That might be hard to tell, until about 7:46. Timing is everything.

The elements of a glorious sunset need to come together in just the right quantities at just the right time. When they do, the day ends with a splash of colors that can make it hard to imagine there's anything wrong in the world.

Clouds, dust, pollution – all can play a role in the hues seen as the sun sinks. Yellows, oranges and reds need an assist from the elements in the atmosphere to bring the day to a colorful close.

Bend it like ...

It all starts with sunlight, of course. If you take a prism and shine white light from the sun through it, the light is refracted, or bent. Basically, that's what happens when sunlight strikes a boundary denser than the air.

Sometimes, that boundary is a cloud. Sometimes, it's something else in the sky.

Online: To see what time the sun will set at any location, any day of the year, go to: timeanddate.com. Then click on "Sunrise and Sunset Calculator" and select the location and date.

John Day, the late co-author of “Peterson's First Guide to Clouds and Weather,” “The Book of Clouds” and several other books on the atmosphere, looked at sunsets through the eyes of a cloud physicist.

“In a real-sky situation ... the prisms are the water droplets in the sky associated with clouds,” said Day in an interview shortly before his death last month at age 95 (see Weather Watch column). “If you're looking at a brilliant sunset, the thing you'll be seeing will be the result of sunlight being bent through the underside of the cloud.”

The greatest bending of sunlight, which contains all the colors of the rainbow, occurs with the greens, blues and violets, and the least occurs with the yellows, oranges and reds, which show up in the clouds.

The colors present are often a function of the size, location and thickness of the clouds, Day said. There are many variables, including: the size of the water droplets; the number of droplets; and where the viewer is in relation to the setting sun.

Another key element is the clouds' exposure to sunlight.

“What you have to have is an opening to the west so the sun can shine on the belly of the clouds,” said Charles Pyke, a meteorologist who has studied sunsets. “You'll see nothing but gray if the western horizon is cloudy.”

The higher the clouds, the longer they can reflect the light after the sun sets, said Pyke, who is principal scientist of his own meteorological consulting company based in Arizona.

“If the clouds are no more than 45 degrees from the horizon, they will fade earlier, but they can be a spectacular red,” he said.

The best sunsets generally occur when there are mid-level clouds present, either alto-stratus or alto-cumulus (8,000 to 20,000 feet), Day said. Those clouds often produce the deep, red sunsets.

Sometimes the sun will set on high cirrus clouds, which are actually ice crystals with a wispy, featherlike appearance. When sunlight hits cirrus clouds near sunset, they tend to turn orange or yellow.

Big, puffy cumulus clouds associated with storms and the low-lying stratus clouds often seen in San Diego during spring usually don't produce colorful sunsets.

Certain times of the year are more conducive to sunsets blessed with beautiful clouds. If there is a front moving toward the coast, there's a greater chance for an attractive sunset, Day said. Fronts are most common in San Diego from mid-fall through mid-spring.

But the sky doesn't necessarily need clouds to burst with colors at sunset.

Other elements

Lynn Russell, an atmospheric scientist at the Scripps Institution of Oceanography at UCSD, brings another perspective. Her focus is understanding the chemical and physical behavior of atmospheric particles.

She grew up in Modesto, but she has traveled the globe for atmospheric studies. She has seen the sun set over the South Pacific near Tasmania; in the Arctic north of Greenland; over Lake Ohrid in Macedonia; and near Capetown, South Africa.

Many of her most memorable sunsets have come at sea, particularly one she saw at Wake Island, about halfway between Hawaii and Japan.

“It was over calm, shallow waters, with a bright yellow sun,” she said.

But Russell said a colorful sunset doesn't always come under pristine conditions. Seeing a sensational sunset can often require a person to close his or her eyes to the forces causing it.

“What often makes a good sunset is a clear sky, without too many clouds,” Russell said. “Typically, a little bit of pollution helps. It gives you nice, red colors.”

Pollution scatters the blue and green light of the spectrum, and what's left are the reds, she said.

Some of San Diego County's more colorful sunsets have come under the worst conditions. “There were some really pretty sunsets when we had the fires here,” Russell said.

When Santa Ana winds blow smoke, dust or San Diego County's air pollution out to sea, or when wind patterns blow smog from the Los Angeles basin down here, local sunsets tend to be more colorful, she said. September and October can be good months.

“That's when we're most likely to get the reddish hues that often get highlighted by the pollution,” she said.

But there can be too much of a bad thing. Pollution also reduces visibility.

“There's certainly a trade-off,” she said. “If the haze gets too thick, it's kind of like looking through a gauze or a dirty window.”

Local sunsets can also get a color boost from what's happening halfway around the world. High-altitude dust storms from the Gobi Desert in Mongolia sometimes can merge with pollution from Asia. Low-precipitation storms can then move the so-called “Asian brown cloud” across the Pacific Ocean, and some of the air mass can filter out and pollute our air. That can contribute to redder sunsets here, Russell said.

Volcanic dust very high in the atmosphere can add a sort of pinkish or reddish glow about 20 minutes after the sun sets, Pyke said. Evening skies were particularly colorful around the world for months after the eruptions of Mount Pinatubo in the Philippines in 1991, and El Chichón in Mexico in 1982.

Sometimes, early-evening rocket launches out of Vandenberg Air Force Base north of Lompoc can produce unusual colors way up in the atmosphere long after the sun sets. The rockets' trails can turn green or blue.

With or without volcanic dust or rocket exhaust, at 7:56 tonight, the sun will set on another day in San Diego. The solar beams could encounter water droplets or other airborne elements. If those particles are positioned properly, a free sky show could follow.

Robert Krier: (619) 293-2241; rob.krier@uniontrib.com