54 L. Lindkvist et al. Agricultural and Forest Meteorology 102 2000 51–67
Fig. 2. Topographical map over the study area, the temperature sampling locations are shown.
3. Methods and materials
3.1. Acquisition of climate data Temperature data was sampled at 38 locations for 60
consecutive days during the growing season of 1996. The sampling sites were distributed over a 625 km
2
area of highly complex terrain shown in Fig. 2. The altitude and terrain form associated with each sam-
pling location is presented in Table 2. The locations comprise a variety of local climate environments, all
in open or clear-felled terrain with low vegetation of grass, twigs and plants. The purpose of this sam-
pling scheme was to spatially partition the temperature distributions that evolve in different terrain types in a
region with strong relative relief. Miniature data-loggers TinyTalk, Orion Compo-
nents, Chichester, England with built in temperature sensors thermistors facilitated the measurements.
The unit stores 1800 temperature observations in non-volatile memory with a precision of 0.2
◦
C in the range +46 to −35
◦
C. The time constant is approxi- mately 8 min when the unit is kept in a protective case
of aluminium, i.e. the observed value is averaged over the previous 8 min. A highly reflecting film covers
a simple radiation shelter. The measuring units are placed inside these shelters and mounted 2 m above
the ground. The radiation shelters length 50 cm,
L. Lindkvist et al. Agricultural and Forest Meteorology 102 2000 51–67 55
diameter 10 cm are then orientated in a north–south direction, cut obliquely at the ends and tilted 40
◦
to the horizontal in order to prevent direct sunlight and
stagnation of air from influencing the measurements. A reference station measuring temperature, wind
speed, wind direction and net radiation was estab- lished at 1020 m on Mt. Lill Fig. 2. The measuring
equipment consisted of temperature sensors Platinum 100 , accuracy 0.1
◦
C, a cup anemometer threshold value 1.0 m s
− 1
, a wind vane Young Wind Monitor, R.M. Young Co., Traverse City, MI, USA and a net
radiometer Q-7, Radiation Energy Balance Systems, Seattle, WA with wind effect less than 1 for neg-
ative fluxes up to 7 m s
− 1
. Data were recorded every 10 min and hourly averages were stored on a Camp-
bell data-logger Campbell Scientific, Logan, UT. The measurement period encompassed a wide range
of different weather conditions see Table 1. From the data it was possible to categorise the weather de-
pending on the prevailing cloudiness and wind. The nighttime net radiation was used as criteria for differ-
entiating between clear and cloudy weather conditions as minimum temperature and frost occurrence formed
the main focus of this study. During the night, the net radiation equals the long wave radiation incom-
ing from the sky minus the outgoing long wave radi- ation from the ground. The incoming part is mainly a
function of air temperature, humidity and cloud cover, with cloud cover dominating. Thus, the variability
observed between different nights is mainly a result of variable cloud cover. According to calculations made
Table 1 The weather daily mean temperature and wind speed, frost occasions and nighttime net radiation during the measuring period from 10
July to 5 September 1996 Day of the month 10
Daily mean tempe- No. of stations
Nighttime net Wind speed m s
− 1
July–5 September rature
◦
C with frost
radiation W m
− 2
Mean Max
Min July
991 8.4
− 44
10.7 11.1
10.1 192
8.6 1
− 44
4.6 7.2
3.1 193
7.4 −
7 2.3
3.2 2.0
194 6.0
3 −
33 3.9
5.9 3.2
195 5.0
− 31
6.9 8.8
5.4 196
3.5 −
7 12.7
14.5 10.8
197 2.9
− 16
15.7 17.5
14.5 198
2.2 −
9 13.4
14.6 11.6
199 4.5
− 29
11.4 13.9
11.1 200
8.9 15
− 67
2.9 3.8
1.5
by Perttu 1981 of net long wave radiation for clear nights based on the formulae by Ångström, 1974
and Brunt 1944 typical values of −65 W m
− 2
or lower were obtained
In this study the weather is categorised as clear if the net radiation is below −40 W m
− 2
and cloudy if the net radiation is above −20 W m
− 2
. The 60-day measuring period comprises 27 nights that are classified as clear
while 16 are categorised as cloudy, with the others being intermediate; i.e. a good coverage of different
types of weather conditions was obtained during the measurement period. During the investigated period
frost occurred at 26 occasions, i.e., at least one of the stations in the area had temperatures below 0
◦
C. 3.2. Terrain forms – site description
A ground based satellite aided navigation equip- ment GPS, which provides position fixes to within
30 m Silva XL 1000, SILVA Sweden AB was used in order to spatially define the locations of temperature
sampling sites and the reference station.
