RESEARCH PAPER 4 3
JULY 1961
BY
OWEN
P.
CRAMER
SUMMARY
The variation of fire-weather elements in mountainous t e r r a i n
i s complex a t any one time, and the patterns vary considerably with
time. During periods of serious f i r e weather, this variation becomes
important. Much information i s obtainable by local interpretation of
available forecasts and observations. Optimum use of available information requires some understanding of basic meteorological processes
and a l s o demands adequate tools for application of the principles.
This paper provides information about variation of midafternoon
temperature and relative humidity in mountainous t e r r a i n and explains
the use of two charts that help in estimating the amount of variation.
F o r layers of a i r in contact with the surface, the charts make it possible
to:
1.
Determine the vertical extent of layers in which mixing i s
present (unstable l a y e r s ) and of layers in which there i s no
mixing (stable layers).
2.
Adjust temperature and humidity between elevations within
mixed layers.
3.
Predict afternoon valley temperatures and humidities from
morning observations a t peak stations.
4.
Estimate humidities at any level within the mixed layer beneath cumulus clouds.
R e s e a r c h P a p e r 43
ADJUSTMENT O F
RELATIVE HUMIDITY AND TEMPERATURE
FOR DIFFERENCES IN ELEVATION
by
Owen P. C r a m e r
J u l y 1961
PACIFIC NORTHWEST
FOREST AND RANGE EXPERIMENT STATION
R. W. Cowlin, D i r e c t o r
P o r t l a n d , Oregon
FOREST SERVICE
U. S. DEPARTMENT O F AGRICULTURE
CONTENTS
Page
. . . . . . . . . . . . . . . . . . . . . . . .
BASIC METEOROLOGICAL BACKGROUND . . . . . . . . . . . .
What Is Relative Humidity? . . . . . . . . . . . . . . . . .
Interrelations of Relative Humidity. T e m p e r a t u r e .
and Elevation . . . . . . . . . . . . . . . . . . . . . .
How T e m p e r a t u r e Affects Vertical Distribution
of Moisture . . . . . . . . . . . . . . . . . . . . . . .
TEMPERATURE ESTIMATES . . . . . . . . . . . . . . . . . .
INTRODUCTION
Determining T e m p e r a t u r e S t r u c t u r e f r o m Reported
Values.. Use of C h a r t I
Visible Indicators of T e m p e r a t u r e S t r u c t u r e
Predicting Maximum T e m p e r a t u r e s a t Valley Stations f r o m
Observed Morning T e m p e r a t u r e s a t P e a k Stations
..................
..........
......
RELATIVE HUMIDITY ESTIMATES . . . . . . . . . . . . . . .
Adjustment of Relative Humidity to Different
Elevations.. Use of C h a r t 11
Estimating Relative Humidity i n the Mixed L a y e r
Below Cumulus Clouds
Predicting Afternoon Minimum Humidities in Valley A r e a s
f r o m Morning Observed Humidities a t P e a k Stations
. . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . .
.....
AIRMASS PROPERTIES THAT A F F E C T TEMPERATURE
ANDMOISTURE
........
Description of A i r m a s s e s . . .
How A i r m a s s e s A r e Modified .
Stratification of A i r m a s s e s . .
.
.
.
.
.
.
.
.
............*
.............
. . . . . . . . . . . . .
. . . . . . . . . . . . .
Typical Oregon and Washington S u m m e r A i r m a s s e s . . . . . .
1
2
2
3
4
7
INTRODUCTION
Relative humidity i s important in f o r e s t f i r e control. It influences the
m o i s t u r e content of f u e l s and tGereby affects f o r e s t flammability. Relative
humidity i s consequently part of many fire-danger rating s y s t e m s . It i s used to
indicate when f i r e weather i s s e v e r e enough that logging operations a r e closed
down to prevent f i r e s . P r e d i c t e d relative humidity i s a n essential ingredient f o r
predicting fuel m o i s t u r e . A/ Despite i t s u s e s , relative humidity is difficult to
apply because i t s distribution i s frequently complex. Observations and f o r e c a s t s
give clues to the distribution but cannot be fully utilized unless the principles of
humidity and t e m p e r a t u r e variation a r e understood. These principles a r e m o s t
easily interpreted through the use of graphs.
