H. J. Fowler Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the H. J. Fowler Observatory clear sky chart from 2005-11-05 to 2017-10-06.

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
29%
14869
16%
47091
8%
14837
39%
14837
19%
31877

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
24%
720
18%
2506
8%
716
37%
716
18%
1629
January16th to endNight
19%
768
17%
2688
5%
768
35%
768
15%
1789
February1st to 15thNight
21%
704
14%
2271
6%
701
33%
701
14%
1621
February16th to endNight
20%
631
17%
1893
5%
631
35%
631
18%
1330
March1st to 15thNight
19%
720
15%
2118
5%
720
32%
720
16%
1530
March16th to endNight
27%
768
19%
2088
7%
768
37%
768
23%
1412
April1st to 15thNight
22%
700
17%
1778
4%
700
30%
700
19%
1303
April16th to endNight
25%
540
16%
1620
5%
540
33%
540
17%
1143
May1st to 15thNight
27%
540
13%
1620
7%
540
34%
540
16%
1170
May16th to endNight
25%
417
11%
1539
8%
416
33%
416
13%
1101
June1st to 15thNight
31%
360
9%
1287
8%
360
37%
360
14%
934
June16th to endNight
41%
360
10%
1293
9%
360
47%
360
21%
862
July1st to 15thNight
43%
340
10%
1360
12%
340
48%
340
19%
868
July16th to endNight
47%
376
12%
1418
12%
354
53%
354
21%
882
August1st to 15thNight
45%
515
12%
1586
8%
513
49%
513
23%
1067
August16th to endNight
47%
615
21%
1761
12%
615
51%
615
32%
1079
September1st to 15thNight
42%
720
18%
1896
9%
720
47%
720
33%
1225
September16th to endNight
43%
717
27%
1980
16%
717
52%
717
34%
1195
October1st to 15thNight
36%
684
20%
1941
10%
684
45%
684
25%
1275
October16th to endNight
24%
704
15%
2271
7%
704
35%
704
18%
1553
November1st to 15thNight
32%
700
20%
2277
11%
700
45%
700
21%
1497
November16th to endNight
29%
733
19%
2416
10%
733
40%
733
18%
1599
December1st to 15thNight
21%
750
15%
2685
5%
750
34%
750
15%
1867
December16th to endNight
19%
787
16%
2799
5%
787
33%
787
14%
1946

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
18%7%25%5%3445%6%36%
48102
Night
18%7%35%5%34446%37%
47261
TransparencyDay
47%5%10%21%13%1
47790
Night
47%5%11%21%12%1
47091
SeeingDay
24%10%29%21%10%3
6247
Night
23%9%31%26%8%
14869

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%6%25%5%345%5%6%37%
1432
16th to endDay
16%10%25%5%245%46%37%
1536
February1st to 15thDay
15%11%36%6%25%445%33%
1589
16th to endDay
14%11%36%5%35%45%6%32%
1580
March1st to 15thDay
19%9%35%5%34445%34%
1832
16th to endDay
17%7%34423446%42%
2136
April1st to 15thDay
19%7%35%5%24346%36%
2170
16th to endDay
20%7%245%23446%37%
2340
May1st to 15thDay
25%7%35%5%24346%30%
2332
16th to endDay
22%6%25%5%245%5%6%33%
2584
June1st to 15thDay
26%7%35%435%45%6%28%
2670
16th to endDay
22%5%245%25%5%5%7%32%
2662
July1st to 15thDay
13%5%246%45%6%6%8%36%
2520
16th to endDay
14%5%35%6%46%5%5%8%33%
2654
August1st to 15thDay
14%6%25%5%345%6%9%36%
2350
16th to endDay
12%42442435%8%45%
2438
September1st to 15thDay
13%5%24425%445%45%
2064
16th to endDay
14%413323347%50%
1980
October1st to 15thDay
20%5%24425%437%40%
1816
16th to endDay
21%6%35%5%23445%36%
1600
November1st to 15thDay
20%5%23425%447%37%
1581
16th to endDay
21%6%25%5%22235%42%
1542
December1st to 15thDay
24%8%25%333345%34%
1272
16th to endDay
23%11%26%424435%30%
1422
January1st to 15thNight
15%9%35%434446%37%
2506
16th to endNight
14%9%47%5%3445%6%36%
2688
February1st to 15thNight
12%9%5%7%7%36%5%5%5%31%
2270
16th to endNight
16%10%45%5%34445%37%
1893
March1st to 15thNight
19%10%45%5%2443434%
2105
16th to endNight
17%7%34424345%42%
2088
April1st to 15thNight
19%8%36%5%24445%34%
1778
16th to endNight
24%6%345%3435%5%34%
1620
May1st to 15thNight
22%8%345%3445%6%31%
1620
16th to endNight
22%9%25%5%3445%7%28%
1537
June1st to 15thNight
24%9%36%43435%7%27%
1287
16th to endNight
20%6%25%5%2445%8%36%
1293
July1st to 15thNight
15%6%35%6%3445%10%35%
1440
16th to endNight
14%426%5%35%46%9%38%
1514
August1st to 15thNight
13%6%25%5%36%6%5%6%38%
1596
16th to endNight
12%425%42335%7%48%
1761
September1st to 15thNight
15%5%23424346%47%
1896
16th to endNight
14%314313346%51%
1980
October1st to 15thNight
19%5%15%334446%41%
1941
16th to endNight
22%6%25%5%23345%36%
2271
November1st to 15thNight
19%6%15%5%24446%39%
2277
16th to endNight
18%6%25%5%33446%39%
2416
December1st to 15thNight
22%9%34433435%34%
2685
16th to endNight
22%10%34424346%32%
2799

