9.1.2 - Horizontal Distribution of Pressure
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Introduction to Atmospheric Pressure
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Good morning class! Today we are going to talk about atmospheric pressure. Can anyone tell me what atmospheric pressure is?
Is it the weight of the air above us?
Exactly! Atmospheric pressure is the weight of a column of air above a specific area. It decreases with altitude, meaning the higher you go, the less pressure you feel.
Why is that?
It’s due to gravity pulling air molecules closer to the Earth. So, all that air above us weighs down, creating pressure.
What is the average pressure at sea level?
Great question! At sea level, the average atmospheric pressure is about 1,013.2 millibars.
How do we measure this pressure?
We use instruments like mercury barometers or aneroid barometers. These devices help us understand changes in atmospheric pressure.
What happens when there are differences in pressure?
Great question! Differences in pressure create wind, as air moves from high to low-pressure areas.
To summarize, atmospheric pressure is crucial as it influences wind direction and weather systems.
Isobars and Pressure Gradient Forces
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Let’s talk about isobars. Who can tell me what an isobar is?
Are they the lines connecting points of equal pressure on a weather map?
That's correct! Close isobars indicate a strong pressure gradient, which means stronger winds.
So, what creates these gradients?
Good follow-up! Pressure gradients form due to uneven heating of the Earth's surface, leading to different pressures.
What happens when isobars are spaced far apart?
When isobars are spaced further apart, it means a weak gradient, leading to lighter winds. Understanding this is crucial for predicting weather.
To wrap up, the space between isobars tells us about wind speed, and it’s essential for studying weather patterns.
The Forces Affecting Wind
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Now let’s discuss the forces that affect winds. What do you think influences wind direction?
Is it just pressure differences?
Excellent! Wind is generated by the pressure gradient force, but it’s also influenced by Coriolis and frictional forces.
What does the Coriolis effect do?
The Coriolis effect deflects the wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is due to the Earth's rotation.
How does friction play a role?
Friction slows down the wind, especially near the Earth's surface. It usually extends about 1-3 kilometers up into the atmosphere.
To summarize, wind direction and speed are affected by pressure gradients, Coriolis, and friction.
Cyclones and Anticyclones
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Let’s apply what we’ve learned about pressure and wind to cyclones and anticyclones. Who can define a cyclone?
Is it a weather system with low pressure at its center?
Correct! The winds spiral inwards anticlockwise in a cyclone in the Northern Hemisphere.
What about anticyclones?
Anticyclones are high-pressure systems where winds spiral outwards clockwise. These usually bring clear and calm weather.
What causes the different wind patterns around these systems?
The pressure differences create these wind patterns. In cyclones, air converges and rises, whereas, in anticyclones, air descends and spreads out.
To conclude, understanding these systems helps us predict severe weather events associated with them.
Introduction & Overview
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Quick Overview
Standard
The section details how atmospheric pressure varies horizontally, influencing wind patterns and weather systems. It explains the concepts of isobars, pressure gradient forces, and the effects of various forces like friction and the Coriolis effect on wind direction and speed.
Detailed
Horizontal Distribution of Pressure
This section elaborates on the horizontal variation of atmospheric pressure, a key influencing factor in weather patterns and atmospheric circulation. Atmospheric pressure, which is the weight of air above a given surface area, decreases with height due to gravitational effects, leading to differences that cause wind to flow from high to low-pressure areas.
The section introduces the concept of isobars, lines drawn on weather maps connecting points of equal pressure, which help visualize pressure distributions. Notably, it highlights how small differences in pressure can significantly affect wind direction and strength. For example, when isobars are closely spaced, the pressure gradient is strong, resulting in faster wind speeds.
Key forces affecting wind include the pressure gradient force, Coriolis effect, and frictional force. The Coriolis effect alters a wind's path based on the Earth's rotation, causing winds in the Northern Hemisphere to veer right. The interaction of these forces contributes to the circulation patterns observed around various pressure systems, such as cyclones and anticyclones, which interchangeably create different weather conditions in various hemispheres. Understanding these concepts allows for better predictions of weather events and understanding of global climatic patterns.
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Understanding Pressure Systems
Chapter 1 of 8
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Chapter Content
Small differences in pressure are highly significant in terms of the wind direction and purposes of comparison. The sea level pressure distribution is shown on weather maps.
Detailed Explanation
Pressure systems influence wind direction and weather patterns. Even small differences can create significant changes in wind behavior. Weather maps use sea level pressure to illustrate these distributions, allowing meteorologists to predict wind patterns and weather conditions.
Examples & Analogies
Think of pressure like a crowded room. If one side of the room (high pressure) is full, and the other is empty (low pressure), people will naturally gravitate towards the empty side, just like air moves from high to low pressure.
Identifying High and Low Pressure Systems
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Low-pressure system is enclosed by one or more isobars with the lowest pressure in the centre. High-pressure system is also enclosed by one or more isobars with the highest pressure in the centre.
Detailed Explanation
In meteorology, isobars represent lines of equal pressure. A low-pressure system, characterized by lower pressure at its center, indicates rising air and often leads to cloudy or rainy weather. Conversely, a high-pressure system, with higher pressure centrally located, is associated with descending air, resulting in clear skies.
Examples & Analogies
Imagine a balloon. When you inflate it, the air pressure inside increases, making it harder for new air to enter. This is similar to a high-pressure system. Conversely, when you let some air out, it creates low pressure, which allows outside air to rush in, similar to a low-pressure system.
Global Pressure Distribution Patterns
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The world distribution of sea level pressure in January and July has been shown in Figures 9.2 and 9.3. Near the equator the sea level pressure is low and the area is known as equatorial low. Along 30° N and 30° S are found the high-pressure areas known as the subtropical highs. Further polewards along 60°N and 60°S, the low-pressure belts are termed as the subpolar lows. Near the poles the pressure is high and it is known as the polar high.
