NOAA Technical Memorandum NWS SR – 232 

___________________________________________________________________________ 

IMPACT OF SITING CHANGES ON TEMPERATURE OBSERVATIONS 

AT CYRIL E. KING AIRPORT, SAINT THOMAS, U.S. VIRGIN 

ISLANDS AND LUIS MUÑOZ MARIN INTERNATIONAL AIRPORT, 

SAN JUAN, PR 

Shawn A. Rossi 

National Weather Service 

Weather Forecast Office 

San Juan, Puerto Rico 

Science and Technology Services Division 

Southern Region 

Fort Worth, Texas 

August 2010 

UNITED STATES National Oceanic and National Weather Service 

DEPARTMENT OF COMMERCE Atmospheric Administration John L. Hayes, Assistant 

Gary Locke, Secretary Jane Lubchenco Administrator for Weather Services 
This publication has been reviewed 

and is approved for publication by 

Science and Technology Services Division 

Southern Region 

 David B. Billingsley, Chief 

 Science and Technology Services Division 

 Fort Worth, Texas 

i
TABLE OF CONTENTS

____________________________________________________ 

PAGE 

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

II. Background: The Importance of Siting at Tropical Coastal Locations . . . . . . . . . . . . 3 

III. Case 1: The Cyril E. King Airport, U.S. Virgin Islands . . . . . . . . . . . . . . . . . . . . . . 5 

IV. Case 2: Luis Muñoz Marin International Airport, Puerto Rico . . . . . . . . . . . . . . . . . 8 

V. Implications on Temperature Extremes at Both Airports . . . . . . . . . . . . . . . . . . . . . 15 

VI Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

VII. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

VIII. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

i
I. Introduction 

The proper siting of meteorological instrumentation has long been recognized as a key 

component to preserving the integrity of climate records. William Heberden (1769) wrote a 

famous paper detailing the difference in precipitation values collected in the garden of 

Westminster Abby and on its towers. In this paper, Heberden demonstrated a 50% difference in 

the amount of precipitation collected solely based on the height of the rain gage above the 

ground. In the early 1800’s, Luke Howard, known as a pioneer in studying the urban heat island 

effect, published The Climate of London, Deduced from Meteorological Observations made at 

Different Places in the Neighborhood of the Metropolis (Howard, 1883). In this publication, 

Howard noted the temperature difference between urbanized areas and the surrounding 

countryside. This temperature difference, due to the changes in physical and thermal properties 

associated with urbanization, showed that the temperature in the center of London was 3.7o F 

greater at night than the surrounding rural areas. More recently, siting has been used to raise 

questions regarding the reliability of surface temperature trends in the U.S. as a whole (Watts 

2009). 

In order to preserve the integrity of its network of observers and to avoid unnecessary shifts in 

data, the National Weather Service established a standard for equipment, siting and exposure for 

all of its cooperative weather instrumentation in the late 1800’s (National Weather Service 

2008). In addition, the Office of the Federal Coordinator for Meteorological Services and 

Supporting Research (1994) proscribed federal standards for the siting of automated 

meteorological sensors at airports. 

In this paper, the siting of two Automated Surface Observing Systems (ASOS), one at the Cyril 

E. King Airport on Saint Thomas, U.S. Virgin Islands and the other at the Luis Muñoz Marin 

International Airport in San Juan, PR, are closely examined. Specifically, there was an 

investigation into the temperature trends at the Cyril E. King Airport on Saint Thomas since the 

commissioning of the ASOS in August 1998 (Case 1), and there was also an investigation into 

temperature trends at the Luis Muñoz Marin International Airport in San Juan since the 

completion of taxiway Juliet in 2008 (Case 2). 

In Case 1, it will be shown that the relocation of the observation site at the Cyril E. King Airport 

on Saint Thomas contributed to a significant decrease in the maximum temperatures and a 

significant increase in the minimum temperatures, accounting for a 20% reduction in the daily 

diurnal temperature cycle. In Case 2, it will be shown that the addition of taxiway Juliet at the 

Luis Muñoz Marin International Airport in San Juan (more than 10 years after the 

commissioning of the ASOS), led to a significant increase in the minimum temperatures at the 

site. 

