Chapter - 2 Understanding the Weather

 Chapter - 2 Understanding the Weather

What is Weather:-

• Weather is the condition of the Earth’s atmosphere at a specific time and place.
• Atmosphere is the layer of gases (mainly nitrogen and oxygen) surrounding Earth—what we commonly call air.
• The troposphere is the lowest layer of the atmosphere, where all living land-based organisms exist and where almost all weather occurs.
• The troposphere ranges in thickness from about 6 km at the poles to 18 km at the equator, depending on temperature (cold air contracts, warm air expands).
• Common weather descriptors include hot, cold, rainy, cloudy, humid, snowy, and windy, which reflect how we experience weather through our senses.

The elements of weather:

• Temperature – how hot or cold the air is
• Humidity – the amount of moisture (water vapor) in the air
• Precipitation – water falling from the sky (rain, snow, sleet, hail)
• Wind – the movement of air from one place to another
• Air Pressure – the weight of the air pressing down on the Earth

1. Forecast: A prediction of what the weather will be like in the future, based on observations and data (e.g., "The forecast says it will rain tomorrow").
2. Sleet: A type of precipitation where raindrops freeze into ice pellets before hitting the ground. It’s like frozen rain.
3. Hail: Hard balls or lumps of ice that form in strong thunderstorms and fall from the sky, usually larger and harder than sleet.
4. Water Vapour: The gas form of water found in the air. It forms clouds when it cools and can later fall as rain, snow, or other types of precipitation.

Observing Nature’s Clues

• Over the last few centuries, scientists have developed accurate methods to measure the elements of weather.
• These measurements help meteorologists (weather scientists) predict the weather in advance—sometimes hours, days, or even weeks ahead.
• Weather forecasting isn’t based on guessing or simply looking at the sky.
• Instead, meteorologists use scientific tools and gadgets to collect weather data.

Weather Instrument: 

1. Temperature:- Thermometers are tools used to measure air temperature.

There are different types:

• Standard thermometers: Measure the current (ambient) temperature.
• Max–Min thermometers: Record the highest and lowest temperatures in a day.
• Many thermometers use a colored liquid (like mercury or alcohol) that expands when heated.
• Digital thermometers are now commonly used because they are:-More precise, Able to store and record data

Useful Temperature Statistics:

• Temperature Range = Maximum temperature − Minimum temperature (over 24 hours)
• Mean Daily Temperature = (Maximum temperature + Minimum temperature) ÷ 2

Ambient:-Of the immediate surroundings.

Statistics:- The technique of gathering and analysing information or data in order to be able to detect patterns, understand events or make predictions

2. Precipitation:- Precipitation includes rain, snow, sleet, or hail, but here the focus is on rainfall.

• Rainfall is measured using a tool called a rain gauge.
• A rain gauge works by:- Collecting rain through a funnel into a cylinder.
• Measuring the depth of the collected water using a scale on the side.

Example:

• If the water in the gauge is 5 mm high, it means 5 mm of rain fell in that area.
• So, 30 mm of rainfall means 30 mm depth of water was collected in the rain gauge during the day.

3. Atmospheric Pressure:- Atmospheric pressure is the weight of the air pressing down on the Earth and everything on it.

It can affect how we feel—for example, the air may feel “heavy” before a thunderstorm because of high pressure.

Where it's higher or lower:

• High pressure: Found at sea level or near the coast.
• Low pressure: Found at higher altitudes, like in the mountains.

As you go higher up a mountain, the air becomes:

• Thinner
• Lower in pressure
• Lower in oxygen, which can make breathing harder

Effects on the body:

• At high altitudes (like in mountains), the air is thinner and contains less oxygen.
• Your lungs have to work harder to take in enough oxygen.
• This is why people may feel breathless, dizzy, or tired at high elevations.

Pressure at different places:

• Generally higher near the sea coast.
• Lower at higher altitudes like mountains.
• Not always high at sea level—sometimes, it drops sharply, forming a low-pressure system, or depression.
• Depressions can lead to storms or cyclones.

Instrument used:

• A barometer measures atmospheric pressure.
• There are different types (like mercury or aneroid barometers).
• Unit of measurement: millibar (Mb)

Normal pressure:

• At sea level, it is around 1013 Mb.
• A reading below 1000 Mb indicates a depression.

Wind – Wind is the movement of air from one place to another. It is caused by differences in air pressure between areas.

1. Two Important Factors That Influence Wind

• Air Pressure: Wind moves from high-pressure areas to low-pressure areas.
• Temperature: Warm air rises and cool air sinks, helping create wind.