Afternoon humidity and temperature?/ v a r y f r o m one place to another in
n~ountainoust e r r a i n , p r i m a r i l y because of elevation differences. Such variation
i s particularly m a r k e d where the t e r r a i n extends upward through two o r m o r e
l a y e r s of a i r , each with i t s own t e m p e r a t u r e and humidity properties. Thus,
peaks m a y sometimes be w a r m e r and d r i e r than valley stations, though the r e v e r s e i s m o r e usual. In completely mixed l a y e r s , t e m p e r a t u r e and humidity
change with elevation in a regular m a n n e r - - t e m p e r a t u r e decreasing with elevation and humidity increasing.
This paper d i s c u s s e s relative humidity, the vertical distribution of
m o i s t u r e in the a i r , and the effects of vertical t e m p e r a t u r e s t r u c t u r e on s t r a t i fication and mixing. C h a r t s a r e presented with which predicted o r observed
relative humidity and t e m p e r a t u r e m a y be adjusted t o different elevations under
c e r t a i n conditions.
1'
C r a m e r , Owen P. Predicting m o i s t u r e content of fuel-moistureindicator sticks in the Pacific Northwest. U. S . F o r e s t Serv. P a c . NW. F o r e s t
& Range Expt. Sta. Res. P a p e r 41, 17 pp. , illus. 1961. ( P r o c e s s e d . )
-2 /
T e m p e r a t u r e s and humidities discussed h e r e i n a r e m e a s u r e d by standa r d procedures in exposures fully open to the wind.
BASIC METEOROLOGICAL BACKGROUND
What Is Relative Humidity?
At each t e m p e r a t u r e , a given space can contain a c e r t a i n maximum
amount of water vapor. When this maximum amount of vapor i s present, the
space i s saturated. Any additional m o i s t u r e would condense and a p p e a r a s dew
o r fog. The quantity of water vapor that s a t u r a t e s the space is not influenced by
The
a i r p r e s s u r e o r even the presence of a i r but depends only on t e m p e r a t u r e .
higher the t e m p e r a t u r e , the m o r e water vapor required for saturation. F o r
e v e r y 20' F. i n c r e a s e in t e m p e r a t u r e , the water vapor capacity is approxim a t e l y doubled.
/
The m o i s t u r e p r e s e n t i n air i s usually described a s weight of water
M p o r p e r kilogram of d r y a i r . This description of a t m o s p h e r i c m o i s t u r e i s useful since the proportion of the m i x t u r e does not change with elevation ( p r e s s u r e )
i n thoroughly mixed a i r .
Relative humidity i s a m e a s u r e of the degree of water vapor saturation.
At any t e m p e r a t u r e , relative humidity i s the ratio, e x p r e s s e d in percent, of the
w a t e r vapor actually present to that n e c e s s a r y f o r saturation (fig. 1). Thus, if
2 2 g r a m s of water vapor will s a t u r a t e a kilogram of d r y a i r and only 11 g r a m s
a r e present, the relative humidity i s 50 percent.
20 grams of water vapor
16 grams of water vapor
-.
I
SPACES OCCUPIED BY 1,000 GRAMS OF DRY AIR
**
5 0 % RELATIVE HUMIDITY
AT ,I0 F.
Be
8 0 % RELATIVE HUMIDITY
AT 77' F.
100% RELATIVE HUMIDITY
AT 70 ,,2O F.
Interrelations of Relative Humidity,
Temperature, and Elevation
Relative humidity v a r i e s with changes in t e m p e r a t u r e (fig. 1 ) . It a l s o
v a r i e s with differences in the actual m o i s t u r e content of the a i r (fig. 2). Since
both t e m p e r a t u r e and m o i s t u r e v a r y with elevation, relative humidity i s difficult
to d e s c r i b e f o r the wide ranges of t e m p e r a t u r e and elevation in a n extensive
mountain a r e a .
ELEVATION
4,000
WARME>
\
MARINE
AIR
\\
2,000
SEA
LEVEL
--
\
--
T f v W s m - ~ \
.
'
-- -_
.
COOL,SEA-BREEZE
40
I
I
50
60
I
I
70
TEMPERATURE
80
OF.
90
4
8
MOISTURE
I
I
12
1
6
- G R A M S
I
0
20
40
% RELATIVE
60
80
100
HUMIDITY
( Grams water per 1,000 grams dry air )
Figure 2. - - T e m p e r a t u r e , m o i s t u r e , and relative humidity curves s i m ulated f o r 4:30 p . m . a t a station having a s e a breeze. In this illustration, each of the t h r e e l a y e r s of a i r i s mixed within itself, producing a constant proportion of m o i s t u r e within each l a y e r . Change
in humidity within each l a y e r i s due entirely to t e m p e r a t u r e d i f f e r ence with elevation, which in each mixed l a y e r i s 5,1/2" F. per
1,000 f e e t elevation.
