Inputs
Formula used
Precipitable water (PW) is the liquid-water depth if all vapor in a column condensed.
PW_{m} = \frac{PW_{kg/m^2}}{\rho_w} \quad , \quad PW_{mm} = 1000\,PW_{m}
q = \frac{w}{1+w} \; (\text{when mixing ratio } w \text{ is provided})
q is specific humidity (kg/kg), \(\Delta p\) is pressure thickness (Pa), g is gravity, and \(\rho_w\) is water density.
How to use this calculator
- Select a method: layered profile is best for real soundings.
- Pick your pressure and humidity units.
- Enter 1–5 layers with bottom and top pressures.
- Enter humidity for each layer using your chosen unit.
- Click Calculate to view PW above the form.
- Use CSV or PDF export for reports and comparisons.
Example data table
| Layer | Bottom pressure (hPa) | Top pressure (hPa) | Specific humidity (g/kg) | Typical region |
|---|---|---|---|---|
| L1 | 1000 | 850 | 7.5 | Moist boundary layer |
| L2 | 850 | 700 | 5.0 | Lower troposphere |
| L3 | 700 | 500 | 2.0 | Mid troposphere |
| L4 | 500 | 300 | 0.6 | Upper troposphere |
| L5 | 300 | 200 | 0.2 | Near tropopause |
Professional article
1) Meaning of precipitable water
Precipitable water (PW) is the total mass of water vapor in a vertical air column, expressed as an equivalent liquid depth. Because 1 kg/m² equals 1 mm of water, PW is a direct, intuitive measure of column moisture available for clouds and rain.
2) Profiles matter more than averages
Most vapor sits in the lowest few kilometers, so near‑surface humidity can dominate PW. Mid‑level moisture still matters by supporting deeper clouds and higher precipitation efficiency. Layer inputs let you approximate a radiosonde or model profile with realistic vertical structure, not a single bulk value.
3) Discrete pressure integration
PW is computed from a pressure integral: PW = ∫ q · dp / g. With layer data, this becomes Σ(qi · Δpi / g). Example: for Δp = 15000 Pa and q = 0.006, the layer contributes about 9.2 kg/m², or 9.2 mm.
4) Unit handling and humidity types
Specific humidity q (kg/kg) is used in the physics. If you provide mixing ratio w, it is converted with q = w / (1 + w). Pressure can be entered in hPa or Pa, but Δp is always treated in Pa internally to keep the integration consistent and dimensionally correct.
5) Typical ranges and sanity checks
Very dry air masses can be below 10 mm. Mid‑latitude winter values often fall near 5–20 mm, while warm‑season columns commonly reach 25–50 mm. Tropical maritime environments can exceed 60–70 mm. If results look extreme, recheck units, layer order, and humidity magnitudes.
6) Main uncertainty sources
Errors usually come from missing surface layers, swapped top/bottom pressures, or entering g/kg as kg/kg. Large gaps between layers can also bias PW because moisture changes rapidly near the surface. Adding more physically meaningful layers reduces discretization error and makes the column closer to observations.
7) Forecasting and monitoring uses
PW supports diagnostics for heavy rain, atmospheric rivers, and convective environments. It is monitored using soundings, numerical analyses, and remote sensing such as GNSS and radiometers. PW does not guarantee rainfall, but it helps compare moisture availability across storms, seasons, and regions.
8) Reporting and repeatable comparisons
Use CSV export to build time series or compare multiple soundings, and PDF export for quick sharing. Keep unit settings consistent between runs and document your layer definitions (pressure bounds and humidity type). Consistent inputs make PW comparisons reproducible and easier to interpret alongside rain and radar data.
FAQs
1) What does PW measure in simple terms?
It measures total water vapor in a vertical column, expressed as the liquid-water depth you would get after condensation. It is a column moisture indicator, not a direct rainfall forecast.
2) Why do I see both mm and kg/m²?
They are equivalent for liquid water: 1 kg/m² equals 1 mm of water depth. The calculator reports both to match meteorological and hydrology conventions.
3) Should I use layered or single-layer mode?
Layered mode is better because humidity changes strongly with height. Single-layer mode can be useful for quick estimates when you only have a column-average value.
4) I have mixing ratio in g/kg. What should I select?
Select “g/kg (mixing ratio w)”. The tool converts w to specific humidity using \(q=w/(1+w)\) before integrating over pressure.
5) My PW seems too large. What should I check?
Confirm pressure units (hPa vs Pa), verify bottom pressure is larger than top pressure, and check whether humidity was entered in g/kg but interpreted as kg/kg. Also confirm near-surface layers are realistic.
6) How many layers do I need for good accuracy?
Two to three layers can capture broad structure, while four to five layers better represent sharp moisture gradients. More layers usually improve accuracy if each layer is physically meaningful.
7) Does high PW always mean heavy rain?
No. High PW indicates abundant moisture, but heavy rain also requires lift, condensation efficiency, and storm organization. Use PW alongside instability, convergence, and radar or satellite diagnostics.