Chapter 4: Weather & Climate

(Pg. 54 – 73)

Introduction:

Distinguish between the terms weather & climate. P. 54

• Weather: the state of the atmosphere at any one place or time. (short term)

• Climate: the average weather conditions of a region over the LONG term. (Season, Month Annually)

• Both refer to temperature, rain fall, humidity etc.

Practice: Do the following phrases refer to weather or climate?

1. Annual precipitation of 200 mm. 2. Rain forecasted for tomorrow. 3. Normal July temperature averages 22oC. 4. Temperatures this week will range

between 20 - 30oC. 5. It was unusually hot this summer. 6. Vancouver has an annual frost free period

of 233 days. 7. The greatest annual precipitation 11, 680

mm falls on Mt. Waialeaie in Hawaii. 8. Residents in Florida are preparing to take

shelter from a hurricane.

Climate Weather Climate Weather

Climate Climate

Climate

Weather

In describing today’s weather:

• 9 features are often used!!

1. Temperature 2. Amount of precipitation 3. Type of precipitation 4. Atmospheric pressure 5. Wind direction 6. Wind speed 7. Cloud cover 8. Hours of sunshine 9. Humidity

How does cloud cover influence the range of temperatures from day to night. (2.1.2)

• Nightly cloud cover reduces

the range of temperature from day to night.

• Clouds reflect heat waves Heat below clouds is reflected back to earth.

• Without clouds heat escapes giving colder nights.

Lack of cloud cover Day AND Night… the effect…

• DAY: No clouds = higher day temperatures

Why?

• NO clouds to deflect/block heat from reaching the surface.

• NIGHT: No cloud cover = cold nightly temperatures.

Why?

• Heat escapes, nothing to TRAP it.

• Ex. Day 24o C, Night 15o C

Clear Day AND clouds over BEFORE night…the effect…

• DAY: No clouds = higher day temperatures Why? • NO clouds to deflect/block heat from reaching the surface.

• Cloud cover PRIOR to night = = warmer and muggy

nights. Why? • Heat being TRAPPED during the night…like a blanket

AND precipitation in the clouds.

• Ex. Day 24o C, Night 21o C

Cloudy Day AND NO clouds at Night… the effect…

• DAY: Cloud cover = lower daily temperatures

• Clouds deflect/block out sun & heat.

• NIGHT: NO cloud cover =even COLDER nightly temperatures.

• Already cooler temperatures become even lower.

• Ex. Day 15o C, Night 9o C

Cloudy Day AND cloudy Night…the effect…

• Day: Cloud cover = lower daily temperatures Why? • Clouds deflects/blocks out heat

• Night: Cloud cover = relatively constant temperatures in the night. Why? • Clouds traps current air temperatures and keeps them relatively

constant.

• Ex. Day 15o C, Night 12o C

So ……..

• Clear day/Clear Night: largest temp range (24–15=9)

• Clear day, cloudy night: small/low temp range (24-21=3)

• Cloudy day, clear night: moderate range (15-9=6)

• Cloudy day/cloudy night: small temp range(15-12=3)

Why does only about ½ of the sunlight directed at earth

actually reaches the earth?

• 6% is scattered in the atmosphere

• 24 % is reflected by clouds

• 4 % is reflected by the earth’s surface.

• 17 % is absorbed by Green House gases & dust

• 49% of sun’s radiation actually reaches the earth’s surface (pg. 57)

• While the 4 % that reflects from the earth’s surface is small it does vary with location & with time.

How/Why do you think it varies? • Winter vs. Summer? • Poles Vs. equator? • Land vs. ocean?

• HINT: Precipitation (Snow & water), reflection vs absorption, dust, cloud cover, angle of sun,

The Greenhouse Effect: (2.2.1)

• The Earth system behaves like a greenhouse.

• The atmosphere, like the glass/plastic of a greenhouse traps heat energy resulted increased temps.

• SHORT WAVE insolation reaches the ground and is absorbed.

• The Earth heats up and gives off LONG WAVE terrestrial radiation which is absorbed by the atmosphere causing it to heat up.

The Earth – Sun Relationship: (2.1.1, 2.1.4, 2.1.5 & 2.1.6)

• Because the Earth is a sphere, all areas on the planet do not receive the same intensity insolation.

• The Earth ROTATES on its axis toward EAST or counter clockwise when looking down from the North Pole.

• It takes the Earth 24 HOURS to make 1 complete rotation on its axis.

• Earth REVOLVES around the sun counter clockwise looking down from the North.

It takes 365.25 days to make 1 complete REVOLUTION.

This is why we have a LEAP YEAR every 4 years.

This keeps our calendar in sync with the Earth’s revolution.

SEE FIGURE ON PAGE 56 IN TEXT.

Tilt of the Earth:

• The earth is tilted on its axis 23.5o

• At different times of the year, the North (or South) is pointed towards the sun, while other times pointed away from the sun.

• Causes daylight to be longer or shorter.

• Contributes to the seasons as well.

• During part of the revolution, the North Pole is tilting away from the sun.

• At another time, the North Pole is tilted toward the sun.

• Titling toward and away from the sun affects the intensity of the insolation.

• As we tilt AWAY from the sun, insolation is less intense and we in the Northern Hemisphere experience cooler temps. (our WINTER)

• During our summer, the Northern Hemisphere tilts TOWARD the sun resulting in more intense radiation.

