Weather: Part 107 practice questions

In the METAR wind group, the first three digits are the direction, in tens of degrees, from which the wind is blowing, and FMH-1 requires that direction to be reported with reference to true north. The next two digits are the speed in whole knots, G introduces the gust speed, and KT shows that the speeds are in knots. 31012G24KT therefore means wind from 310° true at 12 knots, gusting to 24 knots. Reading it as the direction the wind blows toward reverses the meaning, and magnetic north is not the reference used in the report.

FMH-1 defines the ceiling as the lowest layer aloft reported as broken or overcast. FEW and SCT layers do not form a ceiling: SCT is a summation amount of 3/8 to 4/8 of the sky, while BKN is 5/8 to 7/8. So the layers at 800 and 1,500 feet are not the ceiling even though they are lower. Layer heights are coded in hundreds of feet above the surface, so BKN030 is a broken layer at 3,000 feet, and that is the ceiling. The overcast layer at 5,000 feet is higher than the first broken layer.

True. FMH-1 says SKC is the abbreviation manual stations use to show that no layers are present, while CLR is the abbreviation automated stations use to show that no layers are detected at or below 12,000 feet. Automated stations are required to evaluate sky condition from the surface to at least 12,000 feet. NWS Instruction 10-813 also tells forecasters that a TAF should show clouds expected above 12,000 feet even when an automated system reports clear skies below 12,000 feet, so CLR is not a report of a cloudless sky.

FMH-1 sets three distance bands for lightning detected by an automated system. Lightning within 5 nautical miles of the airport location point (ALP) is reported as TS in the body of the report with no remark. Lightning between 5 and 10 nautical miles is reported as VCTS, a thunderstorm in the vicinity, also in the body with no remark. Lightning beyond 10 but less than 30 nautical miles appears in the remarks only, as LTG DSNT followed by the direction from the ALP. At 7 nautical miles, the report therefore shows VCTS in the body.

FMH-1 says the wind speed data for the most recent 10 minutes are examined, and gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and lulls; the gust speed is the maximum instantaneous wind speed. The 16-knot and 22-knot figures belong to the squall definition in the same paragraph: a sudden onset of wind in which the speed increases by at least 16 knots and is sustained at 22 knots or more for at least a minute. A squall is reported as present weather rather than as a gust.

FMH-1 codes the peak wind remark as PK WND dddff(f)/(hh)mm: the direction of the peak wind, the peak speed in knots since the last METAR, and, after the solidus, the time it occurred. Only the minutes are needed when the hour can be inferred from the report time, so /52 in a 1756Z report means 1752 UTC. The remark therefore reports a peak wind of 38 knots from 330° at 1752 UTC. The digits after the solidus are a time, not a speed, and the remark records a wind measured since the last METAR rather than a forecast.

A SPECI is an unscheduled report taken when any of the FMH-1 criteria are observed. For ceiling, a SPECI is required when the ceiling forms or dissipates below, decreases to less than, or, if below, increases to equal or exceed 3,000, 1,500, 1,000 or 500 feet, or the lowest instrument approach minimum. A drop from 1,100 to 800 feet crosses 1,000 feet. A drop from 2,800 to 1,800 feet starts below 3,000 and stays above 1,500, so it crosses none. The visibility thresholds are 3, 2 and 1 miles plus the lowest approach minimum, so 6 to 4 miles crosses none. At this airport the approach minimums, 200 feet and 1/2 mile, are not reached by any of the changes.

In a TAF, 141130Z is the issuance date/time group: the day of the month and the time, in UTC, at which the forecast was completed and ready for transmission. The next group, 1412/1512, is the valid period: the day and hour of the start (the 14th at 1200) followed by the day and hour of the end (the 15th at 1200). Times in TAFs are stated in UTC, and the Z appended to the issuance group stands for Zulu, so local time is not used. The 1130 figure is the issuance time, not the start of the valid period.

