WeatherWeatherChapter 14Chapter 14

Science 1206Science 1206

14.2 Weather Systems14.2 Weather Systems

Weather systems are a set of temperature, Weather systems are a set of temperature, wind, pressure, and moisture conditions wind, pressure, and moisture conditions for a certain region that moves as a unit for a certain region that moves as a unit for a period of days. for a period of days.

Air MassAir Mass

An air mass is a body of air that extends over a An air mass is a body of air that extends over a large area and has nearly uniform temperature large area and has nearly uniform temperature and humidity in any horizontal direction. and humidity in any horizontal direction. Places where air masses form are called source Places where air masses form are called source regions, and they are generally flat with light regions, and they are generally flat with light winds. winds. Ideal source regions are those dominated by Ideal source regions are those dominated by large high-pressure areas, such as the arctic large high-pressure areas, such as the arctic plains in winter and the subtropical oceans and plains in winter and the subtropical oceans and desert regions in summer. desert regions in summer.

Air masses continuedAir masses continuedAir masses are classified according to their Air masses are classified according to their source region.source region.

Polar air masses originate over cold regions. Polar air masses originate over cold regions.

Tropical air masses originate over the warm Tropical air masses originate over the warm tropics. tropics.

Continental air masses originate over land, and Continental air masses originate over land, and maritime air masses originate over water. maritime air masses originate over water.

A cold, dry air mass that forms over land is A cold, dry air mass that forms over land is called a continental polar air mass and a hot called a continental polar air mass and a hot moist air mass that forms over water is called a moist air mass that forms over water is called a maritime tropical air mass.maritime tropical air mass.

Low-Pressure SystemsLow-Pressure Systems

FrontsFrontsFronts Fronts are boundaries between two air masses are boundaries between two air masses with different temperatures or densities.with different temperatures or densities.Temperature can change abruptly when fronts Temperature can change abruptly when fronts pass overhead.pass overhead.A A cold frontcold front is the boundary between warm and is the boundary between warm and cool air when the cool air is advancing to replace cool air when the cool air is advancing to replace the warm air. the warm air. At a cold front cold air following warm air At a cold front cold air following warm air undercuts the warm air, heaving it upwards with undercuts the warm air, heaving it upwards with a more violent thrust compared to the steady a more violent thrust compared to the steady rise of air at a rise of air at a warm frontwarm front. .

Low Pressure SystemsLow Pressure SystemsTend to bring cloudy skies Tend to bring cloudy skies and stormy weatherand stormy weather

FrontFront:: the boundary between a the boundary between a cold air mass and a warm air cold air mass and a warm air massmass

Warm frontWarm front:: the leading edge the leading edge of a warm air mass of a warm air mass

  

Cold front:Cold front: the leading edge the leading edge of a cold air massof a cold air mass

Warm FrontWarm Front

A A warm frontwarm front exists when warm air is rising over exists when warm air is rising over cold air. cold air.

The boundary takes the form of a gradual slope The boundary takes the form of a gradual slope and lifting is slow but persistent. and lifting is slow but persistent.

As the air As the air liftslifts into regions of lower into regions of lower pressurepressure, it , it expands, expands, coolscools and and condensescondenses water vapour as water vapour as flat sheet flat sheet cloudcloud (altostratus), from which (altostratus), from which rainrain can can start to fall once cloud has thickened to about start to fall once cloud has thickened to about 2,500 metres from the ground 2,500 metres from the ground

Occluded FrontsOccluded Fronts

Occluded Fronts Occluded Fronts are usually associated with are usually associated with warmwarm and and coldcold fronts separating warm and cold fronts separating warm and cold sectors of air. sectors of air. The lighter warm air rises above the heavier cold The lighter warm air rises above the heavier cold air, more gently at a warm front but more air, more gently at a warm front but more vigorously at the cold front following behind. vigorously at the cold front following behind. Cold fronts usually travel faster than warm Cold fronts usually travel faster than warm fronts, and therefore at some stage of fronts, and therefore at some stage of depression development, the cold front catches depression development, the cold front catches up with the warm front. up with the warm front.

Fronts ContinuedFronts Continued

The air associated with a cold front is usually The air associated with a cold front is usually unstable and conducive to cumulonimbus cloud unstable and conducive to cumulonimbus cloud formation.formation.Usually, rainfall associated with cold fronts is in Usually, rainfall associated with cold fronts is in the form of heavy deluge. More rain may fall in a the form of heavy deluge. More rain may fall in a few minutes as the cold front passes than during few minutes as the cold front passes than during the whole passage of a warm front. the whole passage of a warm front. As the cold front passes, the clouds roll by and As the cold front passes, the clouds roll by and the air temperature may become noticeably the air temperature may become noticeably cooler, with temperatures dropping by 5°C or cooler, with temperatures dropping by 5°C or more within the first hour.more within the first hour.

