Bumping along

Reference AIM MET 3.7 Turbulence Reporting Criteria

Intensity:

• light
• turbulence: momentary, slight changes in altitude/attitude
• chop: slight, rapid, rhythmic bumpiness without appreciable changes in altitude/attitude
• moderate
• turbulence: greater than light, aircraft remains in positive control, variations in IAS
• chop: greater than light, rapid bumps/jolts without appreciable changes in aircraft altitude/attitude
• severe
• large, abrupt changes in altitude/attitude
• large variations in IAS
• momentary loss of control

Frequency:

• occasional (<1/3 of the time)
• intermittent
• continuous (>2/3 of the time)

Report:

• location
• time
• intensity
• in or near cloud
• altitude
• type of aircraft
• duration (if applicable)

High level turbulence not associated with cumuliform clouds should be reported as CAT.

Wind

Reference ACWM Chapters 5 and 11

Wind is the result of changes in pressure gradient.

• flows from High to Low 
• steeper gradient = stronger wind

Coriolis Effect

• the deflection of wind caused by the Earth's rotation
• varies from zero at equator to maximum strength at poles

Curvature Effect

• when isobars are curved, air moves in an arc >> centrifugal force
• the pressure gradient around lows is generally stronger than around highs

Friction

• As you climb from the surface to 3000 feet, the wind veers (clockwise change in direction) and increases.
• As you descend from 3000 feet to the surface, the wind backs (counterclockwise change in direction) and decreases.
• We must also keep in mind that land and water create different amounts of friction.  There is less friction over water, so wind would blow at less of an angle across isobars and at faster speeds. 

Squalls and Gusts

• characteristics of turbulent flight conditions
• Gusts >> rapid peaks and lulls
• Squalls >> sudden increase lasting for a minute or more, then a decrease

Diurnal Effects

• surface winds are usually stronger and gustier during the day
• Sea Breeze during the day (higher pressure over water than land)
• Land Breeze at night

Topographical Effects

• hilly and mountainous terrain can contribute to mechanical turbulence
• Anabatic winds flow upslope during the day when mountain slops facing the sun are heated
• Katabatic winds flow downslope
• warm = Chinook wind, warms at DALR
• cold = glacier wind, cooling by underlying ice

Wind Shear

• increased performance (headwind) >> airspeed increases
• encountering increased performance on glideslope >> would have to reduce power to recapture glidepath, then increase power to maintain due to stronger headwind
• decreased performance (tailwind) >> airspeed decreases

Atmospheric Pressure

Pressure Measurements

• Inches of mercury (Hg) >> for altimetry
• Hectopascals (hPa) and millibars (mb) >> for weather map analysis
• measured using a barometer (digital or aneroid type)

METARs give both an altimeter setting and a sea level pressure.  What's the difference?

• Station Pressure
• measured at the airport; the weight of the air above the station
• Altimeter Setting
• station level pressure reduced to MSL assuming ISA conditions
• MSL Pressure
• station pressure reduced to MSL using the average surface temperature for the last 12 hours
• useful in weather map analysis when patterns across different observation stations must be compared

And just for fun, here's 'Under Pressure' by Queen:

Pressure Levels

In my January 2 post about the Earth's atmosphere, I said "some things you just have to memorize" with regards to which pressures correspond to which levels of the ICAO standard atmosphere.  As it turns out, memorization is not required if you have a CX-2 flight computer (and possibly others, but this is the one I have)!   Here's how:

• press 'FLIGHT'
• select (1) Altitude
• select (3) Std Atmos
• enter the altitude
• read the OAT, as well as pressure in "Hg and mb!

Small wins :)

Lapse Rates and Stability

Adiabatic Processes

• no heat is added or removed from air
• rising air >> lower pressure >> expands >> temperature decreases
• sinking air >> higher pressure >> compresses >> temperature increases

Lapse Rates

• Dry Adiabatic Lapse Rate (DALR): 3 C / 1000 ft
• Saturated Adiabatic Lapse Rate (SALR): 1.5 C / 1000 ft
• Average: 2 C / 1000 ft

• Environmental Lapse Rate (ELR): indicates the temperature of the surrounding air; comparison to lapse rate of air parcel determines if it is stable or unstable
• shallow lapse rate compared to SALR: absolute stability
• lapse rate between DALR and SALR: conditional instability
• steeper lapse rate than DALR: absolute instability

Characteristics of Stable Air

• sustained low visibility (i.e. haze layers, drizzle, fog)
• continuous precipitation
• strato-form clouds (layers)
• steady winds

Characteristics of Unstable Air

• good visibility
• showery precipitation
• cumulo-form clouds (vertical development)
• gusty winds

DO YOU KNOW... why rising saturated air cool less rapidly than rising unsaturated air?

