Sign inRegisterCheck connection

Reading Cumulus Temperature Time-Series Charts

How to read a Cumulus temperature chart: the diurnal cycle, Celsius versus Fahrenheit axes, and rooftop versus Stevenson-screen siting bias.

Back to Cumulus weather software guides

A temperature time-series chart is a record of how air temperature changed over a stated interval at a stated station. It is useful only when three things travel with the line: the unit on the vertical axis, the clock on the horizontal axis, and enough siting metadata to know whether the sensor was measuring a representative air mass or a roof. Cumulus weather-station software historically published such charts from logger samples. This page is a reading guide for that product. It is not a live Mesa feed, not a generic demo image, and not a chart-library project page.

The historical URL /charts/tempchart.php belonged to the Cumulus publishing surface of the original weather-station site. The interactive plot is not rehosted. The scientific problem the URL named remains: how to interpret a backyard temperature trace without mistaking a drawing for a climate series.

Historical context

Personal weather stations in the Cumulus era typically logged temperature at a fixed interval, then rendered a daily or multi-day PNG or dynamic graph for the station website. Operators wanted a picture visitors could scan: overnight minimum, afternoon maximum, and whether the day was rising or falling. The chart was a derived display of observed samples, not a National Weather Service product and not a climate normal.

Neighboring URLs on this host solved different jobs. A sample or demo graph is a figure that may not even be this station. A JPGraph project page is about a plotting library, not about temperature as a measured variable. Analog dashboard gauges collapse the same logger value to a needle. This URL is the time-series view: temperature as a function of time.

What the axes must declare

A temperature chart without a unit is not an observation product. Celsius and Fahrenheit are different scales. Mixing them, or leaving the axis unlabeled so a visitor assumes the local convention, turns a clean logger file into an ambiguous picture.

State the unit on the axis and in any caption. If the station logger stores one unit and the web template converts to another, say so. A 1 °C change is a 1.8 °F change; a chart that looks “quiet” in Fahrenheit can look “busy” in Celsius only because the tick spacing changed, not because the air did. Do not mix tick labels from both scales on one unlabeled axis.

The horizontal axis needs a date, a start and end time, and a timezone or offset. “Today” is not a timestamp. A daily chart that resets at local midnight is a different product from a rolling 24-hour window. If the logger interval is five minutes, the chart should not be drawn as a continuous physical curve that implies unsampled values between points. Connecting samples with a line is a visual aid. It is not extra data.

The diurnal cycle is the first pattern to read

Over a clear, dry day at a well-exposed site, air temperature typically falls toward a morning minimum after sunrise lag, then rises to an afternoon maximum, then falls again. That shape is the diurnal cycle. It is an observed pattern at one site, not a forecast and not proof of a climate trend.

Read the chart for:

  1. Timing of the minimum and maximum relative to sunrise and solar noon, not relative to an unlabeled clock.
  2. Amplitude — the difference between daily maximum and minimum — which depends on cloud, moisture, wind, and surface type as well as on the true air-mass change.
  3. Interruptions — a sudden drop can be a cold frontal passage, a shower, or a sensor wetting and ventilation change. The chart alone does not name the cause.
  4. Nighttime flattening or spikes — rooftop and poorly aspirated sensors often show nocturnal artifacts that a Stevenson-screen series would not.

A missing hour is missing. Do not interpolate it into a smooth wave and then treat the wave as complete. Monthly or seasonal work that starts from these daily traces has to carry the same gaps forward; that problem belongs with monthly station trends, not with a single-day chart.

Rooftop versus Stevenson-screen siting

WMO observing practice treats temperature as a measurement of air in a specified exposure, not as “whatever the cheap sensor on the eaves reports.” Standard surface air temperature is taken in a ventilated screen, historically a Stevenson screen, at a height on the order of 1.25–2 m above a representative ground surface, away from heat sources, artificial paving, and shading obstacles. Siting classification in the WMO instrument guide exists because nearby buildings, roofs, and reflective surfaces change the radiative environment of the screen.

A personal weather station on a roof, against a south wall, over asphalt, or next to an HVAC exhaust is a different instrument system. It can still be a useful local record if the metadata says so. It is not interchangeable with a climate-network thermometer. Typical rooftop biases that a time-series will show, without needing invented magnitudes:

  • daytime heating from roof membranes, chimneys, and parked vehicles;
  • reduced nighttime cooling when the sensor sees warm building mass;
  • spikes when the sun hits an unshielded or poorly shielded housing;
  • step changes when the station is moved a few meters or the shield is replaced.

The chart cannot correct those biases by itself. Color, smoothing, or a prettier library does not convert a rooftop series into a Stevenson-screen series. Document the mount: height, shield type, surface below, distance to walls, and whether the sensor is naturally ventilated or aspirated. If you do not know, write that you do not know. That sentence is part of the observation.

Precision, sampling, and what the line is not

Logger resolution, display rounding, and physical uncertainty are three different numbers. A chart that prints 23.47 °C does not mean the sensor is good to a hundredth of a degree. Plot the precision you can defend. If the instrument is specified to a tenth of a degree under laboratory conditions, the website should not imply hundredths in still, sun-loaded backyard air.

The line is observed temperature at the sensor, sampled at the logger interval, then derived as a graphic. It is not a forecast. It is not an official warning. It is not NCEI climate. A visitor who sees a steep afternoon rise should not be told the National Weather Service “confirms” the backyard roof. If you overlay a model or NWS forecast on the same canvas, label that overlay as forecast. Mixing families on one unlabeled axis is how hobby charts become unusable as evidence.

Practical reading checklist

  • Station identity and location are in the caption or adjacent text.
  • Vertical axis unit is °C or °F, stated once and used consistently.
  • Horizontal axis has dates, times, and a timezone.
  • Sample interval is stated; gaps are visible, not filled as fact.
  • Shield, height, and mount (roof, yard, Stevenson-class screen) are described.
  • The chart is not titled as a sample, demo, or another station’s figure.
  • Derived displays (gauges, banners) are linked as displays, not as extra sensors.

If any of those is missing, treat the PNG as an illustration until the metadata is restored. That is the same caption discipline argued on the sample-graph page.

Modern relevance

Stations still publish temperature traces, and researchers still have to decide whether a backyard series is evidence of air temperature or evidence of a roof. TNET’s public research groups atmospheric observations as one evidence family among others; a temperature chart is usable in that sense only when units, time, and siting travel with the line. See the evidence families behind a connection outlook. The Cumulus legacy hub indexes the related publishing pages.

This URL does not stream current temperature. It teaches how to read the historical product that once did.

Sources