Previously Asked Questions

   One other thing: What do you suppose would happen if one of us wore a suit, blouse, or tie that was exactly the same color of the key wall?  That’s right, the clothing would disappear and become our weather map!  So we must be very careful with the colors that we wear on the air.   I’ll never forget a St. Patrick’s Day when I was working in Seattle.  We used a green key wall and someone gave me a big green shamrock to wear on the air.  Without thinking, I put it on my lapel and went out in front of the camera.   Immediately I knew I had made a mistake when I looked over at the TV screen and saw on my lapel not the shamrock, but a shamrock-shaped hole through my chest!  I am determined to not make that mistake again.

What do the numbers 29.58in/1002mb on your tropical maps tell us? What does that information mean?
Ms. J. Lenox's 4th grade class
McDonough, GA

When we show you the air pressure numbers inside a tropical depression, tropical storm or hurricane, we give the pressure readings in both inches of mercury and in millibars.  Inches of mercury are the familiar standard of air pressure measurement in the United States.  The scale refers to how high the mercury rises in a barometer due to air pressure.  Millibars, also called hectopascals, are the standard method of air pressure measurement for meteorologists.  Unlike an inch of mercury, a millibar is the unit of pressure in the metric system, like pounds per square inch would be the unit used to express air pressure in the U.S.  Since they refer to a direct measurement of pressure, millibars can be used in metric-based mathematics to calculate weather conditions.   One millibar is equal to .02953 inches of mercury.  To give you some perspective, the average pressure at sea level is 29.92 inches of mercury or 1013.25 millibars. On the other hand, the air pressure in a category three hurricane is 27.91 to 28.47 inches on average, or 945 to 964 millibars.  Air pressure decreases with altitude, so the measured air pressure over land is usually less than what the value would be at sea level.  One important thing to remember is that when we refer to the air pressure on surface maps over land, the raw observations at various locations and elevations are adjusted to what the air pressure measurements would be at those locations if the air extended all the way to sea level.   

Nick,

When determining the temperature, is the thermometer in the shade or in the sun?  Isn't the temperature the same whether it is in the sun or in the shade?
Alan
Venice, Florida

Alan,

There are a lot of factors that go into temperature.  The temperature over hard surfaces such as pavement, roofs, and walls will be warmer than the temperature over flat grassy areas.  And direct sunlight can have a lot to do with that temperature difference.   To give an accurate air temperature, you always want to make sure your thermometer is shaded and away from hard surfaces.  This is because direct sunlight heats up most solid objects (including thermometers) more than the air away from them.  That’s why thermometers that are used for official readings are kept in shelters with slits in the sides.   This allows the air to flow through to the thermometer while keeping direct sunlight off the instrument.  Official thermometers are also up about four feet off the ground, over grass (but away from the shade of trees), and away from buildings or pavement, so the heat from those surfaces will not result in an inaccurately warm reading. 

Nick

Dear Nick,

We have two rain gauges.  After a storm, one had 3.3 inches in it the other only measured 2.5 inches.  Why would the reading be different?  How is rain measured by these gauges?  We know it's in inches, but neither of the gauges measures properly with a ruler.  Why is that?  
Krista 

Dear Krista,

It is possible that two rain gauges not far from one another could measure different amounts.  Rain showers may be heavier in one location than in another fewer than 100 yards away.  Or one rain gauge may be near a building or other structure that blocks the rain from it if the wind blows from the direction of the structure.  Another explanation could be that if a few hours pass before you take the measurements, one gauge may have received more evaporation than the other, especially if one gauge is in the sun and another in the shade. 

Tiny variations in the wind, perhaps even caused by the adjacent rain gauge can influence how much rain falls in one versus how much rain is blown over the top or around it.

