Rowan Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rowan Astronomical Observatory clear sky chart from 2005-11-05 to 2017-10-31.

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%
15312
20%
47385
10%
15279
45%
15279
24%
29952

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%
719
21%
2506
9%
716
40%
716
18%
1582
January16th to endNight
19%
768
22%
2523
6%
768
39%
768
21%
1573
February1st to 15thNight
17%
704
18%
2248
4%
701
33%
701
19%
1433
February16th to endNight
19%
631
25%
1893
8%
631
41%
631
21%
1181
March1st to 15thNight
23%
720
20%
2103
8%
720
37%
720
23%
1324
March16th to endNight
21%
768
22%
2025
8%
768
32%
768
25%
1348
April1st to 15thNight
22%
718
18%
1796
4%
718
29%
718
26%
1217
April16th to endNight
28%
720
19%
1800
5%
720
35%
720
28%
1203
May1st to 15thNight
29%
531
17%
1611
7%
531
38%
531
20%
1103
May16th to endNight
44%
374
15%
1497
14%
374
49%
374
18%
947
June1st to 15thNight
44%
360
16%
1440
13%
360
54%
360
21%
870
June16th to endNight
55%
360
17%
1440
18%
360
62%
360
26%
823
July1st to 15thNight
52%
340
12%
1360
14%
340
56%
340
19%
867
July16th to endNight
57%
406
16%
1446
13%
382
63%
382
26%
835
August1st to 15thNight
62%
596
18%
1666
13%
593
64%
593
33%
1044
August16th to endNight
58%
764
18%
1910
10%
764
59%
764
32%
1263
September1st to 15thNight
50%
720
24%
1800
12%
720
55%
720
34%
1177
September16th to endNight
52%
717
26%
1944
20%
717
56%
717
34%
1214
October1st to 15thNight
44%
720
21%
2148
16%
720
52%
720
27%
1349
October16th to endNight
33%
768
28%
2344
15%
768
49%
768
29%
1377
November1st to 15thNight
33%
700
23%
2277
13%
700
48%
700
25%
1377
November16th to endNight
29%
720
20%
2400
11%
720
43%
720
20%
1512
December1st to 15thNight
24%
720
17%
2520
8%
720
37%
720
17%
1629
December16th to endNight
20%
768
21%
2688
6%
768
39%
768
17%
1704

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
14%5%2442445%7%41%
48368
Night
14%5%2442445%6%43%
47558
Cloud CoverDay
13%5%25%5%3445%7%41%
48377
Night
14%6%2442445%7%43%
47564
TransparencyDay
40%6%11%22%16%2
48059
Night
40%5%11%22%17%2
47385
SeeingDay
18%11%27%24%13%4
5948
Night
17%11%31%28%10%
15314

