Friday, 4 November 2016

daily life science

How Does GPS Work?et

graphic showing earth and a constellation of GPS satellites
Earth is surrounded by navigation satellites. Credit: NOAA.
Humans have looked to the skies to find their way since
ancient times. Ancient sailors used the constellations in the night sky to figure out where they were and where they were going.
Today, all we need is a simple hand-held GPS (short for Global Positioning System) receiver to figure out exactly where we are anywhere in the world. But we still need objects high in the sky to figure out where we are and how we get to other places.
Instead of stars, we use satellites. Over 30 navigation satellites are zipping around high above Earth. These satellites can tell us exactly where we are.

What is GPS?
cartoon of a boy holding a GPS receiver with a satellite and ground station in the background.
The Global Positioning System (GPS) is made up of satellites, ground stations, and receivers.
GPS is a system. It’s made up of three parts: satellites, ground stations, and receivers.
Satellites act like the stars in constellations—we know where they are supposed to be at any given time.
The ground stations use radar to make sure they are actually where we think they are.
A receiver, like you might find in your phone or in your parents car, is constantly listening for a signal from these satellites. The receiver figures out how far away they are from some of them.
Once the receiver calculates its distance from four or more satellites, it knows exactly where you are. Presto! From miles up in space your location on the ground can be determined with incredible precision! They can usually determine where you are within a few yards of your actual location. More high-tech receivers, though, can figure out where you are to within a few inches!
The ancient sailors of history would be flabbergasted by the speed and ease of pinpointing your location today.

Why is the sky blue?

It's easy to see that the sky is blue.Have you ever wondered why?

A lot of other smart people have, too. And it took a long time to figure it out!
blue sky and clouds illustration

The light from the sun looks white. But it is really made up of all the colors of the rainbow.
A prism separates white light into the colors of the rainbow.
When white light shines through a prism, the light is separated into all its colors. A prism is a specially shaped crystal.
If you visited The Land of the Magic Windows, you learned that the light you see is just one tiny bit of all the kinds of light energy beaming around the universe--and around you!
Like energy passing through the ocean, light energy travels in waves, too. Some light travels in short, "choppy" waves. Other light travels in long, lazy waves. Blue light waves are shorter than red light waves.
Different colors of light have different wavelengths.
All light travels in a straight line unless something gets in the way and does one of these things:—
  • reflect it (like a mirror)
  • bend it (like a prism)
  • or scatter it (like molecules of the gases in the atmosphere)

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.
Atmosphere scatters blue light more than other colors.
Closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead. As the sunlight has passed through all this air, the air molecules have scattered and rescattered the blue light many times in many directions.
Atmosphere scatters blue light more than other colors
Also, the surface of Earth has reflected and scattered the light. All this scattering mixes the colors together again so we see more white and less blue.

What makes a red sunset?

As the sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.
Red sky at sunset
Red sun at sunset.

Sometimes the whole western sky seems to glow. The sky appears red because larger particles of dust, pollution, and water vapor in the atmosphere reflect and scatter more of the reds and yellows.

How do hurricanes form?

Hurricane Fran
Hurricane Fran. Image made from GOES satellite data.
Hurricanes are the most violent storms on Earth. People call these storms by other names, such as typhoons or cyclones, depending on where they occur. The scientific term for all these storms is tropical cyclone. Only tropical cyclones that form over the Atlantic Ocean or eastern Pacific Ocean are called "hurricanes."
Whatever they are called, tropical cyclones all form the same way.
World map showing area where cyclones occur.
Tropical cyclones are like giant engines that use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. The warm, moist air over the ocean rises upward from near the surface. Because this air moves up and away from the surface, there is less air left near the surface. Another way to say the same thing is that the warm air rises, causing an area of lower air pressure below.
Cumulonimbus cloud
A cumulonimbus cloud. A tropical cyclone has so many of these, they form huge, circular bands.
Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then that "new" air becomes warm and moist and rises, too. As the warm air continues to rise, the surrounding air swirls in to take its place. As the warmed, moist air rises and cools off, the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the surface.
Storms that form north of the equator spin counterclockwise. Storms south of the equator spin clockwise. This difference is because of Earth's rotation on its axis.
As the storm system rotates faster and faster, an eye forms in the center. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.
Tropical cyclone cross-section
If you could slice into a tropical cyclone, it would look something like this. The small red arrows show warm, moist air rising from the ocean's surface, and forming clouds in bands around the eye. The blue arrows show how cool, dry air sinks in the eye and between the bands of clouds. The large red arrows show the rotation of the rising bands of clouds.

When the winds in the rotating storm reach 39 mph, the storm is called a "tropical storm." And when the wind speeds reach 74 mph, the storm is officially a "tropical cyclone," or hurricane.
Tropical cyclones usually weaken when they hit land, because they are no longer being "fed" by the energy from the warm ocean waters. However, they often move far inland, dumping many inches of rain and causing lots of wind damage before they die out completely.
Tropical cyclone categories:
CategoryWind Speed (mph)Damage at LandfallStorm Surge (feet)
5157 or higherCatastrophic19+
The two GOES satellites keep their eyes on hurricanes from far above Earth's surface—22,300 miles above, to be exact! (Learn more about this kind of orbit.)
These satellites, built by NASA and operated by the National Oceanic and Atmospheric Administration (NOAA), save lives by helping weather forecasters predict and warn people where and when these severe storms will hit land.

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