• heat is released during the condensation of water vapour

Practical Meteorology

Practical Meteorology is all about applying all that meteorological theory to actual weather observations and forecasts.  There were a couple things I'd forgotten.   For example, with cloud layers, anything with 5/8 (broken) coverage or greater constitutes a ceiling.  This is not true if the it is a surface based layer, such as fog or snow.  In this case, the coverage must bee 8/8 to be considered a ceiling.  

Also, there are some charts that I don't use on a daily basis, because they cover much larger areas than I need on my regular runs between Montreal and Toronto.  When doing my practice questions, I was getting questions with regards to issuing and validity times wrong, so I knew it was time to consult the AIM.  

Prognosis = Forecast
Analysis = Observation

Reference AIM MET 3.2.1 Aviation Forecasts and Charts

Significant Weather Forecast - PROG Chart

• prepared 4 times daily, based on 00Z and 12Z data
• issued 12 hours prior to validity time (00, 06, 12, 18Z)
• specific flight level range (i.e. 700 - 400 MB = FL100 - FL240; also FL250-630)
• indicate surface positions of lows, highs, and any significant weather (thunderstorms, turbulence, mountain waves, etc)
• a forecast area of turbulence implies a 50% probability of encountering turbulence somewhere within the depicted area
• jet streams are depicted when the core speed is forecast to attain 80 knots or more

Upper Level Forecast - PROG Chart

• issued 4 times daily, 12 hours before the validity times of 00, 06, 12, 18Z
• applicable FL240, FL340, FL390, FL450
• depicts forecast wind and temperatures for the chart level

Reference AIM MET 3.2.3 Weather Charts

Surface Weather Chart

• observed at 00, 06, 12, 18Z
• issued 2-3 hours after observation
• pressure patterns from surface up to 3000 ft
• surface location of fronts, precipitation, obstructions to vision

Upper Level Chart - ANAL

• observed at 00 and 12Z
• issued over 3 hours after observation
• show reported atmospheric conditions at the pressure levels, such as wind speed/direction, temperatures, moisture content
• the contours or altitude gradient can be considered as the slope of the pressure surface

How High Are We, REALLY?

True Altitude was something that always seemed to trip us up in ground school, but it's not actually that difficult.  In fact, if you read the instructions on your E6B, you're already more than half-way there!

True Altitude is particularly important when there are abnormally cold temperatures.  Usually the scenarios presented involve leveling off at a MEA, flying over a ridge, and realizing that you don't have nearly the amount of altitude clearance as you might have expected because it's -40. 

4 simple steps:

1. set air temperature over pressure altitude (make sure you're using the correct side!)
2. read true altitude over calibrated altitude scale (calibrated = planned - height of altimeter source)
3. add result of (2.) to height of altimeter source
4. subtract height of ridge to get ridge clearance

I'm tagging this both SAMRA and SARON because it fits in both Meteorology and Flight Operations.  So it could come up anywhere!

Things I'd Forgotten

Over the past week, I went through 171 'General Meteorology' practice questions... I got 150 (87.7%) of them right.  So there are obviously a few things that I'd forgotten since studying for my IATRA a couple years ago.  A few examples:

Terminology

• Overrunning: condition existing when an air mass is in motion aloft above another air mass of greater density at the surface.  This process that results in expansional cooling, subsequent condensation and formation of cloud. Example: warm air ascending the surface of a warm front. 
• Altimeter Setting: station level pressure reduced to MSL assuming ISA conditions
• Land breeze: blow from land to water during the night
• Stationary front: a frontal zone separating two different air masses, neither of which is strong enough to replace the other
• Riming: the growth of a descending ice particle as it collides with nearby water droplets which then freeze onto the particle

Location of Supercooled Water Droplets:

• in unstable air: lower levels of cloud where temperatures are only a few degrees below freezing
• in stable air: amount of supercooled water increases with height when temperatures are not far below freezing

Microbusts:

• The shaft of a microburst is normally about 2.2 nm wide or less at the surface.
• Downdrafts associated with a microbust could be as strong as 6000 ft / min

Mountain Waves:

• Rotor clouds are normally centred beneath the standing lenticular cloud (i.e. below each wave crest)
• The most powerful rotor is located under the first wave crest

Upper-Air Contour Charts:

• The wind blows along the contours in the same way that the 2000 ft wind blows along the surface isobars (parallel to the height contours, with lower heights on the left)
• the closer the contours, the stronger the wind; and if the height contours are curved, then centrifugal force acts on the wind

And other random facts:

• Snow grains: imply that freezing drizzle is present aloft.
• The vertical extent of CAT associated with a jet stream will be greatest on the low pressure, cold air mass side of the jet stream core.

I'll plan to cover some of these topics in more detail soon!  It's a good thing I like Meteorology... hopefully I'll still be this enthusiastic when I come back to Radio Nav :)

Air Masses

It is bitterly cold tonight, and one of the top news stories this week has been the weather across the entire country.  

