Atmospheric Circulation and Weather Systems
NCERT Solutions • Class 11 Geography • Chapter 91. Multiple Choice Questions
(i) If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be:
(c) 900 mb
Reason: Air pressure decreases with altitude at a rate of roughly 1 mb for every 10 meters (or 100 mb per 1 km) in the lower atmosphere. So, 1000 – 100 = 900 mb.
Reason: Air pressure decreases with altitude at a rate of roughly 1 mb for every 10 meters (or 100 mb per 1 km) in the lower atmosphere. So, 1000 – 100 = 900 mb.
(ii) The Inter Tropical Convergence Zone normally occurs:
(a) near the Equator
Reason: The ITCZ is a low-pressure belt near the equator where the northeast and southeast trade winds converge.
Reason: The ITCZ is a low-pressure belt near the equator where the northeast and southeast trade winds converge.
(iii) The direction of wind around a low pressure in northern hemisphere is:
(c) anti-clockwise
Reason: Due to the Coriolis force, winds deflect to the right in the northern hemisphere, causing cyclonic (low pressure) circulation to be counter-clockwise.
Reason: Due to the Coriolis force, winds deflect to the right in the northern hemisphere, causing cyclonic (low pressure) circulation to be counter-clockwise.
(iv) Which one of the following is the source region for the formation of air masses?
(c) the Siberian Plain
Reason: A source region must be a vast, uniform area (either all land or all sea) with little surface variation. The Siberian Plain in winter is a classic source region for cold, dry polar continental air masses.
Reason: A source region must be a vast, uniform area (either all land or all sea) with little surface variation. The Siberian Plain in winter is a classic source region for cold, dry polar continental air masses.
2. Short Answer Questions
(i) What is the unit used in measuring pressure? Why is station level pressure reduced to sea level?
Unit: Millibar (mb) or Pascal (Pa).
Reason: Pressure decreases rapidly with altitude. To make meaningful comparisons between stations at different heights (e.g., a hill station vs a coastal city), all readings are converted to mean sea level pressure to isolate weather-related pressure changes from altitude effects.
Reason: Pressure decreases rapidly with altitude. To make meaningful comparisons between stations at different heights (e.g., a hill station vs a coastal city), all readings are converted to mean sea level pressure to isolate weather-related pressure changes from altitude effects.
(ii) Why are the winds north-easterlies in the tropics if the pressure gradient is north to south?
This is due to the Coriolis Force.
Winds start blowing from the Subtropical High (North) towards the Equatorial Low (South). However, the rotation of the earth deflects the wind to its right in the northern hemisphere, changing its direction from North $\to$ South to Northeast $\to$ Southwest.
Winds start blowing from the Subtropical High (North) towards the Equatorial Low (South). However, the rotation of the earth deflects the wind to its right in the northern hemisphere, changing its direction from North $\to$ South to Northeast $\to$ Southwest.
(iii) What are geostrophic winds?
Geostrophic winds occur in the upper atmosphere when the Pressure Gradient Force is exactly balanced by the Coriolis Force. As a result, the wind flows parallel to the isobars rather than crossing them from high to low pressure.
(iv) Explain the land and sea breezes.
These are local periodic winds caused by differential heating:
- Sea Breeze (Day): Land heats faster than sea $\to$ Low pressure over land $\to$ Wind blows from Sea to Land.
- Land Breeze (Night): Land cools faster than sea $\to$ High pressure over land $\to$ Wind blows from Land to Sea.
3. Long Answer Questions
(i) Discuss the factors affecting the speed and direction of wind.
Wind is air in motion. Its speed and direction are controlled by three combined forces:
- Pressure Gradient Force (PGF): This is the primary driver. It pushes air from High Pressure to Low Pressure. The steeper the gradient (closer isobars), the faster the wind speed. It acts perpendicular to isobars.
- Coriolis Force: Caused by earth’s rotation. It deflects wind to the right in the Northern Hemisphere and left in the Southern Hemisphere. It affects direction, not speed, and is strongest at poles/zero at the equator.
- Frictional Force: Acts near the surface (below 1 km). Friction from landforms slows down the wind and reduces the Coriolis effect, causing winds to cross isobars at an angle.
(ii) Draw a diagram of general atmospheric circulation. Reasons for subtropical high pressure?
Reasons for Subtropical High Pressure ($30^\circ$ N & S):
This high-pressure belt is formed dynamically, not thermally:
Reasons for Subtropical High Pressure ($30^\circ$ N & S):
This high-pressure belt is formed dynamically, not thermally:
- Subsidence: Air that rises at the equator cools and moves poleward. By the time it reaches $30^\circ$ latitude, it becomes heavy and sinks (subsides). Sinking air compresses and warms up, creating high pressure.
- Pile-up effect: The Coriolis force deflects the poleward moving air into a West-to-East direction (Westerlies aloft), causing a “pile-up” of air mass around $30^\circ$ latitudes, forcing it to sink.
(iii) Why does tropical cyclone originate over the seas? Where do they have torrential rains and why?
Origin over Seas: Tropical cyclones require a continuous supply of moisture to fuel the storm. The latent heat released during the condensation of this moisture provides the massive energy required to drive the cyclone. Hence, they form only over warm oceans (temp $> 27^\circ C$).
Torrential Rains & Winds:
The most violent activity occurs in the Eye Wall (the ring surrounding the calm central Eye).
Why? In the eye wall, spiraling air rises violently with maximum velocity. This intense updraft causes rapid cooling and condensation of massive amounts of moisture, leading to heavy cumulonimbus clouds, torrential rain, and peak wind speeds.
Torrential Rains & Winds:
The most violent activity occurs in the Eye Wall (the ring surrounding the calm central Eye).
Why? In the eye wall, spiraling air rises violently with maximum velocity. This intense updraft causes rapid cooling and condensation of massive amounts of moisture, leading to heavy cumulonimbus clouds, torrential rain, and peak wind speeds.
Project Work
Activity: Weather Forecasting Analysis
(Guidance for Student Activity)
To perform this analysis:
To perform this analysis:
- Source: Use a newspaper weather map (usually shows ‘H’ for High, ‘L’ for Low, and cloud symbols) and a TV weather channel / App.
- Inference:
– Heavy Clouds/Low Pressure: Predicts rain/storms.
– Clear Skies/High Pressure: Predicts sunny/dry weather. - Comparison: Create a simple 7-day table. Record the prediction on Day 1 and the actual weather on Day 2. Calculate accuracy percentage (e.g., 5/7 accurate = 71%).