Weather explained

Temperature

Temperature is how warm or cold the air is. Weather stations measure it in the shade, inside a white, slatted shield called a Stevenson screen that lets air through but keeps the sun off. A thermometer in direct sunshine can read 10 degrees or more too high.

On a clear day the air is coolest just before dawn after a night of losing heat to the sky. It is warmest in mid-afternoon, not at midday because the ground keeps warming the air for a few hours after the sun has reached its highest point. Cloud evens this out: cloudy nights stay milder and cloudy days stay cooler.

Feels like is how cold or warm it seems to your skin. Wind carries body heat away, so a breezy day feels colder than the thermometer says (wind chill). In hot, humid weather sweat doesn’t evaporate well, so it feels hotter (the heat index).

The UK mostly uses degrees Celsius (°C): water freezes at 0°C and boils at 100°C. The US uses Fahrenheit (°F), where water freezes at 32°F. To turn °C into °F roughly, double it and add 30.

Rule of thumb: clear skies and light winds give the coldest nights and the biggest gap between the day’s high and low.

Humidity

Air always holds some water as an invisible gas called water vapour. Relative humidity compares how much it holds with the most it could hold at that temperature. At 100% the air is saturated and can’t take any more, so water starts to appear as dew, mist, fog or cloud.

Warm air can hold much more water than cold air. So, as air cools in the evening, its humidity rises even though no water has been added and it falls again as the air warms up the next day. That’s why humidity is usually highest at dawn and lowest in the afternoon and why it isn’t the best guide to how muggy it feels. For that, look at the dew point.

Rule of thumb: below about 40% the air feels dry; washing dries fastest on a warm, breezy day with low humidity.

Dew point

The dew point is the temperature the air would have to cool to before it filled up with water and dew started to form. Unlike humidity, it doesn’t go up and down with the temperature through the day, so it tells you how much water is really in the air.

The higher the dew point, the more humid it feels. Below about 10°C the air feels dry and fresh: 10 to 16°C is comfortable: 16 to 18°C a little sticky: above 18°C it starts to feel muggy and above 21°C it’s oppressive. When the temperature falls close to the dew point expect dew on the grass, mist or fog. If the dew point is below 0°C, you get frost instead of dew.

Rule of thumb: on a clear evening, the dew point is a rough guide to how low the temperature will fall overnight.

Air pressure

Air has weight. Air pressure is the weight of the air above you. It’s measured with a barometer, in hectopascals (hPa, the same as millibars) in the UK or inches of mercury (inHg) in the US. The average at sea level is about 1013 hPa.

Pressure falls as altitude increases because there is less air above you. To compare stations at different heights, readings are adjusted to what they would be at sea level. This station is at an altitude of 89 m, so about 11 hPa is added to its readings compared to sea level.

High pressure (above about 1020 hPa) means air is slowly sinking. Sinking air warms and dries, so clouds struggle to form: highs usually bring settled weather, warm and sunny in summer but often cold, frosty or foggy in winter. Low pressure (below about 1000 hPa) means air is rising. Rising air cools, and its water forms cloud and rain: lows bring unsettled, often wet and windy weather.

How pressure is changing matters more than the number itself. A fall of more than about 3 hPa in three hours means a low or a front is on its way and the faster it falls, the windier it is likely to get. Rising pressure usually means things are settling down.

Rule of thumb: falling pressure means worse weather is coming and rising pressure, better.

Weather fronts

Large blocks of air, called air masses, take on the character of where they come from: cold and dry from the Arctic, mild and damp from the Atlantic. A front is the boundary where two of them meet. Fronts usually come with lows, and they bring most of our cloud and rain.

A warm front is where mild, moist air is replacing colder air. The mild air slides gently up over the cold air, so the signs come gradually: high wispy cloud, then lower, thicker grey cloud, then hours of steady rain or drizzle while the pressure falls. Once it has passed it feels milder and more humid.

A cold front is where colder air pushes in and sharply lifts the warm air ahead of it. The change is quicker: a band of heavier rain, sometimes with gusts or thunder, then the temperature and dew point drop, the pressure starts to rise and the wind turns, often to come from the west or north-west. Brighter, fresher weather with showers usually follows.

An occluded front forms when a cold front catches up with the warm front ahead of it because cold fronts move faster. The warm air between them is lifted right off the ground, so at ground level cool air meets cool air. Its weather is a mix of the two: thickening cloud and a spell of rain, much like a warm front, then clearing skies but with only small changes in temperature. Occlusions usually mean a low is past its strongest and starting to fade. They are common in the UK because many Atlantic lows are already old by the time they reach us.

On weather maps, a cold front is drawn with blue triangles, a warm front with red semicircles and an occluded front with purple triangles and semicircles side by side. The Now page marks cold and warm fronts it finds in this station’s readings with the same symbols. They are labelled “likely” because one station sees only part of the picture. It doesn’t look for occlusions: their signs are hard to tell apart from an ordinary spell of rain.