The opening of an official National Weather Service rain gauge (and perhaps the one you have too) is ten times the area of the opening of the measuring cylinder.   That means a ten-inch long cylinder would hold one inch of rain.  This allows us to measure rain in smaller increments, usually in hundredths.  NWS rain gauges are also surrounded by wind baffles, which allow more rain to fall into the gauge rather than blow over or past it.  Inexpensive rain gauges often used by weather enthusiasts can have much smaller openings and different ratios that make their measurements more erratic and sensitive to local wind variations.

Of course anyone can make a simple rain gauge without the ten-to-one ratio.  Here's how: With the help of an adult, cut the top part off a clear plastic soda pop bottle so the bottle is a straight cylinder from top to bottom. Put masking tape over the rough edges around the top so they won't cut your fingers. Using a ruler and a dark permanent marking pen, mark a scale on the bottle in inches, half inches, quarter inches, or centimeters. Place your rain gauge outside and away from walls, fences or buildings that could block the rain. After the rain stops, look at your rain gauge and write down how much rain fell at your house that day.

By the way, an inch of rain in your gauge may not seem like a lot, but think about this: An inch of rain means that your area had enough rain to cover the ground one inch deep. That equals more than five-and-a-half gallons of water for every square yard, or more than seventeen MILLION gallons of water per square mile!

Nick

Dear Nick,
How do you forecast the weather? What kinds of machines do you use? How do you forecast the weather ahead of time?

Thank You,
Katie
Charlton, MA 

Katie,
In order to forecast the weather, we first must know what the weather is doing right now. To do this, meteorologists use several tools: thermometers to measure temperature, barometers to measure air pressure, and anemometers to measure wind speed.  Meteorologists also launch weather balloons to get these measurements higher in the atmosphere.  There are also weather satellites orbiting Earth taking pictures of clouds and making other weather measurements.  These pictures help us see where and how weather systems are moving.  The satellites also help us get weather data over the oceans. In addition, we use weather radar to get information about precipitation and storms.   The radar unit shoots a radio signal into a cloud, which bounces back and shows us, among other things, if there is rain or snow falling and how intense the precipitation is.  

But the best tools we have are our eyes, and the eyes of other weather observers. There are people all over the world making weather observations of clouds, precipitation, wind, and temperature.  They relay that information to other meteorologists all over the world so we can know how the weather is changing from place to place. Then we can plot that information on a weather map every hour and see how fast the weather is changing.

When we can see the direction and how fast air masses are moving, as well as where it is raining or snowing, we can get an idea of when and if it might move into the area that we are forecasting for.  

We also take all these weather observations and put the numbers into several computers, which use complicated formulas to come up with a forecast for the next several days. Using these computer forecasts, using our knowledge of how the weather works, and using our knowledge of what the weather does normally under a certain set of circumstances, we make can come up with a forecast of weather for the hours and days to come.
 
Nick

How does rainfall affect temperature?
Lemmonia

Dear Lemmonia,

Rainfall can affect temperatures in a number of ways, or perhaps not at all.  When it's raining, you're going to have clouds, and clouds will block the sunlight from warming the earth.   If it's cloudy for only a short time or late in the day after the earth has already been warmed, this may have no affect on temperatures, but if it's cloudy early in the day and stays that way for a long time, the earth will warm more slowly, so temperatures will be cooler.  In the evening or all night, this may work the other way.  If the earth has already been warmed and then clouds form in the late afternoon or evening, the clouds tend to hold the heat closer to earth and it's warmer, whereas if the sky cleared, the heat would escape into space and temperatures would cool.

Also, humid air can hold onto the heat more than dry air can.  So if it has rained recently making the air more humid, the temperature of the air is potentially higher than if it were not as humid.

One more thing, rain is often accompanied by a front.  If a warm front brings the rain, temperatures may actually warm up after the front passes through.  It it is a cold front that brings the rain, the temperatures mayl cool after the front passes through.  This really isn't related to the rain though, but only to the fronts that bring the rain.

