Climatology: Winds, Pressure Belts and Precipitation

Seven pressure belts (alternating, from equator to pole): Equatorial Low (doldrums), Sub-tropical Highs (horse latitudes 30 deg), Sub-polar Lows (60 deg), Polar Highs. Equatorial and polar belts are thermally induced; sub-tropical and sub-polar belts are dynamically induced. Planetary/permanent winds blow from high to low pressure: Trade winds (sub-tropical high → equatorial low; NE in N hemisphere, SE in S), Westerlies (sub-tropical high → sub-polar low; the 'Roaring Forties, Furious Fifties, Shrieking Sixties' in S hemisphere), and Polar Easterlies. Coriolis force deflects winds RIGHT in the Northern Hemisphere and LEFT in the Southern (Ferrel's Law), and is zero at the equator, maximum at poles. Winds are named after the direction they blow FROM.

NULL in SQL is not zero, not empty string, not "missing-but-treat-as-something". It is the database's honest admission that "we do not know." GATE examiners love this corner of SQL precisely because most students remember the syntax of aggregates but forget what they silently do to NULLs — and that silence is where marks slip away.

Definition: An aggregate function in SQL collapses a set of values from a column into a single summary value — common ones are COUNT, SUM, AVG, MIN, MAX.

Definition: NULL is SQL's marker for an unknown or inapplicable value. It is not equal to 0, not equal to '', and not even equal to another NULL (NULL = NULL evaluates to UNKNOWN).

The one rule that decides everything

All aggregate functions, except COUNT(*), ignore rows where the input column is NULL.

Read that again. The rule is small, but every trick GATE plays in this area is just a consequence of it.

• COUNT(*) counts rows, not column values, so it sees NULLs.
• COUNT(col) counts non-NULL values in col.
• SUM(col), AVG(col), MIN(col), MAX(col) operate only on non-NULL values of col.

Why it matters: this single asymmetry between COUNT(*) and the others is the entire reason AVG can disagree with SUM/total_rows, and the reason an "empty" aggregate sometimes returns NULL and sometimes returns 0.

AVG and the denominator trap

Here is the GATE-classic numerical setup. Suppose a column marks has values {2, 4, NULL} across three rows.

• SUM(marks) = 2 + 4 = 6. (NULLs ignored.)
• COUNT(*) = 3. (Counts all rows.)
• COUNT(marks) = 2. (Counts only non-NULL.)
• AVG(marks) = SUM(marks) / COUNT(marks) = 6 / 2 = 3.

A careless student computes 6 / 3 = 2. That is wrong. AVG divides by the count of non-null inputs, not by the total number of rows.

So if you ever see a question that gives you a table with NULLs and asks for AVG, your first move is to compute COUNT(non-null), not COUNT(*).

Notice the second oddity: when every value is NULL, COUNT returns 0 while SUM, AVG, MIN, MAX all return NULL. Why? Because COUNT is fundamentally about counting things, and you can sensibly count zero of something. But the sum, average, min or max of nothing is undefined — there is no value to report — so the language returns NULL.

Common misconception: students sometimes think SUM(col) over all NULLs gives 0. It does not — it gives NULL. The temptation to "add nothing and get zero" is mathematically reasonable but contradicts the SQL standard. If you need 0 in such cases, wrap the result: COALESCE(SUM(col), 0).

A short worked example

Question: Consider the table Sales(amount) with five rows holding values {100, NULL, 200, NULL, 300}. What do the following queries return?

SELECT COUNT(*), COUNT(amount), SUM(amount), AVG(amount) FROM Sales;

Solution:
Step 1: COUNT(*) counts all five rows → 5.
Step 2: COUNT(amount) counts only non-NULL values: {100, 200, 300} → 3.
Step 3: SUM(amount) adds the non-NULL values → 100 + 200 + 300 = 600.
Step 4: AVG(amount) = SUM / COUNT(amount) = 600 / 3 = 200.
Conclusion: The result row is (5, 3, 600, 200). If you had divided 600 by 5 (the row count), you would have got 120 — that is the classic GATE trap.

Real-world example

Imagine a college DBMS lab with the Students(student_id, attendance_percent) table. Some students haven't started attending yet, so their attendance is recorded as NULL — meaning "not applicable yet," not zero. If the placement cell asks for the average attendance with AVG(attendance_percent), it correctly ignores the NULL rows. If instead they treated NULLs as 0, the average would unfairly pull the cohort's number down — and a student who simply hasn't registered would damage everyone's report. SQL's "ignore NULL" behaviour is what keeps that statistic honest.

A subtler GATE trap: COUNT(*) vs COUNT(col)

This is the trap behind many GATE one-mark questions:

SELECT COUNT(*) - COUNT(col) FROM T;

This expression returns the number of NULLs in column col. Recognising this idiom on sight is worth a mark.

