Gairloch Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gairloch Mountain Observatory clear sky chart from 2005-11-05 to 2017-09-16.

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
21%
15225
8%
46841
4%
15180
25%
15180
10%
37062

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
9%
750
4%
2506
2%
746
12%
746
3%
2179
January16th to endNight
8%
795
4%
2655
1%
795
13%
795
4%
2280
February1st to 15thNight
12%
704
5%
2286
1%
701
16%
701
5%
1907
February16th to endNight
14%
631
7%
1962
1%
631
18%
631
7%
1605
March1st to 15thNight
18%
717
12%
2097
4%
717
26%
717
10%
1631
March16th to endNight
19%
693
12%
2037
4%
693
27%
693
11%
1574
April1st to 15thNight
20%
539
11%
1796
7%
539
27%
539
10%
1350
April16th to endNight
24%
540
15%
1685
9%
540
31%
540
16%
1195
May1st to 15thNight
28%
540
11%
1440
6%
540
31%
540
16%
1090
May16th to endNight
29%
561
9%
1496
3%
560
32%
560
15%
1142
June1st to 15thNight
21%
540
4%
1440
1%
540
23%
540
12%
1155
June16th to endNight
25%
540
6%
1440
2%
540
28%
540
16%
1122
July1st to 15thNight
39%
510
7%
1360
5%
510
40%
510
22%
980
July16th to endNight
34%
565
9%
1419
6%
532
36%
532
17%
1108
August1st to 15thNight
40%
539
10%
1498
7%
535
41%
535
18%
1118
August16th to endNight
44%
573
13%
1854
12%
573
47%
573
20%
1263
September1st to 15thNight
31%
540
13%
1800
9%
540
37%
540
17%
1233
September16th to endNight
29%
537
18%
1804
12%
537
38%
537
18%
1219
October1st to 15thNight
25%
660
12%
1980
6%
660
32%
660
14%
1451
October16th to endNight
14%
721
7%
2205
2%
721
19%
721
7%
1814
November1st to 15thNight
19%
730
7%
2353
3%
730
22%
730
9%
1890
November16th to endNight
15%
750
5%
2520
3%
750
18%
750
5%
2108
December1st to 15thNight
8%
750
3%
2520
1%
750
11%
750
3%
2222
December16th to endNight
8%
800
2%
2688
0%
800
10%
800
2%
2426

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
23%12%5%8%6%34445%21%
47924
Night
21%11%5%8%7%35%445%21%
47012
TransparencyDay
63%5%9%14%6%
47613
Night
63%5%10%14%5%
46841
SeeingDay
32%17%25%16%6%1
7769
Night
30%17%24%20%6%
15228

By Month

Percentage Cloud Cover NE (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
17%15%9%12%10%5%433312%
1432
16th to endDay
16%16%8%15%10%5%6%3349%
1569
February1st to 15thDay
16%13%6%15%12%6%5%33411%
1575
16th to endDay
14%16%9%11%9%3435%417%
1510
March1st to 15thDay
16%13%7%9%10%46%33420%
1850
16th to endDay
17%16%7%10%6%2333421%
2187
April1st to 15thDay
23%16%47%43334423%
2153
16th to endDay
23%13%45%422345%28%
2271
May1st to 15thDay
27%11%36%5%23345%24%
2512
16th to endDay
23%11%46%5%345%47%22%
2618
June1st to 15thDay
28%11%36%6%24345%21%
2520
16th to endDay
28%9%36%42435%6%25%
2520
July1st to 15thDay
19%7%35%435%46%7%31%
2520
16th to endDay
25%9%36%5%2445%7%23%
2653
August1st to 15thDay
24%9%37%4345%6%6%24%
2448
16th to endDay
22%8%36%43445%8%28%
2348
September1st to 15thDay
25%8%36%4235%46%29%
2160
16th to endDay
20%8%36%335%447%31%
1843
October1st to 15thDay
23%10%37%5%2435%8%24%
1650
16th to endDay
30%14%5%8%6%2433414%
1667
November1st to 15thDay
27%18%5%8%6%2423417%
1502
16th to endDay
21%21%7%9%5%34336%12%
1440
December1st to 15thDay
28%17%10%12%7%432335%
1440
16th to endDay
21%19%14%14%8%35%2234
1536
January1st to 15thNight
18%14%10%13%10%5%5%4349%
2506
16th to endNight
18%13%6%13%10%5%8%6%3310%
2655
February1st to 15thNight
17%13%6%13%9%5%7%44413%
2285
16th to endNight
13%16%8%11%9%445%45%16%
1962
March1st to 15thNight
17%10%5%9%8%45%44423%
2084
16th to endNight
18%13%5%8%7%34435%24%
2037
April1st to 15thNight
23%12%5%7%6%24445%23%
1796
16th to endNight
24%8%35%42343334%
1685
May1st to 15thNight
28%10%34423445%27%
1440
16th to endNight
21%9%36%6%45%5%5%7%24%
1494
June1st to 15thNight
26%10%46%6%445%45%22%
1440
16th to endNight
26%7%36%6%25%445%27%
1440
July1st to 15thNight
17%6%36%435%46%7%32%
1440
16th to endNight
23%7%47%5%246%6%7%24%
1515
August1st to 15thNight
21%6%35%6%245%5%6%30%
1509
16th to endNight
17%6%245%25%5%7%6%34%
1854
September1st to 15thNight
20%7%25%5%24448%34%
1800
16th to endNight
19%7%25%5%35%446%34%
1804
October1st to 15thNight
23%8%26%5%24446%29%
1980
16th to endNight
28%11%47%6%45%45%416%
2205
November1st to 15thNight
24%14%48%6%345%45%18%
2353
16th to endNight
24%16%6%8%6%35%45%5%13%
2520
December1st to 15thNight
25%16%8%10%8%35%4448%
2520
16th to endNight
25%16%9%14%8%343336%
2688