Nick

Nick,
Why does the U.S.A. use Fahrenheit when most of the world uses Celsius?
Zachary
Wisconsin

Zachary,
Scientist Gabriel Fahrenheit invented his scale in the 1700s. Zero degrees Fahrenheit was the coldest temperature that he could create with a mixture of ice and salt.  On his scale, the freezing point of water is 32 degrees and the boiling point is 212.  Mr. Fahrenheit invented the mercury thermometer and the scale eventually became widely used in areas such as the U.S. and Britain and Canada.  

A few years later, Swedish astronomer Anders Celsius invented his scale.  He based it on a metric system of measurement instead of a British system. On it, the freezing point of water is zero and the boiling point is 100 degrees, so each degree is one-hundredth of the distance between the freezing point and the boiling point.  The U.S. has not embraced the metric system like the rest of the world has, so we continue to use the Fahrenheit scale out of habit. The Fahrenheit scale doesn't work with metric systems of measurement, so areas of the world that use the metric system use the Celsius scale.  

It is good to be familiar with both scales, since they both are widely used.

Dear Mr. Walker,
I would like to know about weather satellites.  How do they constantly monitor the same area of Earth without moving?
Hugh
Westminster, CO 

Hugh,
The United States has two kinds of weather satellites in Earth’s orbit.  Geostationary satellites orbit at the same speed as Earth’s rotation, so they stay over the same spot all the time.  They are positioned right above the equator about 22,000 miles from Earth. Polar orbiting satellites orbit 530 miles above Earth over the North and South poles.  They are able to get closer views of Earth, as well as photographs of areas near the poles which the geostationary satellites can’t see as well.   

The U.S. launched its first weather satellite in 1960.  Right now, we use two geostationary satellites, one with a view of North and South America and most of the Atlantic Ocean, and the other aimed at North America and the Pacific Ocean.  A third satellite is in orbit as a spare, in case one of the others stops working.  In addition to showing meteorologists how storm systems move, weather satellites also allow us to estimate rainfall and snowfall amounts, and look at snow cover on land and ice cover on the oceans.  On-board cameras take two types of photographs.  A visible photo is taken with a normal camera, so we can’t see anything when that part of Earth is in darkness.  An infrared photograph senses different temperatures to give us its picture of Earth both day and night.  

The National Oceanic and Atmospheric Administration has more information about weather satellites, including those operated by other countries, here:
http://noaasis.noaa.gov/NOAASIS/ml/genlsatl.html

When the wind direction says, for example, Northwest 5-10 miles an hour, does this mean out of the northwest or toward the northwest?
Deb
West Mifflin, PA

Deb,
Wind direction is very important in meteorology because it can determine whether cool or warm air will flow over an area, or whether moist or dry air will influence an area’s weather. When we talk about a northwest wind, what we mean is that the wind is blowing from the northwest (which would mean toward the southeast). Sometimes you might hear us say, "Wind is out of the northwest,” or “a northwesterly wind.”  It all means the same direction.

Nick,
I understand that leaves turn upside down when a storm is coming. Why is that?
Kelli
Stony Point, NC
Kelli,

A common weather proverb says, “When leaves show their undersides, be very sure that rain betides.” It is true that before rain, some leaves, such as those of the poplar tree, curl up or turn upside down.  This is because before the rain comes, the humidity, or amount of moisture in the air, rises.  The moist air makes the stalks of the leaves softer, so they can be more easily lifted by the wind.  Different types of leaves will react differently to changes in the weather.  Horticulturists also say that dandelions, clover, and tulips all fold their petals just before rain. The reason for each is the increasing humidity in the air. 

Bear in mind that this leafy “forecast” is only for the immediate future, that is, over the next few minutes to the next few hours, and only for the immediate area.

Does the weather always move from west to east?
Paul
New York, NY

Paul,
Like so many questions, the answer to this one is: it depends.  In the middle latitudes of the Northern Hemisphere, which include the United States, most of Europe and much of Asia, the prevailing wind flow is from the west.  However, in much of the tropics in the Northern Hemisphere, winds are primarily from the east.    

