Chapter Notes

Understanding the Weather

A change in the weather can feel like a fresh start.

The Big Questions

• How can we measure and monitor the weather around us?
• How do weather predictions help us prepare for events like heavy rain, storms, drought, and heat waves?

Weather and its Elements

Our bodies are very sensitive to the weather. We instinctively reach for warm clothes on a cold winter morning and choose light, cool clothes in the summer. These are our bodies reacting to the weather.

What is weather?

Weather is the condition of the Earth's atmosphere at a specific time and place. The atmosphere is the layer of gases that surrounds our planet. We commonly call these gases 'air'.

Imagine the Earth's atmosphere like a cake with layers. The layer closest to the Earth's surface is the troposphere. This is where we, along with all other land-based plants and animals, live and breathe. Almost all weather phenomena occur in the troposphere.

The troposphere extends 6 to 18 kilometres from the ground. It's thinner at the poles, where the cold air contracts, and thicker in the tropical zone, where the warmer air expands.

We use words like hot, cold, rainy, cloudy, humid, snowy, and windy to describe the weather. These words describe how we experience the different elements of weather.

The elements of weather are:

• Temperature: How hot or cold the atmosphere is.
• Precipitation: Any form of water, such as rain, snow, sleet, or hail, that falls from the sky.
• Atmospheric Pressure: The weight of the air above us, felt on the Earth's surface.
• Wind: The movement of air, including its speed and direction.
• Humidity: The amount of water vapour in the air.

The Need for Standard Measurements

Imagine Krishnan in Chennai is talking to Amir in Kashmir. Krishnan says it's chilly in Chennai after a rainy night. But what Krishnan considers "chilly" might feel pleasant to Amir, who is used to colder temperatures.

To avoid this confusion, we need a common way to measure weather elements. This chapter will explore how we use common standards to measure the weather.

The Importance of Precise Weather Measurement

Measuring the weather precisely is important for many reasons. Knowing the weather in advance helps us plan our activities.

Traditional Weather Prediction

From early times, people have observed nature to forecast the weather. They watched birds flying low, ants carrying eggs, squirrels gathering nuts, frogs croaking, and pine cones opening and closing. This knowledge has been passed down through generations. Even today, many people in India use traditional methods to predict the weather, especially the arrival of the monsoon.

Sleet: Frozen or partly frozen rain.

Hail: Small, hard balls of ice that fall from the sky like rain.

Water vapour: Water in gaseous instead of liquid form.

Forecast: To predict or find out in advance (in our case, the weather).

Observing Nature's Clues

• Ants shifting their eggs to higher ground indicates an expected change in the weather, especially heavy rain.
• Frogs croaking can indicate an expectation of rain.
• The opening and closing of pine cones are natural mechanisms driven by environmental humidity. Pine cones close in humid conditions to protect their seeds and open in dry conditions to release them, ensuring they spread in favorable weather.

Modern Weather Prediction

In recent centuries, scientists have developed precise methods to measure and monitor weather elements. Meteorologists use this information to predict the weather in a specific region after a few hours, days, or even weeks.

Meteorology: The systematic study of weather and its evolution. This study is the basis for weather forecasting.

They use instruments and data, not just guesses! Let's look at some of these instruments.

Weather Instruments

a) Temperature

In Grade 6 Science, you learned about different types of thermometers, like clinical and laboratory thermometers, and temperature scales like Celsius and Fahrenheit.

A temperature of 15 degrees Celsius ($15^{\circ} \mathrm{C}$) is the same as 59 degrees Fahrenheit ($59^{\circ} \mathrm{F}$).

Types of Thermometers

There are different types of thermometers:

• Some measure the ambient temperature (of the immediate surroundings).
• Others record the maximum and minimum temperatures during a day.

Ambient: Of the immediate surroundings.

Traditional thermometers often use a colored liquid that expands when the temperature increases. However, digital thermometers are becoming more popular because they are more precise and can record more data.

Using Temperature Recordings

Temperature recordings can be used to collect useful statistics:

• Range of temperature: The maximum temperature minus the minimum temperature during a particular period (usually 24 hours).
• Mean daily temperature: The maximum temperature plus the minimum temperature of the day, divided by two.

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

India Meteorological Department

The India Meteorological Department (IMD) was established in 1875. Its motto is ādityāt jāyate vriṣhti, which means, "From the sun arises rain." This phrase comes from the ancient text Manusmriti: "From the sun arises rain, from rain comes food, and from food, living beings originate."

b) Precipitation

If the news reports that a place received 30 mm of rainfall in a day, what does that mean? How is rainfall measured?