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
74%5%8%8%2
1433
16th to endDay
78%46%7%2
1569
February1st to 15thDay
74%48%9%2
1578
16th to endDay
67%5%9%9%6%
1510
March1st to 15thDay
66%47%13%7%
1860
16th to endDay
65%49%13%6%
2187
April1st to 15thDay
62%48%12%10%1
2153
16th to endDay
57%6%8%15%10%
2271
May1st to 15thDay
62%5%7%17%6%
2512
16th to endDay
60%6%8%17%6%
2622
June1st to 15thDay
64%5%10%13%5%
2520
16th to endDay
59%6%9%16%8%
2520
July1st to 15thDay
48%7%14%21%8%
2372
16th to endDay
59%6%11%15%5%
2486
August1st to 15thDay
57%8%10%17%5%
2434
16th to endDay
52%7%15%19%5%
2348
September1st to 15thDay
53%5%11%21%7%
2160
16th to endDay
50%6%12%21%8%
1843
October1st to 15thDay
57%5%11%16%9%
1650
16th to endDay
71%5%9%10%3
1667
November1st to 15thDay
70%48%13%2
1502
16th to endDay
73%5%10%7%3
1440
December1st to 15thDay
84%5%6%3
1440
16th to endDay
84%35%42
1536
January1st to 15thNight
76%5%7%8%2
2506
16th to endNight
74%5%8%8%3
2655
February1st to 15thNight
71%48%11%3
2286
16th to endNight
68%5%8%12%4
1962
March1st to 15thNight
60%5%8%17%8%
2097
16th to endNight
60%49%15%8%1
2037
April1st to 15thNight
59%5%8%15%10%
1796
16th to endNight
53%48%17%13%1
1685
May1st to 15thNight
56%411%19%7%
1440
16th to endNight
56%6%11%18%7%
1496
June1st to 15thNight
63%410%15%4
1440
16th to endNight
57%6%9%15%10%
1440
July1st to 15thNight
47%7%16%19%8%
1360
16th to endNight
55%7%12%18%5%
1419
August1st to 15thNight
52%8%15%18%5%
1498
16th to endNight
45%8%16%23%5%
1854
September1st to 15thNight
47%7%11%24%9%
1800
16th to endNight
48%5%12%26%7%
1804
October1st to 15thNight
55%412%19%7%
1980
16th to endNight
67%5%9%13%3
2205
November1st to 15thNight
65%6%13%12%3
2353
16th to endNight
69%6%11%9%2
2520
December1st to 15thNight
76%6%9%6%1
2520
16th to endNight
81%47%41
2688

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
32%25%26%13%2
240
16th to endDay
31%24%28%14%1
256
February1st to 15thDay
29%24%30%13%1
236
16th to endDay
28%29%21%15%5%
212
March1st to 15thDay
26%26%23%12%9%1
250
16th to endDay
30%27%23%12%42
288
April1st to 15thDay
34%25%20%14%5%
270
16th to endDay
30%23%26%15%3
309
May1st to 15thDay
37%17%18%14%8%3
468
16th to endDay
31%16%26%17%5%1
519
June1st to 15thDay
38%14%24%15%5%
495
16th to endDay
32%11%29%16%7%2
495
July1st to 15thDay
24%9%27%25%11%2
475
16th to endDay
33%11%23%20%11%
514
August1st to 15thDay
29%8%25%21%12%1
421
16th to endDay
27%13%28%21%6%2
332
September1st to 15thDay
29%13%34%16%4
315
16th to endDay
23%17%28%18%8%2
238
October1st to 15thDay
26%16%27%15%9%4
225
16th to endDay
40%19%22%9%6%1
240
November1st to 15thDay
35%18%22%13%5%4
235
16th to endDay
36%22%22%12%5%
240
December1st to 15thDay
48%18%20%10%2
240
16th to endDay
36%26%22%11%11
256
January1st to 15thNight
31%24%26%14%3
750
16th to endNight
33%26%22%13%3
795
February1st to 15thNight
28%23%27%16%3
704
16th to endNight
26%24%26%16%5%
631
March1st to 15thNight
26%20%27%19%5%
720
16th to endNight
28%22%23%18%5%1
693
April1st to 15thNight
31%18%20%25%3
539
16th to endNight
30%15%25%21%6%
540
May1st to 15thNight
34%11%21%19%11%1
540
16th to endNight
28%13%24%25%7%
561
June1st to 15thNight
36%9%25%22%5%
540
16th to endNight
28%10%30%23%6%
540
July1st to 15thNight
22%5%27%33%10%
510
16th to endNight
29%5%22%32%9%
565
August1st to 15thNight
25%6%19%32%15%
539
16th to endNight
22%6%21%34%14%
573
September1st to 15thNight
26%9%26%26%10%
540
16th to endNight
26%13%25%25%8%1
537
October1st to 15thNight
28%16%23%21%9%1
660
16th to endNight
36%20%23%14%4
721
November1st to 15thNight
33%17%22%20%41
730
16th to endNight
34%21%22%15%6%
750
December1st to 15thNight
41%23%19%10%4
750
16th to endNight
39%22%22%10%31
800
Page generated at 2017-09-17 13:05:46. Version ac04

Copyright 2017 A. Danko


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