Chapter 8 Weather Instruments, Maps And Charts

Weather refers to the state of the atmosphere at a specific place and time, defined by elements such as temperature, pressure, wind, humidity, precipitation, and cloudiness. These conditions can change rapidly. Accurate information about weather elements is crucial for understanding current weather patterns and making forecasts.

Weather maps and charts are prepared daily by meteorological departments, synthesizing data collected from weather stations globally. In India, weather-related information is collected, published, and used for forecasting by the Indian Meteorological Department (IMD), headquartered in New Delhi (established in 1875, initially at Calcutta). Weather forecasts are vital for various sectors like agriculture, shipping, aviation, and defense, enabling advance planning and safety measures in case of adverse weather.

Glossary terms introduced in the text:

• Weather: Atmospheric conditions at a given place and time regarding pressure, temperature, humidity, precipitation, cloudiness, and wind.
• Weather Forecast: Prediction of future weather conditions in an area for a specific short period.

Weather Observations

Meteorological data is collected at different levels globally to get a comprehensive picture of the atmosphere. These observation efforts are coordinated by the World Meteorological Organization (WMO), a UN agency.

• Surface Observatories: Measure weather elements at or near the ground level.
• Upper Air Observatories: Collect data from higher levels of the atmosphere (e.g., using weather balloons, radiosondes).
• Space-based Observation Platforms: Satellites providing observations from space.

Surface Observatories

Surface observatories are equipped with instruments to measure key weather elements. Observations are taken at globally fixed times (e.g., 00, 03, 06, ... GMT) using internationally standardized instruments and procedures to ensure data comparability. India has a network of observatories classified into categories based on instrumentation and observation frequency (Class-I being the highest). Typical instruments in a Class-I observatory include: maximum and minimum thermometers, anemometer (for wind speed) and wind vane (for wind direction), dry and wet bulb thermometers, rain gauge, and barometer.

Space-based Observations

Weather satellites provide broad and systematic observations of meteorological elements from space. Geostationary satellites, orbiting at about 36,000 km altitude and appearing stationary over a fixed point on the equator, provide continuous monitoring of a large area (e.g., India's INSAT series provides data on temperature, clouds, winds, and weather phenomena). Sun-synchronous polar-orbiting satellites (700-900 km altitude) cover the entire globe over successive passes, providing detailed data for specific areas.

Weather Instruments

Various instruments are specifically designed to measure different weather elements:

Thermometer

Used to measure air temperature. Consists of a sealed glass tube with a liquid (mercury or alcohol) that expands and contracts with temperature changes, indicated on a scale. Common scales are Centigrade ($0^\circ C$ freezing, $100^\circ C$ boiling) and Fahrenheit ($32^\circ F$ freezing, $212^\circ F$ boiling).

• Maximum Thermometer: Records the highest temperature reached during a specific period, usually a day. It has a constriction that prevents the mercury from falling back when the temperature drops, holding the highest reading (Figure 8.1).

Diagram illustrating a maximum thermometer, showing the constriction in the tube that holds the mercury at the highest temperature reached.

• Minimum Thermometer: Records the lowest temperature reached. Typically uses alcohol and has a small index marker that is pulled down by the alcohol column as temperature falls, but stays in place when temperature rises, indicating the minimum reading (Figure 8.2).

Diagram illustrating a minimum thermometer, showing the marker within the alcohol column that records the lowest temperature reached.

• Dry Bulb and Wet Bulb Thermometers: Used together to measure air humidity (Figure 8.3). Two identical thermometers are mounted side-by-side in a Stevenson Screen (Box 8.2). The 'dry bulb' measures the actual air temperature. The 'wet bulb' has its bulb wrapped in moist muslin cloth. Evaporation from the wet cloth cools the wet bulb. The rate of evaporation depends on the amount of water vapor (humidity) in the air.
  • High humidity: Less evaporation, smaller temperature difference between dry and wet bulbs.
  • Low humidity (dry air): More evaporation, larger temperature difference.
  The difference in readings is used with psychrometric tables to determine relative humidity and dew point.

Diagram illustrating a pair of dry and wet bulb thermometers, used together to measure humidity.

Box 8.2 describes the Stevenson Screen, a white, louvered wooden box designed to house thermometers to protect them from direct sunlight and precipitation while allowing air circulation, providing an accurate measure of ambient air temperature. It is mounted on legs about 1.07m (3 ft 6 in) above the ground and its door faces away from the sun (north in Northern Hemisphere, south in Southern Hemisphere) to prevent direct sunlight when opened for reading.

Barometer

Used to measure atmospheric pressure. Air has weight and exerts pressure on the Earth's surface. This pressure varies with altitude, temperature, and humidity. Common types include:

• Mercury Barometer: An accurate standard instrument. Atmospheric pressure is balanced against the weight of a column of mercury in a glass tube. At sea level, the average atmospheric pressure supports a mercury column about 76 cm high (Figure 8.4).

Diagram illustrating the principle of a mercury barometer, showing how atmospheric pressure supports a column of mercury in a tube.

