Kelvin is the SI base unit of thermodynamic temperature and starts at absolute zero (the coldest possible temperature, −273.15°C). Meteorologists and atmospheric scientists use Kelvin in thermodynamic equations (e.g., the ideal gas law, radiative transfer) because these equations break down with negative temperatures on the Celsius scale. Weather reports use °C or °F for public communication, but the underlying science relies on Kelvin. Note that temperature differences are the same in Kelvin and Celsius (a change of 1 K = a change of 1°C).

Standard sea-level atmospheric pressure is 1013.25 hPa (equivalent to 1013.25 mb, 29.92 inHg, or 14.696 psi). Pressure decreases roughly 12 hPa per 100 meters of altitude. High-pressure systems (above ~1020 hPa) typically bring fair weather; low-pressure systems (below ~1000 hPa) indicate storms. Tropical cyclone centers can drop below 900 hPa. In agriculture, rapid pressure drops of 6–8 hPa over 3 hours signal incoming storms and should trigger irrigation system and equipment preparation.

The UK Met Office and WMO define rainfall intensity as: Light rain — < 2.5 mm/hr. Moderate rain — 2.5–7.6 mm/hr. Heavy rain — > 7.6 mm/hr. Very heavy (cloudburst) — > 50 mm/hr. For daily totals: light < 10 mm/day, moderate 10–25 mm/day, heavy 25–75 mm/day, very heavy > 75 mm/day. In agriculture: 25 mm (1 inch) of rain in a day often replenishes a week's irrigation needs for most crops in summer; > 50 mm can saturate soils and cause ponding or runoff.

Wind chill (apparent temperature) accounts for how wind increases the rate of heat loss from exposed skin. The NWS formula (valid for T ≤ 50°F / 10°C, wind ≥ 3 mph): Wind Chill = 35.74 + 0.6215T − 35.75(V°0.16) + 0.4275T(V°0.16), where T is temperature in °F and V is wind speed in mph. At 28°F (−2°C) with 20 mph winds: wind chill ≈ 14°F (−10°C). Wind chill has no effect on inanimate objects — pipes freeze at the actual air temperature, not the wind chill equivalent.

Relative humidity (RH) measures how much water vapor is in the air as a percentage of the maximum possible at that temperature. At 70°F (21°C): RH of 80% feels muggy and uncomfortable — sweat evaporates slowly, making it harder to cool down. At 95°F (35°C) with 80% RH, the heat index is approximately 130°F (54°C) — dangerously hot. For agriculture: spraying pesticides or herbicides above 80% RH increases drift risk and may reduce efficacy; fungal disease pressure (gray mold, powdery mildew) rises sharply above 80% RH sustained overnight.