II. Background: The Importance of Siting at Tropical Coastal Locations

Diurnal temperature ranges are dependent on several factors. The proximity to large bodies of 

water, regular moderate wind speeds and high relative humidities have all been previously 

identified as having a strong influence on the diurnal temperature range of a given site. These 

are all characteristics common at tropical coastal locations. 

3
The fact that tropical coastal locations are particularly sensitive to siting is nothing new. In 

1958, a report in the Miami News (Colbert, 1958) noted the relocation of the official temperature 

sensor for the city of Miami (maintained by the U.S. Weather Bureau) could significantly affect 

the local climate record. The article quoted meteorologist Leonard Pardue of the U.S. Weather 

Bureau, who stated “There is a considerable variation between the temperature at the shoreline 

and a couple of miles inland.” Pardue went on to explain that this difference was likely the cause 

of an estimated two degree increase in temperature from the previous location, which was just a 

few blocks from Biscayne Bay. 

The entire Lesser Antilles, which includes both the U.S. Virgin Islands and Puerto Rico, falls 

into this same category of tropical coastal locations, i.e., locations highly sensitive to siting. 

Across Puerto Rico and the U.S. Virgin Islands, the average diurnal temperature range varies 

from only 10 to 20 degrees, depending on the month of the year and proximity to the coastline. 

As a result, even a small shift in the location of a station can have a large impact on both the 

range of temperatures as well as the actual temperature values recorded at a given site.

III. Case 1: The Cyril E. King Airport, U.S. Virgin Islands

1. Background 

On 10 August 1998 the ASOS at the Cyril E. King Airport on Saint Thomas was commissioned, 

located on a strip of grass between runway 10/28 and the taxiway (Figure 1). Although the new 

location of the automated weather sensor adhered to the guidelines established by the Office of 

Federal Coordinator for Meteorology (2005), over the following years, large temperature 

discrepancies from long term averages, not seen at the surrounding Caribbean Islands, began to 

be noted at the Cyril E. King Airport (Figure 2). 

Figure 1 – Areal view of the Cyril E. King Airport on Saint Thomas, U.S. Virgin Islands. 

(A) Location of the Limited Aviation Weather Reporting Station, official weather sensor for the 

airport, before the commissioning of the Automated Surface Observing System. (B) Location of 

the Automated Surface Observing System, commissioned on 10 August, 1998. 

4
Figure 2 – Secular temperature trends (departures from 1971-2000 normals) for the months of 

January 2000-2010 at the Cyril E. King Airport on Saint Thomas and the Christiansted Airport 

on Saint Croix. Note: A similar pattern in both the maximum and minimum temperatures 

appears to be present, with the magnitude of the deviations substantially larger at the Saint 

Thomas. 

After noting an apparent change in temperatures at the ASOS site at the Cyril E. King Airport on 

Saint Thomas, as compared to neighboring sites, it was recognized that the previous location of 

the LAWRS (see Figure 1, Point A) was on a bluff approximately 40 feet higher and 

approximately one half a mile east of the new site. While this changes fell within the first two 

compatibility table requirements for the relocation of meteorological instrumentation according 

to the NWS Instruction 10-1307 Table C-1 (NWSI 2008), it appears that the new location may 

have failed requirement #3, which states: “geographic setting of relocated and original stations 

are similar and environmental characteristics - including surrounding bodies of water – 

(emphasis added) are similar.” While one might speculate that the new location, as seen from 

space (see Figure 1), might be much more affected by the surrounding Caribbean waters, this is 

not as obvious when viewed from the observation site (Figure 3). 

Figure 3 – The ASOS at the Cyril E. King Airport on Saint Thomas as seen from ground level 

(photograph courtesy of National Climatic Data Center). 

5
2. Data Analysis 

Comparing the temperature trends at the Cyril E. King Airport on Saint Thomas with the nearest 

high quality observation station, which in this case was at the Christiansted Airport on Saint 

Croix (the same site used in Figure 2), the average annual temperature decreased similarly at 

both sites (see column 4 of Table 1). 

However, after separating out the maximum and minimum temperatures at the two sites, it 

became strikingly clear that something had changed between the two stations since the year 2000 

(the time period for which Table 1 was compiled). While the overall temperature trends were 

nearly identical at Saint Croix, which lies at a similar longitude just 40 miles to the south of Saint 

Thomas, the magnitude of these trends were significantly different (not only for the month of 

January, as seen in Figure 2, but similarly for all months of the year (Table 1). After the 

commissioning of the ASOS at the Cyril E. King Airport on Saint Thomas, it appeared that the 

averaged annual maximum temperature decreased 1.4o

F as compared to that for Saint Croix 

while the average annual minimum temperature increased 1.0o

F. 

