Previously Asked Questions

To make a cloud in a bottle, get an empty two-liter plastic soda bottle and a match.  Put about an inch of water into the bottle, have a grownup light the match, hold it inside the bottle for a few seconds, and then drop it inside.  Quickly put the cap on and shake the bottle to fill the air inside with moisture.  Give the bottle a squeeze.  The increased air pressure will warm the air inside.  Then release the bottle and allow the lower air pressure in the bottle to cool the air inside.  As the air cools, the water vapor inside the bottle should quickly condense on the smoke particles, forming a little cloud.

Good luck in your science fair!

Hi Nick,
Could you please tell me where dew comes from?
Thank You.
Terri
Little Rock, AR 

Terri,
Dew comes from moisture in the air. When the air near the ground cools overnight, plants, grass, and hard surfaces such as glass or metal also cool quickly. Those cool surfaces cause moisture in the air near them to condense from water vapor back to liquid water.
You've seen the same thing happen when drops of moisture collect on the outside of your glass of ice water. When the temperature of the surface cools to the dew point temperature--the temperature at which water vapor condenses to liquid moisture--the liquid forms on those surfaces. And that's dew.  By the way, when the temperature at the ground dips below freezing, water vapor changes directly into ice crystals.  That, as you probably have guessed, is frost.

Thanks for your interest in weather!

Can you please explain the difference between dew point & humidity?
Terry
Babbitt, MN

 Dear Terry, 

Dew point and humidity are closely related.  Dew point refers to the temperature at which water vapor will condense into liquid water.  You have seen this happen when water droplets condense onto the outside of a glass of ice water.  When the glass gets colder than the dew point,  water vapor in the air condenses into liquid on the glass.   The higher the dew point temperature, the more moisture there is in the air.  The closer the dew point temperature is to the actual air temperature, the higher the relative humidity will be.  For example, if you notice in the morning that both the air temperature and the dew point temperature are 60 degrees, then relative humidity is 100%, and it is likely that fog is forming.   However, if the dew point temperature is 70 and the air temperature is 85 degrees, there is actually more moisture in the air even though the relative humidity is only  61%.   That’s why we call it “relative” humidity, because the humidity is relative to the air temperature.  That’s why we talk a lot on the air about the dew point, because it gives you more of an idea of how much moisture is in the air, and how sticky or how dry the air will feel to you.

Nick,
I have heard it said that fog takes away snow more quickly than rain or sunshine. Is this true and if so - why?  I am a home schooling mother and would like to be able to teach this to my children.
Rosalie
Southern New Brunswick
Canada  

Rosalie,
I consulted (as I often do) with The Weather Channel's severe weather expert Doctor Greg Forbes on this one.  He tells me that there is some truth to that "fog melts snow faster than rain or sunshine." But it is true only in certain cases.  

If the overall air temperature a few feet above the snow is the same, foggy conditions will melt more snow than sunny conditions.  Here’s why: The energy for melting the snow comes from the air and its contents (fog droplets, raindrops).  On a sunny day, when the relative humidity is low, the air can cool to a temperature below freezing.  With the lowered air temperature, less snow melts.  When skies are clear there tends to be a large swing between daytime and nighttime temperatures.  If the snow is dirty, there can be significant melting during the daytime, but refreezing at night.  In foggy conditions, there is little fluctuation in temperature between day and night, so melting can proceed around the clock.   

But the time of year may throw a monkey wrench into argument.   In December when the sun angle is low and nights are long (especially in northern latitudes) the sun might not melt the snow very fast.  But in the South and elsewhere in March, the sun can be a real snow-eater, with shady areas hanging on to the snow but sunny areas getting bare quickly.   

How does fog melt snow?  In fog, the relative humidity is 100% and any cooling of the air results in more fog drops condensing out, which gives off some heat to the air.  This slows the rate that the air cools.  Also, some of the energy of the above-freezing fog droplets helps melt the snow. 

