Discussions for week 9/10/11
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25_Lecture_Outline.pptxChapter 25: Driving Forces of Weather
© 2017 Pearson Education, Inc.
Chapter 25 Lecture
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This lecture will help you understand:
Atmospheric Moisture
Weather Variables
Cloud Development
Air Masses, Fronts, and Storms
Violent Weather
Weather Forecasting
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Weather
Four factors influence the weather:
Atmospheric moisture
Temperature
Air pressure
Arrangement of land and water features
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Atmospheric Moisture: Temperature and Water Vapor
Temperature is a measure of the average kinetic energy of molecules.
At high temperatures, water molecules are fast with enough energy to bounce out of the liquid state and into the vapor state—evaporation. With increased temperature, there is increased evaporation, and more water vapor in the air. Air with a lot of water vapor is humid.
When temperatures cool, water-vapor molecules slow down and lack the energy to remain in the vapor state—they begin to clump together—condensation. Depending on the temperature, water vapor may condense to form dew, clouds, or fog, and if cloud droplets get big enough—rain; and if cold enough, frost, snow, or freezing rain.
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Atmospheric Moisture: Humidity
No matter how "dry" the air may feel, there is always some amount of water vapor in the air.
Humidity is the mass of water vapor in a given mass of air.
In other words, humidity is the air's water-vapor content.
When air is saturated with water vapor it is at maximum humidity—any additional water vapor will condense to form water droplets.
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Atmospheric Moisture: Water-Vapor Capacity
Saturation occurs when the air's temperature drops, causing water vapor to condense.
The temperature at which saturation occurs is called the dew point.
When the air is saturated (maximum humidity) it has reached its water-vapor capacity.
Saturation and Water-Vapor Capacity are temperature dependent.
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Atmospheric Moisture: Water-Vapor Capacity
The air's capacity for water vapor varies with temperature.
Warm air can accommodate more water vapor than cold air. Higher temperature means more energetic water-vapor molecules— evaporation.
Increased Temperature
Increased Water-Vapor Capacity
As air cools, it accommodates less and less water vapor. Cooler temperature means slower moving water-vapor molecules—condensation.
Decreased Temperature
Decreased Water-Vapor Capacity
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Atmospheric Moisture: Relative Humidity
Relative humidity—the ratio of the air's water-vapor content to its capacity—is the most common way to describe atmospheric moisture.
Relative humidity depends on actual water-vapor content and air temperature
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water-vapor content (humidity)
water-vapor capacity
Relative humidity:
× 100%
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Atmospheric Moisture
In saturated air, condensation and evaporation are in equilibrium.
Evaporation rate depends on temperature.
Condensation rate depends on humidity and temperature.
When evaporation rate equals the condensation rate, the relative humidity is 100%.
When evaporation exceeds condensation, the air is no longer saturated and relative humidity is less than 100%.
If condensation exceeds evaporation, the air is super-saturated and water droplets form.
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Atmospheric Moisture
Warm air has a higher capacity for water vapor than cool air. When air is completely saturated it is at its maximum specific humidity—water-vapor capacity. At saturation, relative humidity is 100%, and the air temperature is the same as the dew point.
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Atmospheric Moisture: Dew Point
Dew point: Temperature at which saturation occurs.
Condensation occurs when the dew point is reached.
Water vapor condenses high in the atmosphere to form clouds.
Water vapor condenses close to the ground surface to form dew, frost, and/or fog.
Dew Point can be used to indicate water-vapor content:
High dew point = high water-vapor content
Low dew point = low water-vapor content
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Atmospheric Moisture: Dew Point
Dew point is always less than, or equal to air temperature
The difference between air temperature and dew point can be used to indicate whether relative humidity is low or high.
When the difference is big—relative humidity is low
When the difference is small—relative humidity is high
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Weather Variables CHECK YOUR NEIGHBOR
As air temperature increases, what happens to relative humidity?
Relative humidity increases.
Relative humidity decreases.
Relative humidity is unaffected by temperature.
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B. Relative humidity decreases.
Weather Variables CHECK YOUR ANSWER
As air temperature increases, what happens to relative humidity?
Relative humidity increases.
Relative humidity decreases.
Relative humidity is unaffected by temperature.
Explanation:
As temperature increases, the air is able to accommodate more water vapor. Water-vapor capacity increases and relative humidity decreases.
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B. Relative humidity decreases.
When dew point is high, what happens to relative humidity?
Relative humidity increases.
Relative humidity decreases.