What DISTINCT does to the picture

When you write COUNT(DISTINCT col) or SUM(DISTINCT col), NULLs are still ignored, but duplicates among non-NULL values are also collapsed. So for a column {2, 2, 4, NULL, NULL}:

• COUNT(col) = 3, COUNT(DISTINCT col) = 2.
• SUM(col) = 8, SUM(DISTINCT col) = 6.

GATE has used this overlap of DISTINCT and NULL handling to build multi-step numerical-type questions. The safe procedure: first drop NULLs, then apply DISTINCT, then aggregate.

GROUP BY and NULL

When you GROUP BY col, all NULLs in col form a single group of their own — SQL treats "unknown = unknown" specially here, even though NULL = NULL is otherwise UNKNOWN. Many beginners expect each NULL to be its own group. It is not.

Why is the standard like this?

The philosophy of NULL is three-valued logic: TRUE, FALSE, UNKNOWN. Comparisons involving NULL return UNKNOWN, which is filtered out of WHERE clauses (only TRUE rows pass). Aggregates were designed to be conservative: an unknown value should not corrupt a sum or pretend to contribute a measurable amount, so it is excluded. COUNT(*) is the lone exception because counting rows is structural, not value-dependent.

Indian summers carry the dry sting of the Loo, while a French farmer reaches for a wool coat when the Mistral rolls down the Rhone valley. The world's local winds are the planet's small-scale moods — and for UPSC Prelims, they are also some of the most reliably asked one-mark factual questions. A neat map of who blows where, hot or cold, dry or moist, can hand you several marks.

Definition — Local wind: A wind of limited regional extent caused by local relief, pressure or temperature differences, distinct from the global planetary winds.
Definition — Jet stream: A fast, narrow, meandering current of air in the upper troposphere, generally flowing west-to-east at altitudes of 9–12 km.
Definition — Sea breeze / Land breeze: Diurnal coastal winds caused by the differential heating and cooling of land and water.

Hot local winds

The Loo sweeps across the northern Indian plains in May and June. It is a hot, dry continental wind that pushes daytime temperatures over 45 °C in Delhi, Rajasthan and western Uttar Pradesh. Heatstroke deaths during a Loo are a public-health emergency every summer.

The Foehn (also spelled Föhn) descends the leeward side of the Alps in Switzerland, Austria and Bavaria. As moist air is forced up the windward slopes, it loses moisture and releases latent heat; on its way down the other side it is warmed adiabatically. The result is a warm, dry wind that can melt snow within hours — useful for ripening grapes in Alpine valleys.

The Chinook is the North American cousin of the Foehn, blowing down the leeward (eastern) side of the Rocky Mountains across the prairies of Alberta, Montana and the Dakotas. Native Americans nicknamed it the "snow-eater" because it can clear pastures of snow overnight — a blessing for cattle ranchers in winter.

The Sirocco is a hot, dusty, sometimes humid wind that originates over the Sahara and blows northward across the Mediterranean to Italy and southern France. When it picks up Saharan dust and crosses the sea, it can deposit reddish dust on European cities — the famous "blood rain".

The Harmattan of West Africa is dry and dust-laden, carrying Saharan dust toward the Gulf of Guinea between November and March. Despite the dust, it gives relief from the heavy tropical humidity, which is why locals call it "the Doctor."

Cold local winds

The Mistral is a cold, dry, gusty wind that pours down the Rhone valley between the Alps and the Massif Central, hitting the Gulf of Lion in southern France. It chills Provence in winter, but also keeps the skies famously clear — one reason painters loved the south of France.

The Bora is a cold, dry, gusty wind that drops from the Dinaric Alps onto the Adriatic coast of Croatia and Slovenia. Like the Mistral, it is a cold-air drainage wind, often violent enough to overturn small boats.

Jet streams

High above all this local drama, the jet streams steer the world's weather. They form where the Hadley, Ferrel and Polar cells meet, and where huge temperature contrasts exist near the tropopause. Two big ones matter for India:

• The Sub-tropical Westerly Jet (STWJ) sits around 25°–35° N. In winter it shifts south and lies over north India south of the Himalayas. It steers Western Disturbances — extra-tropical storms originating over the Mediterranean — across north-west India, giving winter rain to Punjab and snow to Kashmir.
• The Tropical Easterly Jet (TEJ) appears only in summer, around 10°–15° N, over peninsular India and Africa. Its appearance is intimately linked with the burst of the south-west monsoon.

The classical UPSC fact is: the northward shift / withdrawal of the Sub-tropical Westerly Jet and the onset of the Tropical Easterly Jet together "trigger" the monsoon's arrival over India. The Polar Front Jet, lying near 60° latitude, steers mid-latitude cyclones and aircraft routes — which is why an east-bound transatlantic flight is shorter than a west-bound one.