Fortunately, sometimes t h e r e i s o r d e r in this apparent chaos. An o r d e r l y
a r r a n g e m e n t i s found when heating a t the surface o r turbulent wind has s o s t i r r e d
the a i r that i t i s completely mixed through a substantial l a y e r . Mixing usually
o c c u r s to a height of s e v e r a l thousand feet above the ground on m o s t sunny surnm e r afternoons. In mixed, nonsaturated a i r the t e m p e r a t u r e d e c r e a s e s with inc r e a s e in elevation a t a constant r a t e of 5-1 12" F. per 1, 000 feet. Mixing i s a s sumed in a n y l a y e r through which such a t e m p e r a t u r e d e c r e a s e i s observed. The
proportion of water vapor to a i r i s constant in a mixed l a y e r . Under such conditions, relative humidity v a r i e s in a r e g u l a r m a n n e r with elevation. This i s the
b a s i s f o r the a i d presented l a t e r f o r converting relative humidity directly f r o m
one elevation to another.
HOWTemperature Affects Vertical Distribution of Moisture
Air motion distributes moisture vertically. Except where i t i s caused by
mechanically induced turbulence, vertical motion depends upon existing difference of temperature with height in relation to the change in temperature that occ u r s f r o m change in p r e s s u r e a s a i r moves vertically. The existing decrease of
temperature with increase in height i s called the temperature lapse rate. When
the lapse rate i s just 5-1 / 2 O F. per 1,000 feet, i t i s exactly the same a s the r a t e
a t which rising a i r cools from
expansion, due to decreasing
p r e s s u r e with increase in height,
&d descending- a i r heats from
COOLER
EXISTING
WARMER
compression. This lapse r a t e is
PARCEL
AIR
PARCEL
ELEVATION
often used for comparison and i s
(FEET) I
SINKING -TEMF!RISING
called the d r y adiabatic rate.
When such a lapse r a t e exists,
slight horizontal differences in
temperature will cause vertical
motion (fig. 3). This occurs
because a parcel of a i r w a r m e r
than i t s surroundings i s lighter
than the surrounding a i r and will
r i s e . The r e v e r s e i s true of a
parcel cooler than i t s surroundings.
--
I
,-
-
1
-I
SURFACE
80°
Figure 3 . --Vertical motion supported
by slight horizontal temperature differences in a i r with d r y adiabatic
lapse rate.
-
If the lapse r a t e i s g r e a t e r
than d r y adiabatic (unstable o r
superadiabatic), vertical motion
i s accelerated and mixing i s a s sured (fig. 4a). If the lapse r a t e
i s just 5-1 / 2 " F. per 1,000 feet
(neutral o r d r y adiabatic), vertical
motion has occurred o r will occur
and mixing m a y be assumed (fig.
4c). Vertical motion and mixing
a r e suppressed by lapse r a t e s
l e s s than d r y adiabatic (stable)
(fig. 4b).
-
The lapse rate i s variable. It varies from hour to hour and from day to
day (fig. 5). Lapse rate may v a r y a l s o f r o m place to place, particularly where
there a r e major differences in elevation between stations (fig. 6). Differences in
temperature structure of different a i r l a y e r s and differences in the heating effects
of the t e r r a i n below a r e responsible. Conditions existing m u s t be carefully determined before the procedures to be described can be applied.
ELEVATION
TEMPERATURE
OF.
(FEET)
PARCEL RISES
WARMER THAN SURROUNDINGS
PARCEL
-
-
54.0
TEMF! 5 0
55
DOWNWARD
PARCEL SINKS
,
58
OF.
A . --Unstable lapse r a t e - - g r e a t e r than d r y adiabatic--vertical motion a c c e l e r ated- -mixing occurs.
1,300
-
COOLER THAN SURROUNDINGS
-53.5
PARCEL RETURNS
TO ORIGINAL
LEVEL AND
TEMPERATURE
54.6
-
TEMP 50
55
WARMER THAN SURROUNDINGS
"E
B. --Stable lapse r a t e - - l e s s than d r y adiabatic--vertical motion suppressed-no mixing occurs.
-
SAME TEMPERATURE
AS SURROUNDINGS
-52.4
PARCEL AT REST
PARCEL AT REST
TEMF!
50
55
SAME TEMPERATURE
AS SURROUNDINGS
"E
C . --Neutral lapse rate- - d r y adiabatic- -vertical motion tolerated- -mixing has
o r will occur.