Detailed Explanation
The distribution of pressure across the globe changes with seasons. Near the equator, low pressure creates a warm and moist climate. The subtropical highs around 30° N and S lead to deserts due to sinking air. Toward the poles, low pressures create stormy conditions, while high pressure near the poles brings cold, dry air.
Examples & Analogies
Consider global pressure zones like a big pizza. The middle (equatorial low) is soft and warm, while the crust (subtropical highs) is hard and dry. The toppings (subpolar lows and polar highs) show how different climates can be based on pressure. Each slice represents a different weather pattern!
Isobars and Pressure Mapping
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Horizontal distribution of pressure is studied by drawing isobars at constant levels. Isobars are lines connecting places having equal pressure. In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level for comparison.
Detailed Explanation
Isobars are essential tools in understanding weather patterns. By connecting locations of equal pressure, they help visualize how pressure changes across regions, indicating wind flow and weather fronts. To standardize these measurements, pressures are adjusted to sea level, removing altitude effects so that forecasts can be accurately compared.
Examples & Analogies
Think of isobars like a contour map on a hiking trail. Just like the lines on the map indicate elevation changes, isobars show pressure variations, guiding meteorologists in predicting where winds will blow and weather systems will develop.
Pressure Gradient Force
Chapter 5 of 8
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Chapter Content
The differences in atmospheric pressure produces a force. The rate of change of pressure with respect to distance is the pressure gradient. The pressure gradient is strong where the isobars are close to each other and is weak where the isobars are apart.
Detailed Explanation
The pressure gradient force is key to wind generation. When isobars are closely spaced, the pressure difference between them creates a strong gradient, leading to brisk winds. Conversely, wide spaces between isobars indicate a weaker gradient and lighter winds. This force drives air movement, contributing to overall weather patterns.
Examples & Analogies
Imagine blowing up a balloon with a small pinhole. The air rushes out quickly at the pinhole due to the pressure difference—like wind moving from high to low pressure. A larger, more relaxed balloon would release air slowly, akin to weak winds where isobar spacing is wide.
Frictional and Coriolis Forces
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It affects the speed of the wind. It is greatest at the surface and its influence generally extends up to an elevation of 1 - 3 km. Over the sea surface the friction is minimal. The rotation of the earth about its axis affects the direction of the wind.
Detailed Explanation
Wind speed is affected by surface friction, which is strongest near the ground. This force slows the wind and disrupts its direction. The Coriolis effect, caused by Earth's rotation, also influences wind paths, resulting in a rightward deflection in the Northern Hemisphere and a leftward deflection in the Southern Hemisphere.
Examples & Analogies
Think of wind like a car driving on a curved road (Coriolis effect) but also feeling the bumps from the surface (friction). The car naturally shifts to one side due to the turn while the road surface affects its speed. Together, these forces shape how the wind travels.
Forces Influencing Wind Direction
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The air in motion is called wind. The wind blows from high pressure to low pressure. The wind at the surface experiences friction. In addition, rotation of the earth also affects the wind movement.
Detailed Explanation
Wind is the movement of air from areas of high pressure to low pressure. This movement is influenced by friction with the earth's surface, which slows it down. Additionally, Earth’s rotation causes the wind to curve, creating complex wind patterns across the globe.
Examples & Analogies
Imagine a river flowing from a mountain (high pressure) down to a valley (low pressure). As it winds around obstacles (friction), it changes its path. Similarly, surface features and Earth's spin cause the wind to shift its course.
Wind Patterns and Systems
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The wind circulation around a low is called cyclonic circulation. Around a high it is called anti-cyclonic circulation.
Detailed Explanation
Winds circulate in specific patterns due to pressure systems: around low-pressure areas, air spirals inwards, creating cyclones. Conversely, around high-pressure areas, air flows outward, forming anti-cyclones. This circulation influences local weather patterns and can lead to severe weather conditions.
Examples & Analogies
Think of cyclonic circulation like whirlpool water swirling down a drain, pulling everything toward the center. In contrast, anti-cyclonic seems like a fountain shooting water straight up and outwards, spreading it evenly.
Key Concepts
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Atmospheric Pressure: The force exerted by the weight of air above.
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Isobars: Lines that connect locations of equal pressure on a weather map.
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Pressure Gradient Force: The driving force behind wind movement.
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Coriolis Effect: The influence of Earth's rotation on wind direction.
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Cyclones and Anticyclones: Weather systems characterized by low and high pressure respectively.
Examples & Applications
Cyclones form as air converges into low-pressure centers, causing severe weather.
Anticyclones result in clear skies as air descends from high-pressure areas.
Memory Aids
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Rhymes
Pressure is high and the wind is sly, low-pressure spins, as the storms begin, the air comes down, no need to frown!
Stories
Imagine a big balloon filled with air at sea level. As you take it up a mountain, it starts to feel lighter, just like how our bodies feel with less pressure.
Memory Tools
For pressure gradient, think 'High to Low, Wind Will Go!'
Acronyms
COW - Coriolis, Opposes Wind; remember that Coriolis affects the wind's movement!
Flash Cards
Glossary
- Atmospheric Pressure
The weight of a column of air above a given surface area.
- Isobars
Lines on a weather map that connect points of equal atmospheric pressure.
- Pressure Gradient Force
The force that results from the difference in atmospheric pressure, causing air to move.
- Coriolis Effect
The deflection of moving air due to the Earth's rotation, causing winds to turn.
- Frictional Force
The force that slows winds down due to contact with the Earth’s surface.
- Cyclone
A weather system characterized by low pressure at its center, with winds spiraling inward.
- Anticyclone
A high-pressure weather system where winds spiral outward.
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