Weather Records: Choose the Instrument and Diagnose a Bad Reading

A weather table can contain believable numbers and still be misleading. A thermometer in direct sunshine is not measuring air temperature in the same way as a properly shielded thermometer. A blank rain-gauge entry is not automatically a dry day. Before calculating a total or drawing a graph, check what was measured, where, when and in which unit.

This lesson uses fictional station observations to diagnose measurement and recording problems. It does not forecast weather or prescribe an official station installation. Exact siting dimensions and operating procedures must come from the applicable observing authority and instrument instructions, not from an unscaled classroom sketch.

Match the instrument to the quantity

Rainfall amount is measured with a rain gauge and reported as a depth, commonly millimetres, over a stated interval. A collecting gauge channels rain through a known opening into a collector. Its calibrated measuring arrangement relates the collected quantity to rainfall depth over the collecting area. Water depth in an arbitrary bottle is not automatically rainfall depth.

Air temperature is measured with a suitable thermometer or temperature sensor, often reported in degrees Celsius. For comparable air observations, the sensor needs protection from direct solar heating while remaining exposed to representative airflow. A Stevenson screen is a shelter for instruments; it is not itself the thermometer.

A light-coloured screen reduces absorption of sunlight, its roof shields the instruments, and its slats permit ventilation. A sealed box would trap air and respond differently from the surrounding environment. Keep the purpose of shielding separate from the purpose of insulation: the aim is a representative air measurement, not a permanently isolated pocket of air.

Record the interval as carefully as the number

For this exercise, rainfall is normally read and the collecting gauge emptied at 08:00 local station time each day. Each normal entry covers the preceding 24 hours. Air temperature is an instantaneous reading at 08:00, not the day’s maximum, minimum or average. The station time is used consistently; no clock changes occur during this fictional log.

The labels Day 1 to Day 5 identify observation mornings. The rainfall entered on Day 1 began accumulating at 08:00 on Day 0. Clear interval labels prevent a daily value from being mistaken for an event total or a multi-day accumulation. A daily maximum thermometer would represent a different quantity even if it used the same temperature unit.

The five-day log to investigate

For the rainfall calculation below, assume the gauge remained correctly sited and functional, with no overflow, leakage, spillage or material evaporation loss. Those conditions are supplied facts in this exercise. Without them, even the combined accumulation would need further investigation.

Worked case 1: zero is not missing

Day 2 contains a supported zero: the working gauge was checked and no rain was measured over its stated interval. Day 3 contains no observation. Replace neither with the other. Use a clear missing-data label for Day 3 rather than writing 0.0 simply to fill the table.

The Day 4 value of 7.8 mm covers 08:00 Day 2 to 08:00 Day 4. It includes the two daily intervals that would normally appear under Days 3 and 4. We cannot split it into daily totals from this log. Values such as 3.9 mm for each day would be invented equal shares, not observations.

The missing daily breakdown does not erase the trustworthy two-day total under the supplied gauge assumptions. Preserve 7.8 mm as a 48-hour accumulation, with its start and end times. A later chart requiring daily values must flag the unresolved split rather than quietly plot the whole accumulation as Day 4 rainfall.

Worked case 2: repair a documented unit error

The original Day 5 card says 0.6 cm of rainfall-equivalent depth, but the table heading is mm. Since 1 cm = 10 mm, the corrected entry is 0.6 × 10 = 6.0 mm. This is a justified conversion because the original unit is recorded. A suspicious number without that evidence would need checking, not automatic multiplication.

Keep the raw entry and add a correction note identifying the source card and conversion. Do not silently replace the record and lose the reason for the change. This is different from recalibrating the gauge; we are fixing how a known measurement was transcribed into a differently labelled column.

The total over the five consecutive 24-hour periods from 08:00 Day 0 to 08:00 Day 5 is now 4.2 + 0.0 + 7.8 + 6.0 = 18.0 mm. Count the 48-hour amount once. Do not add another 7.8 for the missing day, and do not describe the table as five independently known daily totals.

Worked case 3: diagnose the sun-exposed thermometer

The Day 4 value of 35.0 °C is flagged because the instrument was moved into direct sunshine. Absorbed radiation can make the sensor hotter than the air it is intended to measure. The siting change gives a specific reason to question comparability; the value’s being larger than neighbouring readings is only a clue.

Do not subtract an invented “sun correction”, and do not replace 35.0 with the previous day’s temperature. Retain the raw value with a quality flag and investigate the exposure. Returning the sensor to its proper shelter supports future observations after equilibration; it cannot reconstruct the earlier air temperature.

The four unflagged 08:00 values average (22.0 + 21.5 + 23.0 + 22.5) ÷ 4 = 22.25 °C, or 22.3 °C to one decimal place. Label this the mean of four valid 08:00 observations. It is neither the mean of five valid observations nor the mean temperature of all hours during the five days.

How to explain a siting problem

Use a three-part answer: identify the exposure, explain its mechanism, then state the consequence or uncertainty. A gauge under roof runoff may collect water from a roof as well as direct rain, producing an unrepresentative amount. Under dense foliage, interception may reduce collection while later drips alter timing. Do not assume one universal correction for every obstruction.

For air temperature, direct sunlight, nearby hot surfaces, artificial heat sources and poor airflow can affect representativeness. “Put it somewhere cool” is not a complete solution, because deliberate deep shelter may create another unrepresentative environment. Seek an appropriately exposed site and suitable radiation shielding under the observing procedure.

Official Met Office guidance describes rain-gauge collection and ventilated temperature screens. Its examples are from its observing network; they do not establish a Kenyan installation rule or an exact universal height for this exercise.

Three independent practice scenarios

Practice 1: choose the better rainfall site

Site A is beneath a roof edge that drains into the gauge. Site B is an unobstructed, level area away from roof runoff, with installation to be checked against the instrument’s procedure. Which is preferable, and why?

Solution: B is preferable on the supplied information because A receives extra roof water and would not isolate rainfall through the gauge opening. B still requires the normal installation and exposure checks; “unobstructed” is not a substitute for every specification.

Practice 2: diagnose a missed emptying

A gauge reads 2.4 mm at 08:00 Tuesday, but is accidentally not emptied. At 08:00 Wednesday it reads 5.9 mm. Assume no losses or overflow. How much fell during the intervening 24 hours?

Solution: 5.9 − 2.4 = 3.5 mm. Wednesday’s raw reading includes Tuesday’s retained water. Record the subtraction and the missed-emptying note. If water had spilled, this subtraction would no longer recover a reliable interval total.

Practice 3: a sealed-box “improvement”

A learner seals all the slats of a temperature screen to keep sunlight out, then labels a blank rainfall entry as zero. Explain both errors.

Solution: Sealing the slats prevents the intended air exchange and can make the enclosed air unrepresentative. Radiation protection must retain suitable ventilation. A blank rainfall entry means the measurement is unknown unless a valid observation establishes zero; neatness does not justify inventing a dry interval.

A record worth trusting

Preserve raw observations, units, interval boundaries, instrument identity and explanatory notes. Mark uncertainty visibly and keep corrections traceable. A shorter table with honest gaps is more useful than a complete-looking table whose numbers no longer mean what their headings claim.

Background references

The public references below support the underlying optical or measurement principles. The worked values, diagrams, fictional logs and solutions in this lesson were newly authored and independently checked. These references do not establish a current Kenyan grade allocation.

• Met Office: measuring rainfall
• Met Office: measuring air temperature

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