Walgrove Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Walgrove Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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%
15209
20%
46952
10%
15168
45%
15168
23%
29710

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
19%
718
21%
2449
7%
716
37%
716
16%
1555
January16th to endNight
16%
768
22%
2496
6%
768
36%
768
19%
1548
February1st to 15thNight
16%
704
17%
2287
4%
701
33%
701
18%
1432
February16th to endNight
15%
631
23%
1910
4%
631
37%
631
18%
1220
March1st to 15thNight
23%
720
20%
2023
8%
720
36%
720
21%
1332
March16th to endNight
19%
768
20%
2076
7%
768
31%
768
22%
1396
April1st to 15thNight
21%
718
19%
1796
4%
718
30%
718
25%
1213
April16th to endNight
33%
720
18%
1800
5%
720
39%
720
28%
1215
May1st to 15thNight
33%
534
17%
1614
9%
534
41%
534
21%
1094
May16th to endNight
47%
374
16%
1497
13%
374
54%
374
19%
918
June1st to 15thNight
49%
360
18%
1440
17%
360
58%
360
23%
824
June16th to endNight
56%
360
16%
1440
17%
360
64%
360
25%
831
July1st to 15thNight
55%
340
11%
1360
13%
340
57%
340
17%
878
July16th to endNight
66%
509
15%
1540
13%
476
72%
476
33%
883
August1st to 15thNight
61%
539
17%
1609
12%
536
63%
536
30%
1009
August16th to endNight
60%
761
20%
1907
12%
761
62%
761
34%
1242
September1st to 15thNight
53%
720
24%
1800
13%
720
58%
720
34%
1167
September16th to endNight
53%
693
24%
1895
19%
693
56%
693
30%
1241
October1st to 15thNight
41%
660
22%
1980
18%
660
50%
660
25%
1271
October16th to endNight
33%
704
27%
2112
14%
704
50%
704
28%
1245
November1st to 15thNight
33%
700
24%
2247
13%
700
49%
700
24%
1367
November16th to endNight
25%
720
20%
2466
9%
720
41%
720
18%
1495
December1st to 15thNight
21%
720
17%
2520
6%
720
36%
720
15%
1634
December16th to endNight
18%
768
20%
2688
5%
768
36%
768
17%
1700

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%25%5%3445%7%40%
47968
Night
14%5%2442445%7%43%
47134
Cloud CoverDay
14%5%25%5%345%5%7%40%
47978
Night
14%5%2442445%7%43%
47138
TransparencyDay
41%6%12%21%15%2
47668
Night
39%6%12%22%16%2
46952
SeeingDay
18%12%27%23%14%4
5703
Night
17%13%29%26%11%1
15212

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
15%6%345%35%5%47%39%
1486
16th to endDay
14%6%35%4335%46%40%
1728
February1st to 15thDay
13%6%35%5%4446%7%38%
1575
16th to endDay
13%7%25%5%35%347%40%
1562
March1st to 15thDay
15%6%345%23436%43%
1924
16th to endDay
15%7%35%5%33445%39%
2148
April1st to 15thDay
15%5%25%5%3435%7%41%
2152
16th to endDay
17%5%25%424436%43%
2160
May1st to 15thDay
18%5%24435%45%8%35%
2343
16th to endDay
15%5%25%6%3446%8%37%
2618
June1st to 15thDay
11%5%25%5%35%5%6%10%38%
2520
16th to endDay
10%4245%35%5%6%8%43%
2520
July1st to 15thDay
9%425%6%45%5%7%9%38%
2520
16th to endDay
6%425%5%5%6%7%8%10%36%
2519
August1st to 15thDay
10%425%7%36%6%6%8%37%
2331
16th to endDay
11%5%245%346%6%8%41%
2292
September1st to 15thDay
13%5%245%235%5%7%43%
2160
16th to endDay
15%6%246%235%48%39%
1923
October1st to 15thDay
18%5%24323447%43%
1650
16th to endDay
15%5%245%23446%45%
1760
November1st to 15thDay
15%6%2432436%5%46%
1610
16th to endDay
17%6%24423446%43%
1491
December1st to 15thDay
16%5%25%43445%7%39%
1440
16th to endDay
17%7%25%42435%8%38%
1536
January1st to 15thNight
