DSE Geography | Weather and Climate Tutorial Plan (for tutors) | Pressure gradient, wind deflection, relief and frontal rain

2026-11-02

Tutor lesson-prep guides · Secondary Geography

Students can recite “rising air makes cloud and rain”, yet cannot say why a summit 13 degrees cooler is a lapse-rate result rather than an effect of height. This lesson ties pressure, humidity and rainfall to one vertical profile built from three calculable numbers.

Objectives — Students can: (1) compute a pressure gradient by dividing the pressure difference by distance, and explain that wind blows from high to low pressure and deflects to the right in the northern hemisphere; (2) compute relative humidity from actual and saturation vapour content, and state that condensation begins only at the dew point; (3) use the 6.5 degrees per km environmental lapse rate to find a summit temperature, and explain the lifting mechanism behind convectional, relief and frontal rain.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Energy and the atmosphere	What happens to air volume and density when it is heated	It expands, density falls, it rises
Proportion	At 6.5 degrees per km, how much does 2 km cool	13 degrees
Water cycle	What is the process from liquid water to vapour called	Evaporation (and transpiration)

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Opening question	Write “1000 and 992” and ask which way the air between them moves	Answer aloud with whatever they recall
5-15 min	Prerequisite check	Hand out three written checks (heated air, lapse rate, evaporation)	Written answers, marked on the spot
15-37 min	Three scenes	Teach pressure and wind, vapour and condensation, three rain types	After each scene, one sentence on “rising or sinking”
37-49 min	Worked questions	Model four questions, each as pressure, wind, weather	Copy, then rewrite from memory
49-55 min	Independent practice	Circulate, only asking “is this about pressure or humidity”	Do three questions and self-mark
55-60 min	Exit check	Collect answers and count errors live	Written, no talking

03 Teaching script (read it out)

Scene 1: Scene 1: Pressure, gradient and wind

Start with the definition: pressure is the weight of the air column, about 1000 hPa at sea level. Wind has one cause: air flows from high to low pressure, and the stronger the difference over a shorter distance, the stronger the wind. Use numbers. Station A reads 1000 hPa, station B reads 992 hPa, and they are 80 km apart, so the gradient is 8 divided by 80, that is 0.1 hPa per km. This is the quantitative version of “close isobars mean strong wind”: the tighter the spacing, the larger the difference over the same distance. The second layer is direction: the pressure gradient force pushes air towards the low, the Coriolis effect turns it to the right in the northern hemisphere, so air spirals anticlockwise into a low and clockwise around a high. Require the three-step chain of gradient force plus Coriolis equals actual wind, not the bare sentence “wind blows from high to low”. The third layer is Hong Kong: the summer south-west monsoon brings maritime moisture, the winter north-east monsoon brings dry continental air, so the same map with a different pattern gives a different set of weather.

On an isobar chart, if two pairs of isobars are the same distance apart but one sits over sea and one over hills, is the wind the same? Why?

Scene 2: Scene 2: Vapour, relative humidity and dew point

Humidity is not “how much water is there” but “how much there is compared with the maximum the air can hold at that temperature”. Saturation content rises steeply with temperature: about 9.4 grams per cubic metre at 10 degrees, 12.8 at 15, 17.3 at 20, 23.0 at 25 and 30.4 at 30. If the air actually holds 12 grams, then at 25 degrees relative humidity is 12 over 23.0, about half; cool that same parcel to 15 degrees and the denominator becomes 12.8, so humidity passes nine tenths; cool it to 10 degrees and the capacity of 9.4 is below 12, so the surplus condenses into cloud droplets or dew. Cooling, not adding vapour, is what causes condensation. The dew point is the temperature at which the parcel just becomes saturated, and it is the threshold for fog and cloud base. Give students the one-line rule: the numerator stays fixed, the denominator moves with temperature, and condensation happens the moment the denominator catches the numerator.

A cold air-conditioned room has water beading on the wall. Did humidity rise or did temperature fall?