Table 1 – Averaged annual temperature departures from the 1971-2000 normals on Saint 

Thomas and Saint Croix 

Maximum 

Temperature 

Minimum 

Temperature 

Average 

Temperature 

Saint Thomas -2.2 F +1.1 F -0.6 F 

Saint Croix -0.8 F +0.1 F -0.4 F 

Difference -1.4 F +1.0 F -0.2 F 

The frequency of daily maximum and minimum temperature departures from normal since 200- 

for both sites can be seen in Figure 4. While both plots indicate a trend of higher minimum and 

lower maximum temperatures, the magnitude of this trend at the Cyril E. King Airport is more 

apparent (Figure 4). This shift has likely contributed to the large number of record warm low 

and cool high temperatures recorded on Saint Thomas since 2000 (Tables 4 and 5). 

6
-2.2 F +1.1 F -0.8 F + 0.1F 

Figure 4 – Frequency of daily temperature departures from 1971-2000 normals for maximum 

(red) and minimum (blue) temperatures on Saint Thomas and Saint Croix for the years 2000-

2009. 

 

3. Case 1: Conclusions 

Considering the location of the ASOS at the Cyril E. King Airport, it is likely that the proximity 

to the water has contributed to the noticeable difference in temperatures recorded at the new 

observation site. Specifically, during the heat of the day, the water acts as a buffer (due to its 

high heat capacity) to the solar radiation, resulting in a less heating of the air relative to the 

surrounding land mass. This reduced heat transfer into the atmosphere has effectively reduced 

the daily maximum temperatures at the new site by 1.4o

F. The reverse is true at night. Once the 

sun goes down, the island cools more rapidly than the surrounding water. As the typical light 

southeasterly trade winds observed at this site during the overnight hours carry the warmer air 

over the water across the peninsula where the ASOS is located, observed minimum temperatures 

have increased by an apparent 1.0o

F. While these changes might seem minimal at a mid-latitude 

station, this modification, magnified by the higher relative humidities, regular trade winds and 

smaller diurnal temperature ranges (all typical of coastal tropical locations), has led to nearly a 

20% reduction in the diurnal temperature cycle being recorded at the Cyril E. King Airport since 

1998. 

IV. Case 2: Luis Muñoz Marin International Airport, PR 

While sometimes the physical location of a meteorological sensor does not change, the actual 

siting of the instrument can be significantly affected by changes that occur within its vicinity. 

This appears to be the case with the ASOS at the Luis Muñoz Marin International Airport in San 

Juan, which has recorded some interesting temperature trends since the completion of taxiway 

Juliet in 2008. 

7
1. Background 

The Luis Muñoz Marin International Airport in San Juan became operational in 1955 and serves 

as the island of Puerto Rico’s main international gateway. It is the busiest airport in the 

Caribbean and handles approximately 10 million passengers a year and over nine million tons of 

containerized cargo (Puerto Rico Ports Authority, 2006). Planning for an expansion of the 

airport to accommodate future air traffic and passenger loads, which included the construction of 

a new concourse and the addition of taxiway Juliet began in 2001. While the permits for the 

taxiway were issued in December 2004, construction did not begin until early 2006, and was not 

completed until early 2008. A picture taken prior to the construction of this taxiway can be seen 

in Figure 5 (the ASOS is circled and labeled A). Figures 6 and 7 are larger scale views of the 

area where the ASOS is located before and after the construction of taxiway Juliet, respectively. 

 E 

S 

Figure 5 - The Luis Muñoz International Airport in San Juan, PR. The ASOS equipment is 

circled and labeled A. Note the lush vegetation just to the east and south of the site, separating 

the observation equipment from the westbound lanes of the highway (marked with the red line) 

and the overpass for the eastbound traffic on the far right. 

Construction of taxiway Juliet between 2006 and 2008 brought the asphalt taxiway within 37 feet 

of the ASOS equipment and approximately 61 feet from the temperature sensor. Although the 

Office of the Federal Coordinator for Meteorology specifies that temperature and dew point 

sensors be mounted in such a way “to ensure that measurements are representative of the free air 

circulating in the locality and not influenced by artificial conditions, such as large buildings, 

cooling towers, and expanses of concrete and tarmac,” there is no official minimum distance 

specified that the temperature or dew point sensors must be from either concrete or tarmac. 

