By the time you are finished reading this page, make sure that you understand when the standard hourly observations are collected and for what hour a particular observation qualifies based on its time stamp. Finally, you should be able to open the map of current observations for nationwide official stations. You do not yet need to know how to interpret all of those numbers at each station!!! You will in due time......
Forecasters that like to keep their jobs (do they ever really get fired??) use recent history as the basis for predicting the future. That's because past weather can, and often does, offer clues to 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 heavy mountain snows almost certainly make news a few days later when they arrive over the Middle West, generating fierce thunderstorms that spawn tornadoes. It was Benjamin Franklin that first documented that most storm systems in the United States travel from West to East across the country!
However, even in more benign weather patterns, conscientious forecasters routinely study weather conditions to their west, 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 weather observations are taken once each hour between 50 minutes past the hour and the top of the next hour. For example, a weather observation taken between 2:50 p.m. and 3:00 p.m. qualifies as the 3:00 p.m. observation. When weather conditions rapidly change, however, you may see observations updated more often than hourly. Think about a time when a quick changeover from rain to snow has occurred. If the change happens at 2:15 pm, the 2:00pm weather observation is no longer valid. Now imagine being a pilot landing at the Buffalo Airport at 2:30pm reading the 2:00pm observation report. You would be pretty surprised to see a runway covered in snow as you drop out of the clouds for landing!!
As you might expect, there's an avalanche of surface weather observations each hour from all the airports across the country. In order to simplify life and create easy-to-read weather maps, the National Weather Service organizes hourly observations onto templates called station models. You may remember looking at and interpreting station models from Regents Earth Science class. (See example below) In the remainder of this lesson, you'll learn how to decode surface station models (and thus determine local weather conditions). We will also look at our class weather station and assess the pros and cons of it. We will also discuss proper location for weather stations to avoid misinformation.
Here's the most recent surface map of station models for the contiguous states. Look for Buffalo, NY current conditions after you open the link. You should notice there is an observing site near Buffalo, but not shown right at the northeast tip of Lake Erie where Buffalo is. To see all weather observing sites across the U.S., click the link at the top for the high resolution image. Next click over NYS to zoom in to see all current New York State weather observing station data. After having me check you computer screen to be sure you are at the right locale at the correct website, check out this explanation on decoding station models from the Weather Prediction Center.
Practice - Complete at least 3 of these examples to become proficient at station model decoding.
1) On a seperate sheet of paper, copy the current station model exactly as you see it for Buffalo, NY. (using the link "most recent surface map" above)Then summarize all pertinent current weather information you can attain by looking at the station model for Buffalo, NY. Record the local time of the reading and put your name at the top. Now check your data with a partner to confirm accuracy.
2) Complete the assignment - Decoding Weather Station Models in class. Submit by the end of class on Friday, 2/28.
This page contains some important concepts about temperature. Make sure that you understand temperature scales as well as how to read the temperature from a station model.
Check out the list of world records for highest and lowest temperatures compiled by the World Meteorological Organization (WMO). The North American all-time marks for highest and lowest temperatures are, respectively, 134 degrees Fahrenheit in California's Death Valley (see photograph below and behold this stunning panorama from Zabriskie's Point) and minus 81.4 degrees Fahrenheit at the village of Snag (near Beaver Creek) in the Yukon Territory of Canada). If you ever want to keep pace with current global extremes in temperature, check out this website, which keeps track of all the hourly weather observations around the world. Look here to see the all time high and low temperatures for Buffalo, NY for every day of the year.
For the record, 134 degrees Fahrenheit and minus 81.4 degrees Fahrenheit convert to 56.7 degrees Celsius and minus 63 degrees Celsius, respectively (read more about the Fahrenheit scale and the Celsius scale). To easily convert from Fahrenheit to Celsius (or vice versa), I recommend the National Weather Service's weather calculator.
Some common markers:
- 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
So what exactly is temperature? Air molecules are restless little lumps of matter, continually vibrating, wriggling and bumping into their many neighbors (think of a very crowded, jam-packed dance floor). 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. Scientifically, such a lively "jitterbug" performed by air molecules translates into ample kinetic energy, which is the energy of motion. Thus, air temperature is a measure of the average kinetic energy of air molecules (oxygen and nitrogen are the most abundant gases in the atmosphere).
Think back to looking at the current surface observations at airports around the country. Remember how unuserfriendly that data was? It would take a good 5 minutes or more of staring at number to get a sense of where in the United States it is currently "warm" or "cold" (relative to average of course). We have a solution for that. Here is a simple to interpret current surface temperature map, courtesy of the University of Illinois at Urbana-Champaign. Much easier to read, is it not?
