What is Atmospheric Pressure? 🌍🌡️
- Atmospheric pressure is the force exerted by air on the Earth’s surface due to its weight.
- It is measured in millibars (mb) or Pascals (Pa) using a barometer.
- Standard atmospheric pressure: 1013.25 mb or 1 atm at sea level.
Factors Affecting Atmospheric Pressure
1. Altitude (Height Above Sea Level) ⛰️
✅ Higher Altitude = Lower Pressure (Air is thinner at high elevations).
✅ Lower Altitude = Higher Pressure (Denser air near sea level).
📌 Example: Pressure is lower on Mount Everest (8848m) than in Mumbai (sea level).
2. Temperature 🌡️
✅ Hot air expands and rises → Low Pressure.
✅ Cold air contracts and sinks → High Pressure.
📌 Example: Equator (hot) has low pressure, Poles (cold) have high pressure.
3. Humidity (Moisture in Air) 💧
✅ More water vapor = Lower Pressure (Moist air is lighter).
✅ Less water vapor = Higher Pressure (Dry air is heavier).
📌 Example: Tropical regions (humid) have low pressure, Deserts (dry) have high pressure.
4. Earth’s Rotation & Coriolis Effect 🌎
- Earth’s rotation causes air to move in circular patterns.
- Influences the formation of pressure belts and wind systems.
Pressure Belts of the Earth
| Pressure Belt | Latitude | Pressure Type | Example |
|---|---|---|---|
| Equatorial Low | 0° (Equator) | Low Pressure | Amazon Rainforest |
| Subtropical High | 30° N & S | High Pressure | Sahara Desert |
| Subpolar Low | 60° N & S | Low Pressure | Iceland |
| Polar High | 90° N & S | High Pressure | Antarctica |
Isobars & Pressure Gradient Force
- Isobars: Lines on a weather map that connect places with equal pressure.
- Pressure Gradient Force (PGF): Air moves from high to low pressure, creating winds.
✅ Close Isobars = Strong Winds (Example: Cyclones).
✅ Far Apart Isobars = Calm Winds (Example: Anticyclones).
Types of Pressure Systems
1. High-Pressure System (Anticyclone) ☀️
- Cold, sinking air creates clear skies & dry weather.
- Winds move outward & clockwise (Northern Hemisphere).
- Example: Deserts like Sahara & Thar.
2. Low-Pressure System (Cyclone) 🌪️
- Warm, rising air causes clouds, rain, and storms.
- Winds move inward & counterclockwise (Northern Hemisphere).
- Example: Monsoons in India, Hurricanes, Tornadoes.
Instruments to Measure Atmospheric Pressure
📌 Barometer – Measures pressure in millibars (mb) or Pascals (Pa).
📌 Aneroid Barometer – Compact, used in weather forecasting.
📌 Altimeter – Measures pressure changes with altitude (used in aircraft).
MCQs on Atmospheric Pressure
1. What is the standard atmospheric pressure at sea level?
a) 500 mb
b) 1013.25 mb
c) 2000 mb
d) 750 mb
Answer: b) 1013.25 mb
2. What happens to atmospheric pressure as altitude increases?
a) Increases
b) Decreases
c) Remains constant
d) Varies randomly
Answer: b) Decreases
3. Which pressure belt is found at 30° latitude?
a) Equatorial Low
b) Subtropical High
c) Subpolar Low
d) Polar High
Answer: b) Subtropical High
4. What type of weather is associated with low-pressure areas?
a) Clear skies
b) Dry weather
c) Rain and storms
d) Extreme cold
Answer: c) Rain and storms
5. What instrument is used to measure atmospheric pressure?
a) Thermometer
b) Barometer
c) Anemometer
d) Hygrometer
Answer: b) Barometer
Short Answer Questions
1. What is atmospheric pressure?
Atmospheric pressure is the weight of air pressing down on Earth, measured in millibars (mb) or Pascals (Pa).
2. How does altitude affect atmospheric pressure?
Higher altitude = Lower pressure. Lower altitude = Higher pressure.
3. What is an anticyclone?
An anticyclone is a high-pressure system with clear skies and dry weather.
4. Name the four major pressure belts.
- Equatorial Low (0°)
- Subtropical High (30° N & S)
- Subpolar Low (60° N & S)
- Polar High (90° N & S)
5. Why do deserts have high pressure?
Deserts have high pressure because hot air rises at the equator, cools, then sinks at 30° latitude, creating dry conditions.
Conclusion
Atmospheric pressure plays a key role in weather, wind systems, and climate patterns. Understanding its distribution and effects is crucial for meteorology and competitive exams.
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