Relative humidity is unaffected by dew point.
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Weather Variables CHECK YOUR NEIGHBOR
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A. Relative humidity increases.
Weather Variables CHECK YOUR ANSWER
When dew point is high, what happens to relative humidity?
Relative humidity increases.
Relative humidity decreases.
Relative humidity is unaffected by dew point.
Explanation:
Dew point is the temperature to which the air must be cooled to become saturated. A high dew point indicates a high water-vapor content, which means an increase in relative humidity.
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A. Relative humidity increases.
Weather Variables: Temperature and Pressure
Air pressure: The force exerted by the movement of air molecules into one another. The faster the air molecules move, the greater their kinetic energy and the greater the air pressure.
Warm air exerts more air pressure on its surroundings than cooler air.
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Weather Variables: Temperature and Pressure
The denser the air, the more molecular collisions and the higher the air pressure.
Air pressure, density, and temperature are interrelated.
Adiabatic processes occur when air is expanded or compressed without heat exchange.
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Weather Variables: Adiabatic Processes
With adiabatic expansion, the temperature of a dry (unsaturated) air parcel decreases by about 10ºC for each kilometer rise.
This rate of cooling for dry air is called the dry adiabatic lapse rate.
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Weather Variables: Adiabatic Processes
Chinooks—warm, dry winds—occur when cold air moving down a mountain slope is compressed as it moves to lower elevations and becomes warmer.
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Weather Variables: Adiabatic Processes
Adiabatic processes also occur in moist air.
As rising air cools to its dew point, water vapor condenses to form clouds.
Because the process of condensation releases heat, the surrounding moist air cools at a lesser rate of 6ºC for each kilometer rise.
This rate of cooling for moist air is called the moist adiabatic lapse rate.
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Weather Variables: Atmospheric Stability
In normal conditions, air temperature decreases with altitude.
This rate of cooling varies from place to place, and can vary over the course of a day.
This rate of cooling with altitude is called the environmental lapse rate.
The average environmental lapse rate decreases about 6.5ºC for each kilometer rise in elevation.
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Weather Variables: Atmospheric Stability
If rising air stays warmer than the surrounding air, it will continue to rise instead of returning to its starting position. This is unstable air.
Eventually, the air parcel will expand and cool sufficiently to match the surrounding air. When the temperatures match, the air parcel stops rising, but it does not sink back to its starting position.
Unstable rising air tends to form clouds with vertical development: Cumulus type clouds.
Stable air resists upward vertical motion and tends to form clouds that spread horizontally: Cirrus and stratus type clouds.
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Weather Variables
When upper regions of the atmosphere are warmer than lower regions, which is opposite of what normally occurs, we have a temperature inversion.
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Weather Variables CHECK YOUR NEIGHBOR
What does the image below demonstrate?
Condensation
Air pressure
Temperature inversion
All of the choices
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A. Condensation
Weather Variables CHECK YOUR ANSWER
What does the image below demonstrate?
Condensation
Air pressure
Temperature inversion
All of the choices
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A. Condensation
Cloud Development
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Cloud Development
Clouds develop when condensation rate exceeds evaporation rate above the lifting condensation level.
A rising air parcel cools at the dry adiabatic lapse rate until it reaches saturation. After saturation, the moist adiabatic lapse rate controls how thick the cloud will become.
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Cloud Development
Height of the cloud base and how thick the cloud becomes depend on:
Environmental lapse rate
Dry adiabatic lapse rate
Moist adiabatic lapse rate
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Cloud Development CHECK YOUR NEIGHBOR
Which of the following clouds appears at highest altitude?
Stratus
Nimbostratus
Altocumulus
Cirrus
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D. Cirrus
Cloud Development CHECK YOUR ANSWER
Which of the following clouds appears at highest altitude?
Stratus
Nimbostratus
Altocumulus
Cirrus
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D. Cirrus
What happens to the relative humidity of a rising air parcel at the lifting condensation level?
Relative humidity decreases marking the upper limit of cloud formation.
Relative humidity increases and the air parcel stops rising.
Relative humidity increases to 100% and the air parcel is saturated.
Relative humidity decreases and cloud formation begins.
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Cloud Development CHECK YOUR NEIGHBOR
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C. Relative humidity increases to 100% and the air parcel is saturated.
Cloud Development CHECK YOUR ANSWER
What happens to the relative humidity of a rising air parcel at the lifting condensation level?
Relative humidity decreases marking the upper limit of cloud formation.
Relative humidity increases and the air parcel stops rising.