• Aneroid Barometer: A portable instrument that measures pressure without liquid (Figure 8.5). It uses a sealed metal box with a flexible lid, partially emptied of air. Changes in atmospheric pressure cause the lid to move, and a system of levers magnifies this movement to drive a pointer on a dial calibrated in pressure units (millibars).

Diagram illustrating the components and working principle of an aneroid barometer, using a flexible metal box.

• Barograph: A recording barometer based on the aneroid principle. It uses multiple vacuum boxes and a lever system to move a pen that traces a continuous record of pressure changes on a chart attached to a rotating drum.

Atmospheric pressure is typically measured in millibars (mb).

Wind Vane

A device used to measure the direction of the wind. It is a lightweight pivotable object (usually with an arrowhead and tail fins) mounted on a rod. Wind causes the vane to rotate freely. The arrowhead points into the wind, indicating the direction from which the wind is blowing (Figure 8.6). Directions are expressed using cardinal points (North, South, East, West) or degrees.

Diagram illustrating a wind vane, a weather instrument used to show the direction from which the wind is blowing.

Rain Gauge

An instrument used to measure the amount of liquid precipitation (rainfall) over a specific period. It typically consists of a collecting funnel with a standard diameter (commonly 20 cm) that channels rainfall into a measuring glass or cylinder. Rainfall is measured in millimeters (mm) or centimeters (cm) that have accumulated in the collector (Figure 8.7). Snowfall is measured by melting the snow collected and measuring the equivalent water amount.

Diagram illustrating a rain gauge, an instrument used to collect and measure the amount of rainfall.

Weather Maps And Charts

Weather maps (also called synoptic maps) are graphical representations of weather conditions over a specific area at a particular time. They depict various weather elements and patterns, such as temperature, pressure, winds, precipitation, and cloudiness. Data from weather observatories taken at fixed times are plotted onto these maps using standardized symbols and lines. Weather charts, often synoptic weather charts, are the primary tools used by meteorologists for weather forecasting. They help in identifying and tracking air masses, pressure systems (highs and lows), fronts, and areas of precipitation.

The Indian Meteorological Department prepares and publishes daily weather maps and charts based on data collected from its observatories across India, from ships at sea, and increasingly from satellites and other platforms. This data, often transmitted in code, is processed and plotted to create synoptic charts.

Weather Symbols

To standardize the representation of weather data on maps, meteorologists use a system of conventional weather symbols. These symbols represent various weather elements and phenomena (e.g., cloud cover, wind speed/direction, precipitation type/intensity, current weather conditions) and are standardized by organizations like the World Meteorological Organization (WMO). Each symbol is plotted around a station circle on the map at a fixed position relative to the location of the weather observatory, allowing for a quick and clear representation of the complete weather observation report for that station (Figures 8.8 and 8.9 show some examples of meteorological symbols and wind speed indicators).

Examples of standard meteorological symbols used on weather maps to represent various weather conditions and phenomena.

Illustrations and table showing symbols used to represent wind speed (often in knots or km/hr) and a description of the common effects of different wind speeds (related to the Beaufort Scale).

Mapping The Climatic Data

Climatic data, which are averages of weather elements over longer periods, are also often mapped using line symbols called isometric lines or isopleths. These lines connect points on a map that have the same value for a specific climatic element. Different types of isometric lines are used:

• Isobars: Connect places of equal atmospheric pressure.
• Isotherms: Connect places of equal temperature.
• Isohyets: Connect places of equal amount of rainfall (or other precipitation) over a given period (e.g., annual, monthly).
• Isohels: Connect places of equal duration of sunshine.
• Isonephs: Connect places of equal amount of cloud cover.

These lines help visualize the spatial distribution patterns of climatic elements.

Weather Map Interpretation

Interpreting a weather map involves analyzing the patterns formed by isopleths, symbols, and other plotted data to understand the weather conditions prevailing across the depicted area. This requires knowledge of weather symbols and the ability to infer information from the map's elements.

For example, analyzing isobars helps locate areas of high and low pressure and infer wind direction and strength (winds flow from high to low pressure, deflected by Coriolis force). Isotherms reveal temperature distribution patterns. Isohyets show where precipitation is occurring and its intensity.

Figure 8.10 shows a weather map for India in May. Figure 8.11 shows a weather map for India in January. Figures 8.12 and 8.13 show mean pressure and temperature for January and July respectively.

Example of an Indian weather map for the month of May, showing plotted weather data and pressure patterns.

Example of an Indian weather map for the month of January, showing plotted weather data and pressure patterns.

Map illustrating the average pressure and temperature distribution across India for the month of January.

Map illustrating the average pressure and temperature distribution across India for the month of July.

Interpreting these maps involves observing patterns like the location of high/low pressure centers, the spacing of isobars (indicating wind strength), the direction of wind arrows, the distribution of temperature (isotherms), cloud symbols, and precipitation symbols. Analyzing these patterns helps understand the dominant weather systems and conditions.

Exercises

Choose The Right Answer From The Four Alternatives Given Below

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Answer The Following Questions In About 30 Words

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Answer The Following Questions In About 125 Words

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Map Reading

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