Weather Chapter 14 Science 1206

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

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

Air masses continued Air masses are classified according to their source region. Polar air masses originate over cold regions. Tropical air masses originate over the warm tropics. Continental air masses originate over land, and maritime air masses originate over water. A cold, dry air mass that forms over land is called a continental polar air mass and a hot moist air mass that forms over water is called a maritime tropical air mass.

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

Low Pressure Systems Tend to bring cloudy skies and stormy weather Front:the boundary between a cold air mass and a warm air mass Warm front:the leading edge of a warm air mass Cold front:the leading edge of a cold air mass

Warm Front A warm front exists when warm air is rising over cold air. The boundary takes the form of a gradual slope and lifting is slow but persistent. As the air lifts into regions of lower pressure, it expands, cools and condenses water vapour as flat sheet cloud (altostratus), from which rain can start to fall once cloud has thickened to about 2,500 metres from the ground liftspressurecoolscondensescloudrainliftspressurecoolscondensescloudrain

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

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

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

Continued… Occluded Front –cold front catches up with a warm front; the warm air is lifted above the earth’s surface and is cut off (occluded) from the cooler air below

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

Life Cycle of a Cyclone (fig 2, pg 547) A: Warm and Cold front meet B:Main Stage: Warm air is pushed up, lots of precipitatio n

Life Cycle of a Cyclone (fig 2, pg 547) C: Occluding Stage: Storm weakens as fronts combine D:Final Stage Flow of air dissapates, storm ends

Precipitation ( pg. 556) Precipitation: Water that reaches the ground in either liquid or solid form Recall: The Water Cycle:

When air reaches its saturation point and the water vapor in the air condenses it forms liquid droplets. Droplets join together and become heavy enough to fall to the surface of earth. LiquidSolid Type of precipitation depends on conditions in the atmosphere and on the ground

Page 556

See Fig 2, pg. 556 Hail: –created in cumulonimbus clouds –Frozen raindrops are circulated up and down –Layers of ice are formed Dew: –Water vapour condenses near the ground as the air cools –Cold Days : Frost

Humidity Humidity: –a measure of the amount of water vapor in the air –Warm air can hold more moisture than cold air Relative Humidity: –The measure of the amount of water vapor compared to the Maximum amount possible

Humidity Calculations Figure 1, pg. 558 – Maximum Amounts

Ex 1: What is the relative humidity if 1.9 g/kg of water in air is present at 0 o C? Ex 2: What is the conc of H 2 O in the air at 20 o C if the relative humidity is 50%?

Continued… Saturated Air –Relative humidity is 100% –Over 100% results in condensation Dew Point –Temp at which dew forms –As air gets colder, it holds less moisture –Releases excess as dew

Measuring Relative Humidity (pg. 560) A Psychrometer measures relative humidity –Two thermometers –One dry, one kept wet –Spun in the air, temp drops –The difference between the two is used to find relative humidity

Dry Bulb Wet Bulb Relative Humidity

Effects of Humidity Higher humidity –causes perspiration to evaporate slowly. –Sweating doesn’t cool us as well Low humidy –Air is dryer –Skin may be uncomfortable Humidex: –Reports how hot the humidity makes us feel

Homework Chapter 14 Assignment Weather Log Due –Complete 3 graphs as well

Test Notes Chapter 13 sections 1, 2, 3, 4, 6, 8, 9, 11, 13. Chapter 14 section 2, 4, 5, 6, 9