Weather vs climate
Weather is the state of the atmosphere at a place at one time or over a short period (today) -- temperature, rainfall, wind, cloud and sunshine, changing hour to hour. Climate is the AVERAGE weather, from records over a long period, typically about 30 years, given as monthly or seasonal averages. One hot, sunny day is weather, not proof of a hot climate; climate shows only across many years of data.
Elements, instruments and units
Each element has its own instrument and unit. Temperature -- thermometer (max-min records the day's highest and lowest) -- °C. Rainfall -- rain gauge -- mm. Air pressure -- barometer -- millibars. Humidity -- hygrometer -- %. Wind speed -- anemometer -- m/s. Wind direction (the way it blows FROM) -- wind vane. Sunshine -- sunshine recorder -- hours. Cloud cover -- estimated by eye -- oktas (0-8). Learn the pairs; a mismatch loses the mark.
Stevenson screen and correct siting
The Stevenson screen is a white, louvred box on legs housing the thermometers and hygrometer. White paint reflects the sun; louvres let air flow while keeping out sun and rain; legs raise it to ~1.25 m clear of ground heat; the door faces away from the sun. The barometer is kept indoors (air pressure needs no shielding). The rain gauge, anemometer and wind vane stay in the open air, on level ground clear of buildings and trees so readings are comparable.
Drawn from real examiner reports.
Weather is not climate
The key definition trap. Weather is the atmosphere at one place over a SHORT period (today); climate is the AVERAGE weather over a LONG period, about 30 years. Candidates describe today's hot weather and call it "the climate", or define climate as just "the weather of a place" without the averaging. A full definition needs both -- an AVERAGE, over MANY YEARS.
Digest (## Cross-cutting exam technique): weather vs climate is one of the two most persistent concept-conflation pairs in the natural-environment theme, recurring across sittings.
How it works vs how it is used
Two question types get confused. "How it works" wants the mechanism (a max-min thermometer: alcohol pushes metal markers up the tube). "How a STUDENT USES it" wants the PROCEDURE -- go at a set time, read the scale at eye level, record with the unit, then reset. Answering "used" with mechanism detail scores nothing.
Digest (## 2.4): describing HOW AN INSTRUMENT WORKS rather than HOW A STUDENT USES IT to take a reading -- rain gauge, max-min thermometer, Stevenson screen, wind vane, clinometer (s23 Paper 11 Q4a(ii)-(iii), s22 Paper 41 Q2(f)).
What is housed in the Stevenson screen
A testable misconception: that the screen houses ALL instruments, including the rain gauge, anemometer and wind vane. Only those measuring the AIR itself -- thermometers and hygrometer -- go inside; the others stay in the OPEN air (a boxed gauge catches no rain; a boxed vane feels no wind). The barometer is kept INDOORS, since pressure needs no shielding.
Digest (## 2.4): candidates believe a rain gauge, anemometer or wind vane can be housed inside the Stevenson screen (w23 Paper 22 Q4c).
Wind data: % of time vs m/s speed
Wind data comes as two DIFFERENT statistics; treating them as one loses marks. A "percentage of time" figure (wind from the SW for 45% of readings) says how OFTEN a direction occurred, not how fast. An "m/s speed" figure says how FAST, not the direction. A real comparison keeps them separate, with the right unit for each -- not just the single highest band.
Digest (## 2.4): comparing wind data, candidates conflate a "% of time" statistic with an "m/s speed" statistic, or compare only the single highest band (w23 Paper 22 Q4b).
Match the instrument to the element
Instrument-unit questions punish a mismatched pair. Wind DIRECTION uses a wind vane, not an anemometer (which gives wind SPEED, in m/s). Temperature uses a thermometer (°C); air pressure a barometer (millibars). Rainfall is a rain gauge (mm); humidity a hygrometer (%). Name the instrument matching the exact element asked, with its correct unit.
Cloud cover is estimated, not measured
Cloud cover is ESTIMATED by eye on the okta scale, not read from an instrument like the other elements. An okta is one EIGHTH of the sky, so the scale runs 0 (clear) to 8 (overcast); 4 oktas is half the sky. Candidates wrongly say an instrument measures it, or treat an okta as a tenth or a percentage. It is estimated by eye in eighths.
Wind vane points INTO the wind (FROM)
A testable wrong belief. A wind vane swings freely and turns so its pointed end faces INTO the oncoming wind, so the arrow shows the direction the wind blows FROM, not TO. A "north wind" blows FROM the north (arrow points north), not towards it. Reading the vane the wrong way gives the exact opposite direction to the mark scheme.
Digest (## 2.4): the wind vane's arrow is frequently misread as pointing to where the wind blows TO rather than FROM (w22 Paper 42 Q2c(i)).
Reading procedure, not the mechanism
For "how a student USES [instrument] to take a reading": WHERE they look (eye level, door out of sun), WHEN (same time daily), HOW the scale is read (to the nearest unit), the UNIT (°C, mm, m/s), and any RESET (magnet for a max-min marker).
Describe the data, do not explain it
"Describe" the weather = the highest and lowest values, the overall pattern, and exact figures with units -- no causes unless "explain" is asked. A describe answered with reasons, or an explain as a bare description, is a much-flagged command-word error.
Use exact figures, not vague words
Precision earns marks. Replace "about", "nearly" or "quite windy" with the exact figure and unit (e.g. 27 °C, 15 mm, 9.5 m/s). Where two statistics are given (% of time and m/s speed), quote each with its own unit. Vague quantifiers are what the reports warn against.
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