Thirty Hours at the Weather Station
On a Friday in April, a high school Earth science class examines 30 hours of data from its rooftop weather station and a nearby airport: 6 a.m. - 22 °C, dew point 19 °C, 1014 mb, wind S at 10 km/h, overcast; noon - 24 °C, dew point 19 °C, 1011 mb, SW at 15 km/h; 6 p.m. - 20 °C, dew point 17 °C, 1004 mb, SW at 20 km/h, cumulonimbus, 8 mm rain in one hour; 9 p.m. - 14 °C, dew point 11 °C, 1000 mb, W at 25 km/h gusting 40; midnight - 10 °C, dew point 6 °C, 1002 mb and rising, NW at 20 km/h; 6 a.m. Saturday - 8 °C, dew point 3 °C, 1008 mb, N at 10 km/h, clear. One student says the storm was a hurricane; another says the temperature fell because clouds blocked the Sun; a third says the cold air came from space. The teacher asks the class to use the data to tell the story of what actually passed over the school.
1. Identify the weather feature and the time it passed, citing at least four pieces of evidence from the data.
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The feature is a cold front, and it passed between about 6 p.m. and 9 p.m.
Evidence:
- Wind shift. Wind veered from south/southwest ahead of the front to west and then northwest behind it - the classic signature of a wind shift at a front.
- Temperature. 20 °C at 6 p.m. fell to 14 °C by 9 p.m. and 10 °C by midnight, a drop that continues well after sunset.
- Dew point. Falling from 17 °C to 11 °C to 6 °C means the incoming air is drier, not just cooler; cloud shading would not dry an air mass.
- Pressure. Falling ahead of the front (1014 → 1000 mb), a minimum near 9 p.m., then rising (1002 → 1008 mb) as denser cold air and the trailing high arrive.
- Clouds and rain. Cumulonimbus and an 8 mm hourly total in the warm, moist air ahead of the front, followed by clearing behind it.
The sharpest change is between 6 p.m. and 9 p.m., so the surface front is best placed around 7–8 p.m.
2. Evaluate the three student explanations for the temperature drop and explain the actual cause.
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Hurricane: not supportable. Sustained winds were only 10–25 km/h (gusts 40), pressure only fell to 1000 mb, the system moved through in hours with a single rain band and a clear wind shift, and there was no eye, no tropical origin, and no prolonged heavy rain. This is a mid-latitude cyclone with a cold front, not a tropical cyclone.
Clouds blocked the Sun: clouds can cool a daytime surface, but the temperature kept falling after dark (20 → 14 → 10 → 8 °C), and the dew point fell with it. Shading does not remove water vapor from the air. The drying and cooling together show a different air mass arrived.
Cold air from space: space is a near-vacuum, not a reservoir of air, and the atmosphere is bound to Earth and mixes. The cold air behind the front is a shallow, dense dome that formed at higher latitudes, where sunlight is weaker and the air loses heat over cold ground. Because it is denser, it wedges under the warm, moist air, forcing that air up - which is why the rain is at the front. The correct statement is that the front advected a cooler, drier air mass over the school.
3. Predict conditions for Saturday afternoon and explain the pressure trend (fall, then rise).
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Pressure trend: the fall from 1014 to 1000 mb occurred as the low-pressure trough and the approaching front displaced air; the minimum near 9 p.m. marks the front's passage. The subsequent rise to 1008 mb reflects the arrival of denser, colder air behind the front and the building high-pressure ridge. Rising pressure with a northwest wind and a low dew point is a clearing, stabilizing signature.
Saturday afternoon prediction: continued clearing and rising pressure (roughly 1010–1015 mb), temperature recovering only to about 12–14 °C, dew point around 3–5 °C, northwest winds 10–20 km/h decreasing, good visibility, and no precipitation. The air mass is cool and dry, so it will warm slowly under April sunshine and cool quickly again after sunset.
A defensible answer notes uncertainty: the school has one station and only 30 hours of data, so this is a short-term extrapolation, not a guarantee.