Stationary FrontsStationary Fronts

Stationary FrontsStationary Fronts occurs when two unlike air occurs when two unlike air masses face one another, and there is little masses face one another, and there is little movement of air, the surface which separates movement of air, the surface which separates them. them. The frontal slope is like that of a warm front.The frontal slope is like that of a warm front.The type of weather that exists at a stationary The type of weather that exists at a stationary front is similar to warm front weather.front is similar to warm front weather.Eventually, either one or both of the air masses Eventually, either one or both of the air masses begin to move resulting in a warm front or cold begin to move resulting in a warm front or cold front.front.

Continued…Continued…

Occluded FrontOccluded Front– cold front catches cold front catches

up with a warm up with a warm front; the warm front; the warm air is lifted above air is lifted above the earth’s the earth’s surface and is cut surface and is cut off (occluded) off (occluded) from the cooler from the cooler air belowair below

Stationary front: Stationary front: warm and cold air warm and cold air masses remains still for some timemasses remains still for some time

Life Cycle of a Cyclone (fig 2, Life Cycle of a Cyclone (fig 2, pg 547)pg 547)

A: Warm and A: Warm and Cold front Cold front meetmeet

B:Main Stage:B:Main Stage:

Warm air is Warm air is pushed up, pushed up, lots of lots of precipitationprecipitation

Life Cycle of a Cyclone (fig 2, Life Cycle of a Cyclone (fig 2, pg 547)pg 547)

C: Occluding C: Occluding Stage: Storm Stage: Storm weakens as weakens as fronts combinefronts combine

D:Final StageD:Final Stage

Flow of air Flow of air dissapates, dissapates, storm endsstorm ends

Precipitation (14.5 - pg. 556)Precipitation (14.5 - pg. 556)

Precipitation: Precipitation: Water that Water that reaches the reaches the ground in either ground in either liquid or solid liquid or solid formform

Recall: The Water Recall: The Water Cycle:Cycle:

When air reaches its When air reaches its saturation pointsaturation point and the water vapor and the water vapor in the air condenses it forms liquid droplets.in the air condenses it forms liquid droplets.

Droplets join together and become heavy enough to fall to Droplets join together and become heavy enough to fall to the surface of earth.the surface of earth.

LiquidLiquid SolidSolid

Type of precipitation depends on conditions in the atmosphere and on the ground

Page 556

See Fig 2, pg. 556See Fig 2, pg. 556

Hail: Hail: – created in cumulonimbus cloudscreated in cumulonimbus clouds

– Frozen raindrops are circulated up and Frozen raindrops are circulated up and downdown

– Layers of ice are formedLayers of ice are formed

Dew:Dew:– Water vapour condenses near the Water vapour condenses near the

ground as the air coolsground as the air cools

– Cold Days : FrostCold Days : Frost

HumidityHumidityHumidity:Humidity:– a measure of the amount of a measure of the amount of

water vapor in the airwater vapor in the air– Warm air can hold more Warm air can hold more

moisture than cold airmoisture than cold air

Relative Humidity:Relative Humidity:– The measure of the The measure of the

amount of water vapor amount of water vapor compared to the compared to the MaximumMaximum amount possibleamount possible

Humidity CalculationsHumidity Calculations

Figure 1, pg. 558 – Maximum AmountsFigure 1, pg. 558 – Maximum Amounts

%100ionconcentrat maximum

ionconcentratHumidity Relative x

Ex 1:What is the relative humidity if 1.9 g/kg of water in air is present at 0oC?

Ex 2:What is the conc of H2O in the air at 20oC if the relative humidity is 50%?

Continued…Continued…

Saturated AirSaturated Air– Relative humidity is 100%Relative humidity is 100%– Over 100% results in condensationOver 100% results in condensation

Dew PointDew Point– Temp at which dew formsTemp at which dew forms– As air gets colder, it holds less moistureAs air gets colder, it holds less moisture– Releases excess as dewReleases excess as dew

Measuring Relative Humidity (pg. 560)Measuring Relative Humidity (pg. 560)

A A PsychrometerPsychrometer measures measures relative humidityrelative humidity– Two thermometersTwo thermometers– One dry, one kept wetOne dry, one kept wet– Spun in the air, temp dropsSpun in the air, temp drops– The difference between the The difference between the

two is used to find relative two is used to find relative humidityhumidity

Dry BulbDry Bulb Wet BulbWet Bulb Relative Relative HumidityHumidity

2222 1717

1818 1414

3232 3030

3232 2222

Effects of HumidityEffects of Humidity

Higher humidityHigher humidity– causes perspiration to evaporate slowly.causes perspiration to evaporate slowly.– Sweating doesn’t cool us as wellSweating doesn’t cool us as well

Low humidyLow humidy– Air is dryerAir is dryer– Skin may be uncomfortableSkin may be uncomfortable

Humidex:Humidex:– Reports how hot the humidity makes us feelReports how hot the humidity makes us feel

HomeworkHomework

Chapter 14 AssignmentChapter 14 Assignment

Weather Log DueWeather Log Due– Complete 3 graphs as wellComplete 3 graphs as well

TestTest

NotesNotes

Chapter 13 sections 1, 2, 3, 4, 6, 8, 9, 11, Chapter 13 sections 1, 2, 3, 4, 6, 8, 9, 11, 13.13.

Chapter 14 section 2, 4, 5, 6, 9Chapter 14 section 2, 4, 5, 6, 9