An elevation database was acquired from the Land Surveying of Sweden. Digital elevation information
on a 100 m grid creates the basis for the analysis of different terrain forms in this study. The information
was converted into raster format for further process- ing by a geographical information system GIS. In
this way, a simple classification of the terrain was ob- tained at each sampling site. To accomplish this, values
of slope aspect and slope inclination were compared
56 L. Lindkvist et al. Agricultural and Forest Meteorology 102 2000 51–67
Table 1 Continued Day of the month 10
Daily mean tempe- No. of stations
Nighttime net Wind speed m s
− 1
July–5 September rature
◦
C with frost
radiation W m
− 2
Mean Max
Min 201
10.4 11
− 66
3.2 5.0
1.5 202
10.8 −
30 5.3
6.0 4.5
203 11.4
− 33
3.6 4.8
2.2 204
12.3 −
23 4.2
5.6 2.3
205 12.0
− 30
6.1 6.7
4.2 206
9.2 −
29 4.4
5.6 3.5
207 7.4
13 −
68 3.8
6.5 1.2
208 7.6
− 2
7.1 8.6
5.4 209
5.9 −
31 4.4
8.0 0.5
210 3.8
− 42
7.9 11.1
5.8 211
4.3 −
43 10.6
12.3 9.4
212 6.7
12 −
44 3.5
5.4 1.5
August 213
7.6 7
− 5
9.2 10.6
8.3 214
5.6 −
5 9.2
10.6 8.3
215 4.6
− 7
12.0 13.9
10.3 216
6.7 −
17 7.8
8.2 6.5
217 10.6
− 20
3.4 5.8
1.5 218
13.0 2
− 65
5.4 5.7
5.0 219
12.3 2
− 66
6.6 7.6
5.9 220
11.3 3
− 55
5.7 6.5
4.8 221
10.4 −
24 7.7
8.7 6.9
222 10.4
− 38
8.3 9.0
6.9 223
11.8 4
− 71
8.3 9.0
7.3 224
12.9 −
57 6.1
7.0 5.2
225 13.0
3 −
68 6.8
7.7 6.0
226 13.3
1 −
68 7.4
8.3 6.7
227 12.2
3 −
62 5.5
7.2 3.2
228 12.4
− 40
3.2 4.3
2.7 229
11.6 −
34 6.7
7.4 5.8
230 10.7
2 −
29 2.2
4.2 0.6
231 13.5
− 46
8.5 10.4
7.5 232
14.5 1
− 66
8.1 10.4
6.6 233
14.5 1
− 66
6.9 8.6
5.5 234
11.9 −
53 3.6
4.2 3.3
235 10.4
− 2
5.1 5.9
4.1 236
11.1 −
4 5.6
6.3 4.5
237 10.5
− 1
9.6 11.5
7.0 238
9.8 −
2 5.9
7.2 4.9
239 9.0
− 3
4.7 6.9
2.7 240
9.3 −
3 7.4
8.5 6.4
241 10.1
5 −
64 4.8
6.5 3.0
242 9.4
− 1
7.2 8.5
5.7 243
7.4 1
− 28
5.5 7.3
2.4 September
244 7.0
− 11
9.5 10.9
7.8 245
9.1 2
− 46
9.1 10.5
6.5 246
6.8 −
60 8.7
11.2 7.2
247 2.1
− 32
12.8 14.8
9.9 248
0.5 3
− 68
10.4 13.2
8.2
L. Lindkvist et al. Agricultural and Forest Meteorology 102 2000 51–67 57
along several topographical cross-sections in IDRISI, a GIS software developed by Eastman 1992. The ter-
rain categories that were defined in this way include five principal types: 1 convex terrain: ∩, 2 linear
sloping terrain: ↓, 3 linear flat: ↔, 4 wide concave terrain: ∪ and 5 narrow concave terrain: ∨. In Table
2 information regarding the altitude and type of terrain are included for each sampling location.
Convex terrain peaks and ridges are found mainly at elevations above 800 m. Four areas are identified in
Fig. 2. Two broad peaks above 1100 m Mts. Anå and Lill , a ridge reaching 1000 m north of the peaks and
a broad convex area east of the peaks at 800–950 m. Two types of valleys, narrow and wide concavities,
are present in the same figure. The former varies in width between 1.0 and 2.0 km, whereas the latter is
2.5–4.0 km wide. The narrow type is located between Mt. Anå and Mt. Lill and the wider type follows
the two rivers and surrounds L. Grundsjön and L. Särvsjön.
The areas surrounding L. Grundsjön are considered flat, i.e. less than 3
◦
inclination. A 5–7 km wide level area divides the concavities in the northern part of the
study area. Open level ground is also identified in the southeast corner of Fig. 2. Thus, this terrain type vir-
tually bisects the study area from north to southeast, leaving valleys and hilltops at each side. The remain-
ing terrain type, i.e. slopes more than 3
◦
inclination, connects the convex hilltops and ridges with concav-
ities and flat valley floors. Differences in slope incli- nation are not considered due to the fact that sloping
terrain is known to show only small variations in frost susceptibility compared to the variations that occur
across different terrain types defined in this study.
Table 2 Altitude and dominating terrain form at 38 sampling locations. Each location is assigned one of the five major terrain forms that were
defined according to its curvature; convex, concave, linear sloping and linear flat. The symbols are included in order to simplify the comparison between tables and figures
a
Sampling site 1
2 3
4 5
6 7
8 9
10 11
12 13
14 15
16 17
18 Altitude m a.s.l.
710 700
670 680
870 680
630 600
710 670
680 710
670 720
660 710
830 780
740 Terrain form
↔ ↓
↔ ↓
∩ ↓
↔ ↔
↓ ↓
↓ ↔
∨ ↓
↔ ↓
↔ ∨
∨ Sampling site
19 20
22 23
24 25
26 27
28 29
30 31
32 33
34 35
36 37
38 Altitude m a.s.l.
950 1120
950 1110
680 710
830 995
680 700
790 650
660 700
1080 680
880 780
940 Terrain form
↓ ∩
↓ ∩
↓ ∨
∩ ∩
∪ ↔
∩ ↓
∪ ↓
∩ ↓
∩ ∩
↓
a
↔ : flat area, surface inclined 3
◦
; ↓: slope, surface inclined 3
◦
; ∩: convex area; ∪: wide concave area U-shaped valley; V = narrow concave area V-shaped valley.
4. Results and discussion
