About

Drought is a creeping phenomenon that significantly impacts agricultural yields and water resource management. Early detection of precipitation deficits allows farmers to adjust irrigation schedules and crop selection. This tool calculates the Percent of Normal (PN) index, a widely used simplistic drought indicator that compares current rainfall to historical averages (climatological normals).

While professional meteorologists use complex SPI (Standardized Precipitation Index) models involving gamma distributions, the PN index provides an immediate, accessible snapshot of hydrological stress relative to local expectations.

Formulas

The Percent of Normal index is calculated as:

PN Index:

PN = P_currentP_normal × 100

Where P_current is the accumulated precipitation for the period, and P_normal is the long-term average (typically 30 years) for the same period.

Reference Data

PN Index (%)	Classification	Agricultural Impact
> 150%	Extremely Wet	Flood risk, root rot, delayed planting.
110 - 150%	Moderately Wet	Good for water storage, potential fungal issues.
80 - 110%	Normal	Optimal growth conditions.
50 - 80%	Moderate Drought	Stress on shallow-rooted crops (e.g., corn, veg).
25 - 50%	Severe Drought	Yield loss expected, irrigation required.
< 25%	Extreme Drought	Crop failure, aquifer depletion.

Frequently Asked Questions

In meteorology, a "Normal" is typically the 30-year average of a weather variable for a specific location. You can find this data from your local meteorological service or agricultural extension office.

Yes, but you should use the "Liquid Water Equivalent" (LWE) of the snowfall, not the snow depth. A general rule of thumb is 10 inches of snow equals 1 inch of liquid water, but this varies by temperature.

For agriculture, checking monthly or at critical growth stages (germination, flowering, grain fill) is most effective. Short-term deficits during flowering can be more damaging than long-term deficits during dormancy.