Social Sciences: Class 7 — Chapter 2: Understanding the Weather

Weather is the state of the Earth's atmosphere at a particular time and place. The atmosphere is the layer of gases (commonly called 'air') that surrounds the Earth.

The atmosphere can be compared to a layered cake. The layer closest to the Earth's surface is the troposphere. This is where all land-based plants and animals live, and where almost all weather phenomena take place. The troposphere extends to a height of $6$ to $18\text{ km}$ from the ground; it is thinner at the poles (where cold air contracts) and thicker in the tropical zone (where warm air expands).

We experience weather through its core elements:

• Temperature: How hot or cold the atmosphere is.
• Precipitation: Any form of water that falls from the sky, such as rain, snow, sleet (frozen or partly frozen rain), or hail (small, hard balls of ice).
• Atmospheric Pressure: The weight of the air above us, felt on the Earth's surface.
• Wind: The movement of air, characterized by its speed and direction.
• Humidity: The amount of water vapour (water in gaseous form) present in the air.

Temperature is measured using thermometers on scales like Celsius ($^\circ\text{C}$) or Fahrenheit ($^\circ\text{F}$). For example, a cool temperature of $15^\circ\text{C}$ is equivalent to $59^\circ\text{F}$.

Key temperature statistics include:

• Ambient temperature: The temperature of the immediate surroundings.
• Range of temperature: The maximum temperature minus the minimum temperature during a specific period (usually 24 hours).
• Mean daily temperature: The average of the maximum and minimum temperatures of the day: $$\text{Mean Daily Temperature} = \frac{\text{Maximum Temp} + \text{Minimum Temp}}{2}$$

Note: The India Meteorological Department (IMD), established in 1875, uses the motto "ādityāt jāyate vriṣhti", meaning "From the sun arises rain."

Rainfall is measured using an instrument called a rain gauge. When it rains, water falls into a collecting funnel and accumulates in a measuring tube. A scale measures the depth of the collected water in millimetres (mm).

Atmospheric pressure is the force exerted by the weight of the air above us. It is measured using a barometer, typically in millibars (mb).

• Normal atmospheric pressure at sea level is around $1013\text{ mb}$.
• A pressure reading below $1000\text{ mb}$ indicates a low-pressure system or "depression," which can develop into storms or cyclones.
• Pressure decreases with altitude. At high altitudes, the air is thinner, containing less oxygen, requiring travelers to pause and acclimatise (adjust to the new climate/conditions). For example, at Khardung La in Ladakh ($5600\text{ m}$ above sea level), the pressure is only about $650\text{ mb}$.

Wind is the movement of air from areas of high pressure to areas of low pressure.

• Direction: Measured using a wind vane (or weather vane) or a wind sock (commonly used on airport tarmacs to assist pilots during take-off and landing).
• Speed: Measured using an anemometer, which features rotating metal cups that spin faster as wind speed increases, calculating speed in kilometres per hour (km/h).

Humidity is measured as relative humidity, expressed as a percentage:

• $0\%$ represents air with absolutely no water vapour.
• $100\%$ represents air completely saturated with water vapour.
• Dry weather typically ranges between $20\%$ and $40\%$, while humid weather falls between $60\%$ and $80\%$.
• Relative humidity is measured using a hygrometer.

A weather station brings together multiple instruments to measure and track weather elements simultaneously at regular intervals.

An Automated Weather Station (AWS) is a self-operating system that uses electronic sensors to record and transmit weather data without requiring human intervention. These are crucial for agriculture, aviation, and disaster management.

Meteorologists analyze long-term data to predict weather patterns. Accurate predictions are vital for preparing for extreme events like droughts, floods, heat waves, and cyclones.

The IMD issues color-coded alerts to help the public and local governments prepare:

• Green: No Warning
• Yellow: Watch (Be Updated)
• Orange: Alert (Be Prepared)
• Red: Warning (Take Action)