Sea breeze and land breeze

By day, land heats up faster than the sea. Air over the land rises, lowering surface pressure; the cooler, higher-pressure sea air flows in. That's the sea breeze — sea to land — felt every afternoon on a Goa beach. By night the equation reverses: the land cools faster than the water, surface pressure over land becomes higher, and a land breeze flows from land to sea, often felt by fishermen setting out before dawn.

Why it matters

Air masses and local winds feature in nearly every UPSC Prelims paper, often as match-the-following or assertion-reason questions. Knowing that the Chinook is North American, the Foehn Alpine, the Loo Indian and the Mistral French, and whether each is hot or cold, separates an average candidate from a strong one. The jet-stream–monsoon link also feeds into Mains GS-1 essays and into questions on climate variability.

Real-world example

When IMD says, "A western disturbance over Kashmir will bring rain and snow to north India by Friday," it is really tracking a meandering loop of the Sub-tropical Westerly Jet. When farmers in Punjab smile at a December shower over standing wheat, they are reaping the harvest of a jet stream sitting thousands of metres above them.

Common misconception

Many aspirants confuse the Foehn (Alps) with the Chinook (Rockies) — both are warm dry leeward winds, but in different continents. A second confusion is between the Loo and a "monsoon wind" — the Loo is pre-monsoon, dry and continental, not a monsoon current.

Worked example

Question: Which of the following correctly matches a local wind with its region and character?
(a) Mistral — France — Cold
(b) Sirocco — Sahara to Mediterranean — Hot
(c) Chinook — Rockies leeward — Warm dry
(d) Harmattan — West Africa — Dry dusty
Solution:
Step 1: Mistral pours down the Rhone valley in southern France in winter — cold, dry. ✔
Step 2: Sirocco rises from the Sahara, crosses the Mediterranean — hot, dusty, sometimes humid. ✔
Step 3: Chinook is the "snow-eater" of the Rockies' leeward side — warm, dry. ✔
Step 4: Harmattan blows from the Sahara across West Africa — dry, dusty. ✔
Conclusion: All four pairings are correct — a likely "All of the above" answer.

Cover the answer, recall, then check. 12 cards on pressure belts, planetary winds and rainfall for UPSC Prelims.

Q1. Name the seven pressure belts on Earth.
A1. Equatorial low, two Sub-tropical highs (30°), two Sub-polar lows (60°), and two Polar highs. They shift north/south with the apparent movement of the Sun.

Q2. What is the equatorial low-pressure belt also called and why is it calm?
A2. The Doldrums. Intense heating causes air to rise vertically, so surface horizontal winds are weak/absent — a zone of calm.

Q3. Why are the sub-tropical highs called Horse Latitudes?
A3. Calm, descending, dry air around 30° left sailing ships becalmed; sailors reportedly threw horses overboard to save water — hence Horse Latitudes.

Q4. In which direction do the trade winds blow?
A4. From the sub-tropical highs to the equatorial low — North-East in the Northern Hemisphere and South-East in the Southern Hemisphere, deflected by the Coriolis force.

Q5. State the Coriolis effect (Ferrel's Law).
A5. Due to the Earth's rotation, moving winds and currents are deflected to their right in the Northern Hemisphere and to their left in the Southern Hemisphere.

Q6. What are the Roaring Forties, Furious Fifties and Shrieking Sixties?
A6. Names for the strong Westerlies in the Southern Hemisphere at 40°, 50° and 60°S, where uninterrupted ocean allows very fast, steady winds.

Q7. Name the three types of precipitation by mechanism.
A7. Convectional (rising heated air — equatorial), Orographic/Relief (air forced up mountains — windward side gets rain, leeward is a rain-shadow), and Cyclonic/Frontal (warm and cold air masses meet).

Q8. Match local winds to their regions: Loo, Chinook, Mistral, Sirocco.
A8. Loo — hot dry wind of North India; Chinook — warm "snow-eater" of the Rockies; Mistral — cold wind of France (Rhone valley); Sirocco — hot dust-laden wind from the Sahara to the Mediterranean.

Q9. What are jet streams?
A9. Narrow bands of very fast westerly winds in the upper troposphere. The sub-tropical and polar jet streams strongly influence weather, storm tracks and the Indian monsoon.

Q10. Distinguish permanent, periodic and local winds.
A10. Permanent (planetary) winds blow year-round (trades, westerlies, polar easterlies); periodic winds reverse seasonally/daily (monsoons, land–sea breezes); local winds are small-scale (Loo, Chinook).

Q11. Which cloud type brings steady rainfall and is dark and low?
A11. Nimbus (nimbostratus) clouds. Cumulonimbus brings thunderstorms; cirrus are high wispy ice clouds; stratus are low layered clouds.

Q12. What is the ITCZ?
A12. The Inter-Tropical Convergence Zone — a low-pressure belt near the equator where the trade winds of both hemispheres meet. Its seasonal migration drives the monsoon.