A weather panel puts eight or nine numbers in front of you at once. Most people read two of them — the temperature and the chance of rain — and misread one of those. This is what each figure is actually measuring, what counts as a normal value, and where the common misreadings come from.
The headline temperature is the temperature of the air, measured in shade, at roughly 1.5 to 2 metres above ground, away from buildings and paving. That standard exists because a thermometer in direct sun reads the sun as much as the air and can sit 10–15 °C above the official figure. It is why a phone left on a windowsill disagrees so violently with the forecast, and why a reading of 22 °C can feel punishing on an unshaded street at midday.
Two related figures usually appear alongside it: the daily high and low. The low almost always occurs shortly before sunrise rather than at midnight, because the ground keeps radiating heat away all night and only starts gaining again once the sun is up. The high typically lands in mid-afternoon, an hour or two after the sun's peak, for the mirror-image reason — the ground carries on absorbing more than it loses for a while after solar noon.
These names all describe the same idea: an estimate of what the air does to a human body, rather than what a thermometer records. The body loses heat by radiating it, by warming the air touching the skin, and by evaporating sweat. Weather changes the rate of all three, so the same 30 °C can be pleasant or dangerous.
In warm weather the dominant factor is humidity. Sweat only cools you when it evaporates, and evaporation slows as the air fills with water vapour. Above roughly 27 °C, high humidity pushes the feels-like figure above the true temperature — sometimes by 8 °C or more. In cold weather the dominant factor is wind. Still air lets a thin insulating layer of body-warmed air sit against your skin; wind strips it away and replaces it continuously, so heat leaves far faster. Below roughly 10 °C the feels-like figure drops below the true temperature, steeply at first and then with diminishing returns as wind speed rises.
Two things worth knowing about wind chill. It applies to exposed skin only — a well-wrapped person feels much less of it than the number implies. And it cannot cool an object below the actual air temperature; a car left outside at 2 °C in a gale still only reaches 2 °C, however brutal the wind chill reads. Some models, including the one behind this screen, also fold in direct sunshine, which is why a feels-like figure can sit above air temperature on a cold, bright, windless morning.
Relative humidity is a percentage of a moving target: how much water vapour the air holds, against the maximum it could hold at its current temperature. That maximum roughly doubles for every 10 °C of warming. So relative humidity changes when the air's moisture changes, and it also changes when nothing but the temperature does.
This produces a daily cycle that fools people constantly. On a still summer day, the air at 5 am might read 95% humidity and at 3 pm read 45% — with exactly the same amount of water in it. The air simply got warmer and its capacity grew. A 95% reading at dawn does not mean a muggy day ahead; it usually means a cool, calm night that brought the air close to saturation, which is also why you find dew on the grass.
For comfort, relative humidity on its own tells you very little unless you read the temperature next to it.
The dew point is the temperature to which air must be cooled, at constant pressure, for its water vapour to start condensing. Unlike relative humidity it is an absolute measure: it does not shift just because the afternoon warmed up. That makes it the honest indicator of how heavy the air will feel.
Two practical uses. First, on a clear, calm night, the overnight low often falls close to the dew point and then stalls there, because further cooling releases latent heat as vapour condenses — so the dew point is a decent quick estimate of the morning low. Second, when air temperature and dew point converge, you get condensation: dew, mist, or fog. A forecast showing the two within a degree of each other around dawn is telling you the walk to shul may well be through fog.
This is the most misread figure in any forecast. “40% chance of rain” means: for a given point in the forecast area, during the stated period, there is a 40% probability that at least a measurable amount of precipitation falls. Measurable is a low bar — typically 0.2 mm, about a hundredth of an inch.
Three things it does not mean, all of which are widely believed:
Read hourly probabilities as a shape rather than a set of individual numbers. A row of 20% values across the afternoon usually reflects general unsettledness with scattered showers — you will probably stay dry but should not be surprised. A spike from 15% to 70% and back over three hours is a front moving through, and the timing of that spike is the useful information. Anything at or above 70% is a forecast of rain, not a hint of it.
One caution specific to summer: convective showers, the kind that build on hot afternoons, are the hardest thing in meteorology to place. A 30% afternoon probability in July may mean a storm is near-certain somewhere nearby, with no model on earth able to say whether it lands on your street.
The UV index measures the strength of sunburn-causing ultraviolet radiation reaching the ground. It is an open-ended linear scale starting at zero, where doubling the number means roughly doubling the rate at which skin burns.
The single biggest driver is the sun's angle, which is why the index peaks near solar noon and collapses within a couple of hours either side, and why summer readings dwarf winter ones at the same latitude. Altitude adds roughly 10% per 1,000 metres. Snow reflects up to 80% of UV back at you and water and pale sand a good deal, so exposure can substantially exceed what the index alone suggests. Thin or broken cloud reduces UV far less than it reduces glare and heat — which is precisely why overcast, comfortable days produce so many surprise sunburns.
Note that the index has nothing to do with temperature. A crisp, clear day at altitude in March can carry a higher UV index than a hazy, sweltering one in August.
Wind direction names where the wind comes from, not where it is going. A “westerly” blows out of the west. This trips people up constantly, and it matters, because the direction usually tells you what kind of air is arriving: for much of the northern hemisphere, wind off the sea is milder and damper, wind off a continental interior drier and more extreme, and wind from the north colder.
Speed is measured at a standard 10 metres above open ground, which is why it usually reads higher than what you feel in a built-up street. Sustained speed is an average over several minutes; gusts are brief peaks, often 40–60% higher, and gusts are what actually knock over garden furniture. Roughly: below 20 km/h you notice a breeze; 20–40 km/h moves branches and makes umbrellas awkward; 40–60 km/h is hard to walk into; above 60 km/h expect debris and disruption.
Pressure is the weight of the atmosphere above you, given in hectopascals (hPa) or millibars — the two are identical in value. Standard sea-level pressure is 1013.25 hPa; typical readings run from about 980 to 1040. Forecast figures are corrected to sea level, so that a mountain town and a coastal city can be compared meaningfully.
The absolute number matters less than which way it is moving. Falling pressure means a low-pressure system approaching, with rising, cooling, condensing air — cloud, wind and rain. Rising pressure means air sinking and drying, so clearer and calmer weather. Rapid movement in either direction, more than about 3 hPa in three hours, signals that something is changing quickly. A steady, high, unmoving reading is the classic settled-weather signature, and in winter it is also the setup for persistent fog or trapped cold air.
Cloud cover is the percentage of the sky covered by cloud, historically measured in eighths, or oktas, by an observer standing outside. Under 25% is clear or sunny, 25–50% partly cloudy, 50–85% mostly cloudy, and above 85% overcast.
Cover says nothing about rain — a completely overcast winter day can stay dry for a week — but it strongly affects temperature. Cloud acts as a blanket: cloudy nights stay warmer because heat cannot radiate to space, and cloudy days stay cooler because sunlight never reaches the ground. A forecast pairing a clear night with light winds is the recipe for the coldest possible morning, along with frost and dew.
Individually these numbers are trivia. Read together they describe a situation. A falling barometer, a swing to a southerly wind, rising dew point and a precipitation probability climbing through the afternoon is a front arriving, in that order, and you can usually name the hour. A high, steady pressure reading with a low dew point and clear skies is a settled, cold, bright spell with sharp mornings.
That is why Shabbat Screen shows all of them at once instead of hiding them behind taps. A screen you cannot interact with has to give you the whole picture in a single glance — and once you can read the panel, that glance is enough to know whether the walk to shul needs a coat.