Weather, Climate and Atmosphere
Structure and composition
The atmosphere is layered by temperature: troposphere (weather, environmental lapse rate about 6.5 °C/km), stratosphere (ozone layer), mesosphere and thermosphere. Air is about 78% nitrogen, 21% oxygen, with argon, carbon dioxide and water vapour. Insolation varies with latitude, season and albedo.
Pressure, winds and the ITCZ
Global pressure belts are the equatorial low (ITCZ), subtropical highs, subpolar lows and polar highs. Winds blow from high to low pressure and are deflected by the Coriolis force. The ITCZ migrates with the overhead sun, giving Malawi a single rainy season from November to April, when moist Congo air and relief enhance rainfall; May to October is dry under subtropical high influence.
Rainfall types
Convectional rain follows intense heating, rising air and afternoon thunderstorms. Relief or orographic rain occurs when moist air is forced over highlands, cools and rains on the windward slope, leaving a rain shadow. Frontal rain occurs where warm air rises over cold air at a front. Malawi's rain is mainly convectional and orographic; Mulanje and Nyika are wetter than the Shire Valley.
Instruments and charts
Rain gauge, barometer, thermometer, hygrometer, anemometer and wind vane. Synoptic charts use isobars, fronts and station models.
Worked example
Dedza (about 1,600 m) is wetter than Salima (about 500 m). Reasoning: moist easterly airflow is forced to rise over the escarpment, cools adiabatically, condenses and rains on the windward side; Salima lies in the rain shadow where descending air warms and dries.
Traps
Confusing weather with climate; saying the ITCZ causes frontal rain; misreading isobars (closer spacing means stronger wind); ignoring altitude and aspect.