Denholm Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Denholm Observatory clear sky chart from 2005-11-05 to 2017-09-18.

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
31%
14658
13%
46864
9%
14628
37%
14628
17%
33275

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
16%
720
6%
2506
3%
716
21%
716
7%
2032
January16th to endNight
22%
768
11%
2634
7%
768
28%
768
12%
1950
February1st to 15thNight
18%
702
10%
2287
4%
701
25%
701
10%
1755
February16th to endNight
23%
631
13%
1954
6%
631
33%
631
13%
1453
March1st to 15thNight
21%
720
13%
2094
7%
720
28%
720
15%
1561
March16th to endNight
30%
768
19%
2038
10%
768
37%
768
22%
1391
April1st to 15thNight
24%
718
16%
1796
6%
718
28%
718
20%
1312
April16th to endNight
31%
645
18%
1725
8%
645
37%
645
22%
1210
May1st to 15thNight
35%
360
17%
1440
16%
360
44%
360
17%
939
May16th to endNight
36%
375
13%
1497
10%
374
49%
374
20%
950
June1st to 15thNight
37%
360
9%
1440
8%
360
46%
360
15%
958
June16th to endNight
41%
360
8%
1440
12%
360
46%
360
19%
917
July1st to 15thNight
54%
340
15%
1360
21%
340
60%
340
27%
746
July16th to endNight
57%
376
14%
1418
17%
354
61%
354
26%
799
August1st to 15thNight
55%
449
15%
1520
14%
447
59%
447
26%
939
August16th to endNight
51%
705
20%
1851
11%
705
55%
705
36%
1153
September1st to 15thNight
46%
720
16%
1800
8%
720
49%
720
29%
1236
September16th to endNight
48%
669
22%
1825
14%
669
53%
669
32%
1166
October1st to 15thNight
34%
660
18%
1980
12%
660
43%
660
20%
1373
October16th to endNight
26%
704
11%
2194
6%
704
34%
704
12%
1675
November1st to 15thNight
27%
700
16%
2337
12%
700
35%
700
16%
1642
November16th to endNight
20%
720
10%
2520
6%
720
26%
720
10%
1934
December1st to 15thNight
19%
720
7%
2520
3%
720
24%
720
8%
2025
December16th to endNight
14%
768
9%
2688
4%
768
21%
768
8%
2159

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
17%8%36%5%35%45%6%33%
47938
Night
17%9%46%5%34446%32%
47033
Cloud CoverDay
20%7%35%5%34446%33%
47947
Night
19%9%36%5%34445%33%
47039
TransparencyDay
50%6%11%20%10%1
47625
Night
51%5%11%19%10%
46864
SeeingDay
21%11%28%22%11%4
5470
Night
22%10%28%26%10%
14660

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
21%13%49%8%35%345%21%
1432
16th to endDay
14%15%48%7%45%35%6%23%
1587
February1st to 15thDay
13%13%5%11%7%45%45%423%
1575
16th to endDay
17%13%48%7%25%445%27%
1518
March1st to 15thDay
17%10%46%8%36%5%4428%
1856
16th to endDay
15%8%35%424446%40%
2185
April1st to 15thDay
23%8%36%423345%34%
2152
16th to endDay
20%8%25%5%23445%37%
2244
May1st to 15thDay
19%7%36%5%35%346%33%
2511
16th to endDay
17%5%35%6%35%45%6%35%
2618
June1st to 15thDay
20%6%35%5%36%45%8%30%
2520
16th to endDay
16%6%35%5%35%6%6%7%32%
2520
July1st to 15thDay
12%5%245%36%6%6%8%39%
2520
16th to endDay
11%5%25%436%5%7%9%38%
2654
August1st to 15thDay
10%5%26%5%36%6%6%8%38%
2427
16th to endDay
10%5%25%435%5%6%8%41%
2339
September1st to 15thDay
15%5%25%5%25%447%41%
2160
16th to endDay
14%5%2442345%7%44%
1857
October1st to 15thDay
20%7%36%43445%6%33%
1650
16th to endDay
26%8%46%6%34436%25%
1677
November1st to 15thDay
15%8%35%5%45%5%5%6%33%
1520
16th to endDay
22%11%5%7%6%25%445%24%
1440
December1st to 15thDay
20%15%48%8%45%5%46%16%
1440
16th to endDay
23%14%6%8%7%1433421%
1536
January1st to 15thNight
18%14%6%11%8%34435%18%
2506
16th to endNight
12%16%6%8%6%35%345%27%
2634
February1st to 15thNight
13%13%5%11%8%35%445%23%
2285
16th to endNight
17%10%48%7%3435%5%28%
1954
March1st to 15thNight
18%11%5%8%6%34335%30%
2081
16th to endNight
15%6%45%5%25%34442%
2038
April1st to 15thNight
22%8%35%5%24345%35%
1796
16th to endNight
17%8%26%5%33346%37%
1725
May1st to 15thNight
21%7%25%42345%6%35%
1440
16th to endNight
15%6%25%5%335%5%7%37%
1495
June1st to 15thNight
17%6%26%6%245%5%7%34%
1440
16th to endNight
16%6%24425%5%6%8%37%
1440
July1st to 15thNight
10%314435%5%5%9%46%
1440
16th to endNight
9%5%235%35%5%6%9%45%
1514
August1st to 15thNight
9%5%35%4345%6%9%42%
1530
16th to endNight
9%324425%45%7%49%
1851
September1st to 15thNight
14%5%25%5%3445%5%43%
1800
16th to endNight
16%415%323335%49%
1825
October1st to 15thNight
19%7%35%5%25%45%6%35%
1980
16th to endNight
25%9%36%5%34446%26%
2194
November1st to 15thNight
19%9%36%5%33445%32%
2337
16th to endNight
23%10%48%6%34445%24%
2520
December1st to 15thNight
22%15%48%5%35%5%45%19%
2520
16th to endNight
24%15%5%8%6%2433419%
2688

