William D. McDowell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the William D. McDowell Observatory clear sky chart from 2005-11-05 to 2018-01-12.

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
34%
15494
20%
48352
10%
15463
45%
15463
23%
30951

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
25%
765
20%
2662
9%
761
40%
761
19%
1662
January16th to endNight
20%
768
21%
2592
7%
768
38%
768
19%
1626
February1st to 15thNight
21%
704
18%
2225
6%
701
36%
701
19%
1464
February16th to endNight
22%
631
27%
1893
10%
631
43%
631
23%
1196
March1st to 15thNight
23%
720
19%
2151
7%
720
38%
720
22%
1417
March16th to endNight
25%
768
23%
1993
8%
768
36%
768
25%
1327
April1st to 15thNight
26%
718
17%
1796
4%
718
32%
718
26%
1240
April16th to endNight
30%
708
18%
1788
5%
708
37%
708
26%
1231
May1st to 15thNight
29%
501
18%
1581
8%
501
39%
501
19%
1067
May16th to endNight
43%
374
15%
1497
14%
374
50%
374
20%
923
June1st to 15thNight
41%
360
15%
1440
12%
360
51%
360
18%
915
June16th to endNight
58%
360
15%
1440
17%
360
64%
360
26%
822
July1st to 15thNight
51%
340
13%
1360
14%
340
55%
340
19%
828
July16th to endNight
57%
376
16%
1418
17%
354
64%
354
25%
824
August1st to 15thNight
55%
560
18%
1631
12%
558
58%
558
30%
1018
August16th to endNight
57%
764
19%
1910
10%
764
59%
764
32%
1287
September1st to 15thNight
48%
720
23%
1800
13%
720
51%
720
32%
1189
September16th to endNight
50%
717
25%
1971
19%
717
55%
717
32%
1240
October1st to 15thNight
40%
720
20%
2106
14%
720
47%
720
26%
1350
October16th to endNight
31%
768
24%
2397
13%
768
46%
768
26%
1481
November1st to 15thNight
34%
760
22%
2472
11%
760
48%
760
24%
1534
November16th to endNight
29%
780
22%
2587
11%
780
44%
780
22%
1605
December1st to 15thNight
27%
780
16%
2730
7%
780
39%
780
17%
1809
December16th to endNight
20%
832
19%
2912
5%
832
38%
832
18%
1896

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
16%5%245%2445%7%41%
48988
Night
16%5%2442445%6%43%
48522
Cloud CoverDay
15%5%2442445%7%41%
48997
Night
16%6%24424446%43%
48528
TransparencyDay
41%6%12%21%15%2
48676
Night
41%5%11%22%16%2
48352
SeeingDay
19%11%28%23%13%4
6128
Night
19%9%31%28%10%
15495

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
14%6%245%35%35%5%43%
1520
16th to endDay
16%6%24423446%42%
1629
February1st to 15thDay
16%6%25%5%34346%41%
1634
16th to endDay
15%6%25%43445%6%41%
1580
March1st to 15thDay
16%6%25%6%25%45%7%38%
1796
16th to endDay
17%6%25%423335%42%
2229
April1st to 15thDay
19%6%24323347%41%
2152
16th to endDay
16%425%433347%43%
2172
May1st to 15thDay
22%6%2442445%7%35%
2379
16th to endDay
18%42443445%8%37%
2618
June1st to 15thDay
19%5%244245%6%8%36%
2520
16th to endDay
11%5%25%5%35%6%6%8%39%
2520
July1st to 15thDay
11%4246%45%6%6%9%36%
2520
16th to endDay
11%5%25%5%36%6%6%9%37%
2654
August1st to 15thDay
12%425%6%35%6%6%8%39%
2304
16th to endDay
13%4145%2445%7%43%
2292
September1st to 15thDay
14%5%13425%446%46%
2160
16th to endDay
14%5%25%5%235%46%44%
1989
October1st to 15thDay
17%5%24434346%42%
1851
16th to endDay
19%414423336%46%
1827
November1st to 15thDay
19%5%24323435%45%
1716
16th to endDay
16%5%23423346%47%
1702
December1st to 15thDay
18%7%25%424336%41%
1560
16th to endDay
20%7%3442335%6%38%
1664
January1st to 15thNight
17%5%23433445%43%
2662
16th to endNight
16%8%24424346%42%
2592
February1st to 15thNight
14%8%35%434345%41%
2224
16th to endNight
16%5%24423435%46%
1893
March1st to 15thNight
19%6%24414346%42%
2138
16th to endNight
17%7%24323445%44%
1993
April1st to 15thNight
18%6%24424447%40%
1796
16th to endNight
18%6%24324346%43%
1788
May1st to 15thNight
18%6%36%433446%39%
1581
16th to endNight
19%5%2442435%7%40%
1495
June1st to 15thNight
20%42432445%7%39%
1440
16th to endNight
9%3245%25%5%6%8%45%
1440
July1st to 15thNight
12%425%5%245%8%8%40%
1440
16th to endNight
9%4145%25%6%8%8%40%
1514
August1st to 15thNight
10%3245%245%6%8%46%
1641
16th to endNight
12%425%5%3445%8%43%
1910
September1st to 15thNight
15%424324346%47%
1800
16th to endNight
13%5%24423336%49%
1971
October1st to 15thNight
16%6%25%423345%44%
2106
16th to endNight
17%5%24322345%47%
2397
November1st to 15thNight
16%6%24323435%47%
2472
16th to endNight
17%5%24423446%43%
2587
December1st to 15thNight
20%6%25%434346%37%
2730
16th to endNight
18%6%34424446%40%
2912