Time Stamps on Weather Maps
Keeping that map open, let us take a look at what time this map is from. Note the date of the map and now see where it says the time at the top of the map in Zulu (Z), which is military code for the time at the Prime Meridian, also known as UTC (Universal Time Coordinated) or GMT (Greenwhich Mean Time).
We are in the Eastern Standard Time (EST) zone of the U.S., which is UTC-5 (5 hours behind the time at the Prime Meridian in London, England) Weather maps all over the world use the exact same time to avoid having to make a map for every time zone of the world!! (4 would be needed just for the mainland United States). However, that means that depending on where you are on Earth, you need to know if you are 1, 2, 3, hours behind UTC or if you are 7, 10 or 11 hours ahead of UTC. A location can be anywhere from 12 hours ahead of UTC to 12 hours behind UTC. This World Time Zone Map shows the entire world and their time zones. Take a minute or two looking at it to let it make sense.
Where we are in New York State, we happen to be 5 hours behind (UTC-5).
The other factor you need to take in to account is that weather maps use a 24 hour clock system (sometimes known as Army Time) to avoid needing to add AM or PM to the map or to accidently confuse the two. So to begin, 0000Z is "zero hundred hours" and represents midnight. 1200Z is "twelve hundred hours" and represents noon.
The following examples show what a normal time would be in this 24 hour system.
Midnight = 0000Z (zero hundred hours)
1:00am = 0100Z
3:00am = 0300Z
6:00am = 0600Z
12:00pm = 1200Z
3:00pm = 1500Z
6:00pm = 1800Z
9:00pm = 2100Z
Now, recall that the time on all weather maps is the local time at the Prime Meridian. This means that we need to convert the time given on the map to our local time (EST) in New York. The map below shows an example of 1500Z at the Prime Meridian. What is the local time then for us in New York? 1500Z - 5 hours = 1000. However, we can no longer use Z as the label, because that by definition is the time at Prime Meridian. Therefore we know it is 10 o'clock, but we must add detail for a correct answer.
The complete answer would be 10:00am EST.
Making Sense of Weather Maps - Temperature
Let's focus our attention on just one value from the station models at each airport, the current air temperature. Back in Earth Science you learned how to draw isolines that connect points of equal value. This is an important skill, but what is even more important is the skill of interpreting weather maps after the isolines are already drawn on them. To help you better understand how this happens, let us refresh you ability to draw isolines. He is an example map of air temperature at each airport around the Midwest United States.
Drawing these isotherms (specifically lines connecting points of equal temperature) will help you interpret some more complex weather maps later in the course.
On most maps of temperature, isotherms are drawn for every multiple of ten degrees Fahrenheit. Sometimes, isotherms that are multiples of five degrees Fahrenheit are included for greater detail Remember: An isotherm a line that connects points of equal temperature. So, if you are drawing a 50 degree F isotherm, stations which have a temperature of exactly 50 F will lie directly on the line. Stations with values greater than 50 F will always be on one side of the line, and values less than 50 F will be on the other.
Isoline Practice - Use this online tool to practice drawing the isotherms.
After you finish the 50 degree isotherm, click the isotherm option box at the bottom and chance it to 50F. Then click "Draw Contour and see if you drew yours correctly. Edit if you need to and then try the 40, 60, 70 and 80F isotherms.
- Like a downhill skiier or roller-blader negotiating the course of gates, you should trace your isotherm in a neat and smooth course instead of a jagged, jerky path (it's a good idea to initially trace your contours lightly in pencil just in case you accidentally get off course).
- An isotherm should begin and end at an edge of the map, or, alternatively, loop around and close on itself.
- An isotherm can never split and go two different directions, rather they must be a single continuous line.
- Typically, you must draw isotherms on weather maps where there are "gaps" in the data (it's an undeniable fact of life that there are a limited number of observation sites). To draw isotherms through "gaps" in the data, you must interpolate (estimate the value of) the temperature between two given data points. For example, if you're drawing a 45-degree isotherm and you come upon two data points marked 49 degrees and 44 degrees, you'll want your isotherm to pass closer to the point marked 44 degrees. In other words, use a little common sense.
- Isotherms should be drawn at equal intervals. There are exceptions, of course. During the cold season, for example, it is sometimes prudent to draw the 32-degree isotherm on a surface temperature analysis. As you know, a reading of 32 degrees is significant for winter weather.
