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Forecasters worth their salt routinely use current weather and recent history as the basis for predicting the future. That's because current and past weather can, and often does, offer clues about how the atmosphere will evolve. During winter and early spring, for example, powerful Pacific storm systems that make news on the West Coast by spawning heavy coastal rains and mountain snows often make news a few days later when they arrive over the Middle West, generating fierce thunderstorms that can spawn tornadoes (opens in a new window).

However, even in more benign weather patterns, conscientious forecasters routinely study weather conditions "upstream" of their location (by upstream, I mean where weather systems are coming from), hoping to extrapolate these conditions into the future to get a more accurate beat on the local weather forecast. There's a big payoff to forecasters who are sticklers for such details. Indeed, the wealth of surface observations taken hourly across the nation often tips the atmosphere's hand and gives meteorologists a leg up on important clues to the weather forecast.

At all U.S. airports, standard hourly weather observations are taken once each hour, typically several minutes before the top of the hour. So, for example, the standard 3:00 PM observation might have a time stamp such as 2:53 PM. More formally, standard hourly weather observations are issued between 50 minutes past the hour and the top of the next hour, so a standard 3:00 PM observation could be time stamped between 2:50 PM and 3:00 PM. When weather conditions rapidly change, however, you'll often see special observations, known as SPECI reports, at other times. A "special ob" taken at 3:15 PM, for example, falls under the umbrella of the 3:00 PM observation, even though the standard observation was taken a little before 3:00 PM. In general, all observations time-stamped between (hh-1):50 to hh:49 are part of the hh observation cycle (hh represents any given hour). So, continuing with our example, any observation time-stamped between 2:50 PM and 3:49 PM belongs to the 3 PM observation cycle. The 4:00 PM observation cycle begins at 3:50 PM, and so on.

As you might expect, there's an avalanche of surface weather observations each hour from all the airports across the country (and across the world, for that matter). In order to simplify life and create easy-to-read weather maps, the National Weather Service organizes hourly observations onto templates called station models. In the remainder of this lesson, you'll learn how to decode surface station models (and thus determine local weather conditions). However, before we tackle the rules and conventions for decoding station models, you'll need to know how weather observers all over the world synchronize their watches in order to standardize the times that weather observations are taken.

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"Does anybody really know what time it is? Does anybody really care...?"

Those words come from this section's theme song, a classic from the musical vault—"Does Anybody Really Know What Time It Is (opens in a new window)" by Chicago. Well, I can tell you that meteorologists must know what time it is, and they definitely care about time. Weather is a global phenomenon, and since our world is sliced into individual time zones, meteorologists need a universal standard to keep it all straight.

That standard is Greenwich Mean Time (GMT). "Greenwich" refers to the English village of Greenwich, a borough of London, through which the Prime Meridian (opens in a new window) (zero degrees longitude) passes. The advantage of adhering to one time standard is that observers all over the world can record weather conditions in Greenwich time. Such a universal time system is indispensable for synchronizing when weather observations are collected. If observers worldwide were to record observations in local time, then interpretation would become much more complicated and confusing. Ultimately, it's important to remember that GMT is a time zone, just like any other. It just happens to be the time zone at Greenwich, England, along the Prime Meridian.

GMT goes by a couple of other aliases--"Zulu time" (often shortened to Z-time), or UTC (Coordinated Universal Time). "Zulu" is a funny sounding name, but it's the U.S. Navy's and our civil aviation's version of GMT. The bottom line is that if you see time expressed as GMT, Z-time, or UTC, they're all referring to the same thing--the time in Greenwich, England. Most often, we'll use UTC or Z-time in this course. Meteorologists universally use this time to synchronize the times of weather observations and forecasts, so it's important for us to be able to convert from UTC to other local time zones, as well as from other local time zones to UTC.

You can convert to Local Time at any location by referring to a map of world time zones (opens in a new window) (zones are labeled along the bottom of the map). That's a pretty "busy" map, so let's streamline our discussion a bit. Focus your attention on the map of standard time zones for a large portion of the Western Hemisphere (shown below). Further note that each time zone is labeled with its corresponding time difference from Greenwich, England (expressed in hours UTC). How does this map work?

