(Canadian) METAR question...?

Question

What are the exact required amounts of change in certain weather variables to issue a SPECI? (or where can I find them).

I know that a special is issued when the weather is variable, and say, the winds suddenly pick up, or the ceiling drops, or whatever, but where is the line drawn as to when we can just wait for the next METAR to be issued as opposed to putting out a SPECI?

Thanks.

If you don't understand the question, please don't answer.

Answer 1

You will find it in the TC-AIM MET 3.15.4.

It gives values for the amount of change in a ceiling, visibility, and other weather phenomena requiring the issue of a SPECI report.

According to TC-AIM MET 3.15.5, SPECI reports from an AWOS will always be more frequent (exceeding 6 per hour 5%-6% of the time) due to the precise measurements of the instrumentation, the continuous sampling, and the undirectional view the AWOS has.

Answer 2

Wind Shift

Wind direction changes by 45 degrees or more in less than 15 minutes and the wind speed is 10 knots or more throughout the wind shift.

Visibility

Surface visibility as reported in the body of the report decreases to less than, or if below, increases to equal or exceed:

a.

3 miles

b.

2 miles

c.

1 mile

d.

The lowest standard instrument approach procedure minimum as published in the National Ocean Service (NOS) U.S Instrument Procedures. If none published use ½ mile.

Runway Visual Range (RVR)

The highest value from the designated RVR runway decreases to less than, or if below, increases to equal or exceed 2,400 feet during the preceding 10 minutes. U.S. military stations may not report a SPECI based on RVR.

Tornado, Funnel

Cloud, or Waterspout

a.

is observed.

b.

disappears from sight, or ends.

Thunderstorm

a.

begins (a SPECI is not required to report the beginning of a new thunderstorm if one is currently reported).

b.

ends.

Precipitation

a.

hail begins or ends.

b.

freezing precipitation begins, ends, or changes intensity.

c.

ice pellets begin, end, or change intensity

Squalls

When they occur

Ceiling

The ceiling (rounded off to reportable values) forms or dissipates below, decreases to less than, or if below, increases to equal or exceed:

a.

3,000 feet.

b.

1,500 feet

c.

1,000 feet

d.

500 feet

e.

The lowest standard instrument approach procedure minimum as published in the National Ocean Service (NOS) U.S Instrument Procedures. If none published, use 200 feet.

Sky Condition

A layer of clouds or obscurations aloft is present below 1,000 feet and no layer aloft was reported below 1,000 feet in the preceding METAR or SPECI.

Volcanic Eruption

When an eruption is first noted

Aircraft Mishap

Upon notification of an aircraft mishap, unless there has been an intervening observation

Miscellaneous

Any other meteorological situation designated by the responsible agency of which , in the opinion of the observer, is critical.