SAMRA/SARON Meteorology Questions

Station pressure differs from mean sea level (MSL) pressure in that station pressure is

• measured only at coastal stations, since inland stations are said to be physically unable to report station pressure at all.
• the actual, unadjusted atmospheric pressure measured at the station's own elevation, while MSL pressure has been mathematically reduced to what the pressure would be at sea level.
• the same value as MSL pressure at every station, since pressure is not thought to vary meaningfully with elevation over short distances.
• always higher than MSL pressure at every station, regardless of the station's actual elevation above sea level.

Explanation: Station pressure is simply what the barometer at that location actually reads, unadjusted. Because pressure decreases with elevation, comparing raw station pressures from stations at different elevations wouldn't reveal the actual horizontal pressure pattern -- so station pressure is mathematically reduced to an equivalent sea-level value (MSL pressure) for use on weather maps and in altimeter setting calculations.

TP 690 Section 8 - Atmospheric Pressure, Station Pressure / Mean Sea Level Pressure

A PIREP (or AIREP), a pilot report of observed weather conditions, is particularly valuable because it

• replaces the need for any ground-based weather observation entirely at the reporting station or nearby stations.
• can only report the absence of significant weather, and can never report the presence of an actual hazard.
• provides an actual, in-flight observation (such as turbulence, icing, or cloud tops) that ground-based sensors and forecasts may not otherwise capture.
• is used only for post-flight record-keeping purposes, with no distribution at all to other pilots, dispatchers, or forecasters at any other facility or agency.

Explanation: A PIREP gives forecasters and other pilots real, in-situ information -- actual turbulence intensity and location, icing type and severity, cloud tops, and so on -- that ground-based instruments and models can't directly observe, which is why pilots are encouraged to file them, especially when conditions differ from the forecast (in either direction).

TP 690 Section 8 - Aviation Weather Reports, Pilot Reports (PIREP/AIREP)

A Föhn (or Chinook) wind, experienced on the lee (downwind) side of a mountain range, is characterized by

• a wind entirely unrelated to any moisture or adiabatic temperature change, driven purely by pressure gradient force across the mountain range and nothing else.
• a comparatively cold, moist wind, resulting from air that gained moisture descending the lee slope faster than it lost moisture on the windward side -- the reverse of the warm, dry result a Föhn or Chinook wind actually produces.
• a wind that only occurs on the windward (upwind) side of a mountain range, never on the lee side where the Föhn or Chinook effect is actually observed.
• a comparatively warm, dry wind, resulting from air that lost moisture through precipitation on the windward slope and then warmed adiabatically faster descending the lee slope than it cooled ascending the windward side.

Explanation: A Föhn/Chinook wind's characteristic warmth and dryness comes from an asymmetry in the moist and dry adiabatic lapse rates: air rising on the windward side cools at the (slower) moist adiabatic rate once saturated, dropping much of its moisture as precipitation, but then descends the lee slope dry, warming at the (faster) dry adiabatic rate the whole way down -- so it arrives at the base of the lee slope noticeably warmer and drier than it started, a well-known and sometimes dramatic local temperature effect.

TP 690 Section 8 - Meteorology, Local Winds

Embedded thunderstorms -- thunderstorm cells obscured within a broader layer of stratiform cloud -- present a particular hazard because

• they may not be visible to a pilot relying on outside visual reference alone, making them difficult to detect and avoid without weather radar or similar equipment.
• they occur exclusively at night, with embedded thunderstorms considered a non-issue for any daytime flight.
• they are, by definition, less intense than an isolated, clearly visible thunderstorm cell, making them a comparatively minor operational concern.
• they are always accompanied by clear, cloud-free gaps immediately surrounding the storm cell, making them just as easy to visually identify and avoid as an isolated cell.

Explanation: Because an embedded thunderstorm is hidden within a larger mass of stratiform cloud, a pilot relying purely on visual detection may not see it coming the way an isolated, visually obvious cumulonimbus cell would be seen -- which is exactly why airborne weather radar (or ATC radar advisories) becomes particularly valuable in that situation, and an embedded cell isn't necessarily any less intense than a visible one, isn't confined to night flying, and by definition lacks the clear surrounding gaps that would make it easy to visually pick out.

