A changing weather pattern and a cooling parcel
A school weather record gives successive pressure readings of1008,1004,998 and1002hectopascals. The corresponding temperatures are20,22,15 and13degrees Celsius, and the wind changes from southwesterly to northwesterly near the sharp cooling. The class considers whether this pattern is consistent with a cold front, without treating the short record as a complete forecast. In a separate simplified humidity model a constant-volume air parcel at20degrees contains13.6grams of water vapor per cubic metre. Its supplied saturation vapor density is17.0grams per cubic metre at20degrees and9.4grams per cubic metre at10degrees. No vapor enters or leaves during cooling, and any excess condenses. These figures are exercise data; do not apply them as a universal outdoor weather prediction or substitute a different parcel model.
1. Interpret the pressure, temperature and wind changes cautiously.
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The cooling, wind shift and pressure minimum followed by a rise are consistent with a cold-front passage. This limited record alone does not prove the front location or determine the subsequent weather.
2. Calculate the initial relative humidity in the specified model.
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At the same temperature the supplied density ratio is13.6/17.0=0.80, giving80%. The comparison must use saturation at20degrees, not the later10-degree value.
3. Find the condensed mass per cubic metre on cooling to10degrees.
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At10degrees only9.4g/m³ can remain vapor under the given model. Thus13.6-9.4=4.2g/m³ condenses. This uses the stated fixed volume and no-exchange assumptions.