Earth Science: Weather and the Environment
Weather questions ask you to read a 天気図 or a table of 飽和水蒸気量 and then explain the result with air movement.
Humidity and cloud
- 湿度 = 水蒸気量 ÷ その気温の飽和水蒸気量 × 100 (%).
- 露点 is the temperature at which the air's water vapour equals the saturation value.
- Rising air expands, cools adiabatically, reaches its dew point and condenses on 凝結核 to form cloud. Cooling by contact with a cold surface gives fog.
Pressure and fronts
- Air flows from high to low pressure; closely spaced 等圧線 mean a steep pressure gradient and strong wind.
- 寒冷前線: steep, brief heavy rain, then a sharp temperature drop and a wind shift to north-west. 温暖前線: gentle, prolonged rain, then warming. 停滞前線 (梅雨前線, 秋雨前線) sits over Japan for weeks.
- Winter: the north-west 季節風 picks up moisture over the Sea of Japan and drops snow on the Japan Sea side. Summer: the 太平洋高気圧 brings hot, humid air.
Worked example. At 20 °C the saturation value is 17.3 g/m³ and the air holds 10.4 g/m³. Humidity = 10.4 ÷ 17.3 × 100 ≈ 60 %. The table gives 10.0 g/m³ at 11 °C and 10.7 g/m³ at 12 °C, so the 露点 is about 11–12 °C. Cooled to 10 °C, about 0.4 g/m³ condenses per cubic metre.
Traps. Reporting humidity above 100 % after cooling (the vapour content falls once condensation begins); saying warming raises humidity when warming actually lowers it because the saturation value grows; reading front symbols backwards.