First, we're using the military's 24-hour clock system (opens in a new window). For this system, 0000 hours ("zero hundred hours") corresponds to local midnight, and 1200 hours ("12 hundred hours") represents local noon. Okay, let’s assume that it’s 1500 hours in Greenwich (alternatively, 15 UTC, 15Z or 15 GMT...take your pick!). On a 12-hour clock, the local time in Greenwich would be 3 P.M. At any rate, you can see, across the top of the colorful map above, the corresponding local times at 15Z for each of the represented time zones. For example, at 15Z (1500 hours in Greenwich), it’s 1000 hours (10 A.M.) local time in the eastern United States (Eastern Standard Time is UTC - 5 hours), and 0600 hours (6 A.M.) local time in Alaska (Alaska Standard Time is UTC - 9 hours).

On the flip side, if you lived in Chicago, Illinois and it was 9 A.M. local time (0900 hours), and you wanted to convert to UTC, you would simply add 6 hours because Central Standard Time (where Chicago is located) is 6 hours behind UTC. So, 0900 hours + 6 hours = 1500 hours, or 15 GMT (or 15 UTC or 15Z).

Ultimately, converting from UTC to local time (or the other way) is really no different than figuring out what time it is in California if you live in, say, New York. If it's 5 P.M. local time in New York, we have to subtract 3 hours to get the local time on the West Coast in California, so we know its 2 P.M. local time in California. Converting to or from UTC is no different: It's just addition or subtraction. You have to figure out how many hours difference there is between whatever location you're interested in and UTC.

Many of the time-zone boundaries are parallel to longitude lines, although, for convenience, there are several exceptions (Alaska, for example). Each time zone spans approximately 15 degrees of longitude, which is the longitudinal distance that the Earth rotates in one hour. Of course, you must adjust for Daylight Saving Time (opens in a new window) during the warmer months (from the second Sunday in March to the first Sunday in November in the United States). While 15 UTC corresponds to 10 A.M. Eastern Standard Time (EST) in New York City, from early March to early November it's 11 A.M. Eastern Daylight Time (EDT) in the New York (Eastern Daylight Time is only 4 hours behind UTC). So, when Daylight Saving Time is in effect, the difference between UTC and time zones in the U.S. is one hour less than what's indicated on the map above. By the way, it is bad form to say "Daylight Savings Time." Save yourself the trouble, and don't put the "s" on the end of "saving."

Want to see a few quick examples of time conversions between UTC and local time zones? Check out the short video (4:07) below:

Please note that the International Date Line (opens in a new window) zig-zags across the Pacific Ocean in an attempt not to inconvenience local time keeping (traveling westward across the date line results in the calendar advancing one day). For convenience, the abrupt zig-zag in the International Date Line south of Siberia allows Alaska's long Aleutian Island chain to be in the same time zone as the rest of the state (Alaska Standard Time, AST, is 9 hours behind UTC).

Now that you know how time conversions work, the best way to really get comfortable with knowing what time it is anywhere in the world is to do some practicing. Make sure to spend some time on the Key Skill questions and the Quiz Yourself tool below.

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While you probably think of temperature as "how hot or cold something is," that's a pretty ambiguous definition (since "hot" and "cold" are somewhat subjective). More precisely, temperature is a measure of energy. You see, air molecules are restless little lumps of matter, continually vibrating, wriggling and bumping into their many neighbors. As air temperature increases, the molecular dance becomes increasingly frenetic. At a temperature of 72 degrees Fahrenheit, the average speed of air molecules is about 1,000 miles an hour, which translates into ample kinetic energy (energy of motion). Thus, air temperature is a measure of the average kinetic energy of air molecules (air consists mostly of nitrogen and oxygen molecules (opens in a new window)).

In the United States, we typically express temperature using the Fahrenheit temperature scale (opens in a new window), but most countries in the world use the Celsius temperature scale (opens in a new window) (undoubtedly, you've heard temperature expressed in "degrees Fahrenheit" or "degrees Celsius" before). Undoubtedly, you'll encounter instances when you need to convert between the two scales. In those circumstances, the National Weather Service temperature conversion calculator (opens in a new window) is great!