Controllers of Temperature
For such a mundane variable as air temperature, you would think that it would be easy to measure. Not so! In fact, measuring the true air temperature can be quite tricky. When you place a thermometer outside to measure the air temperature, what you are actually measuring is the temperature of thermometer's sensor -- not necessarily just the air temperature. For example, a thermometer left out in the sun will measure a temperature that is a combination of both the air temperature and the heat generated by the absorption of sunlight on the sensor. Meteorologists, therefore, take great care to zero-in on the true air temperature by housing thermometers in protective shelters originally called Stevenson Screens (but now referred to as a Cotton Region Shelter).
No matter what name you call them, these shelters are painted white to reflect sunlight (too much absorption of direct sunlight could dramatically raise the temperature of the thermometer above the true air temperature). Cotton Region Shelters are also mounted about five feet above the surface away from the strong fluxes of infrared radiation emitted by the ground when it heats up during the day. Their louvers allow ventilation so that the air in contact with the thermometer has essentially the same temperature as the outside air (check out an inside view). These shelters also protect thermometers from getting wet. You'll learn in this lesson that the evaporation of water can dramatically lower the temperature of the thermometer below the true air temperature.
In the following days, you will learn all about the types of processes that affect local temperatures. From seasonal and geographic influences to local radiation and advection processes, you'll discover that a simple temperature prediction can be a product of many different factors. And the next time a Buffalo forecaster (or computer) gets the day's high temperature wrong, you'll have a better understanding of how complex "the temperature" can be.
Determining Temp. Averages and Records
As weather forecasters prepare to paint the picture of temperature on a given day, they always have a little history to temper their temperature forecast -- the "normal" high and low for the date -- in the back of their minds. These so called "normals" are the 30-year average highs and lows for the date. In other words, a city or town's average high temperature on any given date is simply the average of all the observed high temperatures on that date over a thirty-year period. Such 30-year average temperatures compose part of a city's climate record. For the record, climate is a long-term characterization of the weather at a given location, which in addition to statistical averages, includes extreme weather (such as record highs, record rainfall, etc.)
The National Weather Service is the keeper of much of the nation's weather records. Each day, each office of the National Weather Service issues a "climate report," which summarizes the previous day's temperature and precipitation and compares them to the climate record.
Below is a sample display of temperature data that is a routine part of the "Climate Report" at NWS weather stations (in this case, the climate report represented temperature observations at Chicago's O'Hare Airport on February 14, 2012).
Note that the 30-year averagesfor the Chicago example above ("NORMAL VALUE") came from weather observations taken during the period from 1981 to 2010. ("CLIMATE RECORD PERIOD" means that weather records at O'Hare Airport date back to 1871. Keep in mind that the climate period varies from place to place). Take a look at the Climate Record you have open for Buffalo. What are the Climate Record Period and Climate Normal Period?
Every ten years, the averaging period, from which we calculate the "normal value," is updated. The switch to "new normals" supports a long-held contention that there's nothing normal about "normals." Indeed, because some decades are especially cool, while others are particularly warm, the change of the period of record generally results in normals that are different from previous normals. So the notion of "normal" in weather forecasting is a bit of a misnomer. Moreover, the common use of the term "normal" is unfortunate because weather seldom behaves in a "normal" way. In winter, for example, a season renowned for occasionally abrupt swings from deep Arctic chill to mild thaws, it sometimes turns out that a city's average high for the date never actually occurred as a high temperature on the date in question during the previous 30-year period.
Nonetheless, 30-year average temperatures serve as a general guide for forecasters. Indeed, you should give pause if you see a forecasted high temperature that's 25 degrees above the average high for the date. Not that such large departures from 30-year averages are impossible; they are indeed. But they're not a weekly occurrence either. To understand the importance of such statistics in weather forecasting, consider the graph of 30-year averages at Pittsburgh, Pennsylvania (shown below).
The dark curve represents the annual variation in Pittsburgh's average daily high temperature (this curve will serve as a basis for our current discussion). By way of background, the zigzag red curve represents the variation of daily record high temperatures and the zigzag blue curve represents the annual variation of daily record low-maximum temperatures (for example, a record "low-max" can occur on a cloudy, rainy and cool day during summer). So, for instance, if the high temperature for Pittsburgh was only 40 degrees on July 1, it would be a record low-maximum. We analyzed these values for Buffalo in our last assignment.
As you can imagine, there are many factors that play a role in the shape of the mean high temperature curve. For example, what determines the amplitude of the average temperature fluctuation over the entire year? Or, why does the maximum average temperature occur in mid-July and not in mid-June? We'll attempt to answer these questions in the next few sections.