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
14%6%23435%35%5%42%
1520
16th to endDay
15%6%24434346%43%
1629
February1st to 15thDay
15%6%35%5%34346%41%
1634
16th to endDay
15%5%35%32445%7%41%
1580
March1st to 15thDay
17%7%25%5%25%35%7%38%
1806
16th to endDay
17%5%34423445%43%
2228
April1st to 15thDay
19%6%34323447%41%
2152
16th to endDay
16%425%433347%43%
2172
May1st to 15thDay
22%5%25%5%24447%35%
2379
16th to endDay
17%5%344345%5%9%37%
2618
June1st to 15thDay
18%5%3442445%8%36%
2520
16th to endDay
11%5%25%5%35%6%6%8%40%
2520
July1st to 15thDay
10%425%6%36%6%7%9%36%
2520
16th to endDay
10%5%25%5%36%6%6%8%37%
2654
August1st to 15thDay
11%5%25%6%35%6%6%8%39%
2304
16th to endDay
12%4145%25%45%7%43%
2292
September1st to 15thDay
13%5%24434447%46%
2160
16th to endDay
14%5%245%33446%44%
1989
October1st to 15thDay
17%5%25%433347%41%
1851
16th to endDay
18%414423346%45%
1827
November1st to 15thDay
19%5%23413435%45%
1716
16th to endDay
16%42342435%6%46%
1702
December1st to 15thDay
17%7%25%424346%41%
1560
16th to endDay
20%8%24423346%38%
1664
January1st to 15thNight
17%6%23424345%43%
2662
16th to endNight
16%7%34424446%42%
2592
February1st to 15thNight
13%8%35%434345%41%
2217
16th to endNight
16%5%24433445%46%
1893
March1st to 15thNight
18%7%24324345%42%
2151
16th to endNight
16%7%24323445%44%
1993
April1st to 15thNight
18%6%25%41445%7%39%
1796
16th to endNight
18%5%24324437%42%
1788
May1st to 15thNight
18%6%35%6%23437%39%
1581
16th to endNight
18%5%2443335%8%39%
1495
June1st to 15thNight
18%6%3342446%6%39%
1440
16th to endNight
8%424435%5%6%10%44%
1440
July1st to 15thNight
11%425%5%35%5%7%9%39%
1440
16th to endNight
8%5%145%36%6%7%9%40%
1514
August1st to 15thNight
10%325%5%245%6%9%45%
1641
16th to endNight
12%415%5%35%45%8%42%
1910
September1st to 15thNight
15%414424447%46%
1800
16th to endNight
13%5%14423436%48%
1971
October1st to 15thNight
17%5%25%323346%44%
2106
16th to endNight
16%5%25%322345%48%
2397
November1st to 15thNight
16%6%2442333447%
2472
16th to endNight
17%5%24424446%43%
2587
December1st to 15thNight
20%6%25%424446%37%
2730
16th to endNight
18%6%24424446%40%
2912