To give you some weather context, the North American all-time marks for highest and lowest temperatures are, respectively, 134 degrees Fahrenheit (56.7 degrees Celsius) in California's Death Valley (see the photograph below), and -81.4 degrees Fahrenheit (-63 degrees Celsius) at the village of Snag (near Beaver Creek) in the Yukon Territory of Canada (opens in a new window). If you're interested in current global temperature extremes, this website summarizes the extremes (opens in a new window) from all the hourly weather observations around the world.

You may also be familiar with some other common temperature markers:

• 100 degrees Celsius (212 degrees Fahrenheit) is the boiling point of water
• 37 degrees Celsius (98.6 degrees Fahrenheit) corresponds to normal body temperature
• 22.2 degrees Celsius (72 degrees Fahrenheit) represents the "ideal" room temperature
• 0 degrees Celsius (32 degrees Fahrenheit) is the melting point of ice

Note that I referred to 0 degrees Celsius (32 degrees Fahrenheit) as the melting point of ice, and not the freezing point of water. That phrasing was chosen deliberately! Indeed, ice melts at 0 degrees Celsius, but not all water freezes at 32 degrees Fahrenheit! For more details, check out the Explore Further section below, but this fact has some important consequences for how precipitation forms in clouds that we'll get into later in the course. So, when you hear that 0 degrees Celsius or 32 degrees Fahrenheit is the freezing point of water, keep in mind that it's not usually true.

By the way, there are other temperature scales besides Celsius and Fahrenheit. For example, there's the Kelvin scale (opens in a new window) (sometimes called the absolute temperature scale). Please note that the number of kelvins = the number of degrees Celsius + 273.15. So, the melting point of ice is 273.15 kelvins and the boiling point of water, at standard pressure, is 373.15 kelvins (100 degrees Celsius or 212 degrees Fahrenheit). For the record, it's bad form to say "degrees kelvin." Indeed, the proper way to express the units of absolute temperature is simply "kelvins." Also note that the word "kelvins" is never capitalized except where any word would be capitalized, such as at the beginning of a sentence. The Kelvin scale is used commonly in the physical sciences, and in fact it's the most direct way to describe the relationship between the average speed of air molecules and their temperature (higher temperatures = faster average molecule speeds).

Now that you know what temperature is, the next step is to be able to identify and interpret temperatures from a station model, which is covered in the Key Skill section below.

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Meteorologists are very interested in horizontal visibility (the maximum distance away that an observer can see an object located near or on the ground), because it has major implications for transportation. Very poor visibility can cause major traffic accidents (opens in a new window) and airline catastrophes. Obviously, visibility is very important for pilots during take-off and landing, and if you want to see what it's like for a pilot to land in poor visibility, check out this video of a Boeing 737 landing in poor visibility (opens in a new window) in London (view from the cockpit).

Often, visibility can vary in the 360-degree panorama around a weather station. For example, there could be visibility-restricting snow showers just to the north and west of the station, disproportionately reducing visibility in those quadrants. To see what I mean, check out the panoramic view on a wintry day (opens in a new window) with recurrent, scattered snow showers in the vicinity of Penn State's main campus, and note that parts of the nearby ridges can't be seen because of snow showers. In such a situation, an observer reports a "representative visibility." On days when horizontal visibility dramatically varies over the 360-degree panoramic view around an airport or weather station, a trained weather observer determines a single visibility that reasonably describes more than half the 360-degree panorama. In more precise terms, a representative visibility is the greatest distance that objects can be observed and identified over more than 180 degrees of the panoramic view around an airport or weather station.

Horizontal visibility can run the gamut. On a perfectly clear day, you can't see forever, but visibility can reach approximately 100 miles in the mountainous West. On the other hand, visibility can lower to near zero in very dense fog (opens in a new window), fierce blowing and/or falling snow (opens in a new window), blowing sand/dust (opens in a new window), smoke (opens in a new window), etc. Automated weather stations, however, typically report the visibility as 10 miles when no obstructions to visibility are present.

Obstructions to visibility (so called "present weather") such as fog, haze (opens in a new window), and smoke are considered non-precipitating obstructions to visibility, and their reductions to visibility can be very noticeable. For example, check out these photographs of the ridges south of Penn State's main campus on two different summer days -- one with a clean atmosphere (opens in a new window) and one with a hazy atmosphere (opens in a new window). For a dramatic example of a non-precipitating obstruction to visibility, which may be of special interest to aviators in particular, check out the Explore Further section below.

