Summer is usually one of the most anticipated seasons of the year. After all, there is no better time to enjoy the outdoors and the sunshine.
But summer also brings about a number of changes to our planet, from warming temperatures to dangerous thunderstorms to fireflies that flash in unison.
Here are some of the most interesting and intriguing annual summer phenomena and a quick look at why they happen:
Temperatures get warmer — and heatwaves are more common.
Nothing says summer like warm weather. This rise in temperatures is caused by the tilt of the Earth’s axis.
Our planet doesn’t sit perfectly straight as it spins – it has a slight lean of approximately 23.5 degrees with respect to the sun, called the axial tilt. So throughout the year, this axial tilt causes the Northern and Southern Hemispheres to bask in different levels of sunlight, giving rise to the seasons.
In the summer, the sun’s rays hit the Earth at a steep angle, and the light doesn’t spread as much, so any one spot receives more energy from the sun, increasing temperatures. This effect is amplified by the longer summer days.
Heatwaves are most common in summer when high-pressure systems develop across an area, trapping the heat. And, as climate change continues to increase temperatures, scientists predict there will be more frequent and severe heat waves in summer.
The Arctic Circle enjoys 24 hours of light.
- P J Hansen/Flickr
North of the Arctic Circle, or roughly above 66 degrees in latitude, the sun is visible for 24 hours a day for a given time frame depending on how far north you are. And at the North Pole, it will be daylight from March 21 to September 23.
This phenomenon is popularly called the “Midnight Sun,” and it, too, is a result of the axial tilt.
While at the equator the sun seems to rise straight into the sky and set straight down at the horizon, in the far north latitudes the sun appears to travel in a slanting 360-degree circle in the sky. So even when the sun is below the horizon, it isn’t there for very long, nor is it very far below the horizon, keeping the sky alight all night long.
The Southern Hemisphere experiences this phenomenon, too, just on the other side of the calendar.
The stars that we see at night change.
- Ryan Hallock/Flickr
As the earth orbits the sun, the part of the night sky in our view changes. What this means is that most of the star patterns, or constellations, that we see on a summer night are different from those that we see in the winter, according to Space.com.
In other words, in the summer we can see the stars that during the winter were in our daytime sky and thus were invisible to us.
This change doesn’t happen all at once; it’s subtle from night to night. But as the season progresses, Space.com reports, a given star sets about four minutes earlier each night, so over a month, it will set two hours earlier.
Gradually the stars that were low over the western horizon during the early evening hours disappear entirely from our view over a few weeks, and their places are taken by stars that were higher in the sky at sundown.
Since the planet completes a circle every year, patterns and constellations reemerge on schedule every season. For example, the summer is a great time to see the Big Dipper.
Thunderstorms are more frequent.
- Ethan Miller/Getty Images
Thunderstorms – characterized by rain showers, thunder, and lightning – can occur year-round, but they are much more prevalent and generally more severe in the warmer months of summer, according to the National Oceanic and Atmospheric Administration.
This is because moisture and rapidly rising warm air play an important role in forming these storms: High humidity and warm temperatures result in large amounts of warm, moist air rising into the atmosphere.
As the moisture rises, water droplets form as they reach the colder temperatures in high altitudes. Some of the rising moisture freezes in the upper cloud regions.
While evaporation and condensation continue, turbulence causes droplets of moisture to collide, knocking off electrons and creating charge separation. The electrons gather at the lower portion of the cloud (giving a negative charge) and the rising moisture carries a positive charge to the top of the cloud.
The violent process continues until the positive and negative difference is so great that a conductive path is formed, creating lightning – the high-voltage, high-current surge of electrons.
Lightning bolts are so hot at the core that they rapidly heat the air as they pass through it. This, followed by a rapid cooling after the bolt passes, creates intense sound waves that we call thunder.
Fire rainbow clouds are sometimes spotted on summer days.
