← Atlas catalogue · project 120

Biebrza · floodplain wetlands · teaching model

First frost on the floodplain wetlands

Zero degrees is a single line. The four witnesses to the same morning — two temperature readings, the deposit on the grass and the state of the water — need not all stand on the same side of it.

The four mornings below are models carried over from V2: numbers with no date, station or time of the minimum. The page does not forecast frosts and does not judge whether the ice can be crossed.

Diagram of a floodplain wetland cross-section in the morning, with four witnesses On the left, a post with a weather screen at a height of 2 m (witness 1). In the middle, a tussock of sedges with an air sensor 5 cm above the ground (witness 2) and hoarfrost on the tips of the blades (witness 3). On the right, water with ice at the bank (witness 4). Heights are not to scale; this is a diagram, not a map or a photograph. 4 2 m 1 3 5 cm 2
A diagram, not a map or a photograph: where each of the four witnesses stands. Heights are not to scale. In the drawing, blue marks a temperature measurement (markers 1 and 2) and grey marks an observation (markers 3 and 4: hoarfrost and water with ice).

Ask one witness

All four at once

One cold morning asks four different questions: what the air temperature is at 2 m, what it is just above the ground, whether there is a deposit on the grass, and what the water looks like. Each has its own answer — pick one to see what it gives and what it does not settle.

Four mornings on one scale

One scale for all rows: from −5 to +5 °C, one tick is 1 °C, the thick line is 0 °C. On the upper track is the reading from 2 m (), below it the reading from 5 cm (). The deposit and the water are not on the axis because they are not temperature measurements. A missing date or time is not replaced with zero.

Assumption for the exercise: the uncertainty of each reading

This is an assumption you set yourself — not the specification of any sensor and not a figure from a source. It shows when the zero line stops separating the readings unambiguously.

  1. Morning A · modelDoes +2 °C at 2 m rule out hoarfrost?

    Date: none (model morning) · station: none · time of minimum: none

    2 m

    +2 °C

    air reading in the screen

    5 cm

    −3 °C

    air reading near the ground

    Deposit on the grass

    Hoarfrost

    observation; leaf temperature not measured

    Water

    Liquid

    observation; water temperature not measured

    At 2 m +2 °C, at 5 cm −3 °C: two readings on opposite sides of zero. Hoarfrost on the grass, water liquid. Here the hoarfrost coincides with the negative reading near the ground, not with the reading from the screen — but coincidence does not prove a cause.

    Ground frost Below zero at 5 cm and above zero at 2 m — this is how the source defines it [1]. The name describes the arrangement of the readings, not the cause.

  2. Morning B · modelDoes frost always whiten the grass?

    Date: none (model morning) · station: none · time of minimum: none

    2 m

    −2 °C

    air reading in the screen

    5 cm

    −2 °C

    air reading near the ground

    Deposit on the grass

    No deposit

    observation; leaf temperature not measured

    Water

    Ice in a puddle

    shallow puddle on the ground; observation

    A reading of −2 °C at both heights, and no deposit on the grass; ice in a shallow puddle. Frost in the air does not guarantee hoarfrost, and the absence of hoarfrost does not prove dry air or wind.

    Below zero at both heights We still describe the heights separately.

  3. Morning C · model, borderline caseDoes hoarfrost mean the sensor was wrong?

    Date: none (model morning) · station: none · time of minimum: none

    2 m

    −3 °C

    air reading in the screen

    5 cm

    +1 °C

    air reading near the ground

    Deposit on the grass

    Hoarfrost

    observation; leaf temperature not measured

    Water

    Ice at the bank

    oxbow lake; observation

    A reading of −3 °C at 2 m and +1 °C at 5 cm, and yet hoarfrost on the plant and ice at the bank of the oxbow lake. This is a borderline case: the air reading near the ground is not the leaf temperature, so the hoarfrost is not automatically an observer's error here. The arrangement “warmer below, colder above” is unusual in the model and awaits a meteorologist's assessment.