Precipitation can also reduce visibility by varying degrees. The degrees to which precipitation reduces horizontal visibility gives rise to a hierarchy of qualifiers such as light snow, moderate snow, and heavy snow. Indeed, when dealing with snow, the qualifiers of light, moderate, and heavy are actually defined in part by horizontal visibility. Rain can also reduce horizontal visibility, but its qualifiers of light, moderate, and heavy are defined by rainfall rate (not horizontal visibility).

Now that you know the types of conditions that can reduce visibility, let's take a look at visibility is displayed on the station model, which is covered in the Key Skill section below. Before you dive into that section, one thing to note is that non-precipitating obstructions to visibility are displayed as present weather on the station model only if the horizontal visibility is less than or equal to seven miles. Why seven miles? Typically, a radio beacon that aircraft use while landing called the outer marker (opens in a new window) lies four to seven miles away from the start of the runway. So, if there's an obstruction to visibility that prevents pilots from seeing the runway from the outer marker, then the obstruction must appear of the station model.

Meanwhile, when precipitation falls at an airport, it is always depicted on the local station model as present weather no matter how light or how little it affects visibility. This protocol exists because pilots always need to know when precipitation occurs at an airport not only because it restricts horizontal visibility, but because it also lowers the heights of cloud ceilings, both of which come into play during take-off and landing. To summarize when the obstruction to visibility symbol is displayed on a station model, review this flow chart (opens in a new window).

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Everyone will surely recognize that water is an important player in weather, so meteorologists must have weather variables that help them assess moisture. One such variable is dew point temperature. By definition, the dew point is the approximate temperature to which the water vapor (the gaseous form of water) in the air must be cooled (at constant pressure) in order for it to condense into liquid water drops. I emphasize here that dew point is a temperature, so it's typically expressed in degrees Fahrenheit or Celsius.

As it turns out, the dew point temperature is also an absolute measure of the amount of water vapor in the air. The higher the concentration of water vapor, the higher the dew point (and the lower the concentration, the lower the dew point). What constitutes "high" and "low" dew points? At the surface of the earth, the lowest dew points tend to be found during winter, in bitterly cold, dry air masses from the Arctic, where dew points can be well below 0 degrees Fahrenheit. On rare occasions, dew points in such air masses in the northern United States can drop to -50 degrees Fahrenheit or lower! On the flip side, the highest dew points tend to be found during summer in warm, moist, "tropical" air masses. In the summer, these air masses frequently have dew points above 70 degrees Fahrenheit. On occasion in the United States (usually for short periods of time), dew points can even rise into the low 80s, but extremely rarely climb higher than that. If you want to learn more about extreme dew points, check out the Explore Further section toward the bottom of this page.

The fact that dew point serves as an absolute measure of the amount of water vapor in the air sets dew point temperature apart from many of the other variables that describe moisture in the atmosphere. These other variables have their uses, but they also depend on other factors beyond just the amount of water vapor present. We'll talk more about some of these other variables later in the course. Moisture is a fairly complicated topic, so we're just going to scratch the surface for now!

To better understand dew point and its applications, we should start with the characteristics and behavior or water vapor. As mentioned above, water vapor is the gaseous form of water. You probably learned at some point that matter exists in three states -- solid, liquid, and gas. Well, water is one of the rare substances that can exist in all three states naturally in our atmosphere. Water's solid (ice) and liquid forms are evident all around us, but the gaseous form (water vapor) might not be so obvious. Just like other gases (oxygen, nitrogen, carbon dioxide, etc.) water vapor is invisible.

A consequence of this is that standard photographs really don't show water vapor, even if they claim to. For example, check out this image of a steaming tea kettle (opens in a new window). Within the effluent escaping from the spout, where is the water only in vapor form? Hint: It’s not in the part you can see. Although some water molecules are likely in the vapor state mixed within the visible “cloud,” the water that you can see is actually in the form of tiny liquid drops. If you look closely, there appears to be a gap between the tea kettle’s spout and the visible cloud (here's an annotated image of the tea kettle (opens in a new window)). This is where the water exists in a pure vapor state. In fact, this is only a portion of the effluent that is “steam,” or super-heated water vapor.