2. Importance of Wind

• Pilots: Use wind direction for safe take-off and landing.
• Sailors: Rely on wind to navigate boats and ships.
• Farmers: Need to know wind conditions for planting, spraying, and irrigation.
• Wind dries soil faster – stronger wind speed makes moisture evaporate quickly, which can affect crops.

3. Instruments to Measure Wind

Wind Sock:

• Shaped like a cone and placed on tarmacs (airports).
• Shows the direction of wind.
• Used by pilots and in industries releasing gases or smoke.

Anemometer:

• Has 3 or 4 cups that spin when wind blows.
• The faster it spins, the stronger the wind.
• Measures wind speed in km/h using a meter at the base.

Humidity:- Humidity refers to the amount of water vapor present in the air.

• It depends on factors like temperature, wind, pressure, and location.
• Understanding Relative Humidity
• Relative Humidity measures how much water vapor the air holds compared to the maximum it can hold at a certain temperature.
• 0% humidity = no water vapor in the air (not natural).
• 100% humidity = air saturated with water vapor.

Range of Humidity:

• Dry weather: Usually 20% - 40% relative humidity.
• Humid weather: Usually 60% - 80% relative humidity.

How is Humidity Measured?

• Instrument: A hygrometer measures humidity.
• There are several types of hygrometers, based on different principles (e.g., hair hygrometer, electronic hygrometer).

Why is Humidity Important?

• Industry: Important for processes like food preservation and manufacturing.
• Museums: Monitor humidity to preserve exhibits by maintaining a dry environment.

Weather Stations:- A weather station is a place where different instruments are combined to measure and track various aspects of the weather.

• Instruments at the station can measure temperature, humidity, wind speed, wind direction, precipitation, and atmospheric pressure.
• The readings are taken regularly, allowing meteorologists to map and forecast the weather over time.
• Automated Weather Station (AWS)
• AWS is a self-operating system that measures and records weather data automatically.

It uses sensors to track key weather elements:

• Temperature
• Humidity
• Wind speed & direction
• Precipitation
• Atmospheric pressure

Benefits:

• No human intervention is needed.
• Provides accurate and timely weather information.
• Used in fields like agriculture, aviation, navigation, and environmental monitoring.

Predicting the Weather:- Accurate predictions help us to be ready for such events. They also enable local governments to mobilise resources and prepare for any disasters. Meteorologists collect and study weather data using instruments like:

• Thermometers (temperature)
• Barometers (pressure)
• Rain gauges (precipitation)
• Anemometers (wind speed)
• Hygrometers (humidity)

NCERT QUESTIONS

1. Match the instrument with the weather element it measures.  Instrument used Element of the Weather

(1) Hygrometer                                                               (a) Precipitation

(2) Anemometer                                                             (b) Atmospheric pressure

(3) Barometer                                                                 (c) Wind direction and speed

(4) Thermometer                                                             (d) Humidity

(5) Rain gauge                                                                 (e) Temperature

Answer:- (1 - d)  (2 - c) (3 - b) (4 - e) (5 - a)

2. Jyotsna is deciding what clothes to pack for her school trip to Mumbai in June. She looks at the weather forecast, which predicts 29°C and 84% humidity. What would be your advice to her?

Answer:- Weather Forecast for Mumbai in June:

• Temperature: 29°C (Warm)
• Humidity: 84% (Very high)

Advice on What to Pack:

• Lightweight, breathable clothes – Cotton or linen clothes will help her stay cool and comfortable.
• Loose-fitting garments – Tight clothes may feel sticky and uncomfortable in high humidity.
• Raincoat or umbrella – June marks the beginning of the monsoon in Mumbai, so it often rains.
• Comfortable footwear – Waterproof sandals or shoes that dry quickly are best.
• Hat or cap – For sun protection when it's not raining.
• Water bottle – Staying hydrated is important in humid weather.
• Towel or face wipes – To stay fresh and dry off sweat.

3. Imagine that a small group of students is setting up a rain gauge. Here are some options for the site.

1. The school vegetable garden.

2. The terrace of the school building.

3. Open ground with elevated platform.

4. Compound wall of school.

5. Verandah of the school laboratory.

Discuss in your group and finalise the site. Write down the reasons for your decision.

Answer:- Open ground with elevated platform

Reason:

It is in an open area without obstructions like trees or buildings, ensuring accurate rainfall measurement. The elevated platform prevents splashes or flooding from affecting the readings.