However, according to NWS Cooperative Observer siting instructions (NWS 2010), “the sensor 

should be at least 100 feet from any paved or concrete surface.” In the case of the San Juan 

ASOS, the siting was done over 10 years before the addition of taxiway Juliet, at which time it 

was in full compliance with NWS Instruction 10-1307, and a relocation northward at this point 

would position the ASOS within 100 feet of runway 10/28. (It should also be noted that the 

8
primary purpose of ASOS equipment is to serve the aviation community and is not solely for 

climatological purposes). 

Figure 6 – Pre-Construction: An aerial view of the ASOS (white circle) at the Luis Muñoz Marin 

International Airport in San Juan with the future location of taxiway Juliet highlighted by the 

thin white line (photo courtesy of the Puerto Rico Ports Authority). 

In order to investigate the impact that the addition of taxiway Juliet had on the temperature 

observation at the Luis Muñoz Marin International Airport, two high quality sites were chosen 

for comparison. Using the ASOS at the Christiansted Airport on Saint Croix and the ASOS at 

the Cyril E. King Airport on Saint Thomas (and recognizing the shift in data on Saint Thomas 

began in August 1998 as noted in Case 1 above), it was decided to compare the temperatures 

from 2000-2005 to those from 2008-2010. By computing the difference between 2000-2005 and 

2008-2010, and leaving out the two years (2006-2007) during which taxiway Juliet was under 

construction, a standardized comparison of the sites could be performed. To further simplify the 

comparison, the differences for the stations on Saint Thomas and Saint Croix were combined 

(and hereafter are referred to as the U.S. Virgin Islands). It should be noted that the analysis was 

performed for Saint Croix and Saint Thomas individually, and the results were nearly identical 

and used as justification for their combination. 

9
Figure 7 – Post Construction: An aerial view of the ASOS (white circle) at the Luis Muñoz 

Marin International Airport in San Juan with the location of completed taxiway Juliet 

highlighted by the thin white line. 

2. Data Analysis 

Figure 8 is a frequency plot of the deviations in average temperatures since the completion of 

taxiway Juliet. Compared the temperatures at the Luis Muñoz Marin International Airport with 

those at the U.S. Virgin Islands, it is apparent that the average temperature since 2008 has been 

greater in San Juan than at the U.S. Virgin Islands. While the raw data shows an overall increase 

in average temperature of 0.8o

F (Table 3), a clear shift in the frequency of days with higher 

temperatures can be noted by simply subtracting the two data sets used in Figure 8. These 

differences (Figure 9) show a clear tendency towards fewer cool days and more warm days in 

San Juan. 

The addition of taxiway Juliet has affected the minimum temperatures significantly more than 

the maximum temperatures (Table 3). The minimum temperatures have increased approximately 

1.4o

F since the completion of the taxiway, while the maximum temperatures have experienced a 

much more modest increase of 0.3oF. 

10
Figure 8 – The annual distribution of deviations in average daily temperatures in San Juan and 

the U.S. Virgin Islands from their 2000-2005 average. 

“Warm” days 

Figure 9 – The annual shift in temperatures in San Juan since the addition of taxiway Juliet. 

Table 3 – Averaged temperature trends between 2000-2005 and 2008-2010 at the Luis Muñoz 

Marin International Airport in San Juan compared to that at the two U.S. Virgin Islands sites. 

Maximum Temp Minimum Temp Average Temp 

U.S. Virgin Islands -0.1 F -0.3 F -0.2 F 

San Juan +0.2 F +1.1 F +0.6 F 

Difference +0.3 F +1.4 F +0.8 F 

“Cool” days 

11
Using the noted difference that the addition of taxiway Juliet had on both the maximum and 

minimum temperatures at the Luis Muñoz Marin International Airport as justification, an 

examination of individual frequencies of the maximum and minimum temperatures was 

performed. Those results (Figures 10-12) indicate how the maximum and minimum 

temperatures have shifted at the ASOS in San Juan since the taxiway’s completion. 

