library(PASWR2)
levels(PAMTEMP$month)
 [1] "Apr" "Aug" "Dec" "Feb" "Jan" "Jul" "Jun" "Mar" "May" "Nov" "Oct"
[12] "Sep"
PAMTEMP$month <- factor(PAMTEMP$month, levels = month.abb[1:12])
levels(PAMTEMP$month)
 [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov"
[12] "Dec"
library(plyr)
SEL <- ddply(PAMTEMP, .(year), summarize, Tmax = max(tmax), Tmin = min(tmin))
head(SEL)
  year Tmax Tmin
1 1990 37.0 -5.6
2 1991 38.2 -5.2
3 1992 36.4 -4.4
4 1993 37.6 -4.5
5 1994 37.2 -6.8
6 1995 40.0 -5.0
#
ggplot(data = SEL, aes(x = year, y = Tmax)) + 
  geom_line(color = "red") +
  geom_line(aes(x = year, y = Tmin), color = "blue") +
  theme_bw() +
  labs(y = "Temperature (Celcius)") + 
  geom_smooth(method = "lm") + 
  geom_smooth(aes(x = year, y = Tmin), method = "lm")

summary(lm(Tmax ~ year, data = SEL))

Call:
lm(formula = Tmax ~ year, data = SEL)

Residuals:
    Min      1Q  Median      3Q     Max 
-2.5376 -0.8475 -0.3030  0.6229  2.6723 

Coefficients:
             Estimate Std. Error t value Pr(>|t|)
(Intercept) 27.482251  92.351247   0.298    0.769
year         0.004935   0.046175   0.107    0.916

Residual standard error: 1.281 on 19 degrees of freedom
Multiple R-squared:  0.0006008, Adjusted R-squared:  -0.052 
F-statistic: 0.01142 on 1 and 19 DF,  p-value: 0.916
summary(lm(Tmin ~ year, data = SEL))

Call:
lm(formula = Tmin ~ year, data = SEL)

Residuals:
    Min      1Q  Median      3Q     Max 
-4.6261 -0.5147  0.2952  0.7377  3.9001 

Coefficients:
             Estimate Std. Error t value Pr(>|t|)
(Intercept)  47.11991  136.65578   0.345    0.734
year         -0.02623    0.06833  -0.384    0.705

Residual standard error: 1.896 on 19 degrees of freedom
Multiple R-squared:  0.007699,  Adjusted R-squared:  -0.04453 
F-statistic: 0.1474 on 1 and 19 DF,  p-value: 0.7053
PAMTEMP[which.max(PAMTEMP$precip), ]
     tmax tmin precip day month year tmean
1455  8.6    4   69.2  25   Dec 1993   6.3

The largest precipitation on a single day was 69.2 mm on December 25, 1993.