But even over the U.S. we often see storm systems moving in ways other than the familiar west-to-east direction.  One reason for this is that Earth’s rotation tends to turn winds in the Northern Hemisphere clockwise around high pressure areas and counter-clockwise around low pressure areas.  So while large-scale storm systems may move basically from west to east, it’s often more like northwest to southeast or southwest to northeast.  And in the case of smaller-scale storms, they can move in any direction, depending on local winds and air pressure differences.

Nick

Why is air pressure measured in inches?
Nina
Eatontown, NJ

Nina,

Although scientists give air pressure readings in other units of measurement called “millibars” or “hectopascals,” the National Weather Service still gives air pressure in “inches of mercury” because we have become so familiar with that measurement.   

The term “inches of mercury” dates back to the first barometers, which consisted of a glass tube in a dish of mercury.  There is no air in the tube, so the pressure of the air on the mercury in the dish causes the mercury to rise in the tube.  At Earth’s surface, that pressure forces the mercury up about 27 to 31 thirty inches into the tube, depending on the actual air pressure. The barometric pressure reading would be the actual distance in inches that the mercury rises in the tube. 

We still use mercury barometers, though nowadays we also have other barometers that don’t use mercury.  Even so, the familiar unit of measurement is still with us.

Nick,

What is the difference between scattered and isolated showers?
Lee
Alpharetta, GA

Dear Lee,
Have you ever been in a car with blue sky overhead only to suddenly ride into a heavy rain storm?  Then after a few minutes, you drive right out of the rain into clear skies again.  These showers or thunderstorms are what we call “scattered.”

In the spring and summer, much of the country can see afternoon thunderstorms that seem, to many people, to come out of nowhere.  Clear skies suddenly become filled with tall puffy clouds which produce thunderstorms that can bring short, but sometimes heavy amounts of rain.  These storms come from quickly rising pockets of warm air, which carry water vapor high into the sky where the moisture condenses into thunderstorm clouds.   Usually the clouds are narrow, and the rain doesn’t come down over a wide area, but over only a few miles.  The storms “pop up” here and there, and due to their scattered nature, it’s hard to tell exactly where they will form at any given time. 

When there are very very few of these storms, we call them “isolated.”  A forecast for “isolated thunderstorms” means that most of the area will not see them, but a few storms may come and go quickly during the day, and they may last for only a few minutes or less.  Don’t cancel your baseball game on a forecast of isolated thunderstorms, but it’s a good idea to have a plan, just in case one of those storms moves over you. 

Nick

Why is it difficult to distinguish snow, light drizzle, melting snow, and moderate rain using radar?
Heidi
Cedartown, GA

Weather radar is a wonderful tool for meteorologists, but as you say, it does have its limitations.  As you may know, radars send out radio signals from an antenna.  Those radio waves bounce off objects in the air, such as raindrops, snow crystals, hail stones and other precipitation.  Then the reflected radio waves travel back to the antenna and are electronically converted into pictures and colors showing the location and the intensity of   precipitation.  On radar images that The Weather Channel uses, a light green color shows light precipitation, and red shows heavy precipitation. We call this radar reflectivity. 

When interpreting radar images, something we have to keep in mind is that different types of precipitation sometimes have about the same reflectivity.  For example, the diameter of drizzle drops is very small, but they are dense enough to show up on the radar screen.  Snowflakes are bigger, but they are not as dense as liquid water, so the reflectivity is about the same as that of drizzle.  Untrained observers might therefore interpret the snow on the radar as drizzle.   When snow is melting, big wet snowflakes have more reflectivity and may show up on the radar looking like giant raindrops, and that might lead some radar observers to think moderate rain is occurring instead of wet snow.  The same would be true of wet hail; it might sometimes be mistaken for heavy rain.  This is why meteorologists must use ground observations, temperature readings and other tools together with radar to accurately know what kind of precipitation is occurring. 

Just to let you know that in the ever-changing world of technology, there is a new type of radar that can distinguish between precipitation types.  It’s called “multiparameter” or dual-polarization” radar.  It should be in wide use in a few years. 

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