- Jeff Kubina/Flickr
Fire rainbows, technically known as circumhorizontal arcs, are distinctive clouds, easily recognizable because of their bright rainbow colors and flamelike shape.
These ice halos are formed by hexagonal, plate-shaped ice crystals that occur in high-level cirrus clouds. The halo is so large that the arc appears parallel to the horizon.
The clouds occur mainly during the summer months at particular latitudes because the sun has to be at a precise position in the sky – an elevation of 58 degrees or higher – for the rainbow colors to appear. When the sun is at this position, sunlight enters the clouds’ ice crystals and is refracted (split) into individual colors, just like light when it passes through a prism.
The observer’s location is also important, which is why fire rainbow clouds cannot be seen in latitudes north or south of 55 degrees.
And after sunset, night-shining clouds sometimes appear.
Also known as noctilucent clouds, night-shining clouds are tendril-like with a blueish tint. They are usually visible against darkening summer skies.
These clouds hang at the edge of space and are formed from tiny ice crystals approximately 50 miles above the earth in the upper part of the mesosphere.
The ice crystals only form this high when the air gets extremely cold (negative 190 degrees Fahrenheit), and that condition occurs in the high mesosphere only during the summer months near the poles.
This happens because the polar regions heat up during the summer, and the relatively warm, wet air rises, expanding rapidly, which causes it to cool rapidly as well.
The ice particles that make up these clouds actually contain meteoric smoke, which comprises tiny, floating remnants of meteors that burned up entering Earth’s atmosphere, according to NASA.
Fireflies congregate at dusk and synchronize their flashing light patterns.
Large numbers of fireflies gather for a few weeks every year and can be spotted at dusk, blinking in perfect unison – creating a beautiful, dramatic spectacle.
Fireflies, also called lightning bugs, are beetles, and their light patterns are part of their mating display – the patterns are unique to each species of firefly, helping males and females recognize each other.
Their light is an example of bioluminescence, and, according to the National Park Service, it occurs when fireflies combine the chemical luciferin and oxygen with the enzyme luciferase in a part of their abdomens.
Tiny turtles hatch from their shells and make a quick dash to the ocean.
- REUTERS/Chaiwat Subprasom
In the summer, pregnant sea turtles crawl out of the ocean onto beaches, dig a nest in the sand, and lay as many as 100 eggs before retreating into the water.
These eggs will then incubate in the warm sand for about 60 days, according to NOAA, though the exact incubation time varies with the species of sea turtle.
The hatchlings then begin to break out of their eggs, though they remain in their nests for several days, absorbing their yolk for energy. Then, en masse, the hatchlings climb out of their nests in a coordinated effort and rush to the ocean while trying to avoid waiting predators on the beach.
Turtle hatchlings are often seen this time of year in Florida, Costa Rica, Mexico, and Turkey.
Whales are spotted feeding along northern coastlines.
Summer is prime whale-watching season because a large number of whales migrate to their feeding grounds along northern coastlines.
For example, every summer, humpback whales feed in the high-latitude waters near the Gulf of Maine in the Atlantic Ocean, as well as the Gulf of Alaska in the Pacific, according to NOAA. As baleen whales, they eat mostly tiny crustaceans such as krill and plankton.
This species of whale is popular with whale watchers because they perform aerial displays while they feed. They are often seen breaching the water or slapping the surface with their long pectoral fins, tails, or heads to gather of krill and plankton.
Minke, fin, and right whales are also sometimes spotted in these same feeding grounds.
Salmon swim upriver to their birthplace to spawn.
- Christoph Strässler/Flickr
Every year, salmon migrate from the ocean and swim upriver to their birthplace to spawn. Depending on the river and the species of salmon, this phenomenon starts in the summer and continues into the fall. Once the fish have made this long, exhausting journey and have spawned, they die.
This annual phenomenon attracts a number of hungry predators, including bald eagles and grizzly bears, that converge on the rivers to catch their next meal. For example, in Alaska’s Katmai National Park, brown bears gather at waterfalls in July to catch the swimming fish.