SCIENCE · INVESTIGATE AND EXPLAIN

Assemble a multi-variable front argument

Compare wind, temperature, dew point and pressure through a documented cold-front window.

Learning outcome: Make a timed, bounded interpretation and distinguish cold-front and warm-front geometry.

Starting recap

Front advance direction differs from local wind FROM direction; a single station does not map the boundary.

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Try

22:53 to 23:53 UTC gives -20.6 degrees C temperature and +7.5 hPa sea-level pressure.

Open the experiment

Predict

Does one temperature drop uniquely establish a front?

Your prediction and reason

Experiment

Compare 21:53/22:53, 22:53/23:00 and 22:53/23:53, then read the separate NWS contextual report.

Compare a recorded weather sequence

Historical NOAA station sequence · Ten fixed NOAA/NCEI observations from Denver on December21–22,2022. All times are UTC, and differences are B minus A. Selection never changes observations. Missing pressure stays unavailable, even for the same missing record. Sea-level pressure is a reference-level reduction. Wind bearing is FROM, not direction of travel. Endpoint changes are not forecasts, continuous trajectories, spatial pressure gradients or an automatic front detector.

Record A: 2022-12-21 22:53:00 UTC · routine METAR Record B: 2022-12-21 23:53:00 UTC · routine METAR

Reset keeps your prediction, captures and practice history.

Ten observations, one airport, eight hours

Denver International Airport, December 21–22, 2022. Select endpoints to compare reported weather; every point stays fixed. Times below are UTC. The special 23:00 report is only seven minutes after 22:53. Horizontal positions represent elapsed time, not record number.

Forward time order: B follows A. Signed elapsed time B − A: 60 minutes. Endpoint changes are not rates or predictions.

B minus A; missing evidence is not zero
Quantity	Difference
Temperature	-20.6 °C
Dew point	-6.7 °C
Sea-level pressure	+7.5 hPa
Wind speed	+1.1 m/s
Shortest FROM-bearing change	+10.0 °

Wind bearings name where wind comes FROM, clockwise from true north. The shortest signed bearing difference lies from −180° inclusive to +180° exclusive; opposite bearings use −180°. This endpoint difference does not show which continuous turn the wind actually followed.

All panels keep the same horizontal spacing. Lines guide the eye between reported points; they do not create intermediate measurements. Pressure is reduced to sea level, not the airport’s station pressure. Its fixed 1000–1030 hPa scale does not start at zero. A pressure change over time at one location is not a spatial pressure gradient.

Selected NOAA reports; UTC timestamps, routine METAR or special SPECI
Endpoint / UTC	Type	Temperature °C	Dew point °C	Sea-level hPa	Wind FROM °	Speed m/s
2022-12-21 18:53:00 UTC	Routine METAR	8.9	-14.4	1007.3	180	1.5
2022-12-21 19:53:00 UTC	Routine METAR	10.0	-13.9	1005.9	280	8.2
2022-12-21 20:53:00 UTC	Routine METAR	10.0	-12.2	1003.8	250	8.8
2022-12-21 21:53:00 UTC	Routine METAR	8.3	-11.7	1003.6	310	11.8
A 2022-12-21 22:53:00 UTC	Routine METAR	5.6	-11.1	1006.9	50	11.8
2022-12-21 23:00:00 UTC	Special SPECI	-8.3	-12.2	Missing	50	14.4
B 2022-12-21 23:53:00 UTC	Routine METAR	-15.0	-17.8	1014.4	60	12.9
2022-12-22 00:53:00 UTC	Routine METAR	-18.3	-21.7	1020.5	30	7.7
2022-12-22 01:53:00 UTC	Routine METAR	-20.0	-22.8	1022.1	10	4.6
2022-12-22 02:53:00 UTC	Routine METAR	-21.7	-25.0	1024.2	30	6.2

Inspect source conventions and limits

This is a deliberately selected historical event window, not a random sample, complete archive or climate trend. Temperature, dew point, pressure and speed are decoded from tenths in the source fields. Decimal encoding does not establish measurement accuracy. The pressure sentinel 99999 at 23:00 means missing; its quality flag 9 alone does not mean missing. Quality flag 5 means the value passed all quality-control checks from an NCEI source, not that uncertainty is zero. Wind type N denotes a normal wind-observation type.

