Chapter 2 Understanding The Weather

Our daily experiences, like choosing clothes based on temperature, show how sensitive we are to the prevailing atmospheric conditions. This state of the atmosphere at a specific time and location is called weather.

The Earth is surrounded by layers of gases called the atmosphere. The lowest layer, extending from the surface up to 6-18 kilometres, is the troposphere. This is where most living beings on land reside and where almost all weather phenomena occur. The height of the troposphere varies; it is thinner near the poles (due to colder, denser air) and thicker in tropical regions (due to warmer, expanding air).

Weather is described using terms like hot, cold, rainy, cloudy, humid, snowy, or windy. These terms relate to our experience of the different elements of weather.

The primary elements that define weather are:

• Temperature: Indicates how hot or cold the air is.
• Precipitation: Any form of water falling from the atmosphere to the ground, such as rain, snow, sleet (frozen or partly frozen rain), or hail (small, hard ice balls).
• Atmospheric Pressure: The force exerted by the weight of the air on the Earth's surface.
• Wind: The natural movement of air, characterized by its speed and direction.
• Humidity: The amount of water vapour (water in gaseous form) present in the air.

To accurately describe and compare weather conditions across different locations and times, it is essential to measure these elements using standardized methods and instruments.

For centuries, humans have observed natural signs to predict weather changes, such as animal behaviour (birds flying low, ants moving eggs, squirrels gathering nuts) or plant responses (pine cones opening/closing). This traditional knowledge, particularly useful for forecasting events like monsoons, has been passed down through generations.

Modern meteorology, the scientific study of weather and forecasting, utilizes precise instruments to measure weather elements, enabling more accurate predictions over short and long periods.

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Weather Instruments

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Temperature

Temperature is measured using a thermometer. Different types of thermometers exist, including those that measure the current ambient temperature, and those that record the maximum and minimum temperatures over a period, typically 24 hours. Traditional thermometers often use liquids that expand with heat, while modern digital thermometers offer higher precision and data recording capabilities.

Temperature is commonly measured using scales like the Celsius (°C) scale or the Fahrenheit (°F) scale.

Recording temperature data allows for calculating useful statistics:

• Range of temperature: The difference between the maximum and minimum temperatures recorded during a specific period.
• Mean daily temperature: The average temperature for a day, often calculated as (Maximum Temperature + Minimum Temperature) / 2.

Accurate temperature measurements provide a standardized way to communicate how hot or cold it is, overcoming subjective perceptions that vary from person to person or region to region.

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Precipitation

The amount of precipitation, such as rainfall, is measured using a rain gauge. This instrument typically consists of a funnel that collects rain and directs it into a measuring cylinder. The amount of rain is then determined by reading the depth of water collected in the cylinder, usually measured in millimetres (mm).

Measuring precipitation accurately is vital for various purposes, including agriculture, water resource management, and forecasting floods or droughts.

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Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the column of air above a given point on the Earth's surface. It changes with altitude; pressure is generally higher at lower altitudes (near sea level) where there is more air above, and lower at higher altitudes (like mountains) where the air is thinner and there is less weight above.

Significant drops in atmospheric pressure, known as depressions or low-pressure systems, are often associated with unsettled weather, potentially leading to storms or cyclones. Conversely, high-pressure systems typically bring stable, clear weather.

The instrument used to measure atmospheric pressure is a barometer. Atmospheric pressure is commonly measured in units called millibars (mb). Normal atmospheric pressure at sea level is around 1013 mb. Pressures below this, especially below 1000 mb, often indicate developing weather disturbances.

Measuring atmospheric pressure is essential for weather forecasting, aviation, and understanding how environmental conditions affect human comfort and activities at different altitudes (e.g., breathlessness at high altitudes due to lower oxygen availability). People travelling to high altitudes often need to acclimatise, or adjust to the new conditions, to cope with the lower pressure and reduced oxygen.

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Wind

Wind is the movement of air from an area of high atmospheric pressure to an area of low atmospheric pressure. The direction and speed of wind are important weather elements.

Wind plays a vital role in weather patterns, influencing temperature, humidity distribution, and carrying clouds and precipitation. It is also crucial for activities like aviation and sailing, and for farmers to assess conditions like soil drying rates.

Instruments used to measure wind include:

• Wind Vane (or Weather Vane): Used to determine the direction from which the wind is blowing. It has a pointer that aligns itself with the wind direction. Airport tarmacs often have 'wind socks' which serve a similar purpose for pilots.
• Anemometer: Used to measure wind speed. A common type has rotating cups that spin faster in stronger winds. The speed is typically measured in kilometres per hour (km/h).

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Humidity

Humidity refers to the concentration of water vapour (gaseous water) present in the air. The amount of humidity is influenced by factors such as temperature, wind, pressure, and geographical location (e.g., proximity to water bodies).

Humidity is commonly measured as relative humidity, expressed as a percentage (%). This indicates how much water vapour is currently in the air compared to the maximum amount the air could hold at that specific temperature. 0% represents completely dry air, while 100% represents air saturated with water vapour. Dry weather typically has 20-40% relative humidity, while humid weather is usually between 60-80%.

Understanding humidity is related to the concept of evaporation. When humidity is high, water evaporates more slowly (as the air is already holding a significant amount of water vapour). This is why things dry slowly on humid days, and people tend to sweat more as sweat evaporates less readily, making one feel hotter and stickier.

Humidity is measured using an instrument called a hygrometer. Accurate humidity monitoring is important in various applications, including industrial processes (like food processing) and for preserving sensitive items in places like museums, which require stable, often low, humidity levels.

Weather Stations

A weather station is a facility equipped with multiple instruments for measuring and recording various weather elements at a specific location and time. Collecting readings from these instruments at regular intervals allows meteorologists to monitor current weather conditions and gather data for forecasting.

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An Automated Weather Station

An Automated Weather Station (AWS) is a modern type of weather station that operates automatically using sensors. These systems continuously measure and record weather parameters like temperature, humidity, wind speed and direction, precipitation, and atmospheric pressure without requiring manual human intervention. AWS provide accurate and timely data, which is crucial for applications in agriculture, aviation, navigation, and environmental monitoring.

AWS are particularly valuable in remote or challenging locations, like high-altitude glacial lakes, where they can provide early warnings about changing weather conditions that could pose risks.

Predicting The Weather

Predicting the weather involves meteorologists collecting vast amounts of data from weather instruments over time, analyzing this data using scientific models and techniques, and forecasting how the weather is likely to behave in a specific area in the coming hours, days, or weeks.

Accurate weather predictions are increasingly important, especially as climate change contributes to more frequent and intense extreme weather events such as droughts, floods, heatwaves, cold waves, and cyclones.

Knowing the weather forecast helps individuals, communities, and governments to prepare for potential impacts. Governments and disaster management authorities use predictions to mobilize resources, issue warnings, and take protective measures. For example, fishermen can be warned about rough seas, or coastal populations might be evacuated if a severe cyclone is anticipated. Weather predictions enable proactive responses that can save lives and minimize damage.

Weather warnings are often issued using a color-coded system (like Green for No Warning, Yellow for Watch/Be Updated, Orange for Alert/Be Prepared, and Red for Warning/Take Action) to indicate the level of potential risk associated with expected weather conditions.