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

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?