Percentage Cloud Cover SE (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
20%14%49%8%33435%21%
1432
16th to endDay
14%14%5%8%7%45%5%45%25%
1587
February1st to 15thDay
14%13%410%7%46%45%424%
1575
16th to endDay
17%13%47%7%25%445%27%
1518
March1st to 15thDay
24%8%35%7%35%5%3427%
1866
16th to endDay
23%7%34324345%38%
2184
April1st to 15thDay
22%8%35%423335%36%
2152
16th to endDay
20%6%25%423445%39%
2244
May1st to 15thDay
24%6%35%5%24335%34%
2511
16th to endDay
23%5%245%34446%36%
2618
June1st to 15thDay
25%5%345%35%5%46%28%
2520
16th to endDay
21%6%25%5%35%5%6%7%30%
2520
July1st to 15thDay
12%5%245%35%5%7%7%39%
2520
16th to endDay
11%5%25%5%36%5%6%9%38%
2654
August1st to 15thDay
21%415%5%3445%6%36%
2427
16th to endDay
20%41442345%7%40%
2339
September1st to 15thDay
15%5%25%5%24447%41%
2160
16th to endDay
14%5%2443435%6%44%
1857
October1st to 15thDay
20%6%36%4345%46%33%
1650
16th to endDay
25%9%37%5%34445%26%
1677
November1st to 15thDay
19%7%35%5%3445%6%32%
1520
16th to endDay
24%11%47%5%35%335%24%
1440
December1st to 15thDay
21%14%48%7%446%45%18%
1440
16th to endDay
24%14%6%7%7%2433422%
1536
January1st to 15thNight
18%13%7%10%7%345%4419%
2506
16th to endNight
12%16%6%8%6%34346%26%
2634
February1st to 15thNight
13%12%5%11%7%44445%25%
2278
16th to endNight
17%11%48%7%34345%29%
1954
March1st to 15thNight
27%9%46%5%2332430%
2094
16th to endNight
23%5%35%42433340%
2038
April1st to 15thNight
22%8%34424335%36%
1796
16th to endNight
17%7%25%5%24346%39%
1725
May1st to 15thNight
25%6%24423446%35%
1440
16th to endNight
18%5%35%42345%7%38%
1495
June1st to 15thNight
20%5%36%5%4446%7%33%
1440
16th to endNight
18%6%245%245%5%8%36%
1440
July1st to 15thNight
10%3134445%5%9%46%
1440
16th to endNight
9%4245%26%46%7%46%
1514
August1st to 15thNight
14%425%324448%44%
1530
16th to endNight
16%31432435%5%49%
1851
September1st to 15thNight
14%5%25%43446%5%43%
1800
16th to endNight
16%424423345%49%
1825
October1st to 15thNight
19%8%25%424445%36%
1980
16th to endNight
24%9%36%6%34446%27%
2194
November1st to 15thNight
24%8%35%5%2434432%
2337
16th to endNight
26%10%47%5%33345%24%
2520
December1st to 15thNight
22%14%47%5%345%5%5%21%
2520
16th to endNight
24%14%5%7%7%24235%21%
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
65%5%9%12%6%
1433
16th to endDay
60%413%12%8%
1587
February1st to 15thDay
61%6%10%16%3
1577
16th to endDay
57%5%10%15%9%1
1518
March1st to 15thDay
58%5%11%16%7%
1866
16th to endDay
44%410%19%15%6%
2185
April1st to 15thDay
52%37%19%13%2
2152
16th to endDay
48%47%19%16%3
2244
May1st to 15thDay
50%5%7%20%13%2
2511
16th to endDay
47%7%11%20%12%2
2622
June1st to 15thDay
51%7%10%20%9%
2520
16th to endDay
48%6%12%20%11%1
2520
July1st to 15thDay
37%7%14%27%11%
2371
16th to endDay
40%8%16%24%9%
2487
August1st to 15thDay
42%8%16%21%10%
2413
16th to endDay
36%7%15%28%10%
2339
September1st to 15thDay
41%5%13%27%11%
2160
16th to endDay
39%6%12%32%9%
1857
October1st to 15thDay
50%6%13%20%9%
1650
16th to endDay
61%5%12%13%6%
1677
November1st to 15thDay
49%6%15%19%8%
1520
16th to endDay
