NCERT CLASS 7 CHAPTER 7 YOUTUBE LINK: https://youtu.be/2BIn3EgAkmU

1. WATER
Class 7
NCERT
Chapter-7

2. INTRODUCTION
• Watery part of earth comprises hydrosphere
• 1,250 million cubic km is the estimated total
amount of water on earth’s surface
• 97% of this water is in the oceans
• There are 4
main oceans.

4. TEMPERATURE &SALINITY OF OCEANS
• Temperature decreases from equator towards the
poles
• Average temperature near equator is 26⁰C
• Highest temperatures recorded at enclosed tropical
seas
• Temperature also decreases with depth
• Salt content in ocean water is called salinity; average
salinity is 35per thousand
• Salinity decreases towards equator due to heavy
rainfall
• Dead sea in Asia has highest salinity

5. CIRCULATION OF OCEAN WATER
• Ocean water is always in motion; it moves both
horizontally and vertically
• These movements are due to- prevailing winds;
difference in temperature and salinity; variation
in density
• Circulations takes place in 3 ways- Waves, Tides
and Ocean currents
• Sometimes ocean and tidal currents combine to
form dangerous whirlpools called maelstroms in
the ocean

6. WAVES
• Waves are formed when the wind blows
• The rise and fall of water in wave represents crest
and trough respectively
• The horizontal distance between two crests or
two troughs is Wavelength
• The vertical distance from crest to trough is wave
height
• Tsunami’s are huge oceanic waves formed during
storms, earthquakes or volcanic eruptions

7. TSUNAMI

8. INDIAN OCEAN TSUNAMI(2004)
• On 26 December 2004, a massive undersea earthquake
occurred off the west coast of Sumatra(Indonesia)
• Measured 9.1 on Richter scale
• Displaced the ocean floor by about 20m
• The tsunami devastated shores of Indonesia, Sri Lanka,
India, Thailand and some countries along east coast of
Africa
• Travelling at a speed of 800km/hr and a wave height of
20m
• Indira Point was completely submerged

9. TIDES
• The phenomena of alternate rise and fall in the
level of sea water is called tides
• Importance of tides-
Makes some rivers navigable for ocean going
vessels
Prevents siltation of harbors
Helps in production of common salt
Aid to fishing industries
Tidal power generation

10. TYPES OF TIDES

11. OCEAN CURRENTS
• Movement of ocean water in definite
direction on ocean surface is ocean currents
• Develop due to- variations in temperature and
density of ocean water; prevailing winds;
rotation of earth; shape and position of coasts
• Move at speed of 3-10km/hr
• Broad ocean currents are called drift
• On the basis of temperature they are of two
types- warm currents and cold currents

12. CURRENTS OF PACIFIC OCEAN
• In Northern hemisphere-
North equatorial current
Kuroshio current
North pacific current
California current (cold)
• In Southern hemisphere-
South equatorial current
East Australian current
West wind drift(cold)
Peru current(cold)

13. CURRENTS OF ATLANTIC OCEAN
• In Northern hemisphere-
Gulf stream
North Atlantic drift
Canary current(cold)
Labrador current(cold)
Greenland current(cold) • In Southern hemisphere
Brazil current
West wind drift(cold)
Benguela current(cold)
Falkland current(cold)

14. CURRENTS OF INDIAN OCEAN
• In Northern hemisphere-
North equatorial current
Monsoon drift
• In Southern hemisphere
South equatorial current
Mozambique current
West wind drift(cold)
West Australian
current(cold)

15. EFFECTS OF OCEAN CURRENTS
• Influence distribution of temperature over the
globe
• Warm currents increase moisture-bearing
capacity of winds while cold currents decrease
the same
• Help in navigation
• Junction of warm and cold currents forms the
best fishing ground