Final Exam

1. Where does almost all of our weather happen?

The troposphere holds most of the air and nearly all the water vapour, so clouds and storms form there.

2. Why is the warmest part of a sunny day usually mid-afternoon, not noon?

Heat keeps building as long as incoming sunshine beats outgoing heat, which lasts into the afternoon.

3. Why do clear, calm nights get so cold?

Without a cloud blanket the ground radiates heat away, and with no wind the coldest air pools at the surface.

4. A barometer falling steadily for several hours usually means…

Falling pressure signals an approaching low-pressure system, and a fast fall often means wind.

5. What makes the wind blow?

Air moves from higher to lower pressure. The bigger the difference over a distance, the stronger the wind.

6. A ‘west wind’ is blowing…

Wind direction is always the direction the wind comes from.

7. The dew point is 2°C and tonight’s low is forecast to be 1°C with calm wind. What’s likely?

When the temperature drops to the dew point, moisture condenses into dew, fog or frost.

8. Thin, wispy cirrus cloud thickening from the west often means…

High cloud spreading in ahead of a system is often the first sign of a change.

9. How many oktas is a completely overcast sky?

The sky is divided into eighths. Fully covered is 8 oktas.

10. Freezing rain happens when…

A warm layer aloft melts the snow, and a shallow cold layer at the ground makes the rain freeze on contact.

11. Hail grows bigger when…

A strong updraft holds the ice up longer so more water freezes onto it.

12. When an Arctic cold front passes Lethbridge, you’d typically expect…

Cold Arctic air slides south along the Rockies, often with north winds and a big drop in temperature.

13. The 500 mb level is often called the ‘steering wheel’ because…

Weather systems tend to move along with the flow about 5.5 km up.

14. A trough in the jet stream over the Prairies usually means…

A dip in the jet stream lets colder air move south and often brings cloud and precipitation.

15. Unstable air is air that…

Unstable air keeps rising, which is how cumulus grows into thunderstorms.

16. Why does a chinook warm up so much as it reaches Lethbridge?

Sinking air warms as it’s compressed, and the chinook air lost much of its moisture on the west side of the mountains.

17. What is a chinook arch?

The arch forms in the rising part of mountain waves and is a classic sign of a chinook.

18. Environment Canada issues a severe thunderstorm WARNING. That means…

A watch means conditions are favourable; a warning means it’s happening or imminent.

19. A 40% chance of precipitation means…

POP is the chance that any given spot in the area gets measurable precipitation during the period.

20. For accurate wind readings, a home station’s wind sensor is ideally mounted…

The standard is 10 m above the ground, away from obstructions, which is how the West Lethbridge station is set up.