conterminous US and cover airspace between the surface and 45,000 feet AMSL. They include: A. Synopsis - brief discussion of the synoptic weather affecting the FA area during the 18-hour valid period. B. Clouds and weather - description of the clouds and weather for the first 12-hour period for each state or group of states including: 1. Cloud amount (SCT, BKN, or OVC) for clouds with bases higher than or equal to 1,000 feet AGL and below FL180. 2. Cloud bases and tops (AMSL) associated with the above. 3. Precipitation 4. Visibilities between 3 and 6 SM and obstructions to visibility 5. Sustained surface winds 20 knots or greater C. 12 to 18-hour categorical outlook - IFR, marginal VFR (MVFR), or VFR, including expected precipitation and/or obstructions to visibility.
What are short-range surface prognostic charts?
Called "progs", these charts portray forecasts of selected weather conditions at specified valid times (12, 24, 36, and 48 hour progs). Each valid time is the time at which the forecast conditions are expected to occur, made from a comprehensive set of observed weather conditions. The observed conditions are extended forward in time and become forecasts by considering atmospheric and environmental processes. Forecast information for the surface to 24,000 feet is provided by the low-level significant weather prog chart. Forecast information from above 24,000 to 60,000 feet is provided by the high-level significant weather prog chart.
Name the three types of structural icing that may occur in flight.
Clear ice - forms after initial impact when the remaining liquid portion of the drop flows out over the aircraft surface, gradually freezing as a smooth sheet of solid ice. Rime ice - forms when drops are small, such as those in stratified clouds or light drizzle. The liquid portion remaining after initial impact freezes rapidly before the drop has time to spread out over aircraft surfaces. Mixed ice - forms when drops vary in size or when liquid drops are intermingled with snow or ice particles. The ice particles become imbedded in clear ice, building a very rough accumulation.
What are the three stages of a thunderstorm?
Cumulus stage - Updrafts causing raindrops to increase in size Mature stage - Rain at earth's surface; it falls through or immediately beside the updrafts; lightning; perhaps roll clouds. Dissipation stage - Downdrafts and rain begin to dissipate.
What are radar summary charts?
Computer generated graphical display of a collection of automated radar weather reports (SDs). The chart displays areas of precipitation as well as information about type, intensity, configuration, coverage, echo top, and cell movement of precipitation. Severe weather watches are plotted if they are in effect when the chart is valid. The chart is available hourly with a valid time of 35 minutes past each hour. This chart aids in preflight planning by identifying general areas and movement of precipitation and/or thunderstorms. Displays drops or ice particles of precipitation size only; it does not display clouds and fog. Therefore, since the absence of echoes does not guarantee clear weather, the cloud tops will most likely be higher than the tops of the precipitation echoes detected by radar, the radar summary chart must be used along with other charts, reports, and forecasts for best effectiveness.
What is a Convective SIGMET?
Convective SIGMETs (WST) imples severe or greater turbulence, severe icing and low-level wind shear. They may be issued for any convective situation which the forecasters feels is hazardous to all categories of aircraft. Convective SIGMET bulletins are issued for the Eastern (E), Central (C), and Western (W) United States. Bulletins are issued hourly at H+55. Special bulletins are issued at any time as required and updated at H+55. The text of the bulletin consists of either an observation and a forecast, or just a forecast. The forecast is valid for up to 2 hours. A. Severe thunderstorm due to: surface winds greater than or equal to 50 knots, hail at the surface greater than or equal to 3/4 inches in diameter, or tornadoes B. Embedded thunderstorms C. A line of thunderstorms D. Thunderstorms producing greater than or equal to heavy precipitation that affects 40% or more of an area of at least 3,000 square miles.
What is EFAS?
En route Flight Advisory Service (EFAS) is a service specifically designed to provide enroute aircraft with timely and meaningful weather advisories pertinent to the type of flight intended, route of flight, and altitude. In conjunction with this service, EFAS is also a central collection and distribution point for pilot reports weather information (PIREPs). EFAS provides communications capabilities for aircraft flying at 5,000 feet above ground level to 17,500 feet MSL on a common frequency of 122.0 MHz. It is also known as "Flight Watch".
What is a "flight information service"?
FIS is a method of receiving aviation weather and other operational data in the cockpit that augments traditional pilot voice communication with FSS and ATC facilities. It is not intended to replace traditional and controller/FSS preflight briefings or inflight voice communications.
What is HIWAS?
Hazardous In-flight Weather Advisory Service (HIWAS) is a continuous broadcast of in-flight weather advisories including summarized Aviation Weather Warnings, SIGMETs, Convective SIGMETs, Center Weather Advisories, AIRMETs, and urgent PIREPs. HIWAS is an additional source of hazardous weather information which makes this data available on a continuous basis. Navaids with HIWAS capability are depicted on sectional charts with an "H" in the upper right corner of the identification box. Where implemented, HIWAS alerts are broadcast on all except emergency frequencies once upon receipt by ARTCC, terminal facilities, and AFSS/FSSs.
What are Inflight Aviation Weather Advisories (WST, WS, WA)?
