Weather Instruments and Observations

Objectives:

Weather is the state of the atmosphere at a particular place at a given time. We must describe as closely as possible the state of this mixture of gases with minute quantities of particles (e.g., cloud droplets, ice crystals, aerosols) we call the atmosphere at the time and location where instruments are available. We are interested in knowing for initial descriptive purposes and ultimately for predictive reasons, the atmosphere’s heat energy, density, large-scale motions, water vapor concentration, and liquid or solid water in clouds and precipitation. These quantities translate to the more common parameters of temperature, atmospheric pressure, wind speed and direction, dewpoint (or relative humidity), cloud cover (including height), visibility, precipitation (amount and type), and the general character of the “weather.”

By international convention, weather observations are reported at three-hourly intervals each day, at 0000 UTC, 0300 UTC, etc. [Universal Time Coordinated (UTC), also known as Z time or Greenwich Mean Time (GMT), is the time along the prime meridian, 0 degree longitude.] For aviation and other purposes though, weather observations are routinely reported every hour, twenty-four hours a day. In the U.S., most weather observations are taken by Automated Surface Observing System (ASOS) sensors.

After completing this investigation, you should be able to:

• Describe the Automated Surface Observing System (ASOS) and the data it provides.
• Describe how to access weather observations for the U.S. and the world via the Internet.

Introduction:

Weather observations are a combination of direct measurements by various sensors and determinations made by computer algorithms (programs based on several values). For example, sensors determine the air temperature, dewpoint, atmospheric pressure, ceiling (height to cloud base), precipitation, and wind – speed and direction. The amount of sky cover is determined by the fraction of ceilometer beams that detect clouds or clear air. Visibility is computed from the amount of light scattered back to a sensor from a small volume of air surrounding the sensor. “Weather” type is a combination of temperature, dewpoint, and visibility values and may specify rain, snow, haze, smoke or fog. Figure 1 shows the typical arrangement of sensors and instruments that characterizes the ASOS system.

ASOS consists of an array of instruments including an electronic thermometer, an electronically chilled mirror and light or absorptive humidity sensor to determine the dewpoint, an anemometer and wind vane or sonic anemometer for wind speed and direction, a vertical pulsed laser ceilometer for cloud height and amount, pulsed laser instruments for detecting scattered light for visibility and weather type, a vibrating column for detecting freezing rain, and a heated tipping bucket or weighing rain gauge for rain and snowfall. (For complete technical details of ASOS sensors, see: http://www.nws.noaa.gov/asos/ ().)

Figure 1. ASOS Installation.

The National Weather Service (NWS) provides websites that allow you to obtain the latest weather observation from ASOS instruments at any particular location, http://www.weather.gov/ (). As an example, the following figures show how to obtain the current and recent past observations for St. Louis, Missouri. Figure 2 displays the U.S. map with color-coded areas of various weather watches, warnings and advisories at that time; the latest version appears on your screen. Clicking near the Mississippi River in mid-Missouri on screen brings you to an expanded version of the page from the local National Weather Service Office, St. Louis for much of that state’s weather (Figure 3). On this map, time is given in local time.

Figure 2. U.S. map of weather watches, warnings and advisories at 18Z 13 Dec 2013. (http://www.weather.gov ())

Figure 3. Eastern Missouri and vicinity map of watch, warning and advisory areas at 18Z 13 Dec 2013.

With your cursor, click on St. Louis to go to the forecast page from the St. Louis, MO NWS office. In addition to the wealth of forecast information shown, current local observations are presented. Figure 4 shows the St. Louis forecast page from 11:51 am CST on 13 December 2013.

Figure 4. NWS Seattle forecast webpage for 4 December 2012.

Note the prominent section of Current Conditions at mid-page below the menu bar and headlines. Here are found the latest ASOS observations for the site (Lambert-St. Louis International Airport) nearest the point you selected from the map with your cursor. To the lower right, off the displayed portion of the page, are thumbnail links to the latest radar and satellite images for the area. Any Figure 3 map location in the area can be chosen and the latest observations will be listed from the nearest ASOS site.

Links to additional observations are located in the St. Louis Current Conditions section. The red oval identifies the link, “More Local Wx”, for other observations from area ASOS sites at approximately the same time. In this example, Table 1 shows other area weather observations at times near the St. Louis conditions. Observations are the most recent available with those from within a few minutes of the hour considered to be as of that hour, e.g. 11:51 am is also 1753Z, considered 1800 UTC. Additional observations at intermediate times are additionally transmitted if weather conditions change by specified amounts.