FM followed by the day, hour and minutes (UTC) marks a significant and rapid change, in less than one hour, to a new set of prevailing conditions. Everything after FM152030 applies from 2030 UTC on the 15th to the end of the valid period or the next FM group, and all conditions given before it are superseded, which is why each FM group repeats every element. Temporary fluctuations lasting an hour or less in each instance are shown with a TEMPO group, and a 30 percent chance of a thunderstorm or precipitation event with a PROB30 group.

True. NWS Instruction 10-813 says a TEMPO group indicates temporary fluctuations that are expected to have a greater than 50 percent probability of occurrence, to last one hour or less in each instance, and in the aggregate to cover less than half of the stated period. If the condition is expected to last more than one hour, an FM group is used instead; if it is expected for more than half of the period, it is treated as predominant and entered in the initial period or after an FM group. TEMPO groups do not exceed four hours.

False. NWS Instruction 10-813 states that PROB30 is the only PROB group used in NWS TAFs. A PROB30 group forecasts a 30 percent chance of a thunderstorm or precipitation event, together with the wind, visibility and sky condition directly related to that event, and a PROB30 group does not exceed 6 hours. PROB30 groups also do not include weather in the vicinity (VC) or non-convective low-level wind shear.

False. Under NWS Instruction 10-813, fog-restricted visibility from 5/8 to 6 statute miles is coded BR (mist), and FG is used only when fog restricts visibility to less than 5/8 statute mile. A forecast visibility of 2 statute miles in fog is therefore coded BR. BR itself is never used when the forecast visibility is greater than 6 statute miles (P6SM).

P6SM is the TAF code for a visibility forecast of more than 6 statute miles. TAF cloud heights are forecast in hundreds of feet above ground level, and the forecast ceiling is the lowest level at which the cumulative cloud cover equals 5/8 or more of the sky. SCT040 is a scattered layer of 3 to 4 oktas at 4,000 feet, which is not a ceiling. BKN080 is a broken layer of 5 to 7 oktas at 8,000 feet, and the lowest broken layer is the ceiling, so the forecast ceiling is 8,000 feet AGL.

False. In the non-convective low-level wind shear group WShwshwshws/dddffKT, the three digits after WS give the height of the top of the wind shear layer in hundreds of feet AGL, here 1,500 feet, and the five-digit wind after the solidus is the forecast wind at the top of that layer: 290° true at 45 knots. NWS Instruction 10-813 states that this wind group is not a value for the amount of shear. Low-level wind shear is forecast to be present from the surface up to the stated height.

The AWC lists four reasons for a domestic SIGMET, which advises of weather other than convective activity that is potentially hazardous to all aircraft: severe icing, severe or extreme turbulence, dust storms or sand storms lowering visibilities to less than three miles, and volcanic ash. Areas of moderate turbulence other than convectively induced, and areas of sustained surface winds greater than 30 knots, are G-AIRMET hazards. A G-AIRMET covers weather that may be hazardous to aircraft but is less severe than SIGMET weather.

True. The AWC states that any convective SIGMET implies severe or greater turbulence, severe icing, and low-level wind shear, even though the bulletin itself describes thunderstorms. Convective SIGMETs are issued in the conterminous U.S. for conditions such as a line of thunderstorms at least 60 miles long with thunderstorms affecting 40 percent of its length, or embedded or severe thunderstorms expected for more than 30 minutes. The bulletins are issued hourly at 55 minutes past the hour, and the forecast is valid for up to 2 hours.

False. The AWC's description of PIREP elements says the /SK field gives the height of the base of a cloud layer in hundreds of feet MSL using three digits, and the top of a layer in hundreds of feet MSL preceded by TOP. The /FL altitude where a phenomenon was first encountered is also in hundreds of feet MSL. A METAR uses a different reference: FMH-1 codes the height of each sky condition layer in hundreds of feet above the surface.

