Local Climatological Data Reports

INTRODUCTION

Local Climatological Data (LCD) are data observed at principal meteorological stations by trained observers or automated equipment that has been tested and accepted by the controlling agency. The stations are located worldwide and are operated by agencies of the United States government. The controlling agencies are the National Weather Service (NWS), the U.S. Air Force/Air Weather Service (AWS), the U.S. Navy/National Oceanographic Command (NAVOCFANCOM), and the Federal Aviation Administration (FAA).

The data are collected on a wide range of time scales from one minute for some current automated equipment to three observations per day for intermittent periods at some remote, part-time locations. The majority of stations collect hourly observations and special (between hour) observations when weather conditions warrant it. The data are used initially for aviation guidance and safety, weather forecasting, and severe weather warnings. The hourly data, special observations and summaries are generally transmitted over global telecommunication circuits.

In 1992, NWS began automating surface weather observations through the Automated Surface Observing System (ASOS). The measurement of all weather elements included in conventional observations have not been automated. Some of those elements that have not been automated include clouds above 12,000 ft, snowfall and snow depth, sunshine, and the identification of certain types of weather such as tornadoes, thunderstorms, hail, drizzle, smoke, and blowing phenomena. Until new automated sensors are designed to measure these elements, the data will be obtained from other sources. ASOS is considered to be only one source of the total surface observation. The total surface observation concept includes the ASOS system, augmentation of ASOS data by station personnel, supplementary data from networks distinct from ASOS (e.g., severe weather spotter networks, hydrological networks, and cooperative networks), and complementary data from other observing technologies such as satellite, radar, and lightning detection systems.

HOW TO READ THESE REPORTS

Weather statistics on 108 U.S. cities

The information about the history of key cities is easy to find. The reports are planned to be informative, yet easy to read. The terminology used and the standard formats are explained below.

Narrative report

Typically, each report begins with a narrative description of the local area climate prepared by a local climatologist.

The narrative describes the area in terms of terrain, water bodies, and other topographical features as these features exercise key influences on the local weather. They are usually the cause if an area's weather differs sharply from weather in areas only a few miles away. For example, if a lake is near a city, it will always influence the city's weather, and, in fact, may even create climatic differences from one part of the city to another. Mountains, swamps, even plowed fields exercise their influences on air masses as these masses move toward a city.

The report generally discusses temperatures in the city, rainfall tendencies, snowfall history, and other points. It closes with notes about the area's agricultural adaptability. The history of first fall frost and last spring freeze is usually described, along with suggestions about the types of crops for which the area is climatologically suited.

Statistics

The statistics are of two different kinds. The first type distills many years of history to give a profile of the city's weather (e.g., Normals, Means, and Extremes). The second group offers data for individual years, allowing the user to see what variances and/or patterns have occurred.

There are several meteorological terms used in the table. The following notes are designed to clarify the information included in the tables.

Location

Precise geographical location is provided at the top of each Normals, Means, and Extremes table.

Normal

Term applied as to temperature, degree days, and precipitation refers to the value of that particular element averaged over the period of 1961–90. When the station does not have continuous records from an instrument site with the same "exposure," a difference factor between the old site and the new site is used to adjust the observed values to a common series. The difference factor is determined from a period of simultaneous measurements. The base period is revised every ten years by adding the averages for the most recent decade and dropping them for the first decade of the former normals for 1951–80. Normal does not refer to "normalcy" or "expectation," but only to the actual averages for a particular thirty-year period.

Means and extremes (note "a")

Means and extremes are based on the period of years in which observations have been made under comparable conditions of instrument exposure. Data are included for dates through 1995 unless otherwise noted. The Date of an Extreme is the most recent one in cases of repeated occurrences.

Length of observational record

Length of observational record for Means and Extremes is based on the length of January data for the present instrument site exposure (15 equals 15 years). The table does not give the all time high or low value if it was recorded at a different site within the area. The Mean (or average) values for relative humidity, wind sunshine, sky condition, and the mean number of days with the various other weather conditions listed are also based on length of record noted in each instance. Check the first column for each of these rows to read this length of record for each item.

Average of the highest temperature (°F)

Average of the highest temperature on each day of the month and year for the period 1961–90. This value is obtained by taking the sum of the highest temperature for each day of the period adjusted for site exposure if necessary) and dividing by the number of days included.

Average of the lowest temperature (°F)

Average of the lowest temperature on each day of the month and year for the period 1961–90.

Average of all daily temperatures (°F)

Average of all daily temperatures for the month and year for the period 1961–90; computed as being the sum of the minimum and maximum values divided by two. The monthly mean maximum or minimum temperatures are the sum of the daily maximums or minimums divided by the number of days in the month. The monthly average temperature is the sum of the monthly mean maximum and minimum temperatures divided by two.

Extremes—highest temperature (°F)

Highest temperature ever recorded during any month at present site exposure.

Extremes—lowest temperature (°F)

Lowest temperature ever recorded during any month at present site exposure.

Average number of heating degree days

The number of heating degree days (average) for each month and year for the period 1961–90. The statistic is based on the amount that the daily mean temperature falls below 65°F. Each degree of mean temperature below 65 is counted as one heating degree day. If the daily mean temperature is 65°F or higher, the heating degree day value for that day is zero. Monthly and annual sums are calculated for each period and averaged over the appropriate thirty years of record to establish these "normal" values. Compare this with cooling degree days.