Ultimately, water vapor behaves just like any other gas. On a molecular level, water vapor behaves just like oxygen, nitrogen, carbon dioxide, etc. Consider a situation where you had a box of “air” (containing all of the molecules normally found in the atmosphere). This is very much like having a box of various colored marbles. These marbles (because they have a lot of energy) are zooming around, bouncing off the sides of the box and each other. However, each marble is acting independently of the others. This means that in our box of air, the oxygen molecules are acting independently of (and oblivious to) other molecules – including water vapor molecules. The implication of all of this is: Air does not “hold” water vapor, and has no "holding capacity" for water vapor (which are common, but incorrect, phrases that are used to explain water processes). Air isn't like a sponge that can't absorb any more water once the pores of the sponge become filled with water. Indeed, all the air molecules in our box, combined, would only occupy a really tiny fraction of the space in the box, no matter what. So, there's always enough room for more water vapor molecules. We're going to expand on these ideas later in the course when we talk about topics like cloud formation, but I wanted to lay the groundwork for thinking correctly about water vapor now (it will help later on).

Now we need to discuss the processes by which water changes phase (namely to and from water vapor). When transitioning from a gas to a liquid, water undergoes a process called condensation. Likewise, when transitioning from a liquid to a gas, the process is called evaporation. We'll explore these (and other) “phase transitions” in more detail later in the course; however, at this point, I want to emphasize that evaporation and condensation events are taking place all the time, everywhere around you, even if you can't see them. Surprised? Allow me to illustrate.

Take a look at the metal glass roughly half-filled with cold water on the right. The bottom of the glass is obviously coated with a layer of small liquid water drops (often called “dew”), while the top is not. Why is that? Molecules of water in the gas phase (water vapor) are zinging around in the air, but when a water vapor molecule strikes an object (like the side of the glass), it may “stick” (that is, condense on the surface). I say “may” because there is only some chance that the molecule is captured by the surface. If it does stick, then there is another chance that within some time frame, the molecule will become “unstuck” (that is, evaporate from the surface) and return to the gas phase. Thus, on all surfaces, there is a chance of condensation and a chance of evaporation for each gas molecule that encounters a surface, which means that we always have a rate of condensation and a rate of evaporation for every surface.

So, molecules are impacting (condensing) and leaving (evaporating) on both the top and bottom surface of our glass half-covered in dew. But, then why is the bottom covered in tiny liquid drops while the top remains dry? The answer lies in the fact that the rates of condensation and evaporation are not equal everywhere. On the bottom of the glass, the rate of evaporation is less than the rate of condensation; therefore, there is a net increase in liquid water (we say “net condensation”). On the top of the glass, the rate of evaporation is greater than the rate of condensation, meaning that there's a net decrease in liquid water (we say “net evaporation”). Since the glass is about half full of cold water, you might have guessed that temperature is playing a role here (and you're correct). The colder part of the glass has a lower evaporation rate, which allows tiny water drops to grow via net condensation (condensation occurs faster than evaporation does on this part of the glass).

Now with some background about water vapor's behavior, let’s revisit our definition of dew point temperature. We said that the dew point is the approximate temperature to which the water vapor in the air must be cooled in order for it to condense into liquid water drops, and that the dew point temperature is an absolute measure of the amount of water vapor in the air -- the higher the concentration of water vapor, the higher the dew point. Can you now see how these two ideas connect? If the air contains a high concentration of water vapor (dew points are high), then net condensation will occur at a higher temperature (that is, at a high dew point temperature). If water vapor concentrations are very low (dew points are low), then net condensation will not occur until the air is very cold (that is, at a low dew point temperature). If the dew point temperature is less than 32 degrees, the term frost point is, technically, more appropriate than "dew point" because frost (opens in a new window) will form (by a process called deposition, not condensation) instead of dew.

One final practical point about dew point. The higher the concentration of water vapor, the higher the dew point, and by itself, the dew point serves as an indicator of the way the air “feels” – whether it be dry or muggy. Since our skin temperature is regulated to some degree by evaporation of sweat, it would be logical that we would be affected to some degree by the dew point temperature. Certainly, describing how something “feels” can be a bit dicey in a science course because it’s a somewhat subjective topic, but examine the table below for a rough guide on how the air might “feel” based on dew point temperature.