Figure 10 – The annual distribution of deviations in maximum daily temperatures in San Juan 

and the U.S. Virgin Islands (from their 2000-2005 average). 

Figure 11. The annual distribution of deviations in minimum daily temperatures in San Juan and 

the U.S. Virgin Islands (from their 2000-2005 average). 

12
“Warm” lows 

“Warm” highs 

“Cool” highs 

“Cool” lows 

Figure 12 – The annual shift in both maximum and minimum temperatures in San Juan since the 

addition of taxiway Juliet. 

Careful examination of Figures 10 and 11 reveal an increase in the frequency of days in San Juan 

where maximum and minimum temperatures were either near or above their 2000-2005 

averages. However, in the case of the U.S. Virgin Islands, maximum temperatures have 

displayed a near normal distribution since 2008, while the minimum temperatures have exhibited 

a slight bias towards more days with near or below normal temperatures. 

The annual shift in temperature frequencies (Figure 12) shows both the maximum and minimum 

temperatures exhibit a clear bias towards less frequent cool days and more frequent warm days. 

However, this signal appears stronger in the minimum temperatures (plotted in blue) than in the 

maximum temperatures (plotted in red), consistent with the data presented in Table 3. 

This amplified effect on minimum temperatures is consistent with previous studies on the urban 

heat island effect which have demonstrated that temperature changes are greatest at night 

(Howard 1833). In this case, the addition of taxiway Juliet added a slab of asphalt where 

previously generally lush tropical vegetation was present, a modification typical to those 

associated with urbanization. Thus, it is thought that the resultant change in thermal properties 

associated with the addition of the taxiway (which retains heat more effectively than the natural 

vegetation) is likely why the minimum temperatures have shifted more significantly than the 

maximum temperatures at the site. 

3. Case 2: Conclusions 

Noting the proximity of taxiway Juliet to the ASOS observation equipment at the Luis Muñoz 

International Airport, it is apparent that the proximity of the new taxiway to the temperature 

sensor (61 feet) had an effect on the temperature record at the site. In fact, the NWS Instruction 

10-1307, which describes the standard siting criteria for meteorological instrumentation, 

recognizes that ideally a temperature sensor should not be sited within 100 feet of a paved or 

concrete surface. While the ASOS at the Luis Muñoz Marin International Airport was originally 

sited more than 100 feet from the nearest paved surface, the addition of taxiway Juliet has had a 

13
substantial effect on temperatures recorded at the site. Once again, it should be noted that the 

primary purpose of ASOS equipment is to serve the aviation community and not solely for 

climatology purposes. 

Since the completion of the taxiway there has been a noticeable shift if temperatures at the Luis 

Muñoz Marin International Airport when compared to other high quality surrounding 

meteorological stations. Compared with two other sites in the northeastern Caribbean an overall 

increase in the average temperature at the Luis Muñoz Marin Airport of 0.8 F was noted since 

the addition of the taxiway, with daily maximum (minimum) temperatures increasing 

approximately 0.3oF (1.4 F). The greater impact on minimum temperatures is attributed to the 

change in thermal properties of the ground near the observation site, similar to those that have 

been documented with urbanization. 

As was the case with the relocation of the observation site at the Cyril E. King Airport on Saint 

Thomas, the temperature trends observed since the change in siting were likely amplified due to 

the tropical coastal location. Regular light southeasterly winds at night (coming from the 

direction of the added taxiway), along with high relative humidities and a small diurnal 

temperature range, all likely contributed to the impact on the temperatures than may not have 

otherwise been expected from a slab of asphalt 61 feet away. 

V. Implications on Temperature Extremes at Both Airports 

Since 1 January 2008, 143 record temperatures have been tied or broken at the Luis Muñoz 

Marin International Airport in San Juan (through 31May 2010). Of those, 124 (87%) have been 

record warm temperatures, while only 19 (13%) were record cool temperatures. These should be 

compared to the temperatures Christiansted Airport on Saint Croix and Cyril E. King Airport on 

Saint Thomas were the number of temperature records were much smaller (129 and 51, 

respectively) and were more evenly distributed between warm (47% and 53%, respectively) and 

cool (53% and 47%, respectively) records. 

Recognizing that all three stations have roughly a similar period of record (varying from 47 years 

to 54 years) and that all three stations experience very similar synoptic scale weather conditions, 

it should be expected that a similar number of records would be broken at each of the three 

stations over the same time period. 