• SEE FIGURE 4.3 PAGE 56. (Magnifying Glass)

• The tilt of the axis also results in the length of DAYLIGHT and NIGHTTIME varying throughout the year.

Day vs Night & Length of day (daylight!)

• Rotation on the axis causes day and night.

• Tilt on axis causes length of day to be longer or shorter.

• When hemisphere is pointed towards the sun…

• that hemisphere receives more direct, intense heat radiation AND

• More hours of exposure to the sun

• Giving that hemisphere longer days AND

• Warmer temperatures (Summer!!)

Equal Day and Night…BUT…

Differences in Temperatures!? Why?

Intensity and Duration!!

Equinoxes vs. Solstices (2.1.3)

Equinoxes:

• At 2 times during the year the number of day and nights hours is equal. (equal night)

• MARCH 21 is the VERNAL (SPRING) EQUINOX.

• SEPT 22/23 is the AUTUMNAL EQUINOX.

• These dates mark the 1st day of Spring and Fall, respectively, in the Northern Hemisphere.

Solstices:

• In Northern Hemisphere, the sun is directly overhead at noon on JUNE 21 at 23.5oN. (Longest day of the year.)

• 1st day of SUMMER is called the SUMMER SOLSTICE.

• On Dec 22nd, the sun is directly overhead at noon at 23.5oS.

• In the Northern Hemisphere, this is the shortest day of the year. The beginning of WINTER, it is known as the WINTER SOLSTICE.

Season changes (2.1.4)

Seasonal changes are all driven by changes in:

The tilt of the earth on its axis AND

The position of the earth in its revolution around the sun

NOTE: These will ultimately determine:

the amount of available sunlight (called day length or photoperiod)

the sun's intensity (related to the angle at which it strikes the Earth).

Warmer vs cooler

Seasons Summary: SUMMER in the NORTH!!

•June 21st or 22nd.

•North = Summer

•More sun hours

•More direct sun

•Noon Sun 900 @ Tropic of Cancer

•South = Winter

•Less sun hours

•Less direct sun

A

Seasons Summary: FALL / AUTUMN in the

NORTH

•September 21st

•North = Fall

•12 hrs day & 12 hrs night

•Noon Sun 900 @ Equator

•South = Spring

A

B

A

B

C

Seasons Summary: WINTER in the NORTH

•Dec 21st

•North = Winter

•Less sun hours

•Less direct sun

•South = Summer

•More sun hours

•More direct sun

•Noon Sun 900 @ Tropic of Capricorn

A

B

C

D

Seasons Summary: SPRING in the NORTH

•March 21st

•North = Spring

•12 hrs day & 12 hrs night

•Noon Sun 900 @ Equator

•South = Fall

B

Seasons Summary Back to Summer AGAIN

SOUTH

NORTH

NORTH

SOUTH

EQUINOX/SOLSTICE?

SEASONS??? DATE???

Seasons animated: Sun’s Angle and

Intensity

Seasons and Ecliptic Simulator

Factors that Affect Climate

1. Earth-Sun Relationship

2. Latitude: (Obj.2.2.1 & 2.2.3)

0The Earth’s curvature causes the sun’s heat to be unevenly distributed.

0When the Sun’s rays strike the Earth’s spherical body, the region that corresponds to the equator receives more radiation than the region corresponding to the poles.

0The HIGHER the latitude (Northern/Southern Hemisphere) the COLDER it gets.

Intensity and Duration!!

3. Winds: (Obj. 2.3.1, 2.3.3, 2.3.4,

& 2.3.5) 0Wind: is the flow of air from an area of high pressure

to one of low pressure

0Prevailing Winds: is the major , constant winds blowing over the surface of Earth from high to low pressure areas.

0Air moves from HIGH PRESSURE to LOW PRESSURE (Always).

0There are major pressure belts over the globe:

There are three major wind systems:

0Trade Winds, Westerlies and Polar Easterlies.

0Each of the three systems relate to the major pressure belts:

1. Trade winds flow from the tropical high to the equatorial lows

2. The Easterlies from the polar highs to the subpolar lows

3. The Westerlies from the tropical highs to the subpolar lows.

0Winds are named according to the DIRECTION from which they blow. (Note: Coriolis Effect)

0Winds, as a result, influence temperatures.

0They also influence moisture levels.

Coriolis Force:

0Coriolis Force: is the tendency for objects or fluids to be deflected to the RIGHT of their path in the Northern Hemisphere and to the LEFT in the Southern Hemisphere; caused by the rotation of the earth.

0The effect of the Coriolis Force is negligible at the equator but increases in intensity towards the poles.

Animation of the Coriolis effect over Earth's surface

Wind’s Impact on Moisture: (Obj. 2.3.5, 2.3.6, & 2.3.7)

A. Orographic Rainfall:

0 Caused by abrupt changes in relief, air masses rise up mountains.

0Air is forced to rise (WINDWARD)because it is deflected upward by the high mountains.

0As the air rises, it cools and its ability to hold moisture lessens (condensation occurs).

0By the time the air reaches the highest point on the mountain, it has lost most of its water vapour it picked dup over the ocean.

0The air begins to descend down the inland (LEEWARD) side of the mountain, it compresses and becomes warmer.

0The relative humidity of the air lowers, having a drying effect.

0RAIN SHADOW: is an area of relatively low rainfall on the leeward side of the uplands.

0Refer to figure 4.13 on page 66.