The melting from rain versus fog is a hard one to determine.   If it is rain forming higher in the atmosphere and falling into a dry layer near the ground, then rain may evaporate in that drier air and cool.  In that case, fog would be more effective in melting the snow than the rain.  But if the rain is warm and the near-ground relative humidity is high, then the rain is likely to cause more melting than just fog. 

One other factor can reduce the snow pack without melting.    In dry, windy conditions, the snow can just "evaporate" from ice crystals to water vapor.  Technically we call this “sublimation.”  In this case, the snow just seems to disappear, leaving no water behind.

Dear Nick,

I have asked this question a thousand times and no one has ever been able to answer it. My question is this: When it's raining outside, why is it NOT 100% humidity?  Doesn't humidity measure moisture in the air?
Thank you,
Ruth 
St. Paul, Minnesota 

Ruth,
You are right; humidity measures moisture in the air.   But it can be raining without 100 percent humidity.  And, the humidity can measure 100 percent without it raining.   

For clouds to form and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity. Some water from the rain evaporates into the air it's falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%.  

When air is saturated, or 100% humidity, rain may or may not form, since it takes more than saturated air to make rain.  Rain actually forms from two processes.  In one case, the drops simply collide with one another until they are big and heavy enough to cease being suspended in air and they fall to the ground.  The air may be completely saturated, but the drops are so small that they may remain suspended in the air and not fall as rain.  This is one reason why not all clouds produce rain.  In thick fog, the humidity is usually 100 percent, but the fog droplets are so small they don't form raindrops. 

Most rain actually starts from ice crystals in clouds, which draw moisture from very cold water drops.  The ice crystals grow in size and weight until they are big and heavy enough to fall from the cloud.  If the air at the earth's surface is cold enough, they remain in crystal form and fall as snow or melt on the way to the ground and fall as rain.  Again, the air in the cloud may be 100 percent humidity and this process may not happen.  Thus you have saturated air, but no precipitation. 

Sometimes you just have to ask a question at least one thousand and ONE times to get an answer!

How do clouds come down to make fog?
Shannon
Waterford, MI
 

Shannon, 

We usually think of a cloud as something you see up in the sky.  So it might be natural to think that fog is a cloud that has “come down” to the ground. But the moisture that forms a cloud of fog is already at the ground even before you can see it. 

When the air is very full of water vapor (water in invisible gas form), fog will form if a little more moisture is added to the air, or if temperatures cool enough to cause the water vapor to condense into water drops.   At night, a sky with no clouds may allow heat to escape up into space and the air near the ground to cool enough to cause that condensation.  It’s the same process that turns the moisture in your breath into a little cloud on a cold day. 

Fog can form in other ways too.  Sometimes winds blow warm moist air over a cold surface such as water or ice, which causes the moisture to condense into fog.  When cool air moves over a warm lake or pond, moisture may evaporate from the water’s surface and condense in the cool air.  This results in what is called “steam fog,” because it looks like steam rising from the lake. 

By the way, just as fog doesn’t “come down,” it doesn’t really “lift” either, even though we use that word when fog starts to go away.  As the ground begins to warm, the warmer temperatures cause the moisture in the fog nearest the ground to evaporate back to invisible water vapor.  It may still be foggy above our heads, so it looks as if the fog is “lifting,” but it’s actually evaporating and disappearing from the ground up.

What actually causes it to rain?
Don
Stockton, California

Don,
Sometimes we’re so concerned with more complicated issues that we ignore the basics.  That’s why I appreciate your question.  And it’s a good one, because many people assume that once a cloud forms, the cloud droplets simply fall back to Earth.  But a cloud droplet must grow about a million times bigger in order for it to turn into a raindrop.   

That can happen two ways.  The most common is called the “Bergeron Process,” named after the Norwegian meteorologist who first proposed it.  He discovered that cloud droplets can cool below freezing and still remain in liquid form.  In a cloud that contains these “supercooled” drops as well as ice crystals, the liquid drops begin to freeze to the ice crystals, forming larger ice crystals that finally get big enough to fall as snowflakes.  When the snowflakes meet warmer air on the way to the ground, they melt into raindrops. 

