Counter notes, No. 4. Tuesday 29 September 2026, a seven minute read.

Why the cold glass sweats

Our lychee soda leaves the tap at 3 °C, and within a minute the glass is wet all over. This morning the counter reads 24 °C and 68% humidity. Every number below is worked out from those two readings: drag either one and watch the glass.

Plate 1. Lychee morning, ninety seconds after the pour. Beaded all over, and a ring already on the coaster.Dry: in this air nothing condenses on it.

The air is carrying water

Air at 24 °C can hold at most 21.7 grams of water in each cubic metre. At 68% humidity it is carrying 14.8 of them, invisibly, as vapour.

1. Saturation pressure 6.112 e^{17.62 T/(243.12 + T)} hPa, turned into grams with the gas constant for water vapour, 461.5 J/kg K.

Cool that air and the most it can hold drops, until at 17.7 °C it holds exactly what it carries: that is the dew point. Below it the air has to give water up, onto the nearest cold surface. On our counter, that is the glass.

2. Dew point from the Magnus approximation with b = 17.62 and c = 243.12 °C, good to about 0.4 °C between minus 45 and 60 °C.

17.7 °C

is the dew point. The glass is 14.7 degrees under it, so it sweats. It would stay dry only below 25% humidity, which we have never read here.is the dew point, and the glass is above it, so nothing condenses. There is nothing for the bead to mark.

Room 24 °C Humidity 68%

Figure 1. Dew point against humidity for air at 24 °C. The glass sweats wherever the curve runs above the glass line, over the pale field. The bead is this morning's air.

How long it sweats

A glass does not stay at 3 °C. It warms toward the room, and the gap between them halves every 29 minutes. Once the glass climbs past the dew point no new water arrives, and the beads it carries begin to leave.

3. Time constant 42 minutes, timed on our 350 ml tumbler filled to a finger below the rim, on the counter at 24 °C on 12 September.

51 min

of sweating for Lychee morning

Minute 20: the glass is at 11.0 °C and still sweating.

Figure 2. Glass temperature after the pour, T = room − (room − pour) e^−t/42. The field is the time spent under the dew point; the bead is the minute the glass climbs out of it. Point along the curve to read any minute.

So we pour onto paper

In this morning's air the lychee sweats for 51 minutes, longer than most people take over the glass.In air this dry even the lychee starts above the dew point, but most mornings it does not. Everything at our counter follows from those two readings. Cold sodas stand on 2 mm cotton-pulp coasters that drink what the glass gives up; the hojicha, poured at 70 °C, never sweats and goes straight onto the wood. Today's list, in this morning's air:

Pour	From the tap	Sweats for	Served on	Price
Lychee morning	3 °C	51 min	Tumbler, coaster	¥680
Calamansi fizz	4 °C	49 min	Tumbler, coaster	¥640
Yuzu mint	5 °C	47 min	Tumbler, coaster	¥640
Ume spark	7 °C	42 min	Coupe, coaster	¥600
Barley tea, iced	10 °C	34 min	Tumbler, coaster	¥420
Hojicha, hot	70 °C	Never	Cup, bare wood	¥480

Saturday at the counter

Saturday 3 October, 15:00, twelve seats. Bring one reading from your own kitchen and we will work out with you how long your glass sweats at home.

7 of 12 seats taken