Lecture 7 Flashcards
Describe Earth’s energy balance
- there is an equilibrium between incoming radiation and outgoing radiation
- Earth intercepts only a small portion of the Sun’s radiation
- The Sun’s energy drives the hydrologic cycle and all weather phenomena on Earth
- Nearly all of the energy available at Earth’s surface comes from the Sun
What is the atmosphere composed of?
- composed of nitrogen (78%) and oxygen (21%)
- the remaining 1% consists of water vapour, carbon dioxide, and other ‘trace’ gases
- water vapour in the atmosphere can result in cloud development and the formation of precipitation
- water vapour comes from evaporation from the oceans
Where is weather confined in the atmosphere?
- all weather (ie. clouds and precipitation) is confined to the troposphere
- the ozone layer (25km above surface) protects us from the Sun’s harmful UV rays; it is found in the stratosphere
How are clouds named?
Prefix:
- cirro (high cloud)
- alto (mid-level cloud)
- strato (low cloud)
Suffix:
- cumulus (puffy)
- stratus (flat)
“nimb”: clouds that produce precipitation
What is cumulonimbus cloud?
-a cloud that produces lightning, thunder, and heavy rain
What is atmospheric pressure?
- it is a force exerted on the Earth’s surface by the weight of air molecules
- it is also called barometric pressure and is measured with a barometer
- pressure decreases with increasing altitude
What results from differences in pressure at Earth’s surface?
- causes wind to blow
- in the northern hemisphere: at areas of low pressure air circulates counter-clockwise and rises which results in clouds and precipitation, at areas of high pressure air circulates clock-wise and sinks which results in clear skies
What is a front?
- a front marks the boundary between two air masses
- the name of the front describes the type of air behind that front
- at a cold front, dense cold air undercuts warm air
- at a warm front, the less dense warm air overrides cold air
- cold fronts violently force warm air up so they are associated with cumulonimbus clouds
How is tornado formed?
- start as horizontal spinning columns in the clouds
- hot air rising off of hot ground moves the horizontal column vertically
- whole thing is called mesocyclone
What is required for thunderstorm development?
- an unstable environment (steep vertical temperature gradient)
- water vapour
- rising air (or a lifting mechanism, eg. a front)
What are the stages of thunderstorm development?
- cumulus
- mature
- dissipative
- most storms pass through all 3 stages
What is hail?
- it is formed during thunderstorms in cumulonimbus clouds
- updrafts in the cloud repeatedly force a water droplet upward
- the droplet develops a ring of ice around it each time it enters the cold part of the cloud
- the ball of ice eventually becomes heavy enough to fall to the surface
What is lightning?
- a spark of electricity occurring in a cloud
- the majority of lightning strikes within the cloud
- lightning heats the air causing the air to expand thus creating a shockwave (thunder)
- sometimes the atmosphere refracts thunder making it inaudible
What is the main requirement for lightning?
- a cumulonimbus cloud containing a region of opposite charges
- the interaction of ice crystals, hailstones, and water droplets result in a separate distribution of charges in the cloud
What is a tornado?
- a rotating column of air touching the ground that formed within a supercell thunderstorm
- a rotating column not touching the ground is called a funnel cloud
What are characteristics of a tornado?
- on average they are approximately 300m wide
- they travel from the southwest toward the northeast at an average speed of 50km/h
- they tend to exist for less than 30 mins with a defined life cycle
- the most common season for tonadoes is Summer in Canada and Spring in the US
What is the life cycle of a tornado?
Organizational stage:
- wind shear causes rotation to develop
- funnel cloud protrudes from above
- dust and debris rotates beneath
Mature stage:
-most severe damage occurs at this stage
Rope stage:
-the tornado stretches out and weakens
What is a wind shear?
-a change in wind speed or wind direction over a horizontal or a vertical distance
How are tornadoes classified?
- Enhanced Fujita Scale: tornadoes are classified on a scale of EF0 to EF5 based on the damage produced
- EF5 tornadoes: complete devastation; wind speed over 322km/h, less than 1% of all tornadoes are EF5
Where are the tornado alleys located>
- the US experiences the most tornadoes on Earth
- Canada experiences the 2nd most
- US tornado alley: Kansas and Oklahoma
- Canada Tornado alley: Southwestern ontario
Why do tornado alleys exist?
