Two Towns at the Same Latitude
Two towns lie at 45°N. Town M is on a west-facing coast; Town C is 800 km inland. Climate records show:
| Measure | Town M | Town C |
|---|---|---|
| January mean | 7°C | −5°C |
| July mean | 18°C | 22°C |
| Annual range | 11°C | 27°C |
| Mean daily range | 6°C | 14°C |
Prevailing winds at this latitude are from the west. A warm ocean current runs poleward along the coast. Both towns receive similar annual precipitation, but Town M's rain is spread through the year while Town C's is concentrated in summer. A student claims that because the towns are at the same latitude, they receive the same intensity of sunlight, so their temperature difference must be caused only by altitude. Town C is 120 m above sea level; Town M is at sea level. The class is asked to judge the student's claim, explain the data, and predict what would happen to Town M's winter temperature if the prevailing wind reversed.
1. Evaluate the student's altitude explanation quantitatively. Is altitude sufficient to explain the January difference?
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Using an environmental lapse rate of about 6.5°C per km, a 120 m altitude difference gives only about 0.8°C of cooling. Even a moist adiabatic rate gives roughly 0.6°C. The observed January difference is 12°C, so altitude is far too small to explain it.
Same latitude does mean similar solar angle and day length, but surface temperature also depends on surface type, ocean currents, prevailing wind, cloud cover, and continentality. Town M is maritime: westerlies bring air moderated by the warm poleward current, and water has a high heat capacity. Town C is continental: land heats and cools rapidly, and winter radiative cooling is strong. The student's single-cause claim is not supported.
2. Use the data to explain why Town M has a smaller annual range and a smaller mean daily range than Town C.
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Town M's annual range is 11°C; Town C's is 27°C. The ocean mixed layer has a high heat capacity and mixes, so it warms and cools slowly. Westerlies advect that moderated marine air onshore, keeping Town M cooler in summer and warmer in winter. Town C is far from the ocean, so its air has low water-vapour content and low heat capacity; it loses heat efficiently by longwave radiation in winter and heats strongly in summer.
The daily range shows the same contrast: 6°C at Town M versus 14°C at Town C. Maritime air and cloud cover reduce daytime heating and nighttime cooling at Town M. Dry continental air and clearer skies at Town C allow strong daytime heating and rapid nighttime cooling.
3. Predict the effect on Town M's January mean and daily range if the prevailing winds reversed to come from the east (offshore).
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With offshore easterly winds, Town M would receive air from the interior rather than from the ocean. Its January mean would fall, probably by several degrees, moving toward Town C's continental value. The mean daily range would increase because dry continental air allows stronger daytime heating and more rapid nighttime radiative cooling.
Town M would not become exactly like Town C because it is still coastal and the nearby ocean would moderate extremes somewhat. The warm current would also have less direct influence because its heat would no longer be carried onshore by the prevailing wind.