Why is fog likely when the dew point is close to the air temperature?

The air is nearly saturated, so a small drop in temperature condenses vapour into droplets.

Why is a summit usually cooler than the foot of the hill?

Rising air expands and cools at about 0.6 degree per 100 m.

S5 Geography Weather and Climate, Paper 4: seasons and humidity

2026-11-02

Paper 4 works through lapse rate and relative humidity, adds the dew point, the winter monsoon, wet season rainfall and daily range, and closes with how a rising vapour capacity lowers relative humidity.

Focus　Lapse rate times height gives the fall, relative humidity is vapour over capacity, the dew point gap over the 100 m cooling rate gives cloud height, and a larger capacity with unchanged vapour lowers humidity.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Geography: Weather and Climate
Level	S5

02　Questions (10 items)

FoundationThe air at the foot of a hill is 25 degrees Celsius and cools 0.6 degree per 100 m. What is the temperature at 2400 m?
FoundationAir holds 15 grams of vapour per cubic metre and the saturation amount is 20 grams. What is the relative humidity?
FoundationWhat is the dew point?
FoundationWhich monsoon prevails in Hong Kong in winter and what weather does it bring?
IntermediateThe air is 28 degrees with a dew point of 16 degrees and the saturated air cools 0.8 degree per 100 m. At what height does cloud form?
IntermediateAnnual rainfall is 2300 mm and 78 per cent falls from May to September. How much falls in the wet season?
IntermediateA day reaches 24.5 degrees and falls to 16.8 degrees. What is the daily range?
ChallengeA summit reads 12 degrees and the foot 27.6 degrees with a lapse rate of 0.6 degree per 100 m. How high is the hill?
ChallengeVapour stays at 12 grams per cubic metre while the air warms from 20 to 30 degrees and capacity rises from 17 to 30 grams. How does relative humidity change?
Error redoError: a student says Hong Kong has the northeast monsoon in summer. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	10.6 degrees Celsius	Two thousand four hundred metres is twenty four steps of 100 m, so 14.4 degrees are lost and 25 minus 14.4 is 10.6.
2	75 per cent	Fifteen over twenty times 100 is 75, so the air holds three quarters of its capacity.
3	The temperature at which air becomes saturated and vapour starts to condense	Once air cools to the dew point any surplus vapour condenses into mist or cloud, and the closer it is to the air temperature the damper the air.
4	The northeast monsoon from the continental interior, which is dry and cooler	The winter monsoon blows from the continental high towards the sea, so the air is dry, cool and often clear.
5	1500 m	Twelve degrees must be lost to reach the dew point, and at 0.8 degree per 100 m that is fifteen steps, or 1500 m.
6	1794 mm	Twenty three hundred times 0.78 is 1794, leaving only about 72 mm a month for the other seven months.
7	7.7 degrees Celsius	Daily range is the same day maximum minus minimum, and 24.5 minus 16.8 is 7.7.
8	2600 m	A difference of 15.6 degrees divided by 0.6 gives twenty six steps of 100 m, which is 2600 m.
9	From about 70.6 per cent to 40 per cent	Twelve over seventeen is 70.6 per cent and twelve over thirty is 40 per cent, so unchanged vapour with a larger capacity drops the humidity.
10	Summer brings the southwest monsoon from the sea, which is humid, unstable and wet, while the northeast monsoon belongs to winter	The wind reverses with the season, and the summer onshore flow carries the moisture that makes the wet season.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

Book an S5 Geography tutor Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.