 In an attempt to create an impartial comparison between the two sites, it was decided to tabulate 

the number of records prior to the construction of taxiway Juliet at the Luis Muñoz Marin 

International Airport in San Juan, once again using the years 2000-2005as a comparison (see 

Table 5). 

14
Table 4 – New record temperatures recorded at San Juan, Saint Croix, and Saint Thomas 

between 1 January 2008 and 25 March 2010. 

Record 

High 

Temper

atures 

Record Low 

Temperatures

Record 

Warm Low 

Temperatures

Record Cool 

High 

Temperature 

Total 

Records 

San Juan 21 0 103 19 143 

Saint Croix 13 9 11 27 51 

Saint Thomas 0 9 68 68 129 

The bias towards an increasing number record warm low temperatures and record cool high 

temperatures at the Cyril E. King Airport on Saint Thomas remained similar for both time 

periods, and noticeably different from the distribution of record temperatures in San Juan and on 

Saint Croix (note the red and blue arrows highlighting the significantly higher number of record 

warm low temperatures and record cool temperatures recorded on Saint Thomas in Tables 4 and 

5). The second point to note is the distribution of record temperatures in San Juan and on Saint 

Croix were similar prior to the construction of taxiway Juliet, with a marked increase in record 

warm low temperatures observed in San Juan since the construction of the taxiway (black 

arrows in Tables 4 and 5). 

Table 5 – Same as Table 4, but for the period 1 January 2000 through 31 December 2005. 

Record 

High 

Temper

atures 

Record Low 

Temperatures

Record 

Warm Low 

Temperatures

Record Cool 

High 

Temperature 

Total 

Records 

San Juan 29 17 75 58 179 

Saint Croix 15 36 48 66 165 

Saint Thomas 0 16 120 167 303 

As noted in previous studies of climate change (e.g., Karl, et al. 2008), a small change in the 

average of a parameter can result in much larger changes in its extreme values (Figure 13). After 

a shift in temperatures at a given station, what was once considered to be an extreme event 

becomes just an above average event, with the corresponding number of new records occurring 

at a much higher frequency than in the recent past. 

This is exactly what we have observed at both the Luis Muñoz Marin International Airport in 

San Juan and the Cyril E. King Airport on Saint Thomas. The number of temperature records 

broken on San Juan and Saint Thomas since 1 January 2008 were nearly double those broken on 

15
Saint Croix, with a significant bias in the new temperature records towards their respective shifts 

in the temperature means. 

Figure 13. Effects that a shift in the average of a parameter, such as caused by a change in 

instrument siting, can have on the probability of occurrence of extreme values (taken from Karl, 

et al,. 2008). 

In Case 1, the Cyril E. King Airport on Saint Thomas, due to the relocation of the observation 

site closer to the water, observed maximum (minimum) temperatures had become approximately 

1.4o

F cooler (1.0oF warmer) since the siting of the ASOS. Because this change occurred towards 

the end of 1998, it is not surprising to see a higher proportion of record cool maximum and 

record warm minimum temperatures since then (Tables 4 and 5) when compared to the entire 

period of record. 

In Case 2, the addition of taxiway Juliet at the Luis Muñoz Marin International Airport in San 

Juan, an observed increase in the minimum temperatures of 1.4o

F has been recorded, with a 

sharp increase in the record number of warm low temperatures recorded at the airport since 

1 January 2008. Using the years 2000-2005 for a comparison, there has been nearly a three fold 

increase in the number of new record warm low temperatures in San Juan compared to those 

recorded on Saint Thomas and Saint Croix combined. 

These changes in the number of record temperature at the Luis Muñoz Marin International 

Airport and Cyril E. King Airport are exactly what one would expect with a shift in climate or, in 

the case of the two ASOSs, a shift in temperatures due to changes in siting (Karl 2008). 

While the shear number of records broken in San Juan and on Saint Thomas alone is notable, 

there have also been some other extraordinary records broken as a consequence. In 2009, the 

number of days in which the low temperature did not drop below 80 degrees in San Juan was 59, 

shattering the previous record of 32 set back in 1981. In addition, the two longest strings of days 

where the temperature did not drop below 80 were also observed in 2010, with strings of eight 

days in October 2009 and seven in August 2009 observed. The previous record was six days 

(June 1981). Not surprisingly, these record low temperatures also helped contribute to an 

average annual low temperature of 75.9o

F recorded in 2009, the warmest annual minimum 

temperature recorded in San Juan since record keeping began on 1 January 1956. 