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%2445%5%41%
1432
16th to endDay
15%6%34413446%44%
1698
February1st to 15thDay
15%6%25%42445%5%41%
1612
16th to endDay
13%7%46%5%35%5%5%5%39%
1580
March1st to 15thDay
15%5%26%425%36%6%41%
1845
16th to endDay
18%5%235%23446%41%
2199
April1st to 15thDay
16%6%3342345%6%43%
2152
16th to endDay
14%5%245%235%5%7%43%
2160
May1st to 15thDay
16%6%25%5%35%5%5%7%36%
2349
16th to endDay
14%5%25%43445%7%41%
2618
June1st to 15thDay
12%5%25%5%35%6%7%8%37%
2520
16th to endDay
10%325%436%5%7%9%39%
2520
July1st to 15thDay
8%5%25%6%36%5%7%9%38%
2520
16th to endDay
7%5%25%5%47%6%7%11%37%
2621
August1st to 15thDay
10%425%5%35%5%5%8%40%
2271
16th to endDay
11%5%245%25%5%6%10%41%
2292
September1st to 15thDay
12%425%434447%46%
2160
16th to endDay
13%5%25%5%3445%8%41%
2016
October1st to 15thDay
17%424324446%44%
1809
16th to endDay
16%5%23323336%48%
1878
November1st to 15thDay
16%6%23423346%46%
1581
16th to endDay
16%424424346%46%
1559
December1st to 15thDay
16%5%25%42445%8%40%
1440
16th to endDay
20%5%2442435%7%38%
1536
January1st to 15thNight
17%5%2443445%5%41%
2506
16th to endNight
16%6%24423446%42%
2523
February1st to 15thNight
14%7%35%42335%6%41%
2247
16th to endNight
15%5%24324446%45%
1893
March1st to 15thNight
17%5%1441335%6%45%
2090
16th to endNight
16%7%25%42333445%
2025
April1st to 15thNight
14%6%25%42445%6%43%
1796
16th to endNight
16%5%235%23447%43%
1800
May1st to 15thNight
17%7%35%5%3445%5%38%
1611
16th to endNight
14%5%25%5%35%5%5%7%38%
1495
June1st to 15thNight
13%5%2443335%9%41%
1440
16th to endNight
7%5%34435%46%11%42%
1440
July1st to 15thNight
9%5%35%5%46%6%6%9%37%
1440
16th to endNight
7%4246%35%5%6%10%43%
1544
August1st to 15thNight
8%5%35%425%5%6%7%46%
1677
16th to endNight
11%5%25%5%25%5%48%43%
1910
September1st to 15thNight
12%425%324447%48%
1800
16th to endNight
12%6%24424446%48%
1944
October1st to 15thNight
17%5%1442345%6%45%
2148
16th to endNight
15%413323346%51%
2344
November1st to 15thNight
14%6%34413335%49%
2277
16th to endNight
16%425%43446%6%40%
2400
December1st to 15thNight
18%6%35%4245%5%6%38%
2520
16th to endNight
17%6%244245%5%6%39%
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
14%6%34435%445%42%
1432
16th to endDay
14%7%34424445%45%
1698
February1st to 15thDay
15%7%25%4245%5%6%41%
1612
16th to endDay
13%7%36%4245%46%40%
1580
March1st to 15thDay
15%5%25%5%25%445%42%
1855
16th to endDay
17%6%2442435%7%41%
2198
April1st to 15thDay
15%6%24424346%43%
2152
16th to endDay
14%4244245%5%6%43%
2160
May1st to 15thDay
15%6%35%5%35%5%5%7%36%
2349
16th to endDay
14%415%5%2446%8%41%
2618
June1st to 15thDay
12%6%245%35%6%7%8%37%
2520
16th to endDay
9%425%5%35%6%8%8%39%
2520
July1st to 15thDay
8%5%26%6%46%5%6%8%39%
2520
16th to endDay
7%425%6%46%6%8%10%37%
2621
August1st to 15thDay
11%5%26%5%345%6%9%39%
2271
16th to endDay
11%5%24435%5%6%10%40%
2292
September1st to 15thDay
12%5%244345%47%46%
2160
16th to endDay
13%5%26%5%45%346%42%
2016
October1st to 15thDay
16%6%2342345%6%44%
1809
16th to endDay
16%5%14323336%49%
1878
November1st to 15thDay
17%6%13322446%47%
1581
16th to endDay
15%415%424346%46%
1559
December1st to 15thDay
16%5%3442445%6%41%
1440
16th to endDay
19%6%235%2435%7%38%
1536
January1st to 15thNight
17%5%25%4245%46%41%
2506
16th to endNight
16%6%2442445%6%43%
2523
February1st to 15thNight
14%8%25%32445%6%41%
2240
16th to endNight
15%6%23324446%46%
1893
March1st to 15thNight
17%5%14324347%44%
2103
16th to endNight
17%6%345%33335%44%
2025
April1st to 15thNight
14%6%25%43335%7%43%
1796
16th to endNight
15%6%25%42335%7%43%
1800
May1st to 15thNight
16%7%36%5%34446%37%
1611
16th to endNight
13%6%35%5%25%5%6%6%39%
1495
June1st to 15thNight
13%5%2444436%9%41%
1440
16th to endNight
7%5%25%5%245%6%10%43%
1440
July1st to 15thNight
8%5%26%5%37%6%7%9%37%
1440
16th to endNight
6%425%6%25%6%6%12%40%
1544
August1st to 15thNight
8%5%25%435%5%5%9%45%
1677
16th to endNight
10%5%15%4346%5%8%43%
1910
September1st to 15thNight
12%5%14424447%47%
1800
16th to endNight
12%6%24414448%47%
1944
October1st to 15thNight
16%5%2442435%6%45%
2148
16th to endNight
14%423323346%50%
2344
