It's hard to beat the feeling of the warmth of the winter sun on your face. There's a word for it too: apricity. Uplifting, hopeful, calming — even a few short moments to bask in an unexpected, weak but glorious gleam of winter sunlight is wonderful.

A brief burst of apricity warms the spirits and your skin. It reminds me of distant warm, long, summer days, and the promise of spring in a few months. If it has been bleak and cold, apricity is that much more welcome.

From the UK's position in the Northern hemisphere, with the sun low on the horizon, you can let the sun hit full in your face in the middle of the day without craning your neck or leaning back. It's never too hot to seek shade, and often just the right brightness that facing it dead on with your eyes closed is ideal. Together with the long shadows of winter, the effect can have you swept up with the rays, letting them soak into your skin and transporting you away from the cold under your feet.

From the latin aprico, 'to warm in the sun', and in turn aperio, 'lying open', it has been around since at least 1623. It feels to me like we'd benefit from its resurgence.

Apricity

The two most commonly used temperature scales are Celsius and Fahrenheit. The Celsius scale, named after Swedish astronomer Anders Celsius, sets 0C at the freezing point of water and 100C as the boiling point of water at 1 atmosphere of pressure — normal air pressure at sea level. The Fahrenheit scale has 32F as the freezing point of water and 212F as the boiling point. With 180F between freezing and boiling that means each degree Celsius is 1.8 degrees Fahrenheit.

Several accounts of the creation of the Fahrenheit scale exist but the general gist is that Daniel Fahrenheit set 0F at the freezing point of a brine mixture, 32F at the freezing point of water, and another point at body temperature around 96F (now 98.6F). He divided up the scale between these points giving a scale with helpful resolution.

It's interesting to think that for most of history, there was no comparable way to say how hot or cold something was.

Having spent time on both sides of the Atlantic I've tried to get at least somewhat familiar with both of them. To convert between them in my head, at least around most air temperatures, I use a quick approximation. As 32F is 0C, and for each one Celsius you move almost two Fahrenheit, you can roughly go from Fahrenheit to Celsius by subtracting 32 and dividing by 2 (or a little less to be closer). So 50F is approximately 9C (actually 10C), or 86F is approximately 27C (actually 29.4C). It's not perfect, but it's often good enough if you're trying to work out if it'll be hot out. To go from Celsius to Fahrenheit, it's the reverse, times by 2 and add 32.

There are also at least two simple anchor points that can help just by swapping the digits around:

Both scales are now defined relative to the Kelvin scale, which sets the lowest point of 0K at absolute zero, where basically all movement stops. Zero on the Kelvin scale is around -273C. As such, the Kelvin scale, though helpful for scientists, is not very practical for discussing the weather.

Fahrenheit and Celsius

Heat islands are urban areas that can have higher temperatures than surrounding rural areas with more greenery.

Heat islands are caused by several factors, including:

the types of materials we use in cities for buildings, pavement, and roads that can absorb more heat than natural surfaces such as leaves or grass

human additions to the sun's heat, such as air conditioning, vehicles, cooking, or machinery

the shapes of our cities that may restrict airflow to carry heat away

the mass of our buildings that can absorb heat and release it gradually, even through the night

More natural, often rural areas with greenery may reflect more light, release moisture and provide shade.

There are ways to help reduce the effect of heat islands, such as planting more urban trees or using green roofs.

Deciduous trees have the handy feature of providing shade for a house in the summer while letting light through in the winter when the leaves fall.

A study using 2015 data found that "doubling tree cover in European cities could cut the number of heat-related deaths during summer months by nearly 40 per cent." In Columbia's Medellín, creating 30 Green Corridors helped to reduce temperatures by 2°C in the first three years of the program.

Learn more about the heat island effect on the EPA site.

Heat islands

The Earth’s tilt causes the changing seasons. As the Earth revolves around the sun, the tilt of its axis—23.5 degrees—causes the two hemispheres, North and South, to be either tilted towards the sun or away from it at different times of the year.

In Winter, because light from the sun hits at a greater angle and closer to the poles, the solar energy gets spread over a larger area. This makes it weaker and cooler and gives rise to colder weather (aka winter).

In summer, the sun's light hits more directly and is closer to the equator, so it’s concentrated on a smaller area, leading to hotter days.

Because of the tile of the earth, seasons are reversed in the two hemispheres, so if it’s winter in the Northern Hemisphere, it’s summer in the Southern and vice-versa. The further from the equator you go, the bigger the effect of the seasons. As an example, temperatures in Ecuador stay relatively constant throughout the year, while the poles experience extremes of 24-hour daylight or 24-hour night and great freezes and thaws.

A version of this sketch, along with a number of other planetary and seasonal ideas, features in my book Big Ideas Little Pictures

Seasons

The Goldilocks Zone is the range around a star that’s not too hot or too cold to support liquid water. Liquid water is vital to sustaining life like we have on Earth, so fortunately for us, Earth sits in the Goldilocks Zone.

If we lived too close to the sun, we’d roast. The average temperatures on Mercury and Venus are hot enough to boil water (and then some).

And on Mars, the next planet farther out than Earth, the average temperature sits at -85 degrees F (-65 degrees C)— cold enough to freeze water solid.

If you want to look for planets that might support life as we know it in distant star systems, looking for worlds occupying Goldilocks Zones is a promising start.

This sketch, along with plenty of others in the "Starry-Eyed Surprises" section, features in my book Big Ideas Little Pictures

Know your space objects: comets, asteroids, meteors, meteorites

The Goldilocks Zone

…regulate temperature and (maybe) to keep feathers clean while eating. Keeping the feathers clean was my inspiration for this sketchplanation - there’s something about the cold practicality of not messing up your feathers while you’re tucking into some carrion. However, it looks like it is more likely to be for the more prosaic reason of regulating temperature and keeping cool. As Dominic McCafferty, at the University of Glasgow points out:

‘…other groups of scavengers, particularly giant petrels, have feathered heads. Large mammalian scavengers also don’t have bald heads, so why vultures?’

Should you be interested, read more at the University of Glasgow’s Why do vultures have bald heads?

Temperature sketches