Driftway Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Driftway Observatory 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
27%
14839
17%
46445
8%
14806
38%
14806
17%
31096

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
14%
720
18%
2506
5%
716
29%
716
13%
1647
January16th to endNight
12%
768
17%
2688
4%
768
30%
768
11%
1763
February1st to 15thNight
13%
704
16%
2150
3%
701
28%
701
13%
1471
February16th to endNight
16%
631
23%
1893
5%
631
37%
631
19%
1241
March1st to 15thNight
20%
720
16%
2147
6%
720
31%
720
14%
1472
March16th to endNight
22%
768
21%
2046
7%
768
34%
768
20%
1399
April1st to 15thNight
22%
718
17%
1796
5%
718
29%
718
20%
1286
April16th to endNight
26%
593
16%
1673
4%
593
34%
593
19%
1180
May1st to 15thNight
26%
540
14%
1620
6%
540
33%
540
15%
1173
May16th to endNight
28%
457
10%
1580
6%
457
34%
457
12%
1137
June1st to 15thNight
35%
360
12%
1440
13%
360
41%
360
14%
976
June16th to endNight
46%
360
11%
1440
12%
360
52%
360
18%
892
July1st to 15thNight
44%
340
11%
1360
14%
340
47%
340
17%
863
July16th to endNight
46%
378
14%
1420
16%
356
54%
356
17%
913
August1st to 15thNight
53%
528
16%
1575
11%
524
56%
524
26%
998
August16th to endNight
56%
665
18%
1715
10%
665
59%
665
30%
1131
September1st to 15thNight
46%
660
20%
1717
11%
660
50%
660
29%
1141
September16th to endNight
44%
657
22%
1815
13%
657
51%
657
26%
1173
October1st to 15thNight
30%
660
21%
1820
11%
660
41%
660
22%
1236
October16th to endNight
24%
704
16%
2288
7%
704
37%
704
18%
1484
November1st to 15thNight
25%
700
17%
2277
9%
700
37%
700
16%
1508
November16th to endNight
22%
720
19%
2340
8%
720
36%
720
16%
1523
December1st to 15thNight
18%
720
16%
2451
6%
720
31%
720
11%
1649
December16th to endNight
12%
768
18%
2688
4%
768
29%
768
10%
1840

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%6%24424447%40%
47530
Night
18%7%25%424446%39%
46617
Cloud CoverDay
17%6%2442445%7%40%
47540
Night
17%7%25%42445%6%38%
46622
TransparencyDay
42%6%12%21%14%2
47220
Night
45%5%12%20%13%2
46445
SeeingDay
19%16%26%24%11%2
5832
Night
19%23%24%22%8%
14840