November1st to 15thNight
14%5%24323345%49%
2277
16th to endNight
15%5%245%35%5%6%6%40%
2400
December1st to 15thNight
18%5%35%42445%6%38%
2520
16th to endNight
17%6%35%42445%6%39%
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
42%5%12%21%13%4
1433
16th to endDay
39%410%26%15%2
1698
February1st to 15thDay
42%5%11%24%12%2
1615
16th to endDay
45%310%25%12%2
1580
March1st to 15thDay
42%5%12%24%13%2
1855
16th to endDay
42%5%9%22%18%2
2199
April1st to 15thDay
41%49%23%15%5%
2152
16th to endDay
38%6%10%18%21%4
2160
May1st to 15thDay
45%6%10%19%15%2
2349
16th to endDay
40%7%14%23%13%1
2622
June1st to 15thDay
41%9%12%24%11%1
2520
16th to endDay
37%9%13%22%15%1
2520
July1st to 15thDay
40%7%12%21%14%2
2371
16th to endDay
36%9%14%25%12%1
2456
August1st to 15thDay
39%7%12%21%16%2
2258
16th to endDay
36%8%13%18%18%3
2292
September1st to 15thDay
37%5%11%21%22%2
2160
16th to endDay
42%5%7%19%23%1
2016
October1st to 15thDay
40%5%11%22%18%1
1809
16th to endDay
37%48%27%19%1
1878
November1st to 15thDay
39%410%24%18%2
1581
16th to endDay
39%39%27%16%2
1559
December1st to 15thDay
41%5%15%23%11%1
1440
16th to endDay
45%414%20%13%1
1536
January1st to 15thNight
43%411%21%15%3
2506
16th to endNight
41%410%25%15%1
2523
February1st to 15thNight
42%412%23%14%2
2248
16th to endNight
39%49%23%18%3
1893
March1st to 15thNight
39%313%23%16%2
2103
16th to endNight
43%310%20%18%3
2025
April1st to 15thNight
41%410%21%17%4
1796
16th to endNight
40%5%9%18%22%4
1800
May1st to 15thNight
46%6%8%18%18%2
1611
16th to endNight
43%8%14%15%15%2
1497
June1st to 15thNight
39%7%12%21%17%
1440
16th to endNight
35%6%13%26%15%2
1440
July1st to 15thNight
41%9%14%18%13%2
1360
16th to endNight
32%9%16%25%14%1
1446
August1st to 15thNight
33%8%14%22%20%1
1666
16th to endNight
37%7%12%21%19%2
1910
September1st to 15thNight
35%6%10%23%21%2
1800
16th to endNight
35%48%22%25%2
1944
October1st to 15thNight
38%5%12%22%17%3
2148
16th to endNight
34%49%29%19%2
2344
November1st to 15thNight
38%410%20%18%7%
2277
16th to endNight
42%5%10%23%15%2
2400
December1st to 15thNight
45%5%12%22%13%1
2520
16th to endNight
43%5%13%22%12%1
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
20%33%35%5%5%1
60
16th to endDay
27%29%28%14%
91
February1st to 15thDay
23%33%28%14%
128
16th to endDay
18%22%38%17%2
239
March1st to 15thDay
18%24%30%20%5%
270
16th to endDay
19%21%33%17%5%3
288
April1st to 15thDay
24%15%34%15%8%2
270
16th to endDay
20%11%32%20%12%2
270
May1st to 15thDay
30%17%23%15%10%3
288
16th to endDay
19%6%22%30%16%4
331
June1st to 15thDay
15%11%26%28%13%4
315
16th to endDay
14%6%30%28%15%3
315
July1st to 15thDay
15%324%35%16%4
305
16th to endDay
9%221%34%24%7%
326
August1st to 15thDay
13%419%32%21%8%
313
16th to endDay
12%21%29%23%12%
332
September1st to 15thDay
13%9%23%23%19%10%
315
16th to endDay
18%5%24%25%14%12%
314
October1st to 15thDay
23%423%26%16%5%
315
16th to endDay
16%13%32%22%13%1
325
November1st to 15thDay
18%13%28%23%13%2
235
16th to endDay
16%18%34%18%10%
179
December1st to 15thDay
21%25%30%11%11%
60
16th to endDay
32%21%32%12%
64
January1st to 15thNight
19%17%35%23%3
720
16th to endNight
20%15%40%21%1
768
February1st to 15thNight
19%17%41%20%
704
16th to endNight
19%21%37%20%2
631
March1st to 15thNight
20%18%35%21%4
720
16th to endNight
20%13%41%17%7%
768
April1st to 15thNight
18%18%38%21%4
718
16th to endNight
18%12%35%27%6%
720
May1st to 15thNight
20%9%36%26%7%
531
16th to endNight
16%426%39%12%1
375
June1st to 15thNight
17%426%39%11%
360
16th to endNight
7%327%48%12%
360
July1st to 15thNight
11%324%43%15%
340
16th to endNight
8%222%40%23%2
406
August1st to 15thNight
11%116%46%21%1
596
16th to endNight
14%218%36%26%2
764
September1st to 15thNight
15%423%37%18%1
720
16th to endNight
16%320%32%22%4
717
October1st to 15thNight
21%7%22%29%15%3
720
16th to endNight
19%12%32%24%10%
768
November1st to 15thNight
17%12%32%25%12%1
700
16th to endNight
19%14%31%24%9%
720
December1st to 15thNight
21%13%33%21%9%
720
16th to endNight
20%18%34%23%3
768
Page generated at 2017-11-01 17:23:42. Version ac04

Copyright 2017 A. Danko


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