Table 1. Area weather observations that were linked from St. Louis NWS webpage.

1. Which of the following weather parameters is not reported in the regional observations shown in Table 1: [(temperature)(dewpoint)(wind)(ceiling)(pressure)]?

Dewpoint, by definition, is the temperature to which the air, at constant pressure, needs to be cooled to produce saturation. Therefore, the dewpoint must always be equal to or less than the temperature. “Humidity” reported in the table is the relative humidity or ratio of water vapor actually present in the air to the maximum vapor capacity for that temperature expressed in percent.

2. This relationship can be seen with the help of Table 1. Because the relative humidity would be 100% if the temperature and dewpoint were the same, the data in Table 1 infers that the greater the difference in temperature and dewpoint, the [(lower)(higher)] the relative humidity.

Another arrangement of providing weather conditions from the St. Louis airport site is a table of observations for the previous three days found by clicking the “3 Day History” link marked by the green oval in Figure 4. A portion of the table of St. Louis observations for the three days preceding 11:51 AM CST 13 December 2013 is shown in Table 2.

Table 2. A portion of the 3-day listing of weather observations for St. Louis from 11:51 AM CST 13 December 2013.

3. Comparing the regional values from Table 1 with Table 2 of only the St. Louis values from each hour for 3 days, shows the same weather parameters are reported in both tables except: [(visibility)(wind speed and direction)(temperature and dewpoint)(relative humidity)].

4. From the portion of the Table 2 values listed, the highest temperature reported for St. Louis was [(28)(32)(36)(41)] °F.

5. The lowest dewpoint during the hourly observations shown in the table was [(14)(19)(20)] °F.

6. During the period shown, the maximum sea level pressure was [(1020.2)(1023.8)(1026.4)] mb.

7. The “Weather” condition of the hourly observations showed that the amount of sky covered by clouds [(steadily decreased)(remained the same)(steadily increased)].

The following Table 3 is from the AMS course website and is a partial listing of the State Surface Data - Text for MO (Missouri) at 18Z (1800 UTC), the same time as the latest NWS observation line given above in Table 2. In this tabular listing for MO, the hour (HH) and the station identifier (STN) are followed by the temperature (TMP) and dewpoint (DEW) in whole degrees Fahrenheit, wind: direction (DIR) in tens of degrees from true north, speed (SPD) and gusts (GST) in knots (nautical miles per hour), and sea level pressure (PMSL) in tenths of hectoPascals [equal to millibars (mb)]. The “sky cover” or amount of cloudiness is reported as clear (CLR) - no clouds, scattered (SCT) – less than half covered, broken (BKN) - more than half covered, or overcast (OVC) – completely cloudy for each of the low (CLDL), middle (CLDM) and high (CLDH) levels. Present weather (WTHR) is the weather condition that may limit horizontal visibility for pilots: R (rain), ZR (freezing rain), S (snow), F (fog), H (haze) with “-“ following a letter meaning “light”. Compare the NWS report for St. Louis (STL) in Table 2 with the report for the same time (18Z equals 12 noon CST) for STL from this Table 3 listing.

Table 3. AMS State Surface Data – Text for MO at 18Z 13 DEC 2013.

Data for MO

18Z 13 DEC 2013

HH	STN	TMP	DEW	DIR	SPD	GST	CLDL	CLDM	CLDH	ALT	PMSL	PTD	WTHR	PCPN	SNOW
18	JLN	37	31	19	17	22		OVC		29.98	1015.7	-2.3		0.09
18	STL	36	19	16	3		BKN	OVC		30.12			R-
18	SGF	34	30	16	15	23	-SCT	OVC		29.98	1016.4	-3.1	R-	0.37
18	COU	33	31	17	10		OVC			30.03	1017.7	-2.1	ZR-F	0.28
18	IRK	35	26	19	8		OVC			30.00	1017.2	-1.7	R-	T
18	TBN	33	32	16	6		OVC			30.03	1018.3	-3.0	R-F	0.241
18	JEF	33	31	12	8		BKN	OVC		30.05	1017.2	-2.6	RF	0.28
18	MCI	36	32	16	9			OVC		29.92	1013.8	-1.8	R-F	0.03
18	MKC	38	32	17	6		OVC			29.94	1014.2	-1.9	H	0.01
18	SZL	36	33	18	11		OVC			29.96	1015.6	-2.4	R-	0.29
18	STJ	35	30	16	7			SCT		29.93	1014.2	-2.1	H	T
18	DMO	37	34	17	10		BKN	OVC		29.99	1016.3	-2.3	S-F	0.24

8. Note that the two reports do not report all the same weather conditions. One condition not listed in both reports is [(wind speed)(relative humidity)(dewpoint)].

