Campbell-Stokes Recorder

The Campbell–Stokes Recorder is a meteorological instrument used to measure the duration of sunshine during a given period, typically over a day. It operates by focusing the sun’s rays onto a specially prepared card strip, where the concentrated light burns a trace that indicates the total number of sunshine hours. The instrument has been a standard device in weather observation worldwide since the late nineteenth century, valued for its simplicity, durability, and visual clarity of results.

Historical Background

The Campbell–Stokes Recorder was developed from earlier designs by John Francis Campbell in 1853 and subsequently improved by Sir George Gabriel Stokes in 1879. Campbell’s original version consisted of a glass sphere that concentrated sunlight onto a recording card, but Stokes refined the design by introducing a more accurate and durable metal frame and card holder system.
The British Meteorological Office adopted the Campbell–Stokes Recorder as its standard sunshine recorder in the late nineteenth century, and it has since become widely used across the world by national meteorological agencies.

Construction and Components

The instrument consists of three principal components:

1. Glass Sphere:
   • Made of solid, transparent glass, usually with a diameter of about 10 cm.
   • Functions as a lens that focuses incoming solar radiation onto a small point.
   • The position of the focused point varies with the apparent movement of the sun across the sky.
2. Metal Bowl or Frame:
   • The glass sphere is mounted securely in a brass or bronze bowl with a concave shape that holds a specially calibrated card.
   • The bowl has grooves designed to hold the recording cards in different positions according to the solar altitude and season (summer, equinox, and winter).
3. Sunshine Card:
   • A thick, heat-sensitive cardboard strip graduated with time markings (usually in hours).
   • As the sun moves, the focused beam scorches a continuous or broken burn trace along the card.
   • The length of the burn line corresponds to the duration of bright sunshine.

Principle of Operation

The Campbell–Stokes Recorder works on the principle of solar concentration and thermal recording. When direct sunlight falls on the glass sphere:

• The sphere acts as a convex lens, focusing solar radiation onto a narrow line on the card placed behind it.
• As the Earth rotates, the sun’s position changes, and the point of focus moves gradually across the card.
• Whenever the sunlight is strong enough (above approximately 120 W/m² of direct solar radiation), it burns or scorches the card’s surface.
• When the sun is obscured by clouds, the burning ceases, producing a gap in the trace.

By measuring the total length of the burn trace, observers can determine the total hours of bright sunshine for the day.

Types of Recording Cards

Because the sun’s altitude varies seasonally, the angle at which the focused rays strike the card also changes. To accommodate this, the recorder uses three different types of cards:

1. Summer Card (High Sun): Used when the sun is high in the sky, typically from April to August in the Northern Hemisphere.
2. Equinoctial Card (Mid Sun): Used around the equinox periods, from February to April and from August to October.
3. Winter Card (Low Sun): Used when the sun is low in the sky, typically from October to February.

Each card fits into a differently angled slot in the metal bowl to ensure accurate recording of the sun’s path.

Installation and Exposure

For accurate observation, the Campbell–Stokes Recorder must be installed:

• On a solid, level base, such as a stone or concrete pillar.
• In an open area free from obstructions like trees, buildings, or towers, to ensure uninterrupted sunlight.
• With the bowl oriented precisely north–south, so that the time markings correspond correctly to the apparent solar movement.

Regular cleaning of the glass sphere is essential to prevent dust and dirt from reducing solar intensity and affecting burn clarity.

Interpretation of Records

At the end of each day, the observer examines the burn trace on the card. The process involves:

1. Identifying continuous and broken sections corresponding to periods of direct sunlight and cloud cover.
2. Measuring the total burnt length using a scale printed on the card, which represents time in hours and minutes.
3. Recording the sunshine duration as the sum of all burned segments for the 24-hour period.

In modern meteorological practice, sunshine duration is defined as the period during which direct solar irradiance exceeds 120 W/m², a threshold approximated well by the Campbell–Stokes Recorder.

Advantages

The Campbell–Stokes Recorder has remained in widespread use for over a century due to several practical advantages:

• Simplicity: No moving parts or electrical components; it functions purely through solar energy.
• Durability: Constructed from robust materials capable of withstanding harsh outdoor conditions.
• Low maintenance: Requires minimal attention apart from occasional cleaning and card replacement.
• Visual record: Produces a permanent physical trace that can be archived and re-examined.
• Field reliability: Suitable for use in remote locations without access to power or electronic instruments.

Limitations

Despite its reliability, the instrument also has certain drawbacks:

• Subjectivity: Reading the burn trace involves human interpretation, which may vary slightly between observers.
• Sensitivity to weather: In hazy or partially cloudy conditions, weak sunlight may not produce a burn even though it would be measurable by electronic sensors.
• No measurement of solar intensity: It records only duration, not the total energy received.
• Wear and tear: Prolonged use can dull the sphere’s surface or deform the card holder, affecting accuracy.
• Seasonal adjustments: Requires manual replacement of cards and repositioning according to the solar altitude.

Modern Developments

In contemporary meteorology, automatic sunshine duration sensors and pyranometers have gradually replaced the Campbell–Stokes Recorder in many stations. Modern instruments use photoelectric cells or silicon sensors to detect direct solar radiation digitally, providing continuous, objective, and high-resolution data.
Nevertheless, the Campbell–Stokes Recorder continues to be used in many parts of the world for historical consistency and long-term climatological comparison. Its simplicity and low cost make it particularly useful in developing regions and educational settings.

Geographical and Climatological Significance

Data collected using the Campbell–Stokes Recorder have contributed greatly to understanding global sunshine distribution and climate classification. Sunshine duration records are essential for:

• Determining solar radiation potential for agriculture and renewable energy studies.
• Analysing climate variability and urban air pollution effects on solar exposure.
• Assessing human comfort indices and crop productivity in relation to sunlight availability.