The Earth Grid and Seasons

1
The Earth Grid and Seasons
2
Latitude and Longitude

• Since the Earths surface is approximately
  spherical in shape and weather is global, we need
  a way to identify uniquely any location on the
  surface of the Earth.
• Most meteorological applications use latitude and
  longitude to identify the location of any point
  on the Earths surface.

3
Latitude

• Latitude is defined as the angle between a
  point on the surface of the Earth, the center of
  the Earth, and the equatorial plane.
• The equatorial plane divides the Earth equally
  into northern and southern halves.

4
NP
Latitude
Equatorial Plane
SP
5
Latitude (Cont.)

• Connecting places with the same latitude produces
  concentric circles.
• Since latitude circles are parallel to each
  other, they are sometimes called parallels.

6
(No Transcript)
7
Significant Latitudes

• North Pole 90 N
• Arctic Circle 66.5N
• Tropic of Cancer 23.5N
• Equator 0 N
• Tropic of Capricorn 23.5S
• Antarctic Circle 66.5S
• South Pole 90 S

8
Longitude

• Longitude is the angle between the Prime
  Meridian, the North Pole and the meridian running
  through a point on the surface of the Earth.
• The Prime Meridian is the 0 meridian that runs
  through Greenwich, England

9
(No Transcript)
10
The Earth Grid

• The combination of latitude and longitude allows
  us to provide a unique designation for the
  location of any point on the surface of the Earth.

11
(No Transcript)
12
Locations of Certain U.S. Cities

• Columbus, Ohio 40 N, 83 W
• Boston, MA 42 N, 71 W
• New Orleans, LA 30 N, 90 W
• Miami, FL 25.5N, 80 W
• Denver, CO 40 N, 105W
• Seattle, WA 47 N, 122W
• Honolulu, HI 21 N, 158W
• Point Barrow, AK 71 N, 157W

13
Earth/Sun Relationships

• The Earth revolves around the Sun in a slightly
  elliptical orbit that takes approximately 365.25
  days to complete. This defines the length of a
  year on earth.

14
Earth/Sun Relationships (Cont.)

• The Earth makes its closest approach to the Sun
  in early January when it is approximately
  1.47x1011 m from the Sun.
• The Earth is farthest from the Sun in early July
  when it is approximately 1.52x1011 m from the
  Sun.

15
July
January
1.47x1011 m
1.52x1011 m
16
Earths Rotation

• The Earth rotates around its axis completely once
  every 86165.5 s (23.93 hours). This determines
  the length of a day.

17
Local Solar Time (LST)

• The Local Solar Time (LST) is based on the
  definition of noon at a given location as being
  the instant when the Sun is at its highest
  position (greatest altitude above the horizon) in
  the sky.
• The Local Standard Time (LT) is the time defined
  for a large region based on its general location.

18
Local Solar Time (Cont.)

• Local Standard Time Meridian (LSTM) is a
  reference meridian of longitude used for a
  particular time zone.
• LSTM 15 x dtGMT
• where
• dtGMT is the difference between LT and
  Greenwich Mean Time (GMT)

19
Local Solar Time (Cont.)

• For Columbus, Ohio
• LSTM 15 x 5 75

20
Local Solar Time (Cont.)

• The Equation of Time (EoT) is an empirical
  equation that factors in the eccentricity of the
  Earths elliptical orbit and the tilt of the
  Earths axis.

21
Local Solar Time (Cont.)

• EoT (9.87 min) sin(2B) (7.53 min) cos(B) -
  (1.5 min) sin(B)
• where
• B (360/365 days) x (d 81 days)
• and
• d is the number of days since the start of the
  year

22
Local Solar Time (Cont.)

• On April 1 d 91 days.
• B (360/365 days) x (91days 81days)
• B 9.863
• EoT (9.87 min) sin (2 x 9.863)
• (7.53 min) cos (9.863)
• (1.5 min) sin (9.863)

23
Local Solar Time (Cont.)

• EoT 3.331 min 0.985 min 0.257 min
• EoT 2.089 min 125.34 s

24
Equation of Time (EoT)
25
Local Solar Time (Cont.)

• The Time Correction Factor (TCF) accounts for the
  difference in location between any point and the
  LSTM.
• TCF (4 min/)(LSTM Longitude) EoT

26
Local Solar Time (Cont.)

• TCF (4 min/) x (75 - 83) 2.089 min
• TCF -29.91 min

27
Local Solar Time (Cont.)

• LST LT TCF
• LST LT 29.91 min

28
Seasons

• If the distance from the Sun were the only thing
  that affected the seasons of the year, we would
  be warmest in January, when the Earth is closest
  to the Sun, and we would be coldest in July when
  the Earth is farthest from the Sun.
• Obviously, some other factor is more important in
  determining our seasons.

29
Seasons (Cont.)

• Our seasonal variations in solar radiation are
  caused by the tilt of the Earths axis of
  rotation.
• The Earths axis of rotation is currently tilted
  at an angle of 23.5 from the vertical.

30
North Pole
23.5
South Pole
31
Seasons (Cont.)

• This means the northern half of the Earth is
• pointed away from the Sun and during the
• rest of the year it is point toward the Sun.

32
December Solstice
NP

33
December Solstice

• Occurs around December 22
• Sun is directly overhead at latitude 23.5S at
  noon Local Solar Time
• Beginning of astronomical winter in the northern
  hemisphere and summer in the southern hemisphere
• Is called the winter solstice in the northern
  hemisphere

34
December Solstice (Cont.)

• Locations north of the Equator have their
  shortest period of daylight.
• Columbus, Ohio has 9.1 hours of daylight and 14.9
  hours of darkness.

35

NP
March (vernal) equinox
36
March (Vernal) Equinox

• Occurs around March 20
• Sun is directly overhead at the Equator at noon
  Local Solar Time
• Beginning of astronomical spring in the northern
  hemisphere and astronomical autumn in the
  southern hemisphere
• Is called the spring (vernal) equinox in the
  northern hemisphere

37
March Equinox (Cont.)

• All locations have 12 hours of daylight and 12
  hours of darkness.

38

NP
June Solstice
39
June Solstice

• Occurs around June 22
• Sun appears directly overhead at latitude 23.5N
  at noon Local Solar Time
• Beginning of astronomical summer in the northern
  hemisphere and winter in the southern hemisphere
• Is called the summer solstice in the northern
  hemisphere

40
June Solstice (Cont.)

• Locations north of the Equator have their longest
  period of daylight
• Columbus, Ohio has 14.9 hours of daylight and 9.1
  hours of darkness

41
September (autumnal) equinox
NP

42
September (Autumnal) Equinox

• Occurs around September 23
• Sun is directly overhead at the Equator at noon
  Local Solar Time
• Beginning of astronomical autumn in the northern
  hemisphere and astronomical spring in the
  southern hemisphere
• Is called the autumnal (fall) equinox in the
  northern hemisphere

43
September Equinox (Cont.)

• All locations on the Earth have 12 hours of
  daylight and 12 hours of darkness

44
Meteorological Seasons

• To make it easier to keep records of the seasons
  meteorologists define them on the basis of months
  of the year.
• In the northern hemisphere
• Winter - December, January, February
• Spring - March, April, May
• Summer - June, July, August
• Autumn - September, October, November