Figure 4. --Effects of lapse r a t e s on vertically displaced parcels of a i r . P a r c e l
s t a r t s a t s a m e temperature in each instance and i s displaced the same vertical distance. The differing lapse r a t e s of the surrounding a i r a r e shown by
the heavy solid line in the left-hand diagrams and the side scale in the righthand diagrams. The dashed reference line in the left diagrams i s the d r y
adiabatic lapse r a t e of 5-1/2O F. per 1,000 feet of elevation.
E L E V A T I O N
9,000 F E E T
\
\
BETWEEN DAYS
\
\
\
WARMER
AIR
C
\
\
5,000
CURRENT DAY'S
PREVIOUS DAY'S
4,000 -
3,000
WARMER
2,000
SEA
\
SEA
LEVEL
LEVEL
40
50
60
T
70
80
E M P E R A T U R E
90
OF.
100
.\
B
1,000
40
I
I
50
60
:
:
\
\COOL,
70
80
MARITIME AIR
90
T E M P E R A T U R E
Figure 5. - - T e m p e r a t u r e - elevation (lapse r a t e ) c u r v e s illustrating change in t e m p e r a t u r e a t various
heights; A , with t i m e of day, and B, between days. Dashed straight line i s r e f e r e n c e d r y adiabatic
lapse r a t e of 5-1 1 2 " F . per 1 , 0 0 0 i e e t elevation.
100
ELEVATION
9,000 FEET
1
I
1,000
SEA
LEVEL
A. COOL, MARITIME AIR
often foggy
6. COOL A1 R
from nighttime radiation and cool air drainage from the
slopes forming a nocturnal inversion.
Figure 6 . --Typical s t r a t a of a i r over western Oregon and w e s t e r n
Washington on a s u m m e r morning.
TEMPERATURE ESTIMATES
When m o i s t u r e is evenly distributed, a s within mixed l a y e r s , relative
humidity m a y be graphically adjusted between elevations within such l a y e r s . To
do this, mixed l a y e r s m u s t be identified. This i s done by analysis of the vertical
temperature structure.
Certain typical variations of the surface mixing l a y e r need to be considered. Its depth v a r i e s with elevation of the country over which i t f o r m s , being
g r e a t e s t over the high country. Horizontal t e m p e r a t u r e differences m a y consequently be expected within the mixing l a y e r between mountains and lowlands.
T e m p e r a t u r e a l s o v a r i e s in the surface l a y e r with distance f r o m the coast. To
minimize e r r o r f r o m horizontal t e m p e r a t u r e gradient, stations compared should
be no m o r e than 20 m i l e s a p a r t . Considerably l e s s east-west leeway i s advisable
close t o the coast in the lower 3, 500 feet. In general, t h e r e will be m o r e leeway
along o r within a mountain range than a c r o s s it and parallel to the coast than a t
right angles t o it.
Determining T e m p e r a t u r e S t r u c t u r e f r o m Reported
Values--Use of C h a r t I
Vertical distribution of t e m p e r a t u r e m a y be determined f r o m either obs e r v e d o r predicted values f o r specific elevations. T e m p e r a t u r e s , observed
simultaneously a t a l l points, a r e plotted on C h a r t I (fig. 7) opposite t h e i r elevations.21 If the observations fall in a line parallel to one of the sloping l i n e s , a
d r y adiabatic l a p s e r a t e i s indicated and the l a y e r i s a s s u m e d to be mixed. If the
slope i s f l a t t e r than the sloping lines on the chart, a g r e a t e r than adiabatic l a p s e
r a t e i s present and mixing i s a s s u r e d .
f
Visible Indicators of T e m p e r a t u r e S t r u c t u r e
+
Since t e m p e r a t u r e observations m a y not always be available, s o m e t i m e s
i t m a y be n e c e s s a r y to r e s o r t to c a r e f u l interpretation of the visible indicators
of t e m p e r a t u r e s t r u c t u r e , and the resulting v e r t i c a l motion and mixing. C a r e
m u s t be used in applying the indicators only to the l a y e r s in which they o c c u r ,
since indicators of mixing and no mixing m a y be present in the s a m e vicinity but
in d i f f e r e n t l a ye r s .
Indicators of unstable l a p s e r a t e and presence of v e r t i c a l mixing. --When
mixing o c c u r s , smoke, h a z e , and dust a r e not concentrated in the lower levels of
the mixed l a y e r but become evenly distributed vertically with no horizontal l a y e r s
(fig. 8). Visibility a t a l l levels tends to be equally good. Clouds of smoke and
dust m a y be caught in rising o r descending c u r r e n t s , o r m a y just d r i f t a p a r t v e r tically. If rising portions of the mixing a i r r e a c h saturation, cumulus type clouds
will f o r m . These clouds will have f l a t b a s e s a t a common level. Dust devils indicate a highly unstable l a p s e r a t e n e a r the ground. T e m p e r a t u r e and humidity
m a y be adjusted between elevations within a n y l a y e r f o r which the above indicators
of mixing a r e present.