    Below zero at 2 m, above zero at 5 cm A negative reading in the screen does not mean a negative reading near the ground.

  4. Morning D · model, the edge of zeroWhat does an exact zero do?

    Date: none (model morning) · station: none · time of minimum: none

    2 m

    0 °C

    air reading in the screen

    5 cm

    0 °C

    air reading near the ground

    Deposit on the grass

    No deposit

    observation; leaf temperature not measured

    Water

    Liquid

    observation; water temperature not measured

    Exactly 0 °C at both heights, no deposit, water liquid. Zero lies on the line, not on either side of it — a reading of 0 °C alone does not say whether the frost has begun.

    Exactly 0 °C at both heights A point on the zero line: neither below nor above.

Three routes to white and ice

Hoarfrost on grass and ice in a puddle are different transformations. The arrows show concepts, not a timeline; the diagram does not work out freezing time or the thickness of the cover (source [2]).

  1. water vapourdepositionhoarfrost

    Vapour turns straight into ice on a sufficiently cold surface. There is no liquid state along the way.

  2. liquid waterfreezingice

    A liquid becomes a solid: this is how ice forms in a puddle and at the bank. The transformation alone says nothing about load-bearing capacity.

  3. water vapourcondensationdewfreezingfrozen dew

    Two stages: first droplets, and only then their freezing. This is not the same as hoarfrost.

What each witness gives, and what it does not settle

Thermometer in the screen, 2 m

Gives: a reading of the air temperature at a height of 2 m in a weather screen — one height, one value [1].

Does not settle: the temperature near the ground, the leaf temperature or the state of the water. The minimum may have occurred at a different hour than the minimum at 5 cm.

Sensor near the ground, 5 cm

Gives: a reading of the air temperature just above the ground, at 5 cm [1].

Does not settle: the temperature of the leaf surface (it is not the same quantity), the cause of the difference from 2 m — neither radiative inversion nor an inflow of cold air — or whether hoarfrost formed.

Deposit on the grass

Gives: an observation of plant surfaces: hoarfrost (ice from vapour), dew or no deposit.

Does not settle: the temperature of the plant or the humidity of the air. The absence of hoarfrost does not prove dry air or wind, and hoarfrost with a positive air reading is not automatically an observer's error.

It is not on the temperature axis because it is an observation, not a temperature measurement.

Water and ice

Gives: an observation of the state of the water: liquid, ice at the bank, ice in a puddle.

Does not settle: the water temperature, or the thickness or load-bearing capacity of the ice. The presence or absence of ice is neither permission nor a refusal to enter — do not walk on the ice of the floodplain wetlands.

It is not on the temperature axis because it is an observation, not a temperature measurement.

Limits of this comparison

  • The four mornings are models kept from V2, not data from the Biebrza. They have no date, station or time of the minimum; these values are unknown, not equal to zero.
  • Comparing the minima at 2 m and 5 cm does not prove radiative inversion or an inflow of cold air, because the minima may occur at different hours. A reading at 5 cm is not the leaf temperature. The definition of ground frost (below 0 °C at 5 cm and above 0 °C at 2 m) is carried over from V2 according to source [1]; it needs confirmation by a meteorologist, as do the interpretation of hoarfrost with a positive reading and case C.
  • The drawing is a diagram. Ice, or its absence, is not a basis for judging the load-bearing capacity of the cover. The page is not permission to enter the floodplain wetland, nor a navigation aid.
  • What was removed compared with V2: a form for one's own observation with automatic diagnosis, a local log of entries (no data is kept in the browser any more) and three tabs of the transformations laboratory. The first was replaced by four fixed mornings visible at once, the second was dropped, the third was reduced to the single diagram above. Kept were the numbers of the four mornings, the logic of comparing the signs of the readings, and the principle that an air reading does not replace an observation of the surface.