Temperature

Why places differ

Four things set a place's temperature. Latitude: near the equator the Sun is high and its energy is concentrated; near the poles the same sunlight is spread over more ground. Altitude: air cools by about 6.5 °C for every 1,000 m you climb. The sea: water warms and cools slowly, so coasts have milder winters and cooler summers than the middle of a continent. Cities: concrete and roads store heat, so a city can be a few degrees warmer than the countryside at night.

Why there are seasons

Seasons happen because the Earth's axis is tilted by about 23.4°, not because of its distance from the Sun. In June the Northern Hemisphere leans towards the Sun, so sunlight arrives more directly and days are longer: summer. In December it leans away: winter. The Earth is actually closest to the Sun in early January, during the northern winter.

Feels like

Your body feels wind and humidity as well as temperature. Wind carries heat away from skin, so a windy 5 °C feels colder. In heat, humid air slows the evaporation of sweat, so a humid 32 °C feels hotter.

Where the numbers come from

A forecast model divides the atmosphere into a 3D grid and uses the laws of physics to step the weather forward in time, starting from millions of observations from weather stations, balloons, aircraft and satellites. Forecasts are most reliable for the next day or two and less certain after that.

What the toggles show

Temperature field (eye button)

Shades the whole map by temperature, so you can see warm and cold air masses and the sharp boundaries between them, called fronts. Off by default so the map stays clear.

Sea

Colours the oceans by their surface temperature from NASA's daily satellite analysis. The air temperature field is hidden while it is on, so air and sea are never mixed in one colour scale. Tap the sea to read the temperature at that spot.

Change from normal

Shows how much warmer or cooler the sea is than the 2003–2014 average for the same day of the year. The box in the middle of the Pacific is where El Niño is measured; tap it for the El Niño card.

26.5 °C line

Marks where the sea is warm enough for typhoons and hurricanes to form.

Sun

Shades the night side of the globe, marks the point where the Sun is overhead, and draws the tropics and polar circles. Pick a solstice or equinox to see how the sunlit half changes through the year; city cards then show the day length and the height of the noon Sun.