Indicators of stable l a p s e r a t e and absence of v e r t i c a l mixing. --Where v e r tical mixing i s not occurring, smoke, haze, and dust concentrate in the lower
levels where they often f o r m horizontal l a y e r s (fig. 9). Visible horizontal l a y e r s
a l s o f o r m n e a r the b a s e of a n inversion l a y e r , i. e . , a l a y e r through which the
t e m p e r a t u r e i n c r e a s e s with elevation. Valley fog and s t r a t u s clouds commonly
f o r m beneath a n inversion l a y e r and indicate stable l a p s e r a t e s a t and immediately
above the l e v e l s where they a r e present. Low cumulus clouds f o r m f l a t tops at:
a n inversion. In l a y e r s having indications of stratification, a stable l a p s e r a t e
e x i s t s and v e r t i c a l motion m a y be assurned to be absent. Within such l a y e r s ,
relative humidity and t e m p e r a t u r e cannot be reliably adjusted between elevations.
2'
F o r routine local u s e of C h a r t s I and I1 (pp. 9 and 1 3 ) , the elevations of thk
usual stations f r o m which observations a r e received and f o r which predictions a r e
m a d e should be m a r k e d and identified. The c h a r t s should then be placed under a
t r a n s p a r e n t cover on which the c u r r e n t t e m p e r a t u r e s o r humidities m a y be plotted
and l a t e r e r a s e d ,
ELEVATION
CHART
I.
TEMPERATURE
-
ELEVATION CHART
FEET)
10,000
SEA
LEVEL
SLOPING LINES REPRESENT DRY ADIABATIC LAPSE RATE
--
5 1/2"F. PER 1,000 FEET ELEVATION
F i g u r e 7.
CUMULUS
OF
FAIR
WEATHER
TOWERING
CUMULUS
UNIFORM
VISIBILI
COLUMN
D
F i g u r e 8. - - I n d i c a t o r s of unstable a i r .
STRATIFIED
WAVE
CLOUDS
F i g u r e 9 . - - I n d i c a t o r s of s t a b l e a i r .
Predicting Maximum T e m p e r a t u r e s a t Valley Stations
f r o m Observed Morning T e m p e r a t u r e s a t P e a k Stations
Nighttime cooling i s l e s s a t peak stations than a t valley stations. Consequently, morning t e m p e r a t u r e s a t peak stations a r e b e t t e r indicators of t e m p e r a t u r e s l a t e r in the day when both peak and valley stations l i e within the s a m e
mixed l a y e r . On C h a r t I (fig. 7), starting a t the peak station's elevation and
morning t e m p e r a t u r e , the sloping lines a r e paralleled t o the valley elevation, and
the t e m p e r a t u r e i s read. To this i s added the expected i n c r e a s e in t e m p e r a t u r e
during the day a t the mountain station. This m a y be judged on the b a s i s of the
f o r e c a s t and the previous day1s pbservations of morning and maximum t e m p e r a t u r e s . The 'sum i s the expected m a x i m u m t e m p e r a t u r e a t the valley station (fig.
10, left side).
RELATIVE HUMIDITY ESTIMATES
Adjustment of Relative Humidity to Different Elevations-Use of Chart I1
Where t h e r e i s v e r t i c a l mixing, C h a r t 11 m a y be used t o a d j u s t relative
humidity f o r elevation (fig. 11). On that c h a r t , the intersection of the humidity
(horizontal s c a l e ) and i t s elevation ( v e r t i c a l s c a l e ) i s found.41 F r o m this point,
the sloping lines a r e paralleled to the d e s i r e d elevation. The humidity a t the
new elevation i s r e a d f r o m the horizontal s c a l e . F o r example, relative humidity
of 58 percent a t 2,600 f e e t becomes 50 percent a t 1 , 6 0 0 feet (fig. 12).
Humidities f o r elevations above o r below a mixed l a y e r cannot be a c c u r a t e l y estimated by this method. Rough approximations of humidities between
mixed l a y e r s , a s f o r example within a n inversion, m a y be obtained graphically
a s shown in figure 13. This should be done only with comparatively thin, unmixed l a y e r s between elevations f o r which humidities a r e available.