NOAA integrated surface database · Exact source query · Official field-format documentation · NWS December 2022 event context.

The NWS report describes a late-December-21 Arctic front and a 37°F one-hour airport temperature drop. Our 22:53–23:53 UTC pair drops 20.6°C, equivalent to 37.08°F of temperature difference. In MST this is 15:53–16:53 December 21. The agency account may use the same airport observations; it is not an independent replication. These ten points alone do not establish the exact crossing time, regional boundary geometry or next hour’s weather.

Pressure differences start an acceleration; rotation changes its direction

Predict on paper: at two locations at the same height, which way does the pressure-gradient force point? Then face east in imagination: which side is your right? These original diagrams distinguish a force from the air's current motion. They are qualitative, not trajectories calculated from the station records.

Check the forces and try a new direction

At equal height the pressure-gradient force points toward lower pressure. The eastward velocity arrow is not a force arrow. In Earth's rotating frame, the horizontal Coriolis deflection is to a moving parcel's right in the Northern Hemisphere and left in the Southern Hemisphere. For eastward motion those are south and north. For northward motion they are east and west. Rotation does not start still air moving; the horizontal Coriolis effect vanishes at the equator.

Near-surface friction slows motion and changes the balance of forces, allowing wind to cross isobars toward lower pressure. Actual wind need not point directly down the pressure gradient. A rising pressure reading at one station gives a temporal tendency, not a map of the pressure gradient.

Unequal heating helps sustain broad circulation

Predict: how can circulation move energy away from low latitudes? Annotate the rising and sinking branches below, then explain why this diagram cannot predict tomorrow's wind at a particular airport.

Check the circulation interpretation

Unequal absorbed solar heating creates energy contrasts; atmospheric motion helps transport energy. In this simplified mean pattern, air rises near the equator, moves poleward aloft and sinks in broad subtropical regions. Rotation influences the surface trade winds and midlatitude westerlies. The Ferrel circulation is maintained through weather systems and eddies; the three regions are not identical heat engines.

The polar region has broad sinking and equatorward near-surface flow; a rising branch lies near the subpolar region. Southern Hemisphere broad branches have a corresponding latitude pattern, while Coriolis deflection changes side. These are seasonal and zonal averages: land, oceans, seasons and individual weather systems shift and disrupt the pattern. No parcel must travel around every drawn loop.

A front advances across the ground; its advance is not the local wind

Predict: in each cross-section, which air mass is advancing at the surface? Trace where warmer air can be lifted. The schematic boundaries and arrows below are explanatory constructions, not a spatial reconstruction from the Denver time series.

Check the front evidence and its limits

A cold front has advancing colder air undercutting warmer air. A warm front has advancing warmer air riding over retreating colder air. The arrow beneath each section shows boundary advance across the ground; a weather vane reports the direction from which local air arrives. Those are different quantities. The slopes are not to scale, and clouds or rain are not guaranteed merely by drawing a boundary.

To argue for a passage, compare temperature, dew point, wind and pressure over a stated interval. One cooling value cannot identify a front. A single station series cannot determine boundary slope, regional position or propagation speed. Use neighboring observations and analyzed weather maps to investigate those spatial claims.

Sources: NOAA explanation of wind and forces and NOAA global circulation overview. These original diagrams illustrate mechanisms; the separate recorded-weather panel supplies the observational evidence.

B minus A: 60 minutes, temperature -20.6 °C, dew point -6.7 °C, sea-level pressure 7.5 hPa. B follows A.

weather-fronts-v1; 2022-12-21T22:53:00Z to 2022-12-21T23:53:00Z. Wind speed difference 1.1 m/s; shortest FROM-bearing difference 10°. Source flags are not uncertainty intervals; one station does not determine a regional front geometry or future weather.

