🌦️ Weather Systems for Agriculture

What you’ll learn

1. Weather Is a Schedule, Not a MoodEstablish the through-line: a farm responds to a few measurable variables (heat, cold, water), and distinguish weather from climate.A grower reads weather as measurable quantities — accumulated heat, minimum temperature, and the soil-water balance — not as a mood. Climate is the long-run average that sets the growing season (bounded by frost dates) and is planned against; weather is the day-to-day reality reacted to. The course teaches reading heat, cold, and water.
2. Counting Heat: Growing Degree DaysTeach growing degree days: base and cap temperatures, the daily calculation, and the season heat budget.Crops develop by accumulated heat, not calendar days. Corn GDD = (Tmax+Tmin)/2 − 50, with the high capped at 86°F and the low floored at 50°F. Warm summer days bank ~18–24 GDD; a ~110-day hybrid needs ~2,670 GDU to maturity. Hybrids are sold by GDU so growers match a variety to their region's reliable heat.
3. The Killing Cold: FrostDistinguish radiation from advection frost, explain radiation-frost formation, and match each to its defence.Radiation frost forms on clear, calm nights as the ground radiates heat to space and cold air settles unmixed at the surface; it is local, overnight, and fightable with wind machines or sprinklers (latent heat). Advection frost is an incoming cold air mass that freezes everything for days regardless of sky or wind — countered only by hardy crops.
4. The Water Budget: EvapotranspirationModel soil water as an account with rainfall/irrigation deposits and evapotranspiration withdrawals; give ET rates.The root zone is a water account: rain and irrigation are deposits, evapotranspiration (soil evaporation plus leaf transpiration) is the daily withdrawal, and drainage/runoff is lost. Reference ET0 runs ~2–4 mm/day in mild weather and 5–8 mm/day when hot, dry, and windy; actual crop use is ETc = ET0 × Kc. Scheduling irrigation is arithmetic.
5. When the Rain Comes and Doesn't: Timing and DroughtShow that rainfall timing beats rainfall total, define agricultural drought, and note seasonal forecasting.Two seasons with equal total rainfall can yield feast or famine depending on timing: rain in sensitive windows (like maize pollination) matters far more than one early deluge that runs off. Agricultural drought is a shortfall at the wrong moment, not just low annual totals. Seasonal forecasts and ENSO patterns shift the odds but remain probabilistic.
6. Reading the Land: MicroclimateExplain microclimate: cold-air drainage and frost pockets, slope aspect, and dew point as a frost warning.Land reshapes weather into microclimates. On calm, clear nights dense cold air drains downhill and pools in hollows, forming frost pockets several degrees colder than slopes; sun-facing slopes bank more heat. Dew forms by the same radiative cooling, and a small temperature–dewpoint spread near freezing warns of frost risk that night.
7. Violent Weather and the Farmer's PlaybookContrast scheduling (heat/cold/water) with risk management (hail/wind), and resolve the through-line.Sudden, severe hazards — hail, damaging wind, deluges — are low-probability, high-severity events managed by spreading and transferring risk: diversification, sturdy varieties, dodging forecast days, and insurance. Gradual variables are managed by scheduling. The farmer converts each weather signal into a decision, changing what the weather can do to the crop.

Questions this course answers

Grounded in trusted sources

• Corn growing degree days: base 50°F (10°C), cap 86°F (30°C); GDD = (Tmax+Tmin)/2 − 50 — Purdue Extension: https://www.agry.purdue.edu/ext/corn/news/timeless/hybridmaturity.html
• A ~110-day corn hybrid needs ~2,670 GDU to physiological maturity (black layer); ~18–24 GDD accrue on warm summer days — Ohioline (OSU) AGF-101: https://ohioline.osu.edu/factsheet/agf-101
• Reference evapotranspiration (ET0) ~2–4 mm/day in mild periods, 5–8 mm/day when hot, windy and sunny; ETc = ET0 × Kc — FAO Irrigation & Drainage Paper 56: https://www.fao.org/4/x0490e/x0490e00.htm
• USGS/USDA: radiation vs advection frost and protection methods — general agricultural meteorology references

Every Wunder lesson is built from real, reputable sources — never invented.

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