The other way rain can form is through a process called “collision and coalescence.”  This simply means the cloud droplets bump into each other (collision) and stick together (coalescence) until they’re big and heavy enough to fall to the ground.  This is how rain forms in clouds that aren’t cold enough for ice to form. 

Thanks for the question!
Nick

Hello Nick,
My husband was wondering how much rain does it take to equal a foot of snow?
We were driving in a torrential downpour that dropped five to six inches of rain that evening. He asked how much snow we would've received had it been snow instead of rain. I had no idea so I came home to seek the answer online.   Thank you for your help.  
Nancy

Nancy,
A little bit of moisture goes a long way when it comes to snow.  The average ratio is about ten inches of snow per inch of rain.  However, this ratio is quite variable. If the air is warm, it may take the equivalent of as much as five inches of rain to get ten inches of wet snow, since the snow may be packing or melting quickly.  In extremely cold air, the ratio may be only about half an inch of rain equivalent for ten inches of “dry” fluffy snow.  Our severe weather expert Dr. Greg Forbes says he has seen ratios as much as 40 to one in some lake effect snows, or 40 inches of snow for one inch of water equivalent!  Just be glad all the moisture you saw in your heavy rain wasn’t in a winter storm!

Nick,
I am doing a weather project.  Can you give me some tips on how to make a weather measuring instrument?

Lynn
Rexford Knysna, South Africa

Lynn,

One of the most useful and easy-to-make weather instruments is a rain gauge.  That’s because the rain gauges that meteorologists use are basically fairly simple devices.   An “official” rain gauge is little more than a large cylinder-shaped cup that collects rain when it falls. Several times a day, weather observers insert a measuring stick (which is just a kind of big ruler) into the cup, or they look at a measuring scale printed on the side of the rain gauge. They record how much rain falls over a certain period of time to the nearest one-hundredth of an inch.

You can make your own rain gauge and measure how much rain falls at your house. 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.   It’s interesting to see just how much rain you get over a period of time.

Dear Nick,
We have two rain gauges.  I dumped the rain from one (it had 3.3 inches in it) into the other, and that rain gauge 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?  
Sincerely,
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.

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,
My boyfriend and I were having a discussion about rain. Our question is this: Can it rain at 26 degrees?  Or at any temperature below 32 F for that matter? Your help is greatly appreciated to settle our debate.
Nicole
New York City

Nicole,
The falling precipitation could be rain at 26 F, but if it is liquid before it hits the ground, chances are that it would freeze as soon as it hits something, like the ground, or streets or trees.  So meteorologists refer to this type of precipitation as “freezing rain.”   This happens when the air temperature is above freezing somewhere between the clouds and the ground, but at freezing temperatures or below freezing at ground level.

Water drops can remain liquid in our atmosphere even at temperatures below freezing.  These drops are called “supercooled.”  Drizzle, which is composed of small liquid droplets, can form as liquid and remain unfrozen even when temperatures are continually below freezing.  In these cases, the clouds form as tiny liquid drops, even though the air temperature is below 32 F.  This happens in relatively shallow clouds in which no part of the cloud has temperatures too far below freezing, so the cloud doesn’t contain any snow.  Sometimes the drops grow large enough to become freezing drizzle at the ground, or liquid drizzle if there is warm air below.  Or it might even be crunchy round snow pellets if the air is very cold below the cloud.  

In thicker clouds, rain usually starts as snow, which melts on the way down to turn to rain. If it never melts, you get snow. If it melts, then re-freezes in a layer of sub-freezing air closer to the ground, you get sleet. If the layer of cold air is very shallow, you can get freezing rain as described above. If that layer of sub-freezing air is very shallow and hasn't been around long enough to make the ground freeze, you'll get rain until the surfaces fall below freezing.