Seeing through the fog: the science behind Hollywood's fog and the real thing.

DID YOU KNOW?

* Fog and mist are made of the same thing: tiny water droplets suspended in the air. How are they different? Fog is thicker. When the water droplets reduce visibility to below 1 kilometer (0.62 miles), it's considered a fog. Mist reduces visibility to between 1 and 2 km (0.62 and 1.25 mi).

* The Grand Banks, the body of water off the east coast of Newfoundland, Canada, is considered the foggiest place on Earth. It is shrouded in fog for approximately 40 percent of the winter and 84 percent of the summer. Argentia, a nearby coastal town in Newfoundland, experiences an average of 206 days of fog per year.

CRITICAL THINKING:

* A bathroom usually becomes foggy after a hot shower. How is this process similar to fog formation in nature?

CROSS-CURRICULAR CONNECTIONS:

COMPUTER SKILLS: Turn students into meteorologists. Have them do research to create a PowerPoint presentation about the local weather. Then, have them use the PowerPoint to present a TV-style weather report.

RESOURCES

* To learn more about the water cycle, including fog and condensation, check out this excellent Web site from the U.S. Geological Survey: http://ga.water.usgs.gov/edu/watercycle.html

* For information about the chemistry of dry ice, visit: www.emsb.qc.ca/laurenhill/science/dryice.html

Antonio Bay seems like an idyllic seaside village. A lighthouse overlooks a scenic shoreline, and boats bob along the coast. But one day, a thick fog engulfs the bay area. Soon after, the fog transforms into ghosts, terrorizing the town. Ahhhhh! Okay, Antonio Bay does not really exist. The creepy scenario is just the plot of The Fog, a horror film that hit theaters last fall.

To avoid fog-filled nightmares, learn how real fog forms and why scientists think this movie's spooky fog is ... well ... kind of silly.

WHAT IS FOG?

Basically, fog is a cloud. "It's just like the [clouds] in the sky, but fog happens to be on or near Earth's surface," says Robert Tardif, a meteorologist who studies weather at the National Center for Atmospheric Research in Boulder, Colorado. Some places are more likely to experience fog than others (see map, p. 12).

Two main factors contribute to making fog or clouds: cooling temperatures and sufficient humidity, or the amount of water vapor in the air.

Air contains water molecules. It also contains numerous microscopic particles called aerosols. At any given temperature, water molecules constantly attach and detach from the surfaces of aerosols. But when air temperature cools, more water molecules attach to the particles than detach from them. When moist air cools to its dew point, there are so many water molecules condensed on each aerosol that the particles become visible water droplets. These droplets form clouds. When this process occurs near the ground, the clouds are known as fog.

SEA FOG

Fog can form under a variety of conditions. For example, The Fog takes place near an ocean. These types of regions commonly experience sea fog.

When warm, humid air blows across a cool ocean surface, the warmer air loses its heat to the colder ocean. If the air cools to its dew point, sea fog forms. Wind often carries sea fog toward the coast. "If you stand by the water, you can see the fog moving toward you like a giant wall," says Tardif.

But in the movie, the giant bank of sea fog moved against the wind to reach Antonio Bay. "That's scientifically unsound," says Tardif. "I guess the moviemakers needed something unreasonable to scare people."

GROUND FOG

The Fog also featured wisps of low-lying fog that snaked around buildings. "That's most likely a simulation of ground fog," says Tardif.

This type of fog, also known as radiation fog, usually forms during calm, clear nights. During the day, Earth's surface absorbs energy from the sun in the form of radiation (high-energy waves). But at night, some of this energy is released back into space. With no clouds to trap the radiation, the region's ground surface cools down quickly. This helps to chill moisture-laden air neat" Earth's surface to its dew point. "Ground fog usually starts low and builds up vertically," says Tardif.

In horror movies, ground fog can seep under closed windows. "[In real life] if you have your windows closed, the water droplets in the fog would just hit the glass," says Tardif. Even if your windows were wide open, it's unlikely that your room would fog up.

That's because indoor temperatures are usually warmer than the air outside. "When the water droplets suddenly enter a warmer environment, they will evaporate into vapor [and the fog will disappear]," says Tardif.

HOLLYWOOD FOG

Some of the smoky effects in The Fog were computer-generated images. But during filming, dry ice was also used to make artificial fog. Why is solid carbon dioxide (C[O.sub.2]) good at fogging up a movie set?

Most forms of matter can exist in three phases: solid, liquid, and gas. For example: Water can change from ice to liquid to steam. Dry ice, however, goes directly from solid C[O.sub.2] to an invisible and odorless C[O.sub.2] gas. "There's no liquid phase in between," explains Kenneth E. Smith, chair of environmental chemistry at the American Chemical Society. "Above -78.5[degrees]C (-109.3[degrees]F), or its sublimation point, dry ice vaporizes."

To make the invisible C[O.sub.2] vapors appear as "fog," water vapor must be present. As the chilly dry ice vaporizes, it causes the temperature of the surrounding moisture-filled air to drop swiftly to its dew point. The result: fog.

But just like naturally made fog, this "fake fog" can't transform into ghostly figures or seep under windows. So is there anything to fear?

If dry-ice fog is used in a poorly ventilated space, C[O.sub.2] can build up in the air and smother out the air's oxygen. "Actors might not get enough oxygen to breathe. They might suffocate," says Smith. That just may be the scariest thing about movie fog.

HANDS-ON SCIENCE (No Lab Required)

After reading "Seeing Through the Fog," try this activity to see how fog forms.

MATERIALS

1 medium-size glass jar (Example: a spaghetti sauce or jam jar) e ruler * 1 sheet of black construction paper * pencil * scissors * clear tape * 1 sheet of cheesecloth * hot water * timer * 1 thick rubber band * 1 tray of ice cubes

PROCEDURE

1 Measure the height and the diameter of a glass jar. Hint: To find the diameter, measure across the widest part of the jar's opening.

2 Draw a rectangle on a piece of construction paper using the following dimensions: the jar's height as the length, and the diameter as the width.

3 Cut out the rectangle and place it along the outside surface of the jar. The paper should cover half of the jar.

4 Use clear tape to attach the paper to the jar.

5 Draw a square on the cheesecloth with the following dimensions: Each side is 2.5 centimeters (1 inch) wider than the diameter of the jar.

6 Cut out the cheesecloth square and set it aside.

7 Fill the jar halfway with hot water. Let the jar sit for 30 seconds.

8 Pour out half of the hot water from the jar.

9 Use the cheesecloth square to cover the jar's opening. Then, use a rubber band to tightly secure the cloth around the opening.

10 Place enough ice cubes on the cheesecloth to cover the jar's opening.

11 Look through the clear side of the jar toward the construction paper. Observe what forms inside the jar.

CONCLUSIONS

Why did fog form in the jar?

ANSWER

Fog formed inside the jar because the ice cubes created a cool air mass at the top of the jar. As the hot water evaporated into vapor, the vapor encountered the cool air. The cool air caused the water vapor to chill and condense, forming fog.