TP 690 Section 8 - Meteorology

QFE, as an altimeter setting concept, differs from QNH in that QFE

• is the standard altimeter setting used throughout Canadian domestic airspace, replacing QNH entirely for both VFR and IFR operations.
• causes the altimeter to read zero (or a value referenced to the aerodrome elevation) when the aircraft is on the ground at that aerodrome, rather than reading the aerodrome's elevation above mean sea level as QNH does.
• is set using the current sea-level pressure reported at a distant reporting station many kilometres away rather than any pressure value measured locally at the aerodrome itself, regardless of that aerodrome's own field elevation.
• represents the same numerical value as QNH at every aerodrome, making the distinction between the two settings purely terminological.

Explanation: QFE is an altimeter setting referenced to aerodrome (or a specific point's) elevation, causing the altimeter to read zero, or a value referenced to that elevation, when the aircraft is on the ground there -- unlike QNH, which is referenced to mean sea level and causes the altimeter to read the aerodrome's elevation above sea level when on the ground; QFE is not the standard Canadian domestic setting, the two values are generally not numerically identical, and QFE is based on local aerodrome pressure, not a distant station's sea-level value.

TC AIM MET - Altimeter Setting Terminology

In addition to its standard temperature lapse rate, the International Standard Atmosphere (ISA) also defines a standard pressure lapse rate, which is generally approximated in the lower atmosphere as

• a rate identical in numerical value to the ISA temperature lapse rate, with pressure and temperature decreasing by the same amount for each 1,000 feet of altitude.
• an increase, rather than a decrease, in pressure with increasing altitude, the reverse of the actual relationship.
• a decrease of roughly one inch of mercury (or approximately 34 hectopascals) for every 1,000 feet of altitude gained, a relationship commonly used as a rule of thumb near sea level.
• a fixed, unchanging pressure value at every altitude, meaning pressure does not vary with altitude under the ISA model.

Explanation: Alongside its standard temperature lapse rate, the ISA model also defines an approximate standard pressure decrease with altitude -- roughly one inch of mercury (about 34 hectopascals) per 1,000 feet near sea level, a relationship that becomes less linear at higher altitudes but is commonly used as a rough rule of thumb for altimetry and performance estimation at lower levels.

TP 690 -- Meteorology: The Atmosphere

An aircraft flying at a constant indicated altitude, using an altimeter setting obtained before departure, that flies into a region of lower actual atmospheric pressure without updating the altimeter setting will

• actually be flying at a true altitude higher than indicated, with the altimeter under-reading the aircraft's true height.
• experience no discrepancy at all between indicated and true altitude, since pressure changes en route have no effect on a properly functioning altimeter, which overlooks how a barometric altimeter actually works, since it remains referenced to the pressure setting last entered and will therefore read inaccurately once the aircraft flies into a region of different actual pressure.
• have the discrepancy automatically corrected by the aircraft's transponder, eliminating any risk to terrain clearance.
• actually be flying at a true altitude lower than indicated, since the altimeter, still referenced to the higher pressure setting, will over-read compared to the aircraft's true height above mean sea level.

Explanation: An altimeter is essentially an aneroid barometer calibrated to indicate altitude based on the pressure setting entered by the crew; if the actual atmospheric pressure along the route decreases below what the altimeter is still set to, the altimeter will over-read -- showing a higher altitude than the aircraft is actually flying at -- meaning true altitude is lower than indicated, which is the basis for the classic caution to obtain and set current altimeter settings, particularly when flying from a region of high pressure into a region of lower pressure where terrain clearance could otherwise be eroded without the crew realizing it.

TP 690 -- Meteorology: Atmospheric Pressure and Altimetry

A maritime air mass generally exhibits a smaller diurnal (day-to-night) temperature range than a continental air mass primarily because

• water's high heat capacity causes it to warm and cool more slowly than land, moderating the temperature of the air mass above it
• maritime air masses never experience any solar heating during the day, as commonly observed in forecast verification
• continental air masses are always found at higher latitudes than maritime air masses, under typical synoptic conditions
• maritime air always contains ice crystals that reflect all incoming solar radiation, under typical synoptic conditions

Explanation: Water has a much higher heat capacity than land and warms and cools far more slowly; an air mass sitting over open water is moderated by this slow temperature change, producing a smaller day-to-night temperature range than a continental air mass over land.

TP 690 -- Meteorology: Maritime versus continental diurnal temperature range