Final answer:
The air to the west of (behind) the cold front at St. Louis compared to Lexington was (b) cooler and (d) drier.
Step-by-step explanation:
When comparing the temperature and dewpoint at St. Louis with those at Lexington, especially in the context of a cold front, it is important to understand the effects of such a front. A cold front usually brings cooler and drier air to the region it passes over. Therefore, the air to the west of (behind) the cold front is generally cooler and drier.
Considering the principles of weather and dew point, when a cold front moves through, the temperature drops significantly, and the dew point also decreases. This happens because colder air can hold less moisture than warmer air. Additionally, the evaporative cooling effect, which helps to regulate body temperature on hot days, is less effective on humid days due to the higher concentration of water in the atmosphere. As humidity levels drop behind a cold front, this evaporation becomes more efficient, which can contribute to the perception of cooler air.
The correct answer to the student's question is that the air to the west of the cold front was cooler and drier. This is due to the fact that cold fronts typically bring in air masses that are not only cooler but also have lower humidity levels, which would result in a lower dew point.