Terrace
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Terrace clear sky chart from 2005-11-05 to 2017-08-12.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
22%
14185
3%
46202
2%
14156
24%
14156
6%
40172

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
13%
750
5%
2739
3%
746
16%
746
4%
2451
January16th to endNight
12%
800
6%
2846
3%
800
16%
800
3%
2543
February1st to 15thNight
17%
730
5%
2455
3%
729
20%
729
6%
2151
February16th to endNight
23%
632
10%
2029
5%
632
29%
632
9%
1648
March1st to 15thNight
16%
720
4%
2118
1%
720
18%
720
6%
1870
March16th to endNight
24%
739
2%
2083
2%
739
24%
739
6%
1868
April1st to 15thNight
24%
539
3%
1673
2%
539
24%
539
6%
1482
April16th to endNight
21%
540
2%
1613
1%
540
23%
540
7%
1388
May1st to 15thNight
30%
365
1%
1266
1%
365
30%
365
6%
1095
May16th to endNight
34%
374
0%
1309
0%
374
34%
374
7%
1104
June1st to 15thNight
41%
360
0%
1164
0%
360
41%
360
10%
947
June16th to endNight
30%
360
0%
1080
0%
360
30%
360
7%
940
July1st to 15thNight
37%
340
0%
1059
0%
340
37%
340
11%
862
July16th to endNight
39%
376
0%
1239
0%
354
41%
354
9%
1008
August1st to 15thNight
55%
454
4%
1329
4%
452
55%
452
16%
989
August16th to endNight
32%
525
2%
1568
2%
525
32%
525
7%
1353
September1st to 15thNight
36%
550
6%
1614
5%
550
36%
550
11%
1316
September16th to endNight
18%
656
4%
1815
3%
656
19%
656
6%
1632
October1st to 15thNight
19%
660
4%
1986
3%
660
20%
660
7%
1718
October16th to endNight
16%
704
1%
2304
1%
704
16%
704
3%
2118
November1st to 15thNight
7%
711
2%
2513
1%
711
8%
711
2%
2348
November16th to endNight
10%
750
3%
2700
1%
750
13%
750
4%
2391
December1st to 15thNight
15%
750
5%
2700
3%
750
19%
750
6%
2303
December16th to endNight
15%
800
4%
3000
1%
800
18%
800
4%
2647

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
22%15%5%9%7%3444417%
47814
Night
21%17%5%9%7%34445%16%
46339
TransparencyDay
74%6%8%7%2
47455
Night
73%7%8%7%2
46202
SeeingDay
33%15%24%18%6%
6624
Night
33%12%25%19%8%
14190

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
24%23%7%8%7%35%4319%
1197
16th to endDay
23%19%6%8%7%35%5%3412%
1377
February1st to 15thDay
19%21%6%10%5%3342418%
1409
16th to endDay
8%14%6%9%8%45%5%5%5%26%
1445
March1st to 15thDay
16%20%6%10%8%5%5%445%12%
1842
16th to endDay
11%18%6%12%10%47%6%4413%
2139
April1st to 15thDay
17%20%7%10%8%3434314%
2274
16th to endDay
22%17%7%11%8%4434311%
2346
May1st to 15thDay
22%14%5%10%8%45%43416%
2685
16th to endDay
17%10%5%10%9%5%6%5%5%6%16%
2804
June1st to 15thDay
20%11%5%9%8%44436%19%
2793
16th to endDay
27%13%6%9%7%3443414%
2880
July1st to 15thDay
22%11%48%6%34345%24%
2835
16th to endDay
25%11%5%9%5%35%34421%
2846
August1st to 15thDay
16%8%47%6%3445%6%32%
2536
16th to endDay
26%12%5%9%7%4443415%
2282
September1st to 15thDay
29%12%47%6%3424419%
2014
16th to endDay
34%16%6%9%5%3432210%
1815
October1st to 15thDay
28%16%5%8%6%3433315%
1642
16th to endDay
31%18%5%8%6%3423311%
1567
November1st to 15thDay
26%21%6%11%9%433228%
1345
16th to endDay
21%19%6%10%7%35%335%13%
1260
December1st to 15thDay
20%20%6%9%6%3333317%
1260
16th to endDay
17%23%6%9%7%34435%14%
1221
January1st to 15thNight
24%23%6%9%7%3423310%
2739
16th to endNight
26%19%6%9%6%35%32410%
2846
February1st to 15thNight
20%20%6%9%7%3433415%
2443
16th to endNight
11%16%5%9%7%5%5%5%46%22%
2029
March1st to 15thNight
16%19%6%8%8%45%5%5%5%14%
2118
16th to endNight
12%18%6%11%8%47%5%5%6%13%
2079
April1st to 15thNight
16%16%5%8%8%45%5%5%417%
1673
16th to endNight
18%13%6%10%8%35%45%6%18%
1613
May1st to 15thNight
18%12%5%8%8%45%5%46%19%
1265
16th to endNight
15%8%39%9%46%6%7%7%22%
1309
June1st to 15thNight
12%10%310%8%45%5%5%7%25%
1164
16th to endNight
25%10%5%8%7%35%5%45%17%
1080
July1st to 15thNight
19%9%49%7%45%5%3427%
1122
16th to endNight
17%11%5%7%6%46%446%24%
1323
August1st to 15thNight
10%6%26%7%36%6%5%7%36%
1336
16th to endNight
20%9%410%7%35%5%6%6%20%
1568
September1st to 15thNight
21%10%48%7%43336%26%
1614
16th to endNight
32%13%49%5%3343413%
1815
October1st to 15thNight
26%13%48%7%345%45%16%
1986
16th to endNight
30%19%5%9%6%343339%
2304
November1st to 15thNight
30%24%6%9%7%332127%
2513
16th to endNight
21%21%6%11%7%3333411%
2700
December1st to 15thNight
19%23%5%9%6%3333317%
2700
16th to endNight
18%23%6%10%7%35%33413%
3000

