Maryland Temperature and Precipitation Trends

A study of temperature and precipitation trends was conducted for a selection of stations throughout Maryland. Stations were chosen for analysis on the basis of period of record, completeness of record, and changes in station location and elevation. A total of eight stations were selected for analysis of temperature trends including Baltimore City, Chestertown, Glenn Dale, Laurel, Oakland, Royal Oak, Salisbury, and Woodstock. Precipitation trends were also analyzed for these stations, as well as for Conowingo Dam.

Long Term Trends for Baltimore City and Woodstock

Of the eight stations studied, Baltimore City and Woodstock had the longest periods of record with Baltimore data available from 1893-1999 and Woodstock data available from 1895-1999. Both Baltimore and Woodstock trends were analyzed for their full periods of record and compared. Monthly mean temperatures and precipitation were plotted versus time and linear trends were computed. The table below shows the slopes of the trend lines.

Baltimore's average monthly maximum temperature increased an average 0.0423 degrees F per year while Woodstock's average maximum monthly temperature increased only an average 0.0127 degrees F per year. The difference between the average monthly minimum temperature trends at Baltimore and Woodstock is even more pronounced; Baltimore's average monthly minimum temperature increased an average 0.0405 degrees F per year compared to Woodstock's average monthly minimum temperature which increased an average 0.0018 degrees F per year. Average monthly temperature trends also show a much larger increase at Baltimore than at Woodstock. Using these slopes we estimate a warming of 4.09 degrees F over 100 years in Baltimore and 0.55 degrees F over 100 years at Woodstock. The considerable differences between the temperature trends at the two stations is likely due to the station locations. While they are not far apart in distance, Baltimore City has undergone significant changes in land use and population as the result of urbanization over the past century. Woodstock, which lies near the border of Howard and Baltimore counties, remains a relatively small community. The urbanization of Baltimore City over the past century have affected its climate, causing it to become significantly warmer than surrounding areas and resulting in greater increases in temperature than at Woodstock. Unlike temperature trends, both Woodstock and Baltimore City have very small precipitation trends. Baltimore's monthly precipitation increased an average 0.0018 inches per year while Woodstock's monthly precipitation increased an average 0.0049 inches per year.

1948-99 Temperature and Precipitation Trends

Besides Woodstock and Baltimore, all of the other stations selected for analysis had records available in digital form for 1948-99 (except Laurel which had records available from 1949-99); Temperature and precipitation trends were analyzed for 1948-99. Baltimore and Woodstock were also analyzed for the 1948-99 period so that results could be compared with those of other stations. The slopes of the resulting trend lines are shown in the table below.

Of the eight stations analyzed for temperature trends, four stations had positive maximum temperature trends and four had negative maximum temperature trends. Baltimore City had the greatest average monthly maximum temperature trend, increasing an average 0.0447 degrees F per year. Woodstock had the second largest average monthly maximum temperature trend, increasing an average 0.0352 degrees F per year. Trends for the other six stations were considerably smaller. The varied trends of average monthly maximum indicate that there was no uniform trend of either increasing or decreasing maximum temperatures over the 1948-99 period for the state.

Unlike the maximum temperature trends, a majority of the eight stations showed significant positive minimum temperature trends. Woodstock had the greatest average monthly minimum temperature trend, increasing an average 0.0647 degrees F per year. The other five stations with positive average monthly minimum temperature trends showed similar increases ranging from an average 0.0256 to 0.0338 degrees F per year. Laurel and Chestertown were the only stations studied which had negative minimum temperature trends. For all stations except Laurel and Chestertown, the minimum temperature trend was greater than the maximum temperature trend, in most cases by nearly 0.03 or 0.04 degrees F per year. This seems to indicate a more general increase in minimum temperatures than in maximum temperatures across the state for the 1948-99 period.