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
41%414%23%12%3
1521
16th to endDay
41%412%24%13%2
1629
February1st to 15thDay
44%412%22%13%2
1637
16th to endDay
41%5%11%26%14%1
1580
March1st to 15thDay
44%415%22%10%2
1806
16th to endDay
43%410%19%18%3
2229
April1st to 15thDay
42%410%22%15%4
2152
16th to endDay
40%49%20%20%3
2172
May1st to 15thDay
48%410%18%16%1
2379
16th to endDay
43%6%14%20%13%1
2622
June1st to 15thDay
44%7%13%21%12%
2520
16th to endDay
39%8%13%23%13%1
2520
July1st to 15thDay
40%9%11%20%16%1
2371
16th to endDay
37%8%15%22%12%2
2487
August1st to 15thDay
38%9%15%18%17%
2290
16th to endDay
37%6%11%22%18%2
2292
September1st to 15thDay
36%7%11%21%22%1
2160
16th to endDay
39%49%20%22%2
1989
October1st to 15thDay
41%412%20%17%1
1851
16th to endDay
39%410%26%17%2
1827
November1st to 15thDay
42%411%20%19%2
1716
16th to endDay
38%311%27%15%2
1702
December1st to 15thDay
43%415%20%13%1
1560
16th to endDay
46%5%14%20%12%
1664
January1st to 15thNight
42%411%22%14%3
2662
16th to endNight
43%310%24%15%2
2592
February1st to 15thNight
44%5%12%21%14%2
2225
16th to endNight
40%48%25%18%3
1893
March1st to 15thNight
43%412%21%13%3
2151
16th to endNight
41%5%10%20%17%4
1993
April1st to 15thNight
43%5%11%21%15%2
1796
16th to endNight
42%5%8%19%21%2
1788
May1st to 15thNight
46%6%9%17%17%2
1581
16th to endNight
42%5%16%18%14%2
1497
June1st to 15thNight
44%6%12%18%16%1
1440
16th to endNight
33%8%14%26%15%1
1440
July1st to 15thNight
38%10%12%22%14%1
1360
16th to endNight
33%9%17%22%12%3
1418
August1st to 15thNight
34%9%13%25%16%2
1631
16th to endNight
37%7%12%21%18%1
1910
September1st to 15thNight
37%6%10%25%19%1
1800
16th to endNight
36%5%6%23%26%1
1971
October1st to 15thNight
40%411%23%16%2
2106
16th to endNight
38%411%25%17%1
2397
November1st to 15thNight
40%49%21%19%4
2472
16th to endNight
41%410%26%14%2
2587
December1st to 15thNight
47%5%13%20%11%1
2730
16th to endNight
45%5%12%22%13%1
2912

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
17%26%32%18%31
82
16th to endDay
16%18%40%17%6%
79
February1st to 15thDay
21%22%30%22%2
150
16th to endDay
17%17%41%18%5%
239
March1st to 15thDay
19%20%31%24%5%
270
16th to endDay
21%16%35%17%7%1
288
April1st to 15thDay
22%11%35%17%10%2
270
16th to endDay
21%10%35%18%10%3
270
May1st to 15thDay
32%14%23%18%7%3
288
16th to endDay
22%6%27%25%15%2
331
June1st to 15thDay
18%11%27%26%13%3
315
16th to endDay
15%9%30%26%14%1
315
July1st to 15thDay
12%5%27%33%16%4
305
16th to endDay
15%5%21%30%23%4
326
August1st to 15thDay
15%322%31%19%7%
313
16th to endDay
14%220%29%23%9%
332
September1st to 15thDay
16%7%27%22%16%9%
315
16th to endDay
18%5%22%23%15%14%
314
October1st to 15thDay
22%423%25%19%4
315
16th to endDay
18%13%30%25%8%1
313
November1st to 15thDay
21%11%26%24%13%2
280
16th to endDay
15%19%32%21%9%1
232
December1st to 15thDay
22%27%30%11%8%
90
16th to endDay
33%21%37%6%1
96
January1st to 15thNight
19%14%36%24%4
765
16th to endNight
22%15%37%22%2
768
February1st to 15thNight
19%14%38%25%2
704
16th to endNight
19%15%38%23%2
631
March1st to 15thNight
22%12%37%20%5%
720
16th to endNight
19%14%35%23%5%
768
April1st to 15thNight
21%12%37%23%5%
718
16th to endNight
20%11%33%25%7%
708
May1st to 15thNight
21%9%33%26%8%
501
16th to endNight
20%426%35%12%
375
June1st to 15thNight
20%7%25%33%13%
360
16th to endNight
10%222%49%15%
360
July1st to 15thNight
11%426%40%16%
340
16th to endNight
11%220%43%20%1
376
August1st to 15thNight
14%119%43%20%
560
16th to endNight
15%116%38%25%2
764
September1st to 15thNight
16%425%34%18%1
720
16th to endNight
16%221%32%22%4
717
October1st to 15thNight
19%7%24%30%14%2
720
16th to endNight
20%10%33%25%9%
768
November1st to 15thNight
19%10%29%26%12%1
760
16th to endNight
19%12%33%24%8%1
780
December1st to 15thNight
24%11%31%25%6%1
780
16th to endNight
22%12%39%22%2
832
Page generated at 2018-01-12 22:01:27. Version ac04

Copyright 2018 A. Danko


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