- they are areas where air masses commonly collide
- they are areas of relatively flat land (this allows for undistrubed rotation)
How do tornadoes occur in Canada’s tornado alley?
- tornadoes in Ontario occur when a southwesternly wind brings warm, moist air from the Gulf of Mexico
- the warm moist air often interacts with cooler lake breezes
Describe the 1974 Super Outbreak of tornadoes
-on this day 148 tornadoes touched down between Ontario and Alabama
When did the largest tornado outbreak in world history occur?
- 2011 from April 25-28
- in the southeast US 358 tornadoes touched down and 324 people were killed (239 in Alabama)
- overall more people were killed by tornadoes in 2011 than any other year since 1925
Describe the Joplin tornado
- May 22 2011
- an EF5 tornado killed 162 people in Joplin Missouri
- this tornado was the most costly in US history (2.2B) and the deadliest in the US since 1947
Describe the Goderich tornado
- a tornado in Goderich killed 1 person and destroyed much of the town core on Aug 21 2011
- it was the first EF3 tornado to touch down in Ontario in 15 years
- the tornado was spotted over Lake Huron on RADAR and a warning was issued 12 minutes before it reached town
Describe the Moore tornado
- on May 20, 2013 an EF4 tornado in Moore, Oklahoma killed 24 people
- a tornado warning was issued for the area 16 minutes in advance
- advances in weather technology have greatly improved warning systems over the past decade
What is fog?
- it is a cloud with its base at the Earth’s surface that reduces visibility
- it occurs at night when the air cools to the dew point (at which point water vapour condenses into droplets)
- fog can also form when warm air moves over a cold body of water
What conditions need to be met for blizzards?
- wind of at least 40km/h
- snow falling or blowing snow occurring
- visibility less than 400m
- all of these must occur for 4 hours
- pretty rare in southwestern ontario
What is lake effect snow?
- lake effect snow is caused by cold air moving over relatively warm water
- heavy snow falls downwind of lakes
- snowbelts are found downwind of the lakes (in winter, the wind is often from the northwest)
- heavy snow falls downwind of lakes
- London and Guelph often receive lake effect snow from Lake Huron causing high annual snowfall
- Windsor occasionally receives lake effect snow from Lake Michigan
What are lake effect clouds?
- all of southern Ontario frequently experiences lake effect clouds in winter
- both lake effect clouds and lake effect snow diminish when the lakes freeze (this can occur by February)
What are haboobs?
- sandstorms that occur in arid and semi-arid regions
- downdrafts on the leading edge of a thunderstorm cause a haboob to form
What are dust devils?
- a small spinning vortex of air formed over hot, dry land
- as hot air rises wind direction may change due to obstacles
- this may result in a spinning column of air
What are ice storms?
- these are mainly caused by freezing rain
- the weight of the ice can pull down trees and power lines
- freezing rain is rain that freezes upon impacting the surface
What are droughts?
- this is an extended period of unusually low precipitation
- droughts affect more people in NA than any other hazard
- they cause water shortages that can lead to crop failure
- in developing countries, this may lead to malnutrition and famine
- droughts are linked to global weather patterns and are a normal part of the climate system
What is wind chill?
-it is a correction factor to a temperature reading caused by the presence of wind making the air feel cooler than the temperature suggests
What is the humidex?
-it is a correction factor to a temperature reading caused by high levels of humidity making the air feel warmer than the temperature suggests
How can we minimize severe weather hazards?
- forecasting has improved dramatically with better technology; 3 day forecasts are very accurate today
- weather satellites detect cloud cover and aid in forecasting hurricanes and mid-latitude cyclones
- RADAR detects precipitation (both the type of precipitation and intensity)
What is a watch?
- an alert covering a wide area
- conditions favour the development of hazardous weather but none has been reported
- ex: tornado watch, winter storm watch
What is a warning?
- an alert that usually covers smaller areas
- it indicates that hazardous weather is currently occurring in the area
- ex: severe thunderstorm warning
What is an advisory?
- used to alert the public of less hazardous weather conditions
- dense fog advisory
In which layer of the atmosphere are most clouds found?
troposphere
