A short-term trend on a personal weather station is the tendency of a variable over a few hours: pressure rising, falling, or steady; temperature rising or falling; humidity climbing toward saturation. That language is older than hobby dashboards. Official surface observations treat the character and amount of pressure change over a specified interval—usually the three hours before the observation—as part of the report, not as decoration beside a needle.
The historical URL /trends sat on the main TNET Weather host among current conditions, forecasts, and NOAA-style daily tables. The original interactive plot is not rehosted, and this page does not publish live Mesa tendency. It is a guide to reading 3-hour (and similar) station tendencies as forecasting vocabulary, and to keeping that vocabulary distinct from a monthly climate chart.
Historical context
Station websites needed a page that answered “which way is the weather going right now?” not “was this July wet?” Weather Display and Cumulus both exposed pressure trend and short temperature change as derived fields from the logger. Templates printed “Falling,” “Rising rapidly,” or a signed three-hour delta next to the barometer. Incoming links to /trends were looking for that near-term motion.
A chart suite shows the full afternoon. Current conditions show the latest sample. A monthly PWS trend summarizes calendar months against sample-size rules and official normals. This URL is the short window that forecasting language actually uses.
What “tendency” means
The National Weather Service glossary defines pressure tendency as the character and amount of atmospheric pressure change during a specified period, usually the three-hour period preceding an observation. Federal METAR practice encodes that as a remarks group: a characteristic (how the pressure behaved) plus an amount in tenths of hectopascals over those three hours. Rapid rise or fall at the time of observation is a separate remark.
Two facts follow for a backyard station:
- The interval is part of the measurement. A one-hour slope and a three-hour slope are different statistics. Software that prints “falling” from a 15-minute difference is using a different definition than METAR. State the interval.
- Character is not the same as net change. Pressure can rise then fall and still end higher than three hours ago. Official characteristic tables distinguish those shapes. A hobby arrow that only shows the sign of the net delta throws the shape away.
Temperature tendency is analogous but not standardized the same way in METAR. A 3-hour temperature change is still a useful observed delta at one sensor. It is not a forecast. It is not a climate trend.
Rising, falling, and steady as forecasting language
Forecasters use short-term pressure motion as a clue about the next several hours, not as a proof of tomorrow’s high.
Falling pressure is associated, in textbook synoptic language, with approaching cyclonic systems, warming aloft in some patterns, or local heating that is not a storm. Rising pressure is associated with building highs, cooler air in many frontal passages, or post-convective recovery. Steady means the three-hour net change is small relative to the instrument and the reduction method.
Those associations are forecast language, not causal laws at a rooftop barometer. A desert afternoon pressure dip from heating is not an approaching hurricane. A clogged port, a bad sea-level reduction, or a logger clock jump will print “falling rapidly” with no atmosphere attached.
Use the words this way:
- Observed: “Station pressure fell 2.1 hPa in the last three hours at this mount.”
- Forecast language (qualified): “A falling 3-hour tendency is the kind of signal forecasters treat as consistent with approaching cyclonic weather, other evidence equal.”
- Not allowed: “The barometer is falling, therefore it will rain,” or “TNET treats falling pressure as X percent of a connection score.”
Official forecasts and warnings remain NWS products. A station tendency page is not a warning service. See weather.gov.
Temperature tendency is a different instrument
A 3-hour temperature rise at sunrise is the diurnal cycle. A 3-hour fall at 16:00 local in the Phoenix area in July can be a thunderstorm outflow, an irrigation or sprinkler wetting, or a sensor in sudden shade. The chart suite shows the shape; the trend field compresses the shape to a sign and a number.
Rules that keep the compression honest:
- Pair temperature tendency with the time of day. An unsigned “−3 °F / 3 h” at dawn is not the same event as −3 °F at mid-afternoon.
- Pair it with wind and rain when those sensors exist. Outflow has a wind and often a rain pulse. A wetting error may not.
- Do not mix indoor and outdoor channels in one “temp trend” token.
- Convert units before comparing stations. A 1 °C three-hour change is 1.8 °F. Hobby pages that print both without saying which fed the arrow confuse the tendency.
Humidity tendency toward 100% with a falling temperature is a saturation approach, not a rain gauge. Dewpoint rising while temperature is flat is a moisture advection or irrigation signal. Those are still observed short-term trends. They are not monthly means.
How a PWS should compute a 3-hour field
Conceptually:
- Store station pressure (and, separately, the reduced sea-level value if you display one) with timestamps.
- Take the sample nearest three hours ago that is within a documented tolerance (for example, three hours ± the logger interval).
- Subtract. Report the delta, the actual interval, and the two timestamps.
- Optionally classify the path (steady rise, fall then rise) if you have enough samples. If you do not, print only the net delta and the word “net.”
- Refuse the field when either sample is missing, stale, or from after a station move.
Sea-level reduction is a derived quantity. Tendency of reduced pressure can differ from tendency of station pressure if temperature used in the reduction changed. If you show one number, name which pressure it is.
Do not smooth the three-hour delta with a 24-hour running mean and still call it a tendency. That is a different filter. Short-term forecasting language is built on the short window.
Distinct from monthly climate and from a forecast page
Monthly trends ask whether this July was warmer than last July at this mount, with missing-day rules and NCEI normals as context. That is climatology-scale station diary. It cannot borrow “falling rapidly.”
A station forecast page may reprint NWS text or a model. Tendency is an input language for humans reading the present. It is not the forecast product. NOAA-style daily tables are daily extrema and totals, another grain again.
If a dashboard prints “Rising” without an interval, treat it as unlabeled folklore until the interval is restored.
Practical checklist
- Interval is printed (3 hours, or whatever the logger actually used).
- Pressure type is named (station versus sea-level).
- Units are hPa or inHg, one system per field.
- Temperature tendency includes time of day and which sensor.
- Missing or stale samples blank the field; they do not hold the last arrow.
- Language stays “observed tendency,” not “it will storm.”
- Monthly charts and 3-hour arrows are not mixed on one unlabeled figure.
Modern relevance
Connection and weather research still needs to distinguish a nowcast-scale atmospheric change from a monthly summary. Pressure and temperature tendency are short-term observed fields. How TNET describes evidence types without publishing internal scoring is on how the service works. The historical observations hub collects the related station pages.
This URL does not animate live arrows. It states what “rising” and “falling” are allowed to mean when the clock is a few hours long.
Sources
- National Weather Service glossary, Pressure Tendency: forecast.weather.gov/glossary.php
- NWS, METAR/TAF decode key (5appp 3-hour pressure tendency group): weather.gov media
- National Weather Service, official forecasts and alerts: weather.gov
- TNET, How the service works