16
VI. Summary 

The importance of siting has long been recognized as a key component in the collection of 

accurate meteorological data. As far back as 1769 siting was recognized to have a significant 

impact on record keeping (Herberden 1769). This paper demonstrated how critical good siting 

can be, especially in coastal tropical locations where diurnal temperature ranges are small and 

temperature is strongly dependent on distance from water. 

In Case 1, the relocation of the observation site at the Cyril E. King Airport on Saint Thomas, 

U.S. Virgin Islands, resulted in nearly a 20% reduction in the diurnal temperature range due to 

the new location slightly closer to the warm Caribbean waters. In Case 2, it was shown that a 

change in landscape 60 feet from the sensor at the Luis Muñoz Marin International Airport in 

San Juan had a profound impact on the temperatures at that site. Due to the change in thermal 

properties associated with the addition of taxiway Juliet, there has been a significant increase in 

the nighttime temperatures recorded there. Additionally, in both cases, these changes also likely 

contributed to a significant increase in the number of new record temperatures being recorded at 

their respective locations. 

While focusing attention on the siting of meteorological instrumentation is not new, it is 

something that needs to be considered when investigating temperature trends at a particular site. 

In the end, what may seem like a small change in location can have a critical impact on the 

climatological record for that site, especially in the tropics where diurnal cycles are small and 

temperature extremes correspondingly vary only a few degrees from averages. With this in 

mind, it becomes extremely important to document not only changes in the location of observing 

sites, but also any changes in the vicinity of the site. 

VII. Acknowledgements 

Thanks to Jeff Cupo, Israel Matos, Victor Murphy, and Michael Staudenmaier, for their reviews 

of this paper and the valuable feedback they provided. Thanks also to Bert Gordon, Russell 

Jackson, and Vidal Santiago for their contributions of photographs for this paper. 

VIII. References 

Colbert, H., 1958: Bureau’s Move Spoils Records: Temperatures Confused. Miami News, 27 

July 1958. 

Heberden, W., 1769: On the different quantities of rain which appear to fall, at different 

heights, over the same spot of ground, Phil. Trans. 59, 359-162. 

Howard, L., 1833: The Climate of London, Deduced from Meteorological Observations made at 

Different Places in the Neighborhood of the Metropolis, Brewster Press, London. Vol 1, 358pp 

and Vol 2, 414 pp. 

Karl, T. R., G. A. Meehl, C. D. Miller, S. J. Hassol, A. W. Waple and W. L. Murray (eds.), 

2008: Weather and Climate Extremes in a Changing Climate, Regions of Focus: 

17
18

North America, Hawaii, Caribbean, and U.S. Pacific Islands. U.S. Climate 

Change Science Program, Synthesis and Assessment Product 3.3, 180 pp. [Available online at: 

Weather and Climate Extremes in a Changing Climate, Regions of Focus:

North America, Hawaii, Caribbean, and U.S. Pacific Islands.] 

National Weather Service, 2008: NWS Instruction 10-1307 -- Cooperative Program 

Management and Operations. 53pp. [Available online at: 

http://www.nws.noaa.gov/directives/sym/pd01013007curr.pdf.] 

Office of the Federal Coordinator for Meteorological Services and Supporting Research, 1994: 

Federal Standard for Siting Meteorological Sensors at Airports, FCM-S4-1994, Washington 

DC, 23 pp. [Available online at: http://www.ofcm.gov/siting/pdf/fcm-s4-1994(Siting).pdf] 

Office of the Federal Coordinator for Meteorology, 2005: Federal Meteorological Handbook 

No. 1 -- Surface Weather Observations and Reports. 104 pp. [Available online at: 

http://www.ofcm.gov/fmh-1/fmh1.htm] 

Puerto Rico Ports Authority, 2006: History of the Port of Puerto Rico (in Spanish) [Available 

online at: http://www.prpa.gobierno.pr/] 

Watts, A., 2009: Is the U.S. Surface Temperature Record Reliable?, The Heartland Institute, 

Chicago, IL, 28 pp. [Available online at: 

http://www.heartland.org/books/PDFs/SurfaceStations.pdf]