Supercell over Laramie
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Wyoming Climate Atlas


13.1 General Description

Average sea-level pressure (SLP) is used to standardize surface pressure maps (Figure 13.1-Casper). On the other hand, station pressure which is based on the station's elevation (Figure 13.2-Casper, Figure 13.4-Cheyenne, Figure 13.6-Lander, and Figure 13.8-Sheridan) is used for aviation altimetry settings for take-offs, landings and minimum cruising heights to avoid mountains. In Wyoming, pressure climatology shows little in the way of variation throughout the year. Maximum pressure usually occurs in July during the afternoon while minimum pressure occurs in March during the morning. The differences between average extremes are usually less than 10 mb or 0.33 inch of mercury (Figure 13.2, Figure 13.4, Figure 13.6, and Figure 13.8). The highest frequency of station pressure values are found in July and ranges from 9 to 13 percent as shown in Figure 13.3, Figure 13.5, Figure 13.7, and Figure 13.9 (note that the scales are different for these figures). This is not unusual since few weather systems move through the state in the summer. Since most weather systems either form north or east of Wyoming, the state seldom experiences intense high or low pressures.

Figure 13.1

Figure 13.1. Casper average monthly sea-level pressure as a function of frequency (%) of occurrence from 1950-1996

Figure 13.2

Figure 13.2. Casper mean hourly station pressure in millibars (mb) (1961-1990) by month. Extreme values are depicted.

Figure 13.3

Figure 13.3. Casper pressure frequency (1961-1990) by month

Figure 13.4

Figure 13.4. Same as Figure 13.2 except for Cheyenne (different scaling)

Figure 13.5

Figure 13.5. Same as Figure 13.3 except for Cheyenne (different scaling)

Figure 13.6

Figure 13.6. Same as Figure 13.2 except for Lander (different scaling)

Figure 13.7

Figure 13.7. Same as Figure 13.3 except for Lander (different scaling)

Figure 13.8

Figure 13.8. Same as Figure 13.2 except for Sheridan (different scaling)

Figure 13.9

Figure 13.9. Same as Figure 13.3 except for Sheridan (different scaling)

13.2 Extreme Pressures

One of the claims made by some climate researchers is that with global warming the frequency of extreme weather events would increase. However, when examining the absolute annual extreme SLP events for Cheyenne, Sheridan, Lander, and Rock Springs from 1948-1996 based on daily averages, one finds that high pressure extremes are trending towards lower pressure (no significance) while low pressure extremes are trending towards higher pressure at the 95 percent significance level (Figure 13.10). These trends suggest a lessening of the thermal gradient between the equator and higher latitudes at least in the northern hemisphere. With less conflicting air masses, severe weather events are less likely to occur. On January 12, 1932 and January 10, 1962, Wyoming experienced its greatest extremes in sea-level pressure in the 20th century. Figure 13.11 shows the surface weather map depicting the lowest pressure and Figure 13.12 shows the highest pressure for those dates.

Figure 13.10

Figure 13.10. Annual SLP extremes by year (1948-96) from daily pressure averages derived from Cheyenne, Sheridan, Lander, and Rock Springs. The extreme pressure events are dated.

Figure 13.11

Figure 13.11. Wyoming's lowest pressure recorded on January 12, 1932 103a

When comparing average annual pressure trends no statistical significance was noted. However, both Cheyenne and Sheridan show a 0.47 mb and 0.64 mb increase while Lander and Rock Springs show a 0.67mb and 0.41 mb decrease respectively in annual average SLP between 1948-1996. Also, comparing correlations between these four stations, the highest correlations were between Cheyenne and Sheridan, and Cheyenne and Rock Springs as noted in Table 13.A.

Figure 13.12

Figure 13.12. Wyoming's highest pressure recorded on January 10, 1962 103a

Table 13.A. Pressure correlations between station pairs (1948-1996)

Station Pairs


Station Pairs








Sheridan-Rock Springs


Cheyenne-Rock Springs


Lander-Rock Springs


13.3 Extratropical Low Pressure Systems

As was shown in Figure 4.8, the number of low pressure systems passing within 100 km of Wyoming varies from less than 10 (1961-63, 70, 78, 79, 83-85, 87, 90-92) to 38 in 1968 (Figure 13.13) and 40 in 1975 (Figure 13.14).

Figure 13.13

Figure 13.13. Wyoming extratropical tracks (1967-68) 104

Why is there such a large inter-annual difference with respect to the number of transiting low pressure systems across Wyoming? Statistics reveal that correlations between the number of lows to various atmospheric indices105 such as ENSO (i.e., El Niño, La Niña) and PDO are not significant at the 95 percent level based on annual index averages. Perhaps global warming is masking any relationships to a specific atmospheric index, or perhaps these indices interact with one another in highly complex non-linear ways thus making any comparisons to the number of transiting lows impossible. However, as noted in Figure 4.8, the trend for transiting lows is declining annually and this may be in response to global warming.

Figure 13.14

Figure 13.14. Wyoming extratropical tracks (1975-76)




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