Regional and national summaries of weather data can be shown in the form of maps where the weather data are displayed for individual stations in coded form, called the surface station model. For the explanation of the station model, see the course website, under Extras, User’s Guide.

9. Figure 5 is a sample national map (U.S. - Data) which displays data collected at 15Z
(10 AM Eastern Standard Time, 9 AM CST, etc.) on 13 DEC 2013. (The identifiers for the reporting stations can be found by clicking on “Available Surface Stations” on the course website. The identifiers shown on that map do not include a “K” that is the first letter of all contiguous United States station identifications.) Surface weather data are plotted on the national map in, on, and around a circle representing the station. Temperature, in Fahrenheit degrees, is plotted at the “11 o’clock” position relative to the station circle. The temperature at Albuquerque, New Mexico, at Figure 5 map time was [(18)(26)(30)] °F.

Figure 5. Contiguous U.S. surface weather map of plotted data at 15Z 13 DEC 2013.

10. The national map displays surface observational data from a sufficient number of stations to determine large-scale weather patterns and features. Temperatures reported on the Figure 5 map show that the area of the nation where the lowest temperatures prevailed was over the [(West coast)(South-central)(North-central)(Southeast)].

11. Figure 6 is a sample regional map, one of nine including Alaska and Hawaii, provided via the course website. This map is labeled [(Southern Plains - Data)(Northwest - Data)(Midwest – Data)]. This map is for the same time as the national map, Figure 5.

Figure 6. Regional Southern Plains surface weather map of plotted data at 15Z 13 DEC 2013.

12. The regional maps display many more stations, allowing for more detailed weather analysis. For a comparison of the station densities on the two maps, the national map has one station plotted in New Mexico whereas the number of stations on the regional map is [(7)(12)(19)].

For a single station, a time sequence of weather data can be displayed. Such a time sequence for a twenty-four hour period of selected weather elements is termed a meteorogram, meteogram, or just “metgram” for short. From the course website, under the Surface section, click on “Meteograms for Selected Cities”. From the resulting meteograms page, select your nearest city from the map or table listing. Display that page.

13. The meteogram is constructed to portray [(6)(12)(25)] hourly observations over the time period shown.

The National Weather Service provides listings of weather data for many stations across the country. You can compare the tabular listings of data with the graphic display of the meteogram. Figure 4 highlights the link to displaying three days of ASOS observations. To see these for your nearest station, go to http://www.weather.gov/ (). Select your area of the country, then on the local NWS Forecast Office map, use the cursor to select the approximate point on the map for your desired data. Then at the local forecast webpage, click the “3 Day History” link under the Current Conditions part of the page, the green oval link in Figure 4 labeled “3 Day History:”.

Current weather observations can be found in a variety of formats from the course website, National Weather Service websites, and others. Investigate these sources and their displays.

Suggestions for further activities:

For those in the contiguous U.S. - Regional maps are made available on the course website so that you can track changes in weather and weather features, such as cold fronts, as they progress across the country. Call up the regional map that best fits your location. Mark your present location on the national map and on the regional map (Figures 5 and 6). You can check the three-letter identifier of the nearest station to your location by selecting the “Available Surface Stations” map on the website – click on the map for a full listing of stations.

For those in Alaska, Hawaii/Pacific or Puerto Rico/Caribbean areas - Regional maps for the coterminous U.S. are available to provide a detailed track of changeable weather and weather features. For areas outside of the “Lower 48” on the course website, scroll down to the area with Alaska, Hawaii and Pacific, Puerto Rico and Caribbean links and click on your region’s link. For Alaska and Hawaii, select “Surface:” state under your region’s heading and mark your location on this state map. For “Puerto Rico – Caribbean:”, select Puerto Rico/Virgin Islands or area observations then mark your present location on this regional map and click on the red dot closest to your location for station information.