Scene 3: Scene 3: Three rain types on one profile

The three types differ only in what makes the air rise. Convectional rain: strong surface heating makes the air unstable, giving afternoon thunderstorms that are small in area and violent. Relief rain: moist air is forced up a slope, so the windward side is wet and the leeward side is dry in a rain shadow. Frontal rain: cold air wedges under warm air, which rises gently along the frontal surface, giving wide and persistent rain. Put numbers on relief rain: sea level 28 degrees, a 2000 m ridge, an environmental lapse rate of 6.5 degrees per km, so the summit is 28 minus 13, that is 15 degrees; the parcel reaches saturation near 18 degrees at about 1550 m, which is the cloud base. Descending the leeward side it warms at the saturated adiabatic rate of about 6 degrees per km, so it arrives drier and hotter. A full answer names the lifting mechanism, the cloud base and the position of the rain belt; missing one loses the mark.

Why is a leeward slope not only drier but also warmer than a windward slope at the same height?

04 Question types and worked steps

Type	Example	Steps	Answer
Data calculation	Station A 1000 hPa, station B 992 hPa, 80 km apart. Find the pressure gradient.	1 pressure difference; 2 divide by distance; 3 unit hPa per km; 4 one line on wind strength.	Example: 8 divided by 80 is 0.1 hPa per km.
Humidity calculation	Temperature 25 degrees, actual vapour 12 grams per cubic metre. Find relative humidity.	1 read saturation 23.0; 2 divide 12 by 23.0; 3 convert to a percentage; 4 state it is unsaturated.	Example: about 52 per cent.
Profile calculation	Sea level 28 degrees, ridge 2000 m, lapse rate 6.5 per km. Find the summit temperature.	1 height into km; 2 multiply by the lapse rate; 3 subtract from sea-level temperature; 4 state the unit.	Example: 15 degrees.
Rain-type identification	A place has short intense afternoon thunderstorms in summer. Identify and explain.	1 surface heating; 2 unstable lifting; 3 cumulonimbus formation; 4 classify as convectional and note the small area.	Example: convectional rain, afternoon peak.

Sanity check (teach this): Sanity checks: a gradient of 0.1 hPa per km comes from 8 divided by 80, so an answer of 8 has forgotten the distance. A summit of 15 degrees must match 28 minus 13, so 21.5 means 6.5 was treated as a total drop. Cloud base near 1550 m is the condensation level, not the ridge height. Relative humidity of about 52 per cent must stay below 100, so 120 per cent means the fraction is inverted. The dew point is the saturation temperature, not “the coldest temperature”.

05 Independent practice (3 questions)

Question	Answer	What to watch
State the cause of wind deflection and the rotation of a northern-hemisphere low.	Pressure gradient force plus Coriolis; lows turn anticlockwise, highs clockwise.	Check whether only “the earth spins” is written with no force chain.
A parcel cools from 25 to 10 degrees. What happens to relative humidity?	It rises from about half to saturation, then surplus vapour condenses.	Check for answers claiming humidity is unchanged.
Compare the rain belts of relief and frontal rain.	Relief rain is concentrated on the windward slope with a leeward rain shadow; frontal rain forms a wide persistent band along the front.	Check whether the two mechanisms have been merged.

06 Common mistakes and what to say

Mistake	Student writes	Say this
Forgetting to divide by distance	Answering “the gradient is 8 hPa”.	“A gradient means change per kilometre, so divide by the distance. 8 over 80 gives 0.1.”
Treating humidity as amount of water	Answering “more vapour was added, so it rained”.	“Condensation happens because the denominator shrinks as air cools. The numerator stays the same.”
Wrong lapse-rate unit	Answering “6.5 degrees per 100 m”.	“It is 6.5 degrees per 1000 m. A factor of ten in the unit is a factor of ten in the answer.”
Wind drawn straight into the low	Answering “wind always blows straight to the centre of a low”.	“In the northern hemisphere it deflects right, so it spirals anticlockwise. Name the Coriolis effect.”

07 Homework (paper, section and question numbers)

Secondary Geography index: finish the three questions on this page, then compare rainfall and runoff in the river landforms chapter — Practice 1-3 on this page

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the unit of a pressure gradient?	hPa per km	Cannot answer = reteach scene 1
What directly causes condensation?	Cooling to the dew point (saturation)	Cannot answer = reteach scene 2
On which slope does relief rain fall?	The windward slope; the leeward side is a rain shadow	Cannot answer = reteach scene 3

09 Tutor FAQ

Which year is this for?

Mainly the S4 Geography weather and climate unit; useful as a refresher before S5 climate types.

Must students memorise the saturation table?

No, but they must know capacity rises with temperature and be able to compare two or three values.

Are calculations really required?

DSE regularly examines pressure gradient and lapse rate, so both formulas should be automatic.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.