Weather Humidity and Cloud Chamber: Two Case Studies in Atmospheric Vapor Pressure and Particle Tracks

MIT OpenCourseWare’s lecture presents a two-case study: first, a data-driven exploration of humidity, dew point and vapor pressure in the atmosphere; second, an introduction to cloud chambers that visualize ionizing radiation tracks. The weather case defines relative humidity, dew point, and saturation vapor pressure, then shows how Weather Underground data can be plotted to reveal the daily evolution of moisture and dew formation. The particle-physics case introduces cloud chambers as expansion instruments that create visible clouds around ionized particles, highlighting historic tracks such as the positron in a cloud chamber. The session emphasizes data manipulation, modeling, and the long-standing link between everyday weather phenomena and fundamental physics.

Introduction

MIT OpenCourseWare delivers a two-case exploration that begins with atmospheric humidity and dew point, then shifts to cloud chambers in particle physics. The teacher defines core concepts, uses a real-data workflow, and frames the cloud chamber as a bridge between meteorology and high-energy physics. The aim is to show how simple models and careful data interpretation illuminate both the everyday world and the invisible realms of radiation tracks.

Weather Case Study: Humidity, Dew Point and Vapor Pressure

The lecture starts with a precise definition of relative humidity (RH) as the water vapor pressure divided by the saturation vapor pressure, typically expressed as a percent. A dew point is described as the temperature at which water vapor would condense given the current vapor pressure, a concept commonly seen in weather reports but with deeper thermodynamic meaning. The dew point is the temperature where the current water vapor pressure equals the saturation vapor pressure, and condensation occurs when the air cools to that point. The instructor motivates this with a grass-dew intuition: when the air temperature drops below the dew point, dew forms as water condenses spontaneously from the vapor phase.

The data example uses Weather Underground data for a typical day, showing a time axis in hours and multiple curves for temperature, dew point, humidity, wind, and pressure. The temperature trace follows a normal diurnal pattern: cooler morning, warmer afternoon, then cooling at night. The dew point gradually rises through the day, signaling increasingly humid air. The lecturer plots two key curves: the saturation vapor pressure as a function of temperature (derived from a model) and the actual vapor pressure inferred from the dew point. He clarifies how to obtain the blue curve from the dew point, and how relative humidity is computed as the ratio of actual to saturation vapor pressure. The visualizations illustrate how dew formation is favored when the daily temperature trajectory and moisture input align to push the air toward saturation, which tends to occur more readily on summer nights than in winter. The discussion ends with a reminder that the analysis is broadly useful beyond weather, including materials processing where vapor pressure and subsaturation matter.

"These concepts of vapor, which is subsaturation, are really important for materials processing and pretty much everywhere else." - MIT OpenCourseWare

Cloud Chamber and Particle Physics: Visualizing the Invisible

The lecture then pivots to cloud chambers, devices used to visualize tracks left by ionizing particles in humid gas. A concise two-minute primer explains that cloud chambers are expansion apparatuses, creating supersaturated conditions that trigger condensation on imperfections or ionized molecules carried by radiation. The volume is filled with humid air, and upon rapid expansion into a vacuum, nucleation occurs around ionizing tracks, revealing visible cloud trails. This classic technique, pioneered by Charles Wilson, earned a Nobel Prize and provided direct visual evidence of charged particle paths. The lecturer presents historical images and videos of cloud chambers, including the first published positron track observed by Carl Anderson, to illustrate how radiation becomes visible through condensation patterns.

Various demonstrations show continuous cloud-chamber activity with different saturating media, such as ethanol in a dry-ice–cooled chamber, producing bright, intricate tracks produced by cosmic rays or thorium sources. The aesthetic beauty of the tracks is framed as a powerful teaching tool for understanding how ionizing radiation interacts with matter and how magnetic fields bend charged particles, enabling experimental discrimination of particle species. The overall point is that visualization enhances comprehension of otherwise invisible phenomena and creates connections between early 20th-century physics and modern scientific techniques.

"Cloud chambers are an expansion apparatus for making visible the tracks of ionizing particles and gases." - MIT OpenCourseWare

From Visualization to Problem Solving: The Cloud Chamber Challenge

In the final portion, the instructor sets up a concrete problem: a cylinder with humid air at 298 K and 1 atm expands adiabatically, with the dew point given as 288 K. The control parameter is the fractional volume change delta V over V_initial. The task is to determine the expansion needed to reach saturation. The solution requires combining several concepts: Dalton’s law for vapor pressure, the dew point relation with saturation vapor pressure, and the adiabatic relation between temperature and volume with gamma for diatomic air. The partial pressure of water vapor is P_H2O = x_H2O P_total, where x_H2O is the mole fraction of water and P_total scales with volume via adiabatic cooling. The saturation pressure is modeled as P_sat = C e^{-B/T}, with T tied to volume through the adiabatic relationship. By parameterizing everything in terms of delta V/V_initial and assuming diatomic air, the problem can be solved numerically or graphically to find the expansion required to reach saturation. The instructor emphasizes that this type of thermodynamic reasoning links unary phase diagrams, gas mixtures, and practical humidity considerations, and it serves as a bridge to Dalton’s law of partial pressures that will be explored in Friday's session.

The discussion also highlights the pedagogical value of fast, adiabatic processes as approximations to real systems, noting that heat transfer is negligible on short timescales and that this assumption underpins many engineering analyses. The weather example and cloud-chamber problem together illustrate how a consistent thermodynamic framework can illuminate both everyday phenomena and fundamental physics. The lecture closes with an invitation to questions and a note that these topics have broad relevance across materials science, meteorology, and high-energy physics.

Quotes and insights from the session emphasize the cross-cutting role of vapor dynamics in science and engineering, and the power of simple models to illuminate complex systems.