Measuring Rainfall

The amount of rainfall is measured using a rain gauge. The rain gauge has a funnel that collects rainwater into a cylinder. A scale on the cylinder measures the depth of the collected rainwater. If the water level is 5 mm, we say that the area received 5 mm of rainfall.

c) Atmospheric Pressure

We are aware of temperature and rainfall, but we also experience atmospheric pressure. Atmospheric pressure is the pressure exerted by the weight of the air above and around us. Sometimes the weather feels 'heavy' before a thunderstorm; this is related to atmospheric pressure.

Atmospheric Pressure and Altitude

Atmospheric pressure is higher near the sea coast and lower in the mountains. As you climb a mountain, the air gets thinner, resulting in lower air pressure and less oxygen. This is why people feel breathless, dizzy, or tired at high altitudes. With less oxygen getting into your blood, your body has to work harder to keep you moving!

Atmospheric Pressure and Weather

Atmospheric pressure is not always high in the plains or on the coast. It can sometimes drop dramatically, causing a depression or low-pressure system, which can develop into a storm or even a cyclone.

Measuring Atmospheric Pressure

A barometer measures atmospheric pressure. There are different types of barometers. The unit of measurement is the millibar (mb). Normal atmospheric pressure at sea level is around 1013 mb. A pressure below 1000 mb indicates a depression.

Acclimatise: Adjust to a new climate or new condition.

High Altitude Considerations

People traveling to high altitudes are advised to pause to allow their bodies to acclimatise. Soldiers serving in high-altitude areas like Khardung La in Ladakh (over 5600 meters above sea level) live and work in very low oxygen conditions. The atmospheric pressure there is about 650 millibars!

d) Wind

Wind is the movement of air from areas of high pressure to areas of low pressure. Speed and direction are important factors when describing wind.

The Importance of Wind

Wind is an important element of weather. Its direction and speed are crucial for weather forecasting. Pilots and sailors need wind data because wind greatly influences flying and sailing. Farmers use wind direction to predict rain and wind speed to determine how quickly the soil will dry.

Measuring Wind

A wind vane (or weather vane) measures wind direction. It has a rotating arm with a pointer at one end and a tail at the other. The tail is pushed by the wind, and the pointer indicates the wind's direction.

A wind sock is a type of wind vane used on airport tarmacs. It shows pilots the wind direction during take-off and landing. Wind socks are also used in industries that release ash or gases.

An anemometer measures wind speed. It has three or four metal cups that rotate on a vertical shaft when the wind blows. The stronger the wind, the faster the rotation. A meter calculates the wind speed in kilometres per hour (km/h).

e) Humidity

Humidity is the amount of water vapour in the air. It depends on temperature, wind, pressure, and location.

Measuring Humidity

Relative humidity measures humidity:

• Air with no water vapour has a humidity of 0%.
• Air saturated with water vapour has a humidity of 100%.
• Dry weather has a relative humidity between 20% and 40%.
• Humid weather has a relative humidity between 60% and 80%.

A hygrometer measures humidity. There are different types of hygrometers. Measuring humidity is important in many industries, such as food processing. Museums also monitor humidity to preserve their exhibits.

Weather Stations

A weather station brings together instruments to measure weather at a particular place and time. Measurements are taken regularly to help map and forecast the weather.

Automated Weather Stations

An Automated Weather Station (AWS) is a self-operating system that uses sensors to measure and record weather data, such as temperature, humidity, wind speed and direction, precipitation, and atmospheric pressure. AWS are used in agriculture, aviation, navigation, and environmental monitoring, providing accurate weather information without human intervention.

AWS at a Glacial Lake of Sikkim

In 2023, the National Disaster Management Authority set up an AWS at a glacial lake in Sikkim at an altitude of more than 4800 meters above sea level. The AWS provides early information about upcoming weather conditions.

Predicting the Weather

Meteorologists collect data using instruments over long periods. They study the data and use scientific methods to predict the weather. These predictions are important because climate change is making extreme weather events more frequent.

The Importance of Accurate Predictions

Accurate predictions help us prepare for extreme weather. Local governments can mobilize resources and prepare for disasters. If stormy weather is expected at sea, fishermen are warned. Coastal areas might be evacuated if a cyclone is expected.

Weather Warnings

Weather maps issued by the India Meteorological Department (IMD) provide warnings about weather conditions. These maps use icons to show different weather conditions.

Before we move on ...

• Temperature, humidity, precipitation, wind, and atmospheric pressure define the weather at a particular place.
• Special instruments measure these elements. The data helps us monitor and predict the weather.
• In different times or situations, one element is dominant: rainfall in July, temperature in May and December, atmospheric pressure during a cyclone, and wind during a loo (strong, hot, and dusty winds in north India in summers) or forest fires.
• Weather is closely linked to climate, which we will discuss in the next chapter.