Question

The temperature of the areas surrounding Farmington before Storm 1 and before Storm 2 was about 8°C, and there was the same amount of water vapor in the air.

Storm 1 had more rainfall than Storm 2.

Farmington Before Storm 1: Photograph of farmland on a clear day, with a blank response box containing a question mark. The air temperature is 8° Celsius. Before Storm 2: The same scene with an air parcel in place of the blank response box. The temperature inside the air parcel is 19° Celsius and outside is 8° Celsius. Arrows labeled wind frame the air parcel, pointing inwards.
What were the conditions before Storm 1, and why did more rain fall with those conditions?


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Answer 1

Before Storm 1 the air over Farmington was cool (about 8 °C) and, because the total amount of water vapor was the same as before Storm 2, the air was much closer to saturation (higher relative humidity).

Why that produced more rain
- Saturation vapor pressure rises rapidly with temperature. For the same absolute water vapor, air at 8 °C has a much higher relative humidity than air at 19 °C. (Roughly speaking, the saturation vapor pressure at 8 °C is ~8 hPa while at 19 °C it’s ~22 hPa, so the same vapor corresponds to a much higher %RH at 8 °C.)
- High relative humidity means only a small additional cooling or lifting is needed to reach the dew point, so condensation and cloud/raindrop formation begin sooner and more strongly.
- In contrast, the 19 °C parcel before Storm 2 was relatively dry (low RH) for its temperature, so it had to cool much more during ascent to reach saturation; less condensation occurred and therefore less rainfall resulted, even if there was some upward motion.

In short: the cooler, near‑saturated conditions before Storm 1 allowed condensation to start earlier and more efficiently, producing heavier rainfall.