Now that you know some basics about dew point and the characteristics and behavior of water vapor, let's shift gears to looking at dew points on station models, which is covered in the Key Skill section below.

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Let me start with the age-old question: "Which phrase do you think describes a cloudier sky -- partly sunny or partly cloudy?" The answer to that question might depend on who you ask. The National Weather Service defines partly sunny (opens in a new window) and defines partly cloudy (opens in a new window) as essentially the same, with the caveat that we wouldn't use "partly sunny" at night, of course. But, in practice, some forecasters use these terms differently because the word "partly" is somewhat vague, so it's not clear-cut. Some folks use "partly sunny" to emphasize that there will be a bit more clouds than sun, and use "partly cloudy" to emphasize that there will be a bit more sun than clouds. With this usage, a partly sunny day is actually cloudier than a partly cloudy day.

Most weather forecasters don't want to get drawn into such an argument of semantics, so when it comes to quantifying the coverage of the sky by clouds, they rely on a specific "pie-chart" system that leaves little room for debate (see table below). The "pie" that makes up the sky coverage observation is divided into eight sections. Clear conditions (0/8 cloud coverage) constitute a perfectly sunny sky, while "overcast" conditions (8/8 coverage) constitute a completely cloudy sky. Those two are pretty straightforward. In between those two extremes, a "few" clouds (1/8 to 2/8 coverage) represent mostly sunny (or mostly clear) conditions. "Scattered" clouds (3/8 to 4/8 cloud coverage) correspond to a partly cloudy or partly sunny sky, with "broken" clouds (5/8 to 7/8 cloud coverage) describing a partly cloudy or partly sunny (5/8 coverage) to mostly cloudy (6/8 to 7/8 coverage) sky. When the sky is nearly overcast except for a few breaks, forecasters refer to the cloud coverage as breaks in the overcast (abbreviated as "BINOVC"). This photograph shows an example of BINOVC conditions (opens in a new window) (note the patches of blue sky toward the bottom left of the photo in an otherwise overcast sky). When the sky is broken or overcast, weather observations will include the corresponding cloud ceiling, which is simply the height of the base of a broken or overcast layer of clouds.

On occasion, the sky cover cannot be seen due to near-surface conditions such as dense fog, heavy rain, blowing snow, etc. For example, check out this webcam shot of Penn State's Beaver Stadium in dense fog (opens in a new window). You can't really see the stadium, and you can't really see the sky, either! In such cases when the observer cannot determine the sky coverage, the condition "sky obscured" is reported. Note: Even if the observer is fairly confident that the sky is overcast, if the ceiling cannot be observed, "sky obscured" would still be reported (also note that the observation is "sky obscured," NOT "sky obstructed" -- a common mistake). Also, when sky obscured conditions exist and vertical visibility is very low, you'll sometimes see references to an indefinite ceiling. This simply means that the near-surface conditions (such as dense fog, blowing snow, etc.) have limited the vertical visibility to the point that the cloud ceiling can't be determined.

I should add that thick haze and smoke can also obscure the sky, preventing weather observers from assessing the specific fraction of cloud cover. Thick smoke, for example, often obscures the sky in the vicinity of major wildfires, such as in this striking photograph of the Pine Gulch Fire (opens in a new window) (Credit: Public Domain) in Colorado in 2020. Now that you know the conventions for reporting sky coverage, let's take a look at how to identify and interpret sky coverage on a station model in the Key Skill section below.

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"Pressure...pushing down on me, pressing down on you..."

Those lyrics come from the song "Under Pressure (opens in a new window)" by Queen (featuring David Bowie) from 1981. As we start our investigations of pressure, we have to start with the basics. For starters, what is pressure? On that matter, Queen basically nailed it. It's a force that pushes down on me and you (and everything else), although pressure isn't easy for us to "feel" with our human senses except under certain circumstances. You've probably noticed the impacts of pressure if your ears have popped while driving up or down a mountain, or if you experience discomfort when air pressure decreases as a storm approaches (as many folks with arthritis or bursitis experience).