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%7%16%424446%41%
1432
16th to endDay
16%6%25%5%23346%43%
1536
February1st to 15thDay
18%6%25%4345%46%38%
1710
16th to endDay
17%7%24434445%41%
1580
March1st to 15thDay
20%6%24433347%38%
1802
16th to endDay
16%8%24423346%42%
2176
April1st to 15thDay
20%5%24424447%38%
2152
16th to endDay
18%5%14424446%40%
2286
May1st to 15thDay
26%7%345%24446%33%
2332
16th to endDay
17%5%34435%448%37%
2545
June1st to 15thDay
23%6%25%42445%7%33%
2520
16th to endDay
13%6%2442446%9%41%
2520
July1st to 15thDay
14%6%25%5%2445%8%39%
2520
16th to endDay
14%5%244345%7%8%39%
2639
August1st to 15thDay
13%4244345%6%8%42%
2288
16th to endDay
12%424434448%46%
2132
September1st to 15thDay
14%5%2442435%5%46%
1911
16th to endDay
15%414424346%47%
1815
October1st to 15thDay
20%5%24413335%44%
1807
16th to endDay
15%4245%23446%45%
1584
November1st to 15thDay
22%6%23323245%41%
1581
16th to endDay
17%5%25%33345%8%40%
1620
December1st to 15thDay
20%7%25%423236%42%
1506
16th to endDay
23%9%24324345%35%
1536
January1st to 15thNight
18%7%24424446%39%
2506
16th to endNight
18%10%35%323336%39%
2688
February1st to 15thNight
16%8%36%5%34346%38%
2149
16th to endNight
15%8%25%5%25%345%42%
1893
March1st to 15thNight
18%6%35%4245%47%37%
2134
16th to endNight
16%7%35%434345%42%
2046
April1st to 15thNight
17%8%25%4345%46%37%
1796
16th to endNight
22%5%35%6%2334439%
1673
May1st to 15thNight
22%8%45%424435%33%
1620
16th to endNight
19%8%36%5%345%5%7%30%
1578
June1st to 15thNight
23%7%35%414437%34%
1440
16th to endNight
18%5%243245%5%7%39%
1440
July1st to 15thNight
17%7%35%3245%48%37%
1440
16th to endNight
14%6%26%6%36%5%6%8%34%
1516
August1st to 15thNight
9%5%25%5%35%6%7%7%41%
1587
16th to endNight
13%5%25%33445%7%43%
1715
September1st to 15thNight
15%425%42435%7%43%
1717
16th to endNight
17%5%1432444447%
1815
October1st to 15thNight
17%6%25%434336%41%
1820
16th to endNight
17%5%25%433446%41%
2288
November1st to 15thNight
18%6%24324345%41%
2277
16th to endNight
17%6%35%42235%7%40%
2340
December1st to 15thNight
21%5%245%24445%39%
2451
16th to endNight
21%7%35%423446%35%
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
17%8%34334446%40%
1432
16th to endDay
16%6%35%43335%6%42%
1536
February1st to 15thDay
18%6%2442445%8%36%
1710
16th to endDay
17%7%2442445%6%40%
1580
March1st to 15thDay
20%5%345%2435%7%38%
1812
16th to endDay
16%7%24423445%42%
2176
April1st to 15thDay
19%6%2332445%6%40%
2152
16th to endDay
17%5%24435%447%40%
2286
May1st to 15thDay
25%7%35%423346%33%
2332
16th to endDay
16%7%345%3445%8%35%
2545
June1st to 15thDay
21%7%344145%5%8%33%
2520
16th to endDay
13%6%243245%6%9%41%
2520
July1st to 15thDay
13%6%35%435%45%8%40%
2520
16th to endDay
13%5%35%435%46%9%38%
2639
August1st to 15thDay
12%41443446%8%44%
2288
16th to endDay
10%5%25%42345%8%46%
2132
September1st to 15thDay
13%5%2332445%7%47%
1911
16th to endDay
14%5%13423446%47%
1815
October1st to 15thDay
19%6%2442344444%
1807
16th to endDay
14%415%5%23447%45%
1584
November1st to 15thDay
21%6%24314245%42%
1581
16th to endDay
16%6%25%425%448%38%
1620
December1st to 15thDay
20%7%25%413336%39%
1506
16th to endDay
23%9%24324346%34%
1536
January1st to 15thNight
18%7%25%424447%37%
2506
16th to endNight
18%10%34323346%37%
2688
February1st to 15thNight
16%8%35%5%2445%6%37%
2142
16th to endNight
16%7%15%5%24347%40%
1893
March1st to 15thNight
19%6%35%42435%8%36%
2147
16th to endNight
16%7%25%424436%42%
2045
April1st to 15thNight
17%9%25%42445%6%37%
1796
16th to endNight
21%6%26%433435%37%
1673
May1st to 15thNight
20%9%46%5%33346%32%
1620
16th to endNight
19%10%35%5%245%47%30%
1578
June1st to 15thNight
24%9%25%22335%7%33%
1440
16th to endNight
17%6%243245%5%8%38%
1440
July1st to 15thNight
16%6%35%426%36%8%37%
1440
16th to endNight
12%6%45%5%46%5%6%9%33%
1516
August1st to 15thNight
9%5%345%35%6%6%8%41%
1587
16th to endNight
13%4245%25%35%7%44%
1715
September1st to 15thNight
14%6%15%42446%7%42%
1717
16th to endNight
15%6%2443334446%
1815
October1st to 15thNight
18%6%25%424445%40%
1820
16th to endNight
17%5%35%433347%40%
2288
November1st to 15thNight