Inflight aviation weather advisories are forecasts to advise en route aircraft of development or potentially hazardous weather in 3 types: the SIGMET, the convective SIGMET, and the AIRMET. All heights are referenced MSL, except in the case of ceilings (CIG), which indicate AGL.
Describe the US low-level significant weather prog chart.
It is a "Day One" forecast of significant weather for the conterminous US, pertaining to the layer from the surface to FL240 (400 mb). With two forecast periods, 12 hours and 24 hours, the chart is composed of four panels. The two lower panels depict the 12- and 24-hour surface progs, and the two upper panels depict the 12- and 24-hour significant weather progs. Issued four times a day at 00Z, 06Z, 12Z, and 18Z. Covered are forecast positions and characteristics of pressure systems, fronts, and precipitation. Much insight can be gained by evaluating the individual fields of pressure patterns, fronts, precipitation, weather flying categories, freezing levels, and turbulence displayed on the chart.
Define the terms: LIFR, IFR, MVFR, and VFR.
LIFR - Low IFR: ceiling less than 500 feet and/or visibility less than 1 mile IFR - ceiling 500 to less than 1,000 feet and/or visibility 1 to less than 3 miles MVFR - Marginal VFR: ceiling 1,000 to 3,000 feet and/or visibility 3 to 5 miles inclusive VFR - Ceiling greater than 3,000 feet and visibility greater than 5 miles; includes sky clear.
State the general characteristics in regard to the flow of air around high and low pressure systems in the Northern Hemisphere.
Low Pressure - inward, upward, and counterclockwise High Pressure - outward, downward, and clockwise
List the effects os table and unstable air on clouds, turbulence, precipitation and visibility.
Stable: Stratiform, Smooth, Steady, Fair to Poor Unstable: Cumuliform, Rough, Showery, Good
What types of weather briefings are available from an AFSS/FSS briefer?
Standard Briefing - Request when you are planning a flight and have not received a previous briefing or have not received preliminary information through mass dissemination media (TIBS, TWEB in Alaska). Abbreviated Briefing - Request when you need information to supplement mass disseminated data, update a previous briefing, or when you need only one or two items. Outlook Briefing - Request whenever your proposed departure is six or more hours from the time of the briefing; for planning purposes only. Inflight Briefing - Request when needed to update a preflight briefing.
What causes the winds aloft to flow parallel to the isobars?
The Coriolis force.
What is a convective outlook chart?
The convective outlook chart depicts the forecast to have potential severe (tornado, wind gust 50 knots or greater, or hail 3/4 inch diameter size or greater) and non-severe (general) convection and specific severe weather threats during the following three days. The chart defines areas of slight risk (SLGT), moderate risk (MDT), or high risk (HIGH) of severe thunderstorms for a 24-hour period beginning at 1200 UTC. The Day 1 and Day 2 Convective Outlooks also depict areas of general thunderstorms (GEN TSTMS), while the Day 1, Day 2, and Day 3 Convective Outlooks may use SEE TEXT for areas where convection may approach or slightly exceed severe criteria.
What action is recommended if you inadvertently encounter icing conditions?
The first course of action should be to leave the area of visible moisture. This might mean descending to an altitude below the cloud bases, climbing to an altitude above the cloud tops, or turning to a different course.
What is the definition of the term freezing levels and how can you determine where that level is?
The freezing level is the lowest altitude in the atmosphere over a given location at which the air temperature reaches 0 C. It is possible to have multiple freezing layers when a temperature inversion occurs above the defined freezing level. A pilot can use current icing products (CIP) and forecast icing products (FIP), as well as freezing level graphics chart to determine the approximate freezing level. Other potential sources of icing information are: area forecasts, PIREPs, AIRMETs, SIGMETs, surface analysis charts, low-level significant weather charts, and winds and temperatures aloft.
What causes radiation fog to form?
The ground cools the adjacent air to the dew point on calm clear nights.
Describe a mid-level significant weather (SIGWX) chart.
The mid-level significant weather chart provides a forecast and an overview of significant en route weather phenomena over a range of flight levels from 10,000 feet MSL to FL450, and associated surface weather features. The chart is a "snapshot" of weather expected at the specified valid time and depicts numerous weather elements that can be hazardous to aviation. The AWC issues the 24-hour mid-level significant weather chart four times daily.
What is the primary means of obtaining a weather briefing?
The primary source is an individual briefing obtaing from a briefer at the AFSS/FSS. These briefings, which are tailored to your specific flight, are available 24 hours a day through the use of the toll-free number 1-800-WX BRIEF.
If the isobars are relatively close together on a surface weather chart or a constant pressure chart, what information will this provide?
The spacing of isobars on these charts defines how steep or shallow a pressure gradient is. When isobars are spaced very close together a steep pressure gradient exists which indicates higher wind speeds. A shallow pressure gradient usually means wind speeds will be less.
What factor primarily determines the type and vertical extent of clouds?
The stability of the atmosphere.
What are PIREPs (UA) and where are they usually found?