Meteorologists are concerned about atmospheric pressure, which is the pressure exerted by air molecules, and you may recall from a high school science class that pressure is defined as a force per unit area. In a more practical sense, the pressure exerted by air molecules at a weather station is approximately the weight of the air in a column that extends from a fixed area on the ground to the top of the atmosphere. At sea level, the weight of a column of air on one square inch of area is roughly 14.7 pounds, resulting in an air pressure of 14.7 pounds per square inch. For perspective, that amounts to a total force of more than two tons on just the area covered by a single base on a baseball field (an 18-inch by 18-inch area). Surprised?

Meteorologists typically don't work with pressure in pounds per square inch, however. For example, many home barometers (opens in a new window) (instruments for measuring atmospheric pressure) measure pressure in inches of mercury, which are based on the mercury barometer (opens in a new window). Mercury barometers measured pressure after air was evacuated from a glass tube, and the open end of the tube was immersed in a reservoir of mercury, allowing air pressure to force mercury to rise in the glass tube. At sea level, the standard height of the mercury column is 29.92 inches (76 centimeters). More commonly, meteorologists often work with pressure in units of millibars (abbreviated "mb"). For reference, an atmospheric pressure of 14.7 pounds per square inch (when the height of a mercury barometer would be 29.92 inches) is equal to about 1013 millibars.

The connection between surface pressure and the weight of a column of air that extends above the surface has many important consequences. For starters, processes that reduce the weight of an air column also act to decrease the surface pressure. On the other hand, processes that add weight to air columns act to increase surface pressure. Evolving horizontal patterns of air pressure are crucial to weather forecasting, which is one of the reasons why forecasters pay such close attention to centers of highest and lowest pressure on weather maps (typically marked by a blue "H" and a red "L", respectively). In a very general sense, low-pressure systems tend to bring inclement weather (clouds and precipitation), while high pressure systems tend to bring "fair" weather (sunshine and relatively calm conditions).

The bottom line here is that when you hear meteorologists refer to a "low pressure system," they are really talking about is a "lightweight." In other words, the air column above the center of a low weighs less than any of the surrounding air columns. On the flip side, a high pressure system is a "heavyweight" because the air column above the center of the high weighs more than any of the surrounding air columns. Now, I should point out that the difference in pressure between a run-of-the mill high-pressure system and a pretty strong low-pressure system is only about five percent. In the image on the right, for example, the difference between the labeled high and low is only 32 millibars (1018 millibars - 986 millibars), so the difference was even less than five percent in this case. Still, these differences have very important consequences for the weather, as you'll learn!

To give you an idea of the range of sea-level pressures across the world, the average sea-level pressure (computed over the entire earth over a long period of time) is roughly 1013 mb. A very strong high pressure system in the winter may measure around 1050 millibars. On the other hand, a representative value for sea-level pressure at the center of a fierce low-pressure system that can cause, for example, heavy snow during winter might be in the neighborhood of 960 to 980 mb.

As a general guideline, nearly all sea-level pressures lie between 950 millibars and 1050 millibars, with most pressure readings falling between 980 and 1040 millibars. Narrowing down the field even further, sea-level pressures often tend to cluster closer to 1013 mb.

There are exceptions, of course. The bottom of the observed range of sea-level pressures is populated by the "kings" of all low-pressure systems on our planet -- hurricanes (called "typhoons" in some parts of the world). Very intense hurricanes can have sea-level pressures down near 900 millibars. In 2017, for example, at its peak intensity, Hurricane Maria (opens in a new window) had a minimum sea-level pressure of 908 millibars. The storm later went on to devastate Puerto Rico, and its fierce winds completely destroyed the island's NEXRAD Doppler radar (this short video highlights Maria's damage to Puerto Rico (opens in a new window), and includes some stunning images of the damage to the radar, if you're interested). A handful of hurricanes and typhoons globally have even had sea-level pressures drop a bit below 900 millibars. On the other extreme, the kings of high-pressure systems that occasionally form over Siberia during the throes of Arctic winter can attain maximum sea-level pressure readings above 1050 or even 1060 millibars.

Ultimately, the pressures associated with very intense hurricanes and very strong high-pressure systems in the winter are pretty rare, so we can use the general guideline above (that nearly all sea-level pressures lie between 950 millibars and 1050 millibars) to help us interpret pressure data from various maps. With that in mind, let's turn to this section's Key Skill -- decoding sea-level pressure on the station model.