Reference ACWM Chapter 6

Definitions:

• Air mass: a body of air whose temperature and humidity characteristics are uniform in the horizontal.
• Source region: a large area of land or ocean of relatively uniform characteristics and above which an air mass can form.
• Front: a transition zone between two air masses

Classification of Air Masses:

• Continental Arctic (cA)
• Maritime Arctic (mA)
• Continental Polar (cP)
• Maritime Polar (mP)
• Continental Tropical (cT)
• Maritime Tropical (mT)

Canadian Air Masses: cA, mA, mP, mT

Air masses may be modified as they migrate from their source regions.

• cold air (i.e. cA) moving south over the Lake Ontario in the winter becomes unstable since it is warmed from below, contributing to snow showers in Buffalo / south shores.
• warm air (i.e. mT) moving north over the Great Lakes in the spring becomes more stable since it is being cooled from below, contributing to stratus cloud, drizzle and fog over the north shores

POP QUIZ!

A south-westerly flow of Maritime Tropical air moving over the Labrador Current would produce what kind of fog?

Thunderstorms Part I

No chit-chat this time, let's get started.

Reference ACWM Chapter 15

Requirements for development of a thunderstorm:

• high relative humidity
• steep lapse rate
• lifting force / triggering action

Life cycle of a thunderstorm (1 - 3 hours):

• cumulus stage: only updrafts are present in the cloud
• mature stage: starts at onset of precipitation at the surface; updrafts and downdrafts
• dissipating stage: presence of downdrafts throughout nearly the whole cell

Classification of thunderstorms:

• airmass: form within a warm, moist air mass; isolated
• frontal: warm front (embedded in stratiform layer; least severe), cold front (form in continuous line; quite severe), trowal (rapid lifting of warm, moist air; moderate severity)
• squall line: most severe, may include heavy hail, destructive winds, tornados; often form 100-200 miles ahead of fast-moving cold fronts
• convective: formed by heating from below or by convergence of wind flow
• orographic: wind forces moist, unstable air up mountain slopes
• nocturnal: unusually warm air aloft, frequently occurs at night or early morning in Midwest / Central Plains

POP QUIZ!

You've inadvertently just flown into a thunderstorm cell.  What do you do?

Discussion of tornados, hurricanes and hazards to follow.

The Foggiest Idea

Fog is a weather hazard which can create all sorts of problems and delays.  Different meteorological scenarios produce different types of fog, and understanding these can help us anticipate when it may form.

Reference ACWM Chapter 10 on Visibility

The requirement for the formation of all types of fog is high relative humidity.  Typically this happens as the air temperature is lowered to the dew point (but not always - with steam fog, the dewpoint is raised to the air temperature).  It also involves condensation nuclei and some physical process which causes condensation to occur. 

Types:

• advection fog: cooling of warm air as it moves over a colder surface (land or water)
• radiation fog: cooling of the earth's surface at night (land)
• upslope fog: adiabatic/expansion cooling 
• steam fog: water vapor is added to very cold air
• ice fog: addition of water vapour to air through fuel combustion which then sublimates into ice crystals
• frontal fog: evaporation of rain that falls from the warm air into the cold air preceding a warm front

POP QUIZ!

What is the difference between fog and mist? (TC AIM MET 3.15, WMO Code, Table 4678)

The Earth's Atmosphere

Being a complete aviation nerd, I have no shame in admitting that Meteorology was one of my favourite subjects in flight school. And whereas I'd like to jump into things like lapse rate and stability, I'll save those for later and start with the basics.  Our trusty Study and Reference Guide starts with these topics about the Earth's Atmosphere:

Properties - from ACWM Chapter 2

• mobility
• capacity for expansion
• capacity for compression

Vertical Structure - in ascending order

• troposhere
• contains all the elements of the weather
• temperature decreases with altitude
• tropopause
• height varies from about 8 km over the poles up to 17 km over the equator
• there is an abrupt change in height over each frontal surface
• higher in the summer than in the winter
• In ISA conditions its height is 36 090 ft and temperature is -56.5 C
• stratosphere
• temperature remains steady and then increases with altitude
• stratopause
• temperature starts to decrease with altitude
• mesosphere
• temperature decreases with altitude to about 275 000 ft
• mesopause
• lowest temperature
• thermosphere
• where aurora are seen
• temperature increases dramatically with altitude

ICAO Standard Atmosphere (ISA)

• At sea level: 15 C and 1013.2 hPa / 29.92"Hg
• Average lapse rate is 1.98 C / 1 000 ft

Some things you just have to memorize:

Pressure Level (mb)	Altitude (ft)
700	10000
500	18000
400	24000
250	34000
200	39000
150	45000

And now... time for a POP QUIZ!

(1) What are lines drawn on weather charts joining places having the same temperature called?

(2) Does TAS increase or decrease with temperature?

(3) If the temperature at FL300 is -50, how would this be expressed in terms of an ISA deviation?