Interactive explanations
See how moisture and heat move through a home
Adjust the conditions, rotate the models and follow the results. These illustrations connect everyday questions about windows, warm air and ventilation to the physics behind them.
Interactive illustration
When does a cold window collect water?
Change the room conditions and the glass temperature independently. The useful comparison is between the surface temperature and the air’s dew point.
Open the 3D view to rotate the model. The diagram and calculations work independently of it.
- Estimated dew point
- 12°C53.6°F
- Window surface
- 10°C50°F
- Surface comparison
- Saturation reached
Window surface is below the dew point
At 10°C, the chosen surface is colder than the air’s estimated 12°C dew point, so condensation is possible. This comparison does not predict the amount of water.
Explore the model
Room air
The air temperature and relative humidity together determine this example’s dew point. The arrows indicate air movement only; their path and speed are not calculated.
Cold window
The surface has its own temperature. A room reading does not tell you how cold the glass is. Compare the chosen surface temperature with the dew point.
Water on the glass
Below the dew point, water vapor can condense on the surface. The drawn droplets mark that condition; their number does not represent an amount of water or a drying time.
What the illustration assumes
These are entered examples, not measurements of your room or an appliance. The calculation uses a liquid-water dew-point approximation and a single nonfreezing surface. It does not model frost, airflow, drying time or building moisture sources.
The “near the dew point” band is within 1°C on either side. It is a teaching aid, not a sensor accuracy rating or a safe operating limit. “Saturation reached” describes the condensation boundary; we do not display stable surface humidity above 100%.
Changing temperature while holding RH fixed selects a new air state. The warming-only button changes both together: it assumes constant pressure with no water added or removed, and leaves the window temperature unchanged. Calculations retain the unrounded values.
Arrows, droplets, colors and animation speeds are schematic. See the NPL explanation of dew point and relative humidity.
Interactive illustration
Follow air, heat and water through a dehumidifier
Explore a compressor dehumidifier’s separate air, refrigerant and water paths. Change the incoming air and cold-coil temperature to see when condensation becomes possible.
Open the 3D view to rotate the model. The diagram and calculations work independently of it.
- Estimated dew point
- 12°C53.6°F
- Cold-coil surface
- 8°C46.4°F
- Surface comparison
- Saturation reached
Cold-coil surface is below the dew point
At 8°C, the chosen surface is colder than the air’s estimated 12°C dew point, so condensation is possible. This comparison does not predict the amount of water.
Explore the model
Cool evaporator
Air first crosses the evaporator. A coil surface below the entering air’s dew point permits condensation. Changing this one temperature illustrates the condition for condensation, not how many pints a machine will collect.
Warm condenser
The air then crosses the condenser and is reheated. The refrigerant releases heat here, including energy absorbed at the evaporator and compressor work. No outlet temperature is predicted by this illustration.
Fan and air path
The fan moves room air across the coils. The arrows trace a schematic air path; fan position and the arrangement of parts vary by appliance. Animation speed is not a measured airflow.
Compressor and refrigerant loop
Refrigerant circulates in a sealed loop through the compressor, condenser, pressure-reducing device and evaporator. It stays separate from the room air and the water collected in the bucket.
Bucket or drain
Water condensed at the evaporator falls into a collection path and is held in a bucket or sent to a suitable drain. The dashed hose shows a possible drainage route, not installation instructions for a particular model.
What the illustration assumes
These are entered examples, not measurements of your room or an appliance. The calculation uses a liquid-water dew-point approximation and a single nonfreezing surface. It does not model frost, airflow, drying time or building moisture sources.
The “near the dew point” band is within 1°C on either side. It is a teaching aid, not a sensor accuracy rating or a safe operating limit. “Saturation reached” describes the condensation boundary; we do not display stable surface humidity above 100%.
The model does not calculate an outlet temperature, fan flow, refrigerant pressure, power use or water-removal rate. Coil and fan arrangements vary between real products. Arrows, droplets, colors and animation speeds are schematic. See the NPL explanation of dew point and relative humidity and Dantherm’s explanation of the dehumidifier air path.
Indoor and outdoor air
Would outside air bring in less moisture?
Compare two air samples at the same pressure. Try a cold, humid day and a warm day with a lower outdoor humidity percentage.
Moisture comparison
Outdoor air contains less moisture
At 50°F and 90% RH, the outdoor air contains less water vapor per mass of dry air than indoor air at 68°F and 60% RH. Bringing it in offers a way to dilute indoor moisture.
Indoor dew point: 53.6°F. Outdoor dew point: 47.2°F.
What this comparison can tell you
The bars compare water vapor per kilogram of dry air at 101.325 kPa, approximately sea-level atmospheric pressure. Temperature inputs cover 32–104°F (0–40°C); relative humidity covers 5–100%.
The calculation uses NPL’s liquid-water Magnus relationship and the ideal-mixture humidity-ratio equation. Differences within 0.2 g/kg are labeled similar: that is a display convention, not a statement of sensor accuracy.
Drier outdoor air can remove moisture as it replaces indoor air. This tool does not estimate airflow, a drying time, the eventual indoor humidity or the heating and cooling cost. Rain, outdoor air quality and the home’s ventilation needs still matter.
How the models work
The illustrations are educational models, not simulations of a particular home or appliance. The calculations use NPL’s published vapor-pressure and dew-point relationships and ASHRAE’s ideal-mixture humidity-ratio equation. Read each model’s explanation for its input limits and assumptions.
The three linked guides give worked examples, explain the heat balance and cite the supporting sources. Sources and calculations checked November 1, 2026. Geometry, colors and any moving droplets are schematic; they do not represent measured airflow or water collection.
The 3D views use Three.js, served locally under its MIT license. Choose a diagram whenever you prefer a still illustration.