Relative humidity increases to 100% and the air parcel is saturated.
Relative humidity decreases and cloud formation begins.
Comment:
The lifting condensation level marks the base of the cloud.
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C. Relative humidity increases to 100% and the air parcel is saturated.
Cloud Development: Precipitation Formation
Each step toward precipitation is part of the collision-coalescence process.
Formation of dust
Updrafts
Growth of stationary drops of water
Falling of raindrops
Vertical development in the cloud is necessary so that enough droplet collisions occur.
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Cloud Development: Precipitation Formation
Raindrops shrink as they fall, because the evaporation rate exceeds the condensation rate once they leave the cloud.
If enough evaporation occurs, raindrops may disappear before they reach the ground this is called virga.
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Air Masses, Fronts, and Storms
Air masses fall into six categories:
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Air Masses, Fronts, and Storms
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Air Masses, Fronts, and Storms
Atmospheric lifting—lifting of air.
Three types:
Convectional lifting—cumulus clouds
Orographic lifting—rain shadow
Frontal lifting—cirrus clouds changing to cumulonimbus clouds
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Air Masses, Fronts, and Storms
Convectional lifting:
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Air Masses, Fronts, and Storms
Orographic lifting:
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Air Masses, Fronts, and Storms
Frontal lifting:
Cold front
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Air Masses, Fronts, and Storms
Frontal lifting:
Warm front
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Air Masses, Fronts, and Storms
Frontal lifting
Occluded front
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Air Masses, Fronts, and Storms: Cyclones
A cyclone is an area of low pressure around which winds flow.
Due to the Coriolis force, winds in a cyclone move:
Counterclockwise in the Northern Hemisphere
Clockwise in the Southern Hemisphere
Air converges in the center (lowest pressure) and is forced to rise upward.
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What is the name for a front that occurs when a cold front and warm front merge?
Convection
Occluded
Stationary
Turbulent
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Air Masses, Fronts, and Storms CHECK YOUR NEIGHBOR
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B. Occluded
Air Masses, Fronts, and Storms CHECK YOUR ANSWER
What is the name for a front that occurs when a cold front and warm front merge?
Convection
Occluded
Stationary
Turbulent
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B. Occluded
Violent Weather
Storms are defined as violent and rapid changes in the weather.
Three major types of severe storms:
Thunderstorms
Tornadoes
Hurricanes
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Violent Weather
Thunderstorms begin with humid air rising, cooling, and condensing into a single cumulus cloud.
When fed by unstable, moist air, a cumulus cloud grows into a thundercloud.
Thunderstorms contain immense amounts of energy.
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Violent Weather
All thunderstorms include thunder and lightning.
The electrical energy flowing from cloud to ground is lightning.
As lightning heats up the air, the air expands and we hear thunder.
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Violent Weather
Tornado: a funnel-shaped column of air rotating around a low-pressure core that reaches from a cumulonimbus cloud to the ground.
A funnel cloud is similar to a tornado, but it does not touch the ground.
Tornadoes are dangerous because of their suction and also the battering from their high winds.
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Violent Weather
Hurricanes are the greatest storms on Earth—energy comes from latent heat released from condensing water vapor.
Rising warm air creates low pressure near the surface, drawing in more moist air.
Winds rotate around a central low-pressure area—the eye of the storm.
There is a continuous supply of energy from tropical waters—a hurricane weakens as fuel is cut off (as it makes land fall or enters an area of cooler water).
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Violent Weather
Strong vertical wind shear can cause warm air to tilt inward and spiral. It can develop into a tropical depression.
If the storm intensifies, it progresses into a tropical storm, with increased wind speeds.
Hurricane—winds up to 300 km/hour.
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Violent Weather CHECK YOUR NEIGHBOR
What is the eye of a hurricane?
Area of high pressure
Area of highest level of precipitation
Area of low pressure
Area where upper-level air descends
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C. Area of low pressure
Violent Weather CHECK YOUR ANSWER
What is the eye of a hurricane?
Area of high pressure
Area of highest level of precipitation
Area of low pressure
Area where upper-level air descends
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C. Area of low pressure
Weather Forecasting
Weather forecasting involves collecting data from all over the world.
Computers can plot and analyze data and predict weather, although the many variables make accuracy difficult.
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Weather Forecasting
Weather symbols are used to represent data for various locations—sky cover, wind direction and speed, dew point, temperature, and pressure. On the weather map, these station models are used to draw lines of equal pressure (isobars), which are used to represent frontal systems.
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