16%6%25%42445%7%40%
2449
16th to endNight
16%6%2343445%5%43%
2496
February1st to 15thNight
14%7%35%42345%6%43%
2285
16th to endNight
13%5%27%5%34446%43%
1910
March1st to 15thNight
15%5%2332445%6%43%
2011
16th to endNight
14%6%26%5%24446%42%
2076
April1st to 15thNight
12%5%345%2445%8%42%
1796
16th to endNight
16%5%2432336%7%43%
1800
May1st to 15thNight
19%7%24324446%39%
1614
16th to endNight
12%6%245%345%47%42%
1495
June1st to 15thNight
10%4244345%5%10%44%
1440
16th to endNight
9%324435%46%12%42%
1440
July1st to 15thNight
9%5%27%5%47%6%6%10%34%
1440
16th to endNight
5%323436%45%13%46%
1647
August1st to 15thNight
9%5%25%5%2446%8%44%
1620
16th to endNight
11%4244245%5%8%45%
1907
September1st to 15thNight
11%5%25%32345%7%47%
1800
16th to endNight
13%6%25%5%24347%45%
1895
October1st to 15thNight
18%5%23333345%44%
1980
16th to endNight
14%412412446%51%
2112
November1st to 15thNight
12%6%25%42333450%
2247
16th to endNight
16%5%2442445%6%43%
2466
December1st to 15thNight
17%5%25%435%5%5%6%38%
2520
16th to endNight
16%6%25%42445%7%40%
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
15%6%34435%447%39%
1486
16th to endDay
14%7%35%5%245%45%41%
1728
February1st to 15thDay
13%7%35%5%3446%6%38%
1575
16th to endDay
13%7%25%5%34346%40%
1562
March1st to 15thDay
14%7%345%23437%42%
1935
16th to endDay
16%6%36%6%33446%39%
2147
April1st to 15thDay
14%5%25%5%3336%8%41%
2152
16th to endDay
17%5%24424445%43%
2160
May1st to 15thDay
18%5%25%435%46%8%36%
2343
16th to endDay
15%4346%3446%8%37%
2618
June1st to 15thDay
11%5%25%5%35%5%6%10%38%
2520
16th to endDay
10%42444446%8%43%
2520
July1st to 15thDay
9%425%6%36%6%7%8%39%
2520
16th to endDay
7%415%5%5%7%8%8%10%37%
2519
August1st to 15thDay
11%426%6%35%7%6%7%37%
2331
16th to endDay
11%4245%346%6%8%41%
2292
September1st to 15thDay
13%425%5%2445%7%43%
2160
16th to endDay
16%5%25%5%23448%39%
1923
October1st to 15thDay
17%5%14323447%44%
1650
16th to endDay
15%5%244235%45%46%
1760
November1st to 15thDay
14%5%2332345%6%46%
1610
16th to endDay
16%6%24423346%43%
1491
December1st to 15thDay
16%426%4345%5%7%39%
1440
16th to endDay
16%7%25%424348%38%
1536
January1st to 15thNight
17%5%25%42446%7%40%
2449
16th to endNight
16%6%2342445%6%42%
2496
February1st to 15thNight
14%6%245%2445%6%43%
2278
16th to endNight
13%6%35%5%24446%43%
1910
March1st to 15thNight
15%6%2443445%6%43%
2023
16th to endNight
14%6%25%5%24436%42%
2076
April1st to 15thNight
12%6%345%3445%7%42%
1796
16th to endNight
16%5%2442435%7%43%
1800
May1st to 15thNight
19%7%24324446%39%
1613
16th to endNight
12%6%245%34447%42%
1495
June1st to 15thNight
10%414425%5%6%9%44%
1440
16th to endNight
9%324345%5%6%12%42%
1440
July1st to 15thNight
9%5%26%6%47%6%6%9%35%
1440
16th to endNight
6%223426%5%6%13%46%
1647
August1st to 15thNight
9%5%25%4335%6%8%44%
1620
16th to endNight
11%425%4245%5%9%45%
1907
September1st to 15thNight
12%5%25%323448%47%
1800
16th to endNight
13%6%25%5%24346%44%
1895
October1st to 15thNight
19%5%2332335%5%44%
1980
16th to endNight
14%313312446%51%
2112
November1st to 15thNight
12%6%35%423335%50%
2247
16th to endNight
16%424425%5%5%6%43%
2466
December1st to 15thNight
17%5%25%5%25%45%5%38%
2520
16th to endNight
16%6%2442345%7%40%
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