Estimating Relative Humidity in the Mixed L a y e r
Below Cumulus Clouds
Cumulus clouds a r e the tops of columns of r i s i n g w a r m a i r cooled to s a t uration by the a s c e n t . The common b a s e level of cumulus i s the elevation of 100percent relative humidity in the mixed a i r . The humidity a t a n y other elevation
m a y be determined by following down the a p p r o p r i a t e curve on C h a r t I1 f r o m 100
percent a t the cloud-base elevation. Elevation of the cloud b a s e m a y b e d e t e r mined where clouds touch a peak, o r i t m a y be estimated (fig. 14). F o r example,
if cumulus cloud b a s e s a r e estimated a t 5, 500 feet, the humidity a t 2, 500 feet
would be about 62 percent.
4'
See footnote 3, p. 8.
CHART I
ELEVATION
CHART 1
ELEVATION
f FEET)
SEA
LEVEL
Figure 10. --Use of morning observation a t peak stations to indicate afternoon t e m p e r a t u r e and relative
humidity a t valley stations. Valley below 3,400 feet i s filled with night accumulation of cold a i r - nocturnal inversion. Considerably l e s s cooling has occurred above this level. Heating and resultant
mixing will d e s t r o y the inversion on a sunny s u m m e r afternoon. Approximate expected t e m p e r a t u r e
and humidity curves a r e determined f r o m adjusted morning observed values a t the peak station.
ION
1
RELATIVE
H U M I D I T Y
(
P E R C E N T
1
FOR MIXED LAYERS AND LAPSE RATE OF 5 1/2OF. PER 1,000 FEET ELEVATION
,
BASED ON MIXING RATIO VALUES AND U. S. STANDARD ATMOSPHERE PRESSURE ALTITUDES FROM PSEUDO-ADIABATIC DIAGRAM WB 1147,
SLOPING CURVES REPRESENT CONSTANT MOISTURE CONTENT
F i g u r e 11.
ASSUMING SEA-LEVEL TEMPERATURE OF 9 0 ° F .
CHART
CHART
I
II
ELEVATION
ELEVATION
( F8,000
EET)
(FEET)
8,000
SEA
LEVEL
SEA
LEVEL
50
60
70
TEMPERATURE
80
OF.
40
50
60
70
% RELATIVE
80
90
100
HUMIDITY
Figure 14. --Estimating relative humidity when cumulus clouds a r e present. Base of the cumulus i n t e r s e c t s peak D a t 5, 900 feet. Cloud type indicates unstable a i r below, and cloud b a s e indicates elevation of 100% relative humidity. This fixes location of the humidity curve for the unstable l a y e r .
Humidities for any other station, such a s A , B , o r C, m a y be determined f r o m the curve. Simil a r l y , elevation of cloud b a s e could be determined f r o m a surface relative humidity. Although cloud
type indicates a d r y adiabatic lapse r a t e , position of t e m p e r a t u r e curve on Chart I m u s t be d e t e r mined by a n observation a t some station such a s B.
Conversely, if the humidity i s known a t a given elevation and cumulus
clouds a r e present, the elevation of the cloud b a s e can be estimated by paralleling the curved lines f r o m the known point to saturation a t the right-hand margin.
Predicting Afternoon Minimum Humidities in Valley
A r e a s f r o m Morning Observed Humidities a t P e a k Stations
It i s often w a r m e r and d r i e r a t peak stations e a r l y in the morning than a t
valley stations. By afternoon, however, warming in the lower l a y e r s frequently
produces convective mixing between valley bottom and peak levels. The morning
m o i s t u r e a t the peak approximates the m o i s t u r e expected by afternoon throughout
the mixed l a y e r and hence a t lower elevations. Two s o u r c e s of e r r o r a r e recognized: (1) no allowance i s made f o r i n c r e a s e in t e m p e r a t u r e , and (2) the lower
l a y e r s m a y have a higher m o i s t u r e content. Effects of these two e r r o r s on the
humidity of the afternoon mixed l a y e r will usually be compensating.
F r o m the peak humidity-elevation point, the sloping lines on Chart I1 a r e
paralleled to the d e s i r e d valley-station elevation and the humidity scale i s r e a d
(fig. 10, right side). This a s s u m e s mixing between peak and valley and does not
apply where a n inversion prevents valley a i r and peak-level a i r f r o m mixing.
Yesterday's maximum t e m p e r a t u r e s plotted on Chart I show whether mixing has
been occurring between the two levels. Depth of this mixing l a y e r changes f r o m
day to day and frequently does not extend much m o r e than 3,000 to 4,000 f e e t
above the valleys.