Average number of cooling degree days

The number of cooling degree days (average) for each month and year for the period 1961–90. The concept of this statistic is the mirror image of the concept of heating degree days and is based on the amount that the daily mean temperature exceeds 65°F. Each degree of mean temperature above 65°F is counted as one cooling degree day. If the daily mean temperature is 65°F or below, the cooling degree day is zero. It should be noted that heating and cooling degree days are calculated independently and do not cancel each other out.

Sunshine

The average percent of daytime hours subject to direct radiation from the sun at the present site. The percentage is given without regard for the intensity of sunshine. That is, thin clouds, light haze, or other minor obstructions to direct solar rays may be present but would not mitigate the full counting of an hour.

Mean sky cover

Average amount of daytime sky obscured by any type of cover expressed in tenths (e.g., 4.8 or, 48%).

Activity limiting weather

Average number of days in month with specified weather conditions (precipitation, storms, etc.) based on present exposure. The symbol * indicates less than half a day.

Cloudiness

Average number of days in month at the present site with various amounts of cloud cover. Clear indicates average daytime cloudiness of 0.3 or less; partly cloudy indicates average daytime cloudiness between 0.4 and 0.7; cloudy indicates average daytime cloudiness of 0.8 or more.

Maximum ≥ 90° days

Average number of days in month and year when the temperatures at the present site is 90°F or above. (70°F or above at Alaskan stations.)

Maximum ≤ 32° days

Average number of days at the present site when the temperatures remained below 32°F at all times.

Minimum ≤ 32° days

Average number of days at the present site when the temperature dropped to a minimum of 32°F or below.

Minimum ≤ 0° days

Average number of days at present site when the minimum temperature was 0°F or below.

Average station pressure

Given in inches (in).

Average relative humidity (percent)

Relative humidity levels at various hours of the day. The time is expressed in terms of the 24-hour clock (00 is midnight, 06 is 6 a.m., 12 is noon, and 18 is 6 p.m.). Values are for present site only.

Average precipitation

Precipitation in inches of water equivalent for each month and year during the period 1961–90. As in the other precipitation data, the values are expressed in inches of depth of the liquid water content of all forms of precipitation even if initially frozen. As in all "normal" values, when the station does not have continuous records from the same instrument site, a "ratio factor" between the new and old exposure is used to adjust the observed values to a common series.

Maximum precipitation (month)

Greatest amount (in inches) of water equivalent ever recorded during any month at present site.

Minimum precipitation (month)

Least amount (in inches) of water equivalent ever recorded during any month at present site.

Maximum precipitation (in 24 hours)

Greatest amount (in inches) of water equivalent ever recorded during any day at present site.

Summaries for snow, ice pellets, hail

Figures include sleet and are similar to those for total precipitation. The values are expressed in inches of actual snow or ice fall. The water equivalent can be estimated roughly by using the rule-of-thumb that 10 in of snow equal 1 in of water.

Wind

Average speed of wind is expressed in miles-per-hour (mph) without regard to direction.

Prevailing wind direction

The most common single wind direction without regard for wind speed or any minimum amount of persistence. The aggregate total of wind from the other directions may be very much greater than that from the "prevailing" direction. Direction is coded in two different ways, some reports use letters, some use numbers. When letters are used, they have the usual meaning, such as WSW, indicating west-south-west. When numbers are used, they are given in tens of degrees clockwise from true north, so that 09 is 90° clockwise from north (east), 18 is 180° (south), 27 is 270° (west), and 36 is 360° (north).

High wind

The greatest speed in miles-per-hour of any "mile" of wind passing the station. The accompanying direction and year of occurrence are also given. A mile of wind passing the station one minute has an average speed of 60 mph; two minutes—30 mph; five minutes—12 mph, etc. The wind cups of the particular instrument involved operate much like the wheel of a car in actuating the car's odometer. The instrument does not record the strength of individual wind gusts, which usually last less than 20 seconds and may be very much greater than the value given here. The fastest mile however does give some idea of the extremes of wind that can be encountered.

Special symbols

Symbols that appear on many of the individual summaries: (*) less than one half; (T ) trace, an amount too small to measure; (—) below zero temperatures are preceded by a minus sign.

Weather averages year-by-year

PRECIPITATION refers to the inches of water equivalent in the total of all forms of liquid or frozen precipitation that fell during each month. Snowfall refers to the actual amount of snow in inches that fell during the month. T (trace) is a precipitation amount of less than 0.005 in (note: in estimating the water equivalent of snow a ratio of 10 in of snow equal 1 in of water is customarily employed).

AVERAGE TEMPERATURE equals the average of the maximum and minimum temperatures for each day of the month for the given year; afternoon temperatures were typically higher than these values and late night/early morning temperatures were typically below them.

HEATING DEGREE DAYS (HDD) provide a well-established index of relative fuel consumption for space heating in a given place—a month of 2,000 HDD requires about twice the amount of space heating energy as one of 1,000 HDD, while 100 HDD will require about the same fuel whether accumulated in two or four days. Regional differences in the Heating Degree Day Index are only partially useful in estimating comparative fuel requirements because the building construction and cultural expectations tend to be different in different parts of the country (e.g., subjective ideas of comfort in relation to temperature will vary). For example, the average standards of efficiency in heating equipment and insulation are generally lower in warmer climates so that fuel requirements tend not to decrease as rapidly as the heating degree days decrease.

COOLING DEGREE DAYS provide only a rough guide to relative energy consumption in air conditioning. A proper air conditioning index will almost certainly require a factor for humidity variation and possibly factors for cloudiness or other weather variables. However, the Cooling Degree Days Index will have some usefulness in indicating relative outdoor comfort and relative indoor air conditioning requirements.