Answer the following questions in about 30 words.
(i) What is the unit used in measuring pressure? Why is the pressure measured at station level reduced to the sea level in preparation of weather maps?
(ii) While the pressure gradient force is from north to south, i.e. from the subtropical high pressure to the equator in the northern hemisphere, why are the winds north easterlies in the tropics.
(iii) What are the geotrophic winds?
(iv) Explain the land and sea breezes.
(i) The unit used to measure air pressure is BAR. More commonly, mb or milli-bar is used to report the figures.
The pressure on weather maps is reported at sea level due to the gravitational effects. At sea level, the air is denser and exerts higher pressure.
(ii) The winds in the northern hemisphere are characterised as north-easterlies because:
• North: The wind follows the pressure gradient. It originates in STH and flows towards the equator.(N->S)
• Easterlies: Due to the rotation effect of Earth (Coriolis Force), the winds get deflected to the right in the Northern Hemisphere. Hence, they seem to originate in the east and flow westwards.
(iii) In a situation where isobars are parallel to each other and are generally straight, the inwards pressure gradient force is balanced out by outwards Coriolis force. Consequently, the winds flow parallel to the isobaric lines. This is known as geotropic winds.
(iv) Land and Sea breeze constitute the local wind systems in coastal areas. This is produced by differential heating of land and sea. As land heats and cools faster than the sea, there is a reversal of direction of the breeze from day to night. During the day, sea breeze blows towards land and at night, the land breeze blows towards sea.