AIRMASS PROPERTIES THAT A F F E C T
TEMPERATURE AND MOISTURE
Many properties of the atmosphere v a r y f r o m day t o day and f r o m place
to place. Much of this variation i s due to changes f r o m one a i r m a s s to another
a i r m a s s with different properties. More gradual change m a y be due to modification of a n a i r m a s s in place. Those who attempt to adjust t e m p e r a t u r e and r e l a tive humidity f r o m one elevation to another should know something about typical
a i r m a s s e s and how they a r e modified and thus change t e m p e r a t u r e and m o i s t u r e
distribution.
Description of A i r m a s s e s
The m o s t common reason f o r day-to-day change in a t m o s p h e r i c m o i s t u r e
and t e m p e r a t u r e s t r u c t u r e i s a change in type of a i r m a s s . An extensive portion
of the atmosphere with distinctive properties of t e m p e r a t u r e , m o i s t u r e , and
stability i s called a n a i r m a s s . These distinctive properties a r e acquired by prolonged passage o r stagnation over a portion of the e a r t h ' s surface. Such a r e a s ,
r e f e r r e d to a s s o u r c e regions, provide names f o r the a i r m a s s e s . Tropical a i r
comes f r o m low latitudes and i s hot, while polar a i r comes f r o m n o r t h e r n latitudes and i s cool. Maritime a i r comes f r o m a n ocean s o u r c e region and c a r r i e s
considerable m o i s t u r e , whereas continental a i r comes f r o m over land and i s d r y .
A i r m a s s e s a r e designated by combinations of these s o u r c e names.
Tropical
continental a i r i s hot and d r y and f o r m s over the i n t e r i o r southwestern United
States. In c o n t r a s t , polar m a r i t i m e a i r , which i s cool and wet, r e a c h e s the P a cific Coast f r o m the n o r t h e r n P a c i f i c . Tropical m a r i t i m e a i r that r e a c h e s the
Northwest m a y originate in either the tropical e a s t e r n Pacific o r in the Gulf of
Mexico and m a y bring with i t considerable thunderstorm activity.
As the winds change direction, they bring different a i r m a s s e s over a n
a r e a . Transition f r o m one a i r m a s s to another m a y be gradual o r m o r e o r l e s s
a b r u p t where the transition i s m a r k e d by a front. T e m p e r a t u r e and humidity m a y
be expected to change in the transition zone, often abruptly through a f r o n t .
How A i r m a s s e s A r e Modified
Typical a i r m a s s e s have acquired t h e i r p r o p e r t i e s in l a y e r s s e v e r a l
m i l e s thick. These p r o p e r t i e s m a y be modified a s they move, particularly in the
lower l a y e r s . A i r m a s s e s moving northward become cooler while those moving
southward become w a r m e r . P a s s a g e over a mountain range such a s the Cascades
m a y produce m a r k e d changes in t e m p e r a t u r e and humidity (fig. 15). A i r a s c e n d ing the windward side cools by expansion during a s c e n t . If i t i s m o i s t , condensation and precipitation m a y occur. Condensation of m o i s t u r e l i b e r a t e s heat, p a r tially counteracting cooling during f u r t h e r a s c e n t ; and precipitation reduces the
m o i s t u r e content. L a t e r , a s the a i r descends on the leeward side, i t w a r m s a t
the d r y adiabatic r a t e (5-112" F. per 1, 000 feet elevation). The a i r on the l e e w a r d side i s w a r m e r and d r i e r than a t the s a m e level on the windward side #because i t h a s l o s t m o i s t u r e and h a s been heated by condensation during the a s c e n t .
Without condensation, t e m p e r a t u r e s and humidities would be identical f o r a n y
elevation on both sides of the ridge.
An important modification p r o c e s s takes place within the typical highp r e s s u r e center. The c h a r a c t e r i s t i c v e r t i c a l motion of a i r within a high i s downward. This brings a i r to lower elevations f r o m v e r y high altitudes, where low
t e m p e r a t u r e s prevent i t f r o m holding appreciable m o i s t u r e . As i t sinks, i t
w a r m s a t the d r y adiabatic r a t e . This p r o c e s s i s called subsidence and m a y occur
over s e v e r a l s t a t e s a t once. Subsiding a i r i s observed a t high elevations f i r s t and
m a y o r m a y not r e a c h the valley bottoms. Downslope winds on the l e e s i d e s of
mountain ranges produce s i m i l a r drying and warming effects. Some of the lowest
humidities observed a t the s u r f a c e have resulted f r o m combined subsidence and
downslope winds. These two p r o c e s s e s often combine during the Oregon and
Washington e a s t winds and the California Santa Ana.