19%6%34423445%41%
2277
16th to endNight
18%6%3442345%8%38%
2340
December1st to 15thNight
21%6%345%24445%38%
2451
16th to endNight
21%8%26%42345%5%34%
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
44%6%13%22%10%2
1433
16th to endDay
43%411%26%13%1
1536
February1st to 15thDay
45%5%15%20%12%1
1713
16th to endDay
42%5%12%23%13%2
1580
March1st to 15thDay
45%6%13%22%9%3
1812
16th to endDay
43%412%20%15%4
2176
April1st to 15thDay
43%5%10%19%17%2
2152
16th to endDay
42%5%10%21%15%3
2286
May1st to 15thDay
52%5%9%17%13%2
2332
16th to endDay
45%6%13%21%10%1
2549
June1st to 15thDay
49%7%11%19%11%
2520
16th to endDay
38%7%14%23%15%
2520
July1st to 15thDay
40%8%12%21%15%1
2371
16th to endDay
39%8%14%22%13%2
2473
August1st to 15thDay
36%7%15%22%16%1
2275
16th to endDay
34%8%10%26%17%1
2132
September1st to 15thDay
35%7%12%21%21%
1911
16th to endDay
36%6%8%24%21%2
1815
October1st to 15thDay
42%411%22%16%2
1807
16th to endDay
38%5%13%23%15%2
1584
November1st to 15thDay
46%5%12%18%15%2
1581
16th to endDay
44%5%12%23%11%2
1620
December1st to 15thDay
45%416%18%13%
1506
16th to endDay
51%5%14%18%10%
1536
January1st to 15thNight
46%5%13%21%12%1
2506
16th to endNight
48%410%21%13%1
2688
February1st to 15thNight
47%5%13%21%10%1
2150
16th to endNight
43%312%22%15%2
1893
March1st to 15thNight
46%6%12%19%12%3
2147
16th to endNight
43%5%9%17%19%4
2046
April1st to 15thNight
47%410%18%15%2
1796
16th to endNight
49%48%18%14%4
1673
May1st to 15thNight
52%5%9%16%13%2
1620
16th to endNight
52%7%13%16%8%2
1580
June1st to 15thNight
50%6%11%17%13%
1440
16th to endNight
41%6%13%21%14%1
1440
July1st to 15thNight
44%6%12%22%13%
1360
16th to endNight
44%8%14%22%8%2
1420
August1st to 15thNight
36%8%14%24%14%1
1575
16th to endNight
37%6%11%27%15%1
1715
September1st to 15thNight
39%5%13%23%16%
1717
16th to endNight
40%46%22%23%2
1815
October1st to 15thNight
44%5%9%22%15%2
1820
16th to endNight
43%5%15%21%12%1
2288
November1st to 15thNight
45%5%12%19%14%2
2277
16th to endNight
44%5%12%22%11%2
2340
December1st to 15thNight
47%5%15%18%12%
2451
16th to endNight
50%5%13%19%10%
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
16%25%36%21%
60
16th to endDay
18%21%35%21%1
64
February1st to 15thDay
23%24%36%13%1
258
16th to endDay
13%25%37%20%3
239
March1st to 15thDay
24%26%26%18%3
270
16th to endDay
17%25%32%18%5%
288
April1st to 15thDay
20%19%30%23%5%
270
16th to endDay
20%18%29%22%8%1
270
May1st to 15thDay
30%18%25%15%7%1
288
16th to endDay
20%12%27%28%10%
331
June1st to 15thDay
22%13%24%24%14%
315
16th to endDay
15%14%26%31%10%1
315
July1st to 15thDay
18%8%18%35%17%1
305
16th to endDay
12%12%18%37%16%2
326
August1st to 15thDay
13%7%18%37%20%2
297
16th to endDay
12%7%13%32%24%8%
268
September1st to 15thDay
16%7%20%30%20%5%
255
16th to endDay
20%8%25%23%12%9%
254
October1st to 15thDay
20%14%22%21%18%2
254
16th to endDay
13%20%29%28%7%
240
November1st to 15thDay
23%20%26%14%11%2
235
16th to endDay
19%27%27%16%8%
240
December1st to 15thDay
17%23%34%18%6%
126
16th to endDay
34%21%31%9%3
64
January1st to 15thNight
17%33%30%15%2
720
16th to endNight
23%33%26%14%2
768
February1st to 15thNight
17%32%31%16%1
704
16th to endNight
18%28%32%17%2
631
March1st to 15thNight
21%28%26%20%3
720
16th to endNight
16%26%29%20%5%
768
April1st to 15thNight
20%22%30%22%3
718
16th to endNight
18%21%27%24%7%
593
May1st to 15thNight
20%20%26%22%8%1
540
16th to endNight
24%20%19%25%10%
458
June1st to 15thNight
22%20%15%28%12%
360
16th to endNight
18%17%13%38%12%
360
July1st to 15thNight
18%12%16%34%17%
340
16th to endNight
14%16%14%35%17%
378
August1st to 15thNight
11%10%17%41%17%
528
16th to endNight
14%10%12%33%24%4
665
September1st to 15thNight
13%11%21%32%17%2
660
16th to endNight
19%12%17%25%20%4
657
October1st to 15thNight
21%16%23%24%11%1
660
16th to endNight
18%22%30%20%7%
704
November1st to 15thNight
19%27%23%18%9%1
700
16th to endNight
17%32%23%18%6%
720
December1st to 15thNight
23%31%23%16%5%
720
16th to endNight
24%33%27%13%1
768
Page generated at 2017-08-13 14:16:57. Version ac04

Copyright 2017 A. Danko


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