Observation to attach to a capture

Captures preserve endpoints A and B in the fixed historical NOAA sequence. Replay compares the same observations; it supplies neither new measurements nor a weather forecast.

Understand

Propose a transition window from several variables

Inspect the full recorded sequence before reading the agency interpretation. Mark a plausible transition interval on paper and list the observations supporting it. Temperature alone is insufficient: cooling can occur through several processes. Look for jointly changing wind, pressure and moisture-related dew point, while preserving missing values and actual time spacing.

The two selectors choose endpoints from one fixed station sequence. They do not move a front or cause a temperature change. Your evidence is a retrospective comparison of archived records, not a controlled experiment or an automated front-detection score.

Compare the lead-in interval

Choose A at 21:53 and B at 22:53 UTC on December 21. Temperature falls from 8.3 to 5.6 °C, a −2.7 °C difference, while reported sea-level pressure rises by 3.3 hPa. Wind direction changes from 310 degrees to 50 degrees FROM true north.

Ordinary subtraction gives −260 degrees, but the shortest signed circular bearing difference is +100 degrees. Crossing north does not require a huge rotation through the opposite side of the compass. The endpoint difference also does not prove that the wind continuously turned through that exact path between observations.

Draw the two FROM bearings and then separate arrows showing the directions of air travel. Front advance direction is another quantity. A local wind bearing at one station is not automatically the direction in which the frontal boundary moves.

Inspect the abrupt cooling without filling gaps

Now compare 22:53 with the special 23:00 observation. Temperature changes from 5.6 to −8.3 °C in seven minutes, a −13.9 °C difference. Wind speed increases from 11.8 to 14.4 m/s while the FROM bearing remains 50 degrees. Pressure at the second endpoint is missing, so no pressure difference can be calculated for this pair.

Extend B to 23:53. Across the full hour, temperature falls by 20.6 °C, dew point falls by 6.7 °C and sea-level pressure rises by 7.5 hPa. Wind speed changes by +1.1 m/s and its shortest bearing difference is +10 degrees. Write at least two different measured quantities into your claim, with the exact interval.

Dew point is moisture-related, but its change is not a new relative-humidity estimate. No humidity formula is applied here. A short temperature difference also cannot be extrapolated indefinitely as a sustained cooling rate.

Explain a front without inventing its measured geometry

In an ideal cold-front sketch, advancing colder air undercuts warmer air, which can rise along the boundary. In a warm-front sketch, warmer air advances over retreating colder air. Label the advancing air, boundary and vertical motion separately. The original diagrams explain mechanisms; one station’s sequence does not measure their full slope, width or regional position.

Cold and warm describe the contrasting air masses and their movement, not an absolute requirement that a warm front be above freezing. Weather and clouds also depend on moisture, stability and other conditions. Avoid making every frontal passage identical to this unusually sharp historical event.

Compare with the agency’s interpretation

The NWS Boulder December 2022 report identifies an Arctic cold front crossing the plains late on December 21 and describes a 37 °F one-hour airport drop. Our selected −20.6 °C difference corresponds to −37.08 °F: multiply a temperature difference by 9/5 without adding 32.

The UTC hour 22:53–23:53 corresponds to 15:53–16:53 Mountain Standard Time, UTC minus seven hours. This consistency strengthens the contextual interpretation, but the agency report may use the same station measurements. It is not an independent replication. Neither the report nor these ten points gives an exact frontal-crossing second for this activity.

Transfer: separate a boundary from one wind arrow

An unfamiliar cold-front diagram shows the front advancing east, while a station behind it reports wind from the northwest. A student says the two directions contradict each other. Explain why they can coexist and what a spatial map would add.

Check the front-passage transfer

Front propagation and local air velocity are different quantities. A northwest FROM wind moves toward the southeast locally, while the boundary can advance east. Simultaneous neighboring observations or an analyzed front map would help establish regional structure; one station cannot reconstruct the entire boundary geometry.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Use multiple quantities while preserving the missing pressure and potential shared source evidence.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Audit a recorded time series Next: Defend limits and transfer

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