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Wind is a weather variable that's pretty easy to notice, from a gentle breeze on a summer day to whipping winds that can cause damage during a storm. Really, wind is just about everywhere -- even in music! Wind has captured the attention of songwriters for years, with numerous songs referencing "wind" in some way ("Blowin' in the Wind (opens in a new window)," "Candle in the Wind (opens in a new window)," "Summer Breeze (opens in a new window)," and "Dust in the Wind (opens in a new window)" are but a handful of examples).

But, just what is the wind? In short, wind is the horizontal movement of air. One of the most fundamental rules that you need to know is that the direction of the wind is always expressed as the direction FROM which the wind blows and NOT the direction toward which the wind blows. Make sure to commit that to memory! So, if the wind blows from the north toward the south, for example, you'll hear a meteorologist say that the wind is "northerly" (or there's a "north" wind), NOT a "southerly" or "south" wind. Meteorologists are always interested in where the air is coming from because it can help with weather forecasting. For example, if a wind is blowing from a region of warm air toward a region of colder air, a weather forecaster would want to know that! If you happen to own a weather vane, remembering this rule should be easy because a wind vane points into the wind and thus toward the direction FROM which the wind blows.

So, wind direction is always the direction from which the wind is blowing. While forecasters commonly brand the wind with a general direction (such as "north" or "southeast"), in practice, they routinely use standard compass angles to fine-tune the wind direction, as shown in the compass below. For sake of illustration, the wind direction from the north blows from a direction of 0 degrees. A wind that blows from the east is a 90-degree wind, while a wind direction of 70 degrees corresponds to a wind that blows from the east-northeast.

Wind speed is simply how fast the air is moving, and it is the sustained wind speed that is routinely included in weather observations. What is "sustained" wind speed? It's the wind speed averaged over a certain time period (usually 1 or 2 minutes). The rotating cup anemometer shown near the top of the page is a popular instrument for measuring sustained wind speed at home weather stations. To determine the sustained wind speed, the revolution rate of a rotating cup anemometer is typically averaged over a one- or two-minute time period and then mathematically converted to a speed.

The wind is sometimes unsteady, however, with brief, sudden increases in wind speed called gusts. As a general rule, gusts last less than 20 seconds. Weather observers typically only report gusts when the wind speed varies by greater than 10 knots (between the peaks and lulls), so wind gusts are only included in routine weather observations when they're noteworthy.

The units of "knots" may not be familiar to you; in the United States, we often talk about wind speed in mile per hour (just like automobile speed). But, in routine weather observations, wind speed is actually expressed in units of knots (nautical miles per hour). For the record, 1 knot = 1.15 miles per hour. To convert between knots and the more familiar "miles per hour," multiply knots by 1.15. You can find many wind speed converters (opens in a new window) online, but if you have to make the conversion in your head, it's much like if you were out at a restaurant and wanted to leave a 15 percent tip. Imagine your bill at the restaurant is $25. To leave a 15 percent tip, first multiply your bill by 10 percent, which gives you $2.50. Then add on half of $2.50 (which is $1.25) to get to your 15 percent tip of $3.75. Your total bill, then, is $25 + $3.75 = $28.75. Converting knots to miles per hour is the same as computing your tip and total bill at a restaurant. A 25-knot wind speed converts to 28.75 miles per hour (which we get by multiplying 25 by 1.15).

Now that we've covered the basics of wind speed and direction, you might be wondering, "What if the wind is calm? What's the wind speed and direction?" Technically, if the wind is calm, then its speed is 2 knots or less (which gets reported as 0 knots) and it does not have a reported direction. Keep these ideas in mind as you concentrate on this section's Key Skill -- determining wind speed and direction from a station model.

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We've covered the primary atmospheric variables that weather forecasters keep tabs on, as well as how they appear on station models. This short video (4:46) shows a couple of examples of decoding station models in very different extreme weather situations, and serves as a "one-stop shop" for the parts of the station model that we covered throughout the lesson -- temperature, dew point, winds, present weather, sea-level pressure, etc. The video includes a couple of time conversions from UTC for good measure, too.

This wraps up the required part of our lesson. The remaining section is optional, and takes a look at the raw observation code used for transmitting weather observations (it's where the data plotted on station models comes from). If you're interested, check it out!