🇺🇦 Українська

♨️ Steam & Condensation — Phase Transition

Water vapour molecules rise and cool. When temperature drops below the dew point Td, molecules lose enough kinetic energy to form liquid droplets — condensation. Relative humidity RH = e/es(T) where es is the saturation vapour pressure (Clausius-Clapeyron).

Scenario

Atmosphere

Stats

Dew point Td—
Cloud base—
Condensed drops0
Vapour molecules0

What this simulation shows

Vapour molecules (white dots) are emitted from the warm surface at the bottom. They rise against gravity with random Brownian motion. The background colour gradient shows the temperature — warm at the bottom, cold at the top. When a molecule's height causes the local temperature to drop below the dew point, it condenses into a droplet (blue dot) that grows as more molecules join it. Droplets large enough fall as rain (green).

The cloud base height is where saturation is reached: zLCL ≈ 125 · (T − Td) metres. Increasing humidity lowers the cloud base; increasing the lapse rate raises it.

Did you know?

The Clausius–Clapeyron equation describes how saturation vapour pressure doubles roughly every 10 °C. This is why a 4 °C warmer atmosphere can hold ~28% more water vapour — intensifying rainfall events in a changing climate.

About the Steam & Condensation Simulator

This simulation models how water vapour rises from a warm surface, cools with altitude and condenses into liquid droplets once it crosses the dew point. Around 200 vapour particles drift upward with Brownian motion against a vertical temperature gradient set by the environmental lapse rate. The dew point is found from the inverse Magnus formula, while saturation vapour pressure uses the Tetens approximation, an empirical fit to the Clausius–Clapeyron relation.

Three scenario buttons preset realistic conditions for fog or dew, a cloud base, and hot steam. The temperature, humidity and lapse-rate sliders then let you tune the atmosphere directly, and the panel reports the dew point, cloud-base height, droplet count and live vapour count. The same physics governs morning mist, cloud formation, the cooling tower at a power station and condensation forming on a cold window pane.

Frequently Asked Questions

What does this simulation actually show?

It shows water vapour molecules emitted from a warm surface rising into progressively colder air. White dots are vapour; when they cross the dew point they turn into blue droplets that grow and eventually fall. The background gradient represents temperature, warm at the bottom and cold at the top.

What is the dew point and how is it calculated?

The dew point is the temperature to which air must be cooled, at constant pressure, for water vapour to begin condensing. Here it is computed from the base temperature and relative humidity using the inverse Magnus formula, with coefficients a = 17.67 and b = 243.5 degrees Celsius.

What do the three controls do?

Base temperature sets the surface warmth from minus ten to fifty degrees Celsius. Humidity sets the relative humidity from ten to one hundred per cent, which fixes the dew point. The lapse rate, between two and twelve degrees Celsius per kilometre, sets how quickly air cools with height and therefore where condensation begins.

How is the cloud base height worked out?

The simulation uses the lifting condensation level approximation: height in kilometres is roughly 0.125 multiplied by the difference between temperature and dew point. In metres this is about 125 times that gap. A dashed blue line marks the level where rising air first becomes saturated.

What is the Clausius–Clapeyron equation?

It describes how the saturation vapour pressure of water rises sharply with temperature, roughly doubling every ten degrees Celsius. The simulator approximates it with the Tetens formula. This steep dependence is why warm air holds far more moisture than cold air.

Why do droplets form higher up rather than near the surface?

Near the warm surface the air temperature stays above the dew point, so vapour remains gaseous. As molecules rise, the lapse rate cools the surrounding air. Once the local temperature falls to or below the local dew point, the vapour loses enough kinetic energy to condense into liquid.

How does humidity change the result?

Higher relative humidity raises the dew point, so air needs less cooling before it saturates. That lowers the cloud base and produces condensation closer to the surface. At very high humidity, around ninety-five per cent, fog and dew form almost immediately.

Is this simulation physically accurate?

It is a qualitative, educational model rather than a research tool. The thermodynamic formulas for dew point, saturation pressure and lifting condensation level are standard meteorological approximations, but the particle motion, droplet growth and rainfall are simplified for clarity and visual effect.

What does the lapse rate represent in real weather?

The environmental lapse rate is the rate at which air temperature drops with altitude. The global average is about 6.5 degrees Celsius per kilometre. Steeper lapse rates make the upper air colder, raising the chance of condensation and influencing how high clouds form.

Where does this physics matter in the real world?

The same principles explain morning dew, valley fog, cloud and rain formation, and the visible plume from a cooling tower. They also govern everyday condensation on cold drinks and windows, and underpin weather forecasting and climate models of a moistening atmosphere.