The two types of PIREPs (routine, or "UA", and urgent, or "UUA") contain information concerning weather as observed by pilots en route. Required elements for all PIREPs are message type, location, time, flight level (MSL), type of aircraft, and at least one weather element encountered (visibility in SM, distance in NM). A PIREP is usually transmitted as an individual report but can be appended to a surface aviation weather report or placed into collectives.
What information does a weather depiction chart provide?
The weather depiction chart is computer generated from METAR reports. This chart gives a broad overview of the observed flying category conditions at the valid time of the chart. The chart begins at 01Z each day, is transmitted at three-hour intervals, and is valid at the time of the plotted data. The plotted data for each station area are; total sky cover, cloud height or ceiling, weather and obstructions to vision and visibilities. The weather depiction chart is an ideal place to begin in preparing for a weather briefing and flight planning. From this chart one can get a "bird's eye" view of areas of favorable and adverse weather conditions at chart time.
What are the different types of AIRMETs?
There are three AIRMET types: Sierra, Tango, and Zulu: A. AIRMET Sierra describes IFR conditions and/or extensive mountain obscurations. B. AIRMET Tango describes moderate turbulence, sustained surface winds of 30 knots or greater, and/or non-convective low-level wind shear. C. AIRMET Zulu describes moderate icing and provides freezing level heights.
What conditions are necessary for structural icing to occur?
Visible moisture and below freezing temperatures at the point moisture strikes the aircraft.
What is a surface analysis chart?
This is a computer prepared chart that covers the contiguous 48 states and adjacent areas. The chart is transmitted every three hours. The surface analysis chart provides a ready means of locating pressure systems and fronts. It also gives an overview of winds, temperatures and dew point temperatures at chart time. When using the chart, keep in mind that weather moves and conditions change. Using the surface analysis chart in conjunction with other information gives a more complete weather picture.
How do you determine the stability of the atmosphere?
Unstable air is indicated when temperature decreases uniformly and rapidly as you climb. If temperature remains unchanged or decreases only slightly with altitude, the air tends to be stable. Instability is likely when air near the surface is warm and moist. Surface heating, cooling aloft, converging or upslope winds, or an invading mass of colder air may lead to instability and cumuliform clouds.
What is upslope fog?
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. Upslope fog is often quite dense and extends to high altitudes.
Describe the winds and temperatures aloft forecasts (FB).
Winds and temperature aloft forecasts are computer prepares forecasts of wind direction, wind speed, and temperature at specified times, altitudes, and locations. They are produced 4 times daily for specified locations in the continental US, Hawaii, Alaska, and coastal waters, and the western Pacific Ocean. Amendments are not issued to the forecasts. Wind forecasts are not issued for altitudes within 1,500 of a location's elevation. Some of the features of FBs are: A. Product header includes date and time observations collected, forecast valid date and time, and time period during which the forecast is to be used. B. Altitudes up to 15,000 feet referenced to MSL; altitudes at or above 18,000 feet are references to flight levels (FL). C. Temperature indicated in degrees Celsius (two digits) for the levels 6,000 through 24,000 feet. Above 24,000 feet, minus sign is omitted since temperatures are always negative at those altitudes. Temperature forecasts are not issued for altitudes within 2,500 feet of a location's elevation. Forecasts for intermediate levels are determined by interpolation. D. Wind direction indicated in tens of degrees (two digits) with reference to true north and wind speed is given in knots (two digits). Light and variable wind or wind speeds of less than 5 knots are expressed by 9900. Forecast wind speeds of 100 through 199 knots are indicated by subtracting 100 from the speed and adding 50 to the coded direction. For example, a forecast of 250 degrees, 145 knots, is encoded 7545. Forecast wind speeds of 200 knots or greater are indicated as a forecast speed of 199 knots. For example, 7799 is decoded as 270 degrees at 199 knots or greater.
Is frost considered to be hazardous to flight? Why?
Yes, because while frost does not change the basic aerodynamic shape of the wing, the roughness of its surfaces spoils the smooth flow of air, thus causing a slowing of airflow. This slowing of the air causes early airflow separation, resulting in a loss of lift. Even a small amount of frost on airfoils may prevent an aircraft from becoming airborne at normal takeoff speed. It is also possible that, once airborne, an aircraft could have insufficient margin of airspeed above stall so that moderate gusts or turning flight could produce incipient or complete stalling.
At what altitude above the surface would the pilot expect the bases of cumuliform clouds if the surface temperature is 82 and the dew point is 62?
You can estimate the heigh of cumuliform cloud bases using surface temp / dew point spread. Unsaturated air in a convective current cools at about 5.4 F per 1,000 feet; dew point decreases at about 1 F. Thus, in a convective current, temperature and dew point converge at about 4.4 F per 1,000 feet. You can get a quick estimate of a convective cloud base in thousands of feet by rounding the values and dividing into the spread. When using Fahrenheit, divide by 4 and multiply by 1,000. This method of estimating is reliable only with instability, clouds and during the warmer part of the day. [(Temp - Dew Point) / 4] * 1000 = Base of Clouds