The AWC explains that each FB Winds/Temps entry has the form ddffttt: dd is the wind direction in tens of degrees, ff is the wind speed in knots, and ttt is the temperature in Celsius. In 2719-05, 27 is 270°, 19 is 19 knots and -05 is -5°C. Reading the first pair as the speed reverses the order of the group, and the temperatures in this product are Celsius, not Fahrenheit. The product gives winds from 3,000 to 53,000 feet, so a separate rule covers speeds over 100 knots, when the first direction digit is greater than 4.

Density altitude is the altitude in the standard atmosphere at which the air has the same density as the air at the point in question. The NWS lists pressure, temperature and humidity as the factors that determine air density. Warmer air is less dense, so a rise in temperature raises the density altitude. Pressure works the other way: as pressure decreases, density altitude increases, so a pressure rise lowers it. Humidity also works the other way from the distractor: water vapor is lighter than air, so moisture decreases air density, and as moisture decreases the air becomes denser, which lowers the density altitude.

An air mass passing over a colder surface does not form convective currents. It becomes a stable air mass with poor surface visibility, because smoke, dust and other particles cannot rise out of it and stay trapped near the surface, and a stable air mass can produce low stratus clouds and fog. The opposite case is an air mass passing over a warmer surface: it is warmed from below, convective currents form, and it becomes unstable with good surface visibility. Moist, unstable air causes cumulus clouds, showers and turbulence.

Advection fog is likely when a layer of warm, moist air moves over a cold surface. Unlike radiation fog, it requires wind: winds of up to 15 knots let it form and intensify, and above 15 knots it usually lifts into low stratus clouds. It is common in coastal areas where sea breezes blow the air over cooler landmasses. Radiation fog forms on clear nights with relatively little to no wind, when the ground cools rapidly. Steam fog forms in the opposite setting, when cold, dry air moves over warm water.

When unsaturated air is lifted, it cools at 5.4°F per 1,000 feet while its dew point decreases at 1°F per 1,000 feet, so temperature and dew point converge at 4.4°F per 1,000 feet. The text says to divide the temperature/dew point spread by 4.4 and multiply by 1,000: (82 − 60) ÷ 4.4 × 1,000 = 5,000 feet AGL. Dividing the 22°F spread by 5.4 gives about 4,100 feet, which ignores the falling dew point. Multiplying the spread by 100 to get 2,200 feet is not the method the text describes.

The NWS weather theory text describes three stages. In the cumulus stage, lifting begins and continuous, strong updrafts keep moisture from falling. Within about 15 minutes the storm reaches the mature stage, the most violent period of its life cycle: drops of rain or ice become too heavy for the cloud to support and fall as rain or hail, which creates downward motion, so warm rising air, cool descending air and violent turbulence all exist within and near the cloud. In the dissipating stage, the downdrafts spread out and replace the updrafts that sustained the storm.

The text states that for a thunderstorm to form, the air must have sufficient water vapor, an unstable lapse rate, and an initial lifting action to start the storm process. A stable lapse rate is the opposite of what is needed: a stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out. A temperature inversion is a layer in which temperature rises with altitude; inversion layers are commonly shallow layers of smooth, stable air close to the ground, and the top acts as a lid that keeps weather trapped below.

True. The NWS weather theory text says hailstones may fall some distance from the storm core and that hail may be encountered in clear air several miles from thunderstorm clouds. Hailstones falling through air warmer than 0°C begin to melt, so precipitation may reach the ground as either hail or rain, and rain at the surface does not mean the absence of hail aloft. The text says possible hail should be anticipated with any thunderstorm, especially beneath the anvil of a large cumulonimbus.

True. The NWS weather theory text describes this sequence for a takeoff into a microburst: a performance-increasing headwind, followed by performance-decreasing downdrafts, and then a rapid shear to a tailwind that can result in terrain impact or flight dangerously close to the ground. A typical microburst occurs in a space of less than one mile horizontally and within 1,000 feet vertically, lasts about 15 minutes, and can produce downdrafts of up to 6,000 feet per minute.