Weather Frequently Asked Questions

How accurate are weather forecasts?

Three day forecasts: Highly reliable. No forecast is infallible, but 1-to-3 day high-resolution deterministic models (like HRDPS) can be trusted for contractor dispatch and fleet scheduling.

Seven day forecasts: Worth consulting for strategic planning. Synoptic wave patterns are generally resolved, but individual storm timing and transition lines can shift by 12–24 hours.

14 day forecasts: Useful only for macro temperature and storm frequency trends (e.g., "Next week will average colder than normal"). Never base specific hourly crew deployments on a 10-day forecast.

What is a synoptic day and why doesn't daily data start at midnight?

tl;dr: Environment and Climate Change Canada (ECCC) defines a synoptic day as starting at 06:00 UTC and ending at 05:59 UTC the following day, nationwide.

Why not midnight-to-midnight? Due to Daylight Saving Time (DST), calendar days in spring are 23 hours long and in fall are 25 hours long. For precipitation totals (rain and snow), this creates an artificial 4% bias in measurement opportunity. Standardizing to 06:00 UTC guarantees every meteorological day is precisely 24 hours.

Furthermore, this aligns observations coast-to-coast: 06:00 UTC corresponds to 1:00 AM EST (standard time) / 2:00 AM EDT in Ottawa and Toronto, 10:00 PM PST in Vancouver, and 3:30 AM in St. John's, NL.

What do you mean by "first frost"?

The official scientific definition used by Environment Canada is the first calendar day when the calibrated thermometer inside the airport's Stevenson screen drops to 0.0°C or below.

"Why did I see frost on my car roof when the official station reported +2.5°C?"

This is caused by Radiative Cooling. On clear, calm nights, dark, flat surfaces (such as vehicle roofs, asphalt shingles, and grass blades) emit longwave thermal infrared radiation directly into outer space. This causes the surface itself to cool several degrees below the ambient air temperature, allowing visible water vapor deposition even when air at 1.5 meters height is +2°C to +3°C.

What are the different types of frost?

1. Hoarfrost: Delicate, feathery ice crystals that deposit when moist air cools below its frost point under calm winds.
2. Rime Frost: Rapid freezing of supercooled fog or cloud droplets onto vertical surfaces, trees, and power lines.
3. Black Frost: Cold dry air drops well below 0°C without visible surface crystals because the dewpoint is too low. Plant vascular tissue freezes and turns blackened.
4. Hard Frost (Killing Frost): When the topsoil surface reaches 0.0°C (usually requiring sustained air temperatures of -3°C or lower), ending the commercial growing season.

What do those "monotonic" tables mean?

A monotonic table is a reverse-chronological "most since" or "least since" historical extreme tracker.

Mathematically, a sequence is monotonic if it never increases or never decreases. In our weather record tables:

• Entries are sorted in reverse chronological order (newest day at the top).
• Each row below must have a recorded value that is less-than-or-equal-to (for cold maximums) or greater-than-or-equal-to (for warm maximums) the entry above it.
• Days that do not break or tie the preceding milestone are skipped entirely.

This allows an observer to immediately answer questions such as: "When was the last time Ottawa stayed below -25°C?" You simply scan the table to find the first historical row below -25.0°C.

What is Cumulative Snow Depth and why is it measured in cm·days?

Cumulative snow depth is calculated by taking the depth of snow on the ground for each day and summing them together.

Why not use an average? Standard averages are distorted by snowless days. If a month has 1 day with 10 cm and 29 days with 0 cm, the average is 0.3 cm, which conceals the real ground cover experience. Using cumulative snow depth accurately indices winter severity across different years without calendar boundary errors.

How are hours of bright sunshine calculated?

Decades ago, Environment Canada used a mechanical Campbell–Stokes recorder (a glass sphere that focused sunlight to scorch a paper card). After ECCC discontinued these optical recorders, our system calculates bright sunshine using hourly Surface Weather Observation sky condition strings:

'a few clouds', 'clear', 'haze', 'mainly clear', 'mainly sunny', 'mostly sunny', 'partly cloudy', 'sunny'

Whenever an hourly observation records one of these conditions, it is indexed as bright sunshine. This heuristic is self-consistent and enables rigorous year-over-year sunshine comparisons.