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
79%6%6%6%1
1197
16th to endDay
78%45%6%31
1377
February1st to 15thDay
72%6%7%5%6%
1409
16th to endDay
62%6%12%13%4
1445
March1st to 15thDay
79%5%6%6%
1842
16th to endDay
76%9%8%5%
2139
April1st to 15thDay
80%5%6%6%1
2274
16th to endDay
82%45%41
2346
May1st to 15thDay
77%5%9%6%1
2685
16th to endDay
73%9%10%5%
2808
June1st to 15thDay
72%6%11%6%3
2793
16th to endDay
79%6%7%6%
2880
July1st to 15thDay
68%7%11%10%2
2669
16th to endDay
69%8%11%9%
2667
August1st to 15thDay
56%9%14%14%4
2518
16th to endDay
75%7%10%5%
2282
September1st to 15thDay
70%7%11%9%1
2014
16th to endDay
82%5%46%1
1815
October1st to 15thDay
74%8%8%7%1
1642
16th to endDay
82%5%5%5%
1567
November1st to 15thDay
87%3322
1345
16th to endDay
77%5%48%4
1260
December1st to 15thDay
72%5%7%8%31
1260
16th to endDay
76%5%7%5%3
1221
January1st to 15thNight
81%35%6%2
2739
16th to endNight
79%6%3431
2846
February1st to 15thNight
74%6%37%6%
2455
16th to endNight
65%6%10%12%4
2029
March1st to 15thNight
75%7%7%7%11
2118
16th to endNight
75%8%9%5%
2083
April1st to 15thNight
74%8%7%7%2
1673
16th to endNight
71%7%11%8%1
1613
May1st to 15thNight
70%10%8%7%2
1266
16th to endNight
66%10%13%7%
1309
June1st to 15thNight
62%10%14%9%3
1164
16th to endNight
70%10%13%4
1080
July1st to 15thNight
63%8%13%12%1
1059
16th to endNight
63%10%17%7%1
1239
August1st to 15thNight
49%12%21%12%4
1329
16th to endNight
67%12%13%6%
1568
September1st to 15thNight
63%8%13%11%3
1614
16th to endNight
78%5%7%6%1
1815
October1st to 15thNight
71%7%9%8%2
1986
16th to endNight
82%6%6%4
2304
November1st to 15thNight
87%3422
2513
16th to endNight
79%446%4
2700
December1st to 15thNight
75%47%8%3
2700
16th to endNight
77%6%5%6%4
3000

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
20%11%30%30%6%1
60
16th to endDay
44%10%21%18%5%
97
February1st to 15thDay
35%15%27%19%2
236
16th to endDay
19%22%29%18%9%
211
March1st to 15thDay
33%30%21%10%3
248
16th to endDay
28%20%27%18%4
288
April1st to 15thDay
33%17%27%14%5%
269
16th to endDay
39%19%23%13%3
276
May1st to 15thDay
36%14%25%18%5%
468
16th to endDay
25%14%24%23%9%1
518
June1st to 15thDay
31%15%26%20%4
495
16th to endDay
36%14%26%15%5%1
495
July1st to 15thDay
32%14%26%22%4
475
16th to endDay
33%14%25%17%7%1
515
August1st to 15thDay
16%11%22%30%16%2
475
16th to endDay
30%16%30%15%5%
275
September1st to 15thDay
42%12%21%14%8%
255
16th to endDay
50%15%16%10%5%
255
October1st to 15thDay
40%14%17%18%7%1
246
16th to endDay
46%14%18%14%5%
223
November1st to 15thDay
50%16%23%333
60
16th to endDay
26%328%28%13%
60
December1st to 15thDay
30%11%16%28%11%1
60
16th to endDay
26%6%20%28%18%
64
January1st to 15thNight
39%15%25%15%3
750
16th to endNight
43%15%24%12%3
800
February1st to 15thNight
36%12%28%18%4
733
16th to endNight
21%15%34%21%8%
632
March1st to 15thNight
33%14%31%14%5%
720
16th to endNight
25%14%28%22%9%
739
April1st to 15thNight
30%12%25%22%8%
539
16th to endNight
31%12%26%21%8%
540
May1st to 15thNight
26%9%26%26%10%
366
16th to endNight
21%7%27%28%14%
375
June1st to 15thNight
21%7%23%28%18%
360
16th to endNight
35%6%21%25%10%
360
July1st to 15thNight
24%8%22%25%17%
340
16th to endNight
26%7%20%28%15%
376
August1st to 15thNight
18%414%30%30%1
454
16th to endNight
32%7%20%26%12%
525
September1st to 15thNight
33%6%18%24%16%
550
16th to endNight
46%13%17%14%8%
656
October1st to 15thNight
38%11%26%15%8%
660
16th to endNight
44%12%20%16%5%
704
November1st to 15thNight
47%16%23%9%2
711
16th to endNight
33%19%30%13%3
750
December1st to 15thNight
34%13%28%19%4
750
16th to endNight
34%15%28%17%4
800
Page generated at 2017-08-13 11:16:30. Version ac04

Copyright 2017 A. Danko


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