62%5%11%12%6%1
1440
December1st to 15thDay
67%7%10%11%2
1440
16th to endDay
67%411%11%5%
1536
January1st to 15thNight
67%6%9%11%5%
2506
16th to endNight
59%410%16%8%
2634
February1st to 15thNight
61%5%10%16%5%
2287
16th to endNight
57%49%18%9%
1954
March1st to 15thNight
57%49%19%8%
2094
16th to endNight
46%310%19%16%4
2038
April1st to 15thNight
51%37%20%15%
1796
16th to endNight
46%5%7%19%17%3
1725
May1st to 15thNight
48%47%19%15%4
1440
16th to endNight
44%6%12%20%14%2
1497
June1st to 15thNight
48%6%12%21%10%1
1440
16th to endNight
43%7%12%20%15%
1440
July1st to 15thNight
33%6%13%30%15%
1360
16th to endNight
34%6%16%26%15%
1418
August1st to 15thNight
37%8%17%24%12%
1520
16th to endNight
32%6%15%32%12%
1851
September1st to 15thNight
41%6%11%28%11%1
1800
16th to endNight
37%411%35%10%
1825
October1st to 15thNight
47%5%11%22%12%
1980
16th to endNight
59%5%11%16%6%
2194
November1st to 15thNight
51%5%13%17%10%
2337
16th to endNight
61%5%11%15%4
2520
December1st to 15thNight
64%7%12%12%2
2520
16th to endNight
68%49%12%5%
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
28%23%26%16%31
60
16th to endDay
30%17%30%20%1
73
February1st to 15thDay
30%25%31%13%
90
16th to endDay
32%22%23%16%5%
177
March1st to 15thDay
26%13%31%22%5%
270
16th to endDay
22%16%29%19%9%2
288
April1st to 15thDay
28%18%26%17%7%1
270
16th to endDay
23%17%26%18%11%4
270
May1st to 15thDay
27%10%29%20%7%4
288
16th to endDay
18%13%33%21%11%1
331
June1st to 15thDay
24%9%31%21%10%2
315
16th to endDay
16%10%33%24%13%2
315
July1st to 15thDay
14%5%29%30%14%5%
305
16th to endDay
13%329%28%19%5%
326
August1st to 15thDay
12%6%25%30%17%7%
313
16th to endDay
12%426%28%15%12%
332
September1st to 15thDay
19%7%22%17%21%12%
315
16th to endDay
15%7%26%22%16%10%
262
October1st to 15thDay
22%9%26%22%15%2
255
16th to endDay
29%18%24%20%7%
257
November1st to 15thDay
18%13%30%22%12%2
174
16th to endDay
31%10%36%18%3
60
December1st to 15thDay
21%18%41%15%11
60
16th to endDay
43%17%34%4
64
January1st to 15thNight
25%16%32%20%5%
720
16th to endNight
22%12%33%25%5%
768
February1st to 15thNight
20%14%37%21%4
704
16th to endNight
24%15%31%22%5%
631
March1st to 15thNight
26%13%31%26%2
720
16th to endNight
20%15%27%26%10%
768
April1st to 15thNight
26%13%29%24%5%
718
16th to endNight
22%14%27%26%8%
645
May1st to 15thNight
25%7%25%30%10%1
360
16th to endNight
18%6%33%26%14%
375
June1st to 15thNight
20%7%28%33%10%
360
16th to endNight
20%327%34%13%
360
July1st to 15thNight
11%225%44%15%
340
16th to endNight
10%223%44%16%1
376
August1st to 15thNight
13%220%39%23%1
449
16th to endNight
10%5%23%35%23%1
705
September1st to 15thNight
16%5%25%29%19%2
720
16th to endNight
17%7%22%27%21%2
669
October1st to 15thNight
23%10%26%26%12%1
660
16th to endNight
30%12%23%25%7%
704
November1st to 15thNight
26%9%30%21%11%
700
16th to endNight
29%15%25%20%9%
720
December1st to 15thNight
31%13%29%21%3
720
16th to endNight
33%13%29%18%41
768
Page generated at 2017-09-19 07:32:29. Version ac04

Copyright 2017 A. Danko


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