Like minimum temperature trends, average temperature trends were positive for six of eight stations. Woodstock had the greatest average monthly temperature trend, increasing an average 0.0508 degrees F per year. The average monthly temperature increased an average 0.0341 degrees F per year at Baltimore for the 1948-99 period. The other four stations with positive average monthly temperature trends showed increases ranging from an average 0.0093 to 0.0173 degrees F per year. Chestertown and Laurel were the only stations with small, negative average monthly temperature trends. The general increasing trend of average monthly average temperatures is due largely to the significant increases in minimum temperatures. At all stations a cooling trend occurred from the 1940's until the late 1970's, followed by a warming trend from the late 1970's to present. The warming has become dominant at six of the eight stations, while the magnitude of the recent warming has not exceeded that of the earlier cooling at Chestertown and Laurel.

Monthly precipitation generally showed slight increases over the 1948-99 period. Woodstock's monthly precipitation increased an average 0.0070 inches per year, and Conowingo Dam's monthly precipitation increased an average 0.0067 inches per year. The monthly precipitation trends at 4 other stations studied were just above 0, showing little increase. The monthly precipitation trends were negative at Glenn Dale, Baltimore City, and Oakland. Though the majority of stations showed a positive monthly precipitation trend, the fact that all the trends are relatively small indicates there is not a strong trend, either positive or negative, across the state.

1977-99 Temperature Trends

Examining the 1948-99 graphs of monthly temperature data shows a pattern of increasing temperature beginning in the late 1970's and continuing today. Analysis of temperature data from 1977-99 shows temperature increases at all stations selected much larger than increases from the 1948-99 period. The table below shows the slopes of trend lines.

All stations studied had strong maximum temperature trends, most greater than 0.1000 degrees F per year. Woodstock had the greatest average monthly maximum tempertaure trend, increasing an average 0.1635 degrees F per year. Chestertown had the second greatest trend with average monthly maximum temperature increasing an average 0.1518 degrees F per year. Only two stations had average increases less than 0.1000 per year, and Oakland, with an increase of an average 0.0520 degrees F per year, was the only station well below 0.1000 degrees F per year. The strong trend of increasing maxmimum temperatures in recent years is indicated by all stations studied, especially when compared with the trends for the 1948-99 stations.

Every station studied also showed a positive minimum temperature trend. The greatest average monthly minimum temperature trend occurred at Glenn Dale, where an average increase of 0.1506 degrees F per year was observed. The remaining stations showed increases greater than 0.0865 degrees F per year except for Chestertown. Chestertown had the smallest increase in average monthly minimum temperature of all stations studied, an average 0.0273 degrees F per year. Like maximum temperatures, minimum temperatures also showed a strong increasing trend. However, at five of the eight stations studied, the increase in minimum temperature was less than the increase in maximum temperature. This result is contrary to what has been seen on a worldwide basis (IPCC, 1996).

Like maximum and minimum temperatures, average temperature trends were also positive for all eight stations. Woodstock had the greatest average monthly temperature trend, increasing an average 0.1293 degrees F per year. The smallest average monthly temperature trend was an increase of 0.0871 degrees F per year observed at Laurel. Over the 23 years from 1977 to 1999 the total warming ranges approximately 2 degrees F at Laurel to approximately 3 degrees F at Woodstock. When compared with the trends for the same locations for the 1948-99 period, temperature trends are much greater for the 1977-99 period, in some cases as much as five times larger. Interestingly, analysis of the full period of record for Batimore and Woodstock showed that Baltimore had greater temperature trends than did Woodstock. However, when viewing only the 1977-99 data, the temperature trends at Woodstock are greater than those at Baltimore City. Since the analysis for the entire period of record reflected the affects of urbanization on Baltimore's climate, the fact that Woodstock has experienced greater recent average increases in temperature suggests that the urbanization effect on Baltimore's climate occurred before the 1977-99 period.

Intergovernmental Panel on Climate Change. Climate Change 1995: the Science of Climate Change. Cambridge University Press, 1996.