Stratification of A i r m a s s e s
Usually the a i r s t r u c t u r e overhead i s not typical of a single a i r m a s s but
i s m o r e like a stack of horizontal l a y e r s , each with i t s own a i r m a s s p r o p e r t i e s
(fig. 9). Winds, usually varying with height, bring these l a y e r s f r o m differing
s o u r c e s . Thus, these l a y e r s d e t e r m i n e the pattern of t e m p e r a t u r e , humidity,
and wind in mountainous t e r r a i n . Intelligent use of fire-weather information in
mountainous t e r r a i n r e q u i r e s a w a r e n e s s of a i r stratification. Degree of
E LEVA
(FEET)
12,000
SEA
LEVEL
SEA
LEVEL
F i g u r e 15. --Modification of a m a r i t i m e a i r m a s s passing over a m a j o r mountain range. A i r a s c e n d s
slope f r o m A, cooling to saturation a t B. Between B and C , cooling i s l e s s rapid due to the l i b e r ation of heat by condensation, and water i s removed by precipitation. At C descent begins and
adiabatic warming c a u s e s dissipation of the clouds. F r o m C to E warming o c c u r s a t the d r y a d i a batic r a t e .
stratification i s m o s t readily indicated by the v e r t i c a l t e m p e r a t u r e s t r u c t u r e .
C h a r t I will be useful in determining the presence, vertical extent, and charact e r i s t i c s of individual l a y e r s .
Typical Oregon and Washington Summer A i r m a s s e s
Although stratification v a r i e s considerably between day and night and a l s o
d i f f e r s f r o m one day to another, west of the Cascade c r e s t in Oregon and Washington t h e r e a r e commonly t h r e e distinct l a y e r s in contact with the t e r r a i n a t
various elevations during s u m m e r f a i r weather. The lowest l a y e r i s m a r i t i m e
a i r which i s comparatively cool and moist. It m a y extend upward to 5, 000 feet,
though this will v a r y considerably. The t e m p e r a t u r e and, consequently, humidity
within this l a y e r change f r o m night to day with the g r e a t e s t changes n e a r the
ground and the l e a s t a t the top of the l a y e r . The lapse r a t e within this l a y e r a l s o
v a r i e s greatly f r o m day to night, but i s usually d r y adiabatic during midafternoon
on sunny days. Along the coast the lower portion of this l a y e r i s cool and often
foggy and i s c a r r i e d into Coast Range valleys a t night by the s e a b r e e z e .
-
Above the m a r i t i m e a i r i s a transition l a y e r between lower and higher
l a y e r s . When the contrast i s great, the l a y e r m a y be a n inversion. This t r a n s i tion l a y e r , often called the m a r i t i m e inversion, i s m o s t pronounced in the m o r n ing
- and, with sufficient warming- in the l a y e r below, m a y actually disappear in the
afternoon. When present, this inversion l a y e r prevents mixing of a i r above and
below it and thus s e p a r a t e s possibly contrasting humidity and wind r e g i m e s .
The uppermost of these t h r e e l a y e r s v a r i e s much l e s s f r o m day to night
than the lower l a y e r s . Low humidity and a lapse r a t e slightly l e s s than d r y adiabatic a r e c h a r a c t e r i s t i c . This a i r usually originates a t high elevations within the
Pacific high-pressure cell and m a y gradually descend to lower elevations over a
period of s e v e r a l days under subsidence conditions. Within this l a y e r w e s t e r l y
winds m a y bring low humidities to mountain stations.
Daytime heating f r o m the ground produces a mixing l a y e r of some depth
on e v e r y sunny s u m m e r day. On some days the mixing l a y e r extends above the
usual level of the inversion. During periods of pronounced s e a b r e e z e that brings
cool a i r f r o m the ocean, only a shallow mixing l a y e r m a y f o r m a t lower elevations. At the s a m e time, a deeper mixing l a y e r m a y f o r m over t e r r a i n that extends above the cool a i r (fig. 16). T e m p e r a t u r e r e p o r t s and C h a r t I can a i d in
identifying this condition.
CHART
I
CHART
II
ELE
ELEVAT
(FEET)
I
-
I
n
I
SEA
LEVEL
T E MPERATURE
OE
% RELATIVE
HUMIDITY
F i g u r e 16. --Variation in depth of mixing l a y e r with distance f r o m the c o a s t and with elevation. T e m - .
p e r a t u r e and humidity c u r v e s a r e not continuous but r e p r e s e n t conditions a t different distances f r o m
the ocean.