Unit 4: Instrumentation and Observational Techniques

1. Conventional Measurements (Surface)

These are the standard instruments used at a surface weather station to measure the conditions we experience on the ground.

• Pressure: Measured with a Barometer.
  • Mercury Barometer: A long glass tube of mercury, very accurate, used as a standard.
  • Aneroid Barometer: A more common, mechanical device with a small, flexible metal box that expands/contracts with pressure changes.
• Temperature: Measured with a Thermometer, often housed in a louvered white box called a Stevenson Screen to shield it from direct sun.
• Humidity: Measured with a Hygrometer or Psychrometer. A psychrometer uses two thermometers: one "dry bulb" (measures air temp) and one "wet bulb" (covered in a wet cloth). The rate of evaporation (and thus cooling) on the wet bulb reveals the humidity.
• Wind Speed: Measured with an Anemometer (typically with rotating cups).
• Wind Direction: Measured with a Wind Vane (points into the wind).
• Sunshine Duration: Measured with a Campbell-Stokes recorder, which is a glass sphere that focuses sunlight onto a paper card, burning a trace.
• Radiation: Measured with a Pyranometer, which measures the total solar radiation (insolation) received.
• Clouds: Measured visually. Observers record the cloud type (e.g., cumulus, stratus) and cloud cover (measured in "Oktas," or eighths of the sky).

2. Upper Air Measurements

Weather is a 3D phenomenon, so measuring conditions in the upper atmosphere is essential for forecasting. This is done with a Radiosonde.

[Image of radiosonde (weather balloon) components]

• Radiosonde: This is a small, disposable instrument package attached to a large helium or hydrogen balloon.
• As the balloon rises, the radiosonde measures pressure, temperature, and humidity.
• It radios this data back to a ground station in real-time.
• By tracking the position of the balloon with GPS or radar, upper air wind speed and direction can also be calculated.
• Balloons are launched twice a day from hundreds of stations worldwide.

3. Application of Radars

RADAR stands for RAdio Detection And Ranging. It is one of the most important tools for "nowcasting" (short-term forecasting) of precipitation.

How it Works

1. A radar dish sends out a short pulse of microwave energy.
2. This pulse travels through the air until it hits an object (e.g., a raindrop, hailstone, or snowflake).
3. A small part of the energy (an "echo") is reflected back to the dish.
4. The radar measures two things:
   • Time Delay: How long the echo took to return. This gives the distance to the rain.
   • Echo Strength: How "bright" the echo is. This gives the intensity of the rain (light rain vs. heavy downpour).

Doppler Radar

Modern weather radars are Doppler radars. They can also detect the Doppler Effect (a shift in the frequency) of the returning echo.

• This shift reveals whether the precipitation is moving towards or away from the radar.
• This is crucial for seeing the *wind patterns* and *rotation* inside a storm, which is how we get warnings for tornadoes and cyclones.

[Image of Doppler weather radar schematic showing reflectivity and velocity]

4. SONAR

SONAR stands for SOund NAvigation and Ranging. It is the underwater equivalent of radar.

Sonar is used in oceanography to map the seafloor, locate shipwrecks, and find schools of fish. It is not used for atmospheric study.

5. Atmospheric Aerosols

Definition and Classification

Aerosols are tiny solid particles or liquid droplets suspended in the atmosphere. They are not gases.

• Natural Aerosols: Sea salt from ocean spray, mineral dust from deserts, pollen, smoke from wildfires, ash from volcanoes.
• Anthropogenic (Human-caused) Aerosols: Sulfates and nitrates from burning fossil fuels, soot (black carbon) from diesel engines.

Properties, Concentration, and Size

Aerosols have two major, competing effects on the climate:

1. Direct Effect (Climate):
   • Most Aerosols (like Sulfates): Are light-colored and reflect sunlight back to space. This has a cooling effect on the planet.
   • Some Aerosols (like Soot): Are dark and absorb sunlight. This has a warming effect on the atmosphere.
2. Indirect Effect (Weather):
   • Aerosols act as the "seeds" for cloud formation. Water vapor cannot condense to form cloud droplets without a particle to condense onto. These particles are called Cloud Condensation Nuclei (CCN).
   • The concentration and size distribution of aerosols determine the properties of clouds. For example, a high concentration of small aerosols can create clouds with many small droplets, which are brighter (reflect more) and less likely to rain.