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
46%5%11%19%13%2
1433
16th to endDay
46%412%22%12%1
1536
February1st to 15thDay
52%410%19%11%1
1592
16th to endDay
51%5%10%16%12%3
1580
March1st to 15thDay
52%311%17%12%2
1842
16th to endDay
43%49%18%18%4
2136
April1st to 15thDay
48%47%19%16%3
2170
16th to endDay
47%5%8%20%13%3
2340
May1st to 15thDay
53%5%7%18%12%2
2332
16th to endDay
50%6%11%18%11%1
2588
June1st to 15thDay
56%5%9%18%8%1
2670
16th to endDay
49%7%10%18%12%
2662
July1st to 15thDay
40%9%12%25%10%1
2371
16th to endDay
46%7%12%21%10%1
2487
August1st to 15thDay
42%8%13%26%9%
2336
16th to endDay
35%5%13%24%18%1
2438
September1st to 15thDay
39%410%26%17%1
2064
16th to endDay
34%38%30%22%1
1980
October1st to 15thDay
44%410%24%14%1
1816
16th to endDay
49%410%23%11%
1600
November1st to 15thDay
46%5%12%20%14%1
1581
16th to endDay
46%313%20%14%1
1542
December1st to 15thDay
51%5%12%20%9%
1272
16th to endDay
56%49%17%11%1
1422
January1st to 15thNight
46%413%23%11%1
2506
16th to endNight
48%5%9%20%14%1
2688
February1st to 15thNight
52%5%11%18%10%1
2271
16th to endNight
49%5%9%19%13%2
1893
March1st to 15thNight
54%38%18%12%2
2118
16th to endNight
44%310%17%18%4
2088
April1st to 15thNight
51%47%17%16%2
1778
16th to endNight
51%5%8%16%14%3
1620
May1st to 15thNight
52%6%7%18%14%1
1620
16th to endNight
54%6%11%17%9%1
1539
June1st to 15thNight
56%412%17%7%1
1287
16th to endNight
45%6%13%21%11%1
1293
July1st to 15thNight
43%7%14%22%11%
1360
16th to endNight
41%7%17%23%9%
1418
August1st to 15thNight
43%8%14%23%10%
1586
16th to endNight
33%7%14%28%15%1
1761
September1st to 15thNight
37%5%10%30%14%1
1896
16th to endNight
33%48%33%17%1
1980
October1st to 15thNight
42%5%11%25%14%1
1941
16th to endNight
49%411%23%10%
2271
November1st to 15thNight
45%412%21%14%1
2277
16th to endNight
45%5%12%21%13%1
2416
December1st to 15thNight
52%48%23%10%
2685
16th to endNight
53%49%20%12%
2799

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
21%15%31%23%8%
60
16th to endDay
26%15%35%17%4
64
February1st to 15thDay
28%12%33%19%5%
222
16th to endDay
20%16%33%23%6%
239
March1st to 15thDay
29%15%32%16%5%
270
16th to endDay
25%16%29%15%7%4
288
April1st to 15thDay
30%13%31%14%8%2
270
16th to endDay
26%18%30%14%7%2
270
May1st to 15thDay
38%9%27%15%7%2
288
16th to endDay
30%9%34%18%6%
331
June1st to 15thDay
28%10%27%20%12%
465
16th to endDay
21%8%28%29%9%1
457
July1st to 15thDay
17%7%31%26%13%3
305
16th to endDay
14%434%25%14%5%
326
August1st to 15thDay
15%8%27%29%16%2
313
16th to endDay
15%9%25%25%17%7%
332
September1st to 15thDay
19%6%29%26%13%5%
315
16th to endDay
17%8%25%20%15%12%
314
October1st to 15thDay
25%11%19%22%16%4
267
16th to endDay
27%11%30%20%8%1
240
November1st to 15thDay
23%8%30%23%11%2
235
16th to endDay
30%12%24%22%9%1
240
December1st to 15thDay
30%6%43%11%8%
72
16th to endDay
39%15%31%14%
64
January1st to 15thNight
20%12%35%27%3
720
16th to endNight
20%15%36%23%3
768
February1st to 15thNight
20%14%32%28%3
704
16th to endNight
21%13%37%22%4
631
March1st to 15thNight
28%12%35%20%3
720
16th to endNight
21%15%29%25%6%
768
April1st to 15thNight
26%14%32%21%4
700
16th to endNight
28%11%30%23%5%
540
May1st to 15thNight
27%10%29%25%7%
540
16th to endNight
25%8%33%27%4
417
June1st to 15thNight
26%6%27%30%8%
360
16th to endNight
20%327%39%9%
360
July1st to 15thNight
19%228%34%14%
340
16th to endNight
15%225%39%16%
376
August1st to 15thNight
13%328%38%14%
515
16th to endNight
15%326%34%18%1
615
September1st to 15thNight
18%330%31%14%1
720
16th to endNight
17%5%30%27%17%2
717
October1st to 15thNight
23%7%27%27%13%1
684
16th to endNight
27%10%31%23%6%
704
November1st to 15thNight
23%8%30%27%9%1
700
16th to endNight
23%12%29%24%9%
733
December1st to 15thNight
29%10%34%21%4
750
16th to endNight
30%11%33%20%3
787
Page generated at 2017-10-07 14:07:53. Version ac04

Copyright 2017 A. Danko


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