Rule of thumb: falling pressure and thickening cloud mean a warm front is coming; a sudden drop in temperature with rising pressure means a cold front has just passed.

Wind

Wind is air moving from where the pressure is higher to where it is lower. The bigger the difference over a short distance, the stronger the wind. The Earth’s spin turns the wind, so in the northern hemisphere it blows anticlockwise around lows and clockwise around highs.

Wind direction is where the wind is coming from: a westerly wind blows from the west. Our most common wind is from the south-west, bringing mild, damp air off the Atlantic. Winds from the north and east are rarer; they bring cold air in winter and often dry, sunny weather in summer.

Wind speed is an average over a few minutes. Wind is never steady, though: gusts are the brief stronger bursts, often half as strong again as the average and it is gusts that bring branches down. Buildings and trees around a station slow the wind and make it gustier.

The Beaufort scale describes wind by what it does:

Rain

Rain is measured by how deep it would be if none of it ran off or soaked in. 1 mm of rain means a layer of water 1 mm deep: on a square metre, that is one litre. A rain gauge catches rain in a funnel and counts it in small steps.

Rain today is the total since midnight. Rain rate is how hard it is raining at that moment, given as how much would fall in an hour if it kept going like that. Light rain is under 2 mm an hour, moderate 2 to 10 mm, heavy 10 to 50 mm, and very heavy over 50 mm. A short, heavy shower can have a high rate but leave only a little rain.

Showers fall from separate heaped clouds, so they start and stop suddenly, with sunny spells between. Steady rain, lasting hours, usually comes from the wide sheets of cloud along a front. The East of England, where this station is located, is one of the driest parts of the UK, with roughly 600 mm in a typical year.

Rule of thumb: 25 mm (an inch) in a day is a lot of rain here; it may cause flooding where drains can’t cope.

Sunshine and UV

Solar radiation is how much of the sun’s energy is reaching the ground, measured in watts per square metre (W/m²). Around midday on a clear summer day here it can pass 900 W/m²; on a dull winter day it may hardly reach 50. Clouds, haze and a low sun all cut it down.

UV (ultraviolet) is the invisible part of sunlight that causes sunburn. The UV index says how strong it is: 1 to 2 is low, 3 to 5 moderate, 6 to 7 high, 8 to 10 very high and 11 or more extreme. From 3 upwards, it’s worth protecting your skin in the middle of the day.

UV depends mostly on how high the sun is, not on how warm it feels. A cool, breezy day in May can have stronger UV than a hot, hazy afternoon in August, and thin cloud lets much of it through. In the UK it peaks around 1 pm in late spring and summer, and stays low all winter.

Rule of thumb: if your shadow is shorter than you are, the UV is strong enough to burn.

Evapotranspiration

Evapotranspiration (ET for short) is the water lost from the ground into the air. Some evaporates straight from wet soil and some is drawn up by plants and given off through their leaves (transpiration). It’s measured like rain, in millimetres, and calculated from the sunshine, temperature, humidity and wind.

Sunny, warm, dry and breezy days lose the most, 5 mm a day or more in summer; in winter it is under 1 mm a day. Compare it with rainfall: when ET is more than the rain, the soil is drying out.

Rule of thumb: for gardeners, a week with 20 mm of ET and no rain means it’s time to water.

The sun and moon

The Earth is tilted so as it goes round the sun each year, our half of the world leans towards the sun in summer and away from it in winter. That gives us the seasons and changing day lengths. Here, the longest day (around 21 June) has about 16¾ hours of daylight, and the shortest (around 21 December) about 7¾. The days lengthen or shorten fastest around the equinoxes in March and September, by about 4 minutes a day.

The moon doesn’t make its own light; we see the half of it lit by the sun. As it goes round the Earth every 29½ days, we see more or less of that lit half. These are its phases: new moon (unseen), then a growing (waxing) crescent, first quarter (half lit), gibbous (more than half), full moon, and then the same in reverse as it shrinks (wanes). From the UK, a waxing moon is lit on its right.

Despite the folklore, the moon has no noticeable effect on the weather. It does drive the tides.

Rule of thumb: a full moon rises around sunset and sets around sunrise.

About the measurements

This is an amateur station: a Davis Vantage Pro2 in Letchworth Garden City. Its sensors send readings to a console by radio every few seconds and a computer saves a record every few minutes. Each record holds the average temperature, wind and so on over those minutes, the strongest gust and the rain that fell.

Throughout this website, the high and low are the most extreme readings in a period, and the mean is the average of every reading in it. Days run from midnight to midnight, UK time.

Readings here won’t always match a forecast or the Met Office’s readings. Official stations follow strict rules on where sensors go, such as over short grass, well away from buildings and trees. A garden station is affected by its surroundings, and weather really does vary from street to street. Readings are only checked automatically, not by a person, so now and then one may be wrong. The Station page shows how the sensors are doing.