43%6%13%20%11%3
1487
16th to endDay
43%5%12%23%11%3
1728
February1st to 15thDay
43%5%11%25%11%2
1577
16th to endDay
43%413%24%11%2
1562
March1st to 15thDay
41%5%12%22%15%2
1935
16th to endDay
46%5%10%17%17%2
2148
April1st to 15thDay
41%5%11%21%17%3
2152
16th to endDay
41%5%8%18%20%5%
2160
May1st to 15thDay
44%7%11%19%14%1
2343
16th to endDay
42%8%13%20%12%1
2622
June1st to 15thDay
39%8%13%23%13%
2520
16th to endDay
37%8%14%20%17%1
2520
July1st to 15thDay
38%10%14%20%14%2
2371
16th to endDay
36%11%16%22%12%1
2363
August1st to 15thDay
41%8%12%19%15%1
2318
16th to endDay
37%8%13%22%15%2
2292
September1st to 15thDay
38%6%11%19%20%2
2160
16th to endDay
42%5%9%20%19%2
1923
October1st to 15thDay
39%6%12%22%17%2
1650
16th to endDay
38%410%26%18%1
1760
November1st to 15thDay
37%5%12%21%18%5%
1610
16th to endDay
41%411%25%14%2
1491
December1st to 15thDay
42%5%15%23%10%2
1440
16th to endDay
44%6%12%22%13%
1536
January1st to 15thNight
42%6%11%20%15%3
2449
16th to endNight
40%412%26%14%1
2496
February1st to 15thNight
41%413%22%14%2
2287
16th to endNight
40%5%10%24%17%2
1910
March1st to 15thNight
40%6%11%26%13%2
2023
16th to endNight
42%5%12%20%16%3
2076
April1st to 15thNight
39%5%12%20%18%3
1796
16th to endNight
39%5%10%19%18%6%
1800
May1st to 15thNight
45%6%10%21%14%2
1614
16th to endNight
38%9%14%18%14%3
1497
June1st to 15thNight
34%8%12%27%15%
1440
16th to endNight
33%8%14%24%16%2
1440
July1st to 15thNight
42%10%15%16%11%4
1360
16th to endNight
27%11%18%23%17%2
1540
August1st to 15thNight
35%8%14%22%18%1
1609
16th to endNight
34%7%12%23%18%3
1907
September1st to 15thNight
36%5%11%21%21%3
1800
16th to endNight
38%6%9%19%22%3
1895
October1st to 15thNight
41%410%22%17%2
1980
16th to endNight
33%410%28%20%1
2112
November1st to 15thNight
37%47%23%19%6%
2247
16th to endNight
39%5%11%23%17%3
2466
December1st to 15thNight
45%6%11%22%12%1
2520
16th to endNight
42%5%15%21%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
27%37%21%11%1
69
16th to endDay
20%34%28%15%1
96
February1st to 15thDay
25%32%35%6%
90
16th to endDay
15%28%40%14%
221
March1st to 15thDay
20%22%31%22%3
270
16th to endDay
22%17%37%16%42
288
April1st to 15thDay
18%17%35%18%8%1
270
16th to endDay
20%11%33%17%13%3
270
May1st to 15thDay
28%18%25%15%9%3
288
16th to endDay
19%6%26%25%19%3
331
June1st to 15thDay
14%9%28%25%18%3
315
16th to endDay
16%7%30%29%11%3
315
July1st to 15thDay
13%425%32%18%4
305
16th to endDay
7%322%32%26%7%
326
August1st to 15thDay
14%419%30%21%9%
313
16th to endDay
12%16%32%23%14%
332
September1st to 15thDay
16%8%19%21%20%13%
315
16th to endDay
21%316%28%18%11%
287
October1st to 15thDay
23%8%18%28%16%4
255
16th to endDay
15%16%30%26%10%1
272
November1st to 15thDay
17%15%28%18%18%2
240
16th to endDay
24%18%33%14%9%
111
December1st to 15thDay
21%28%28%13%5%3
60
16th to endDay
28%17%39%12%3
64
January1st to 15thNight
21%21%32%20%3
720
16th to endNight
20%21%39%17%1
768
February1st to 15thNight
19%20%37%20%1
704
16th to endNight
19%24%36%17%1
631
March1st to 15thNight
20%18%34%22%3
720
16th to endNight
18%17%41%16%5%
768
April1st to 15thNight
16%22%37%19%4
718
16th to endNight
19%13%30%25%10%
720
May1st to 15thNight
24%8%28%27%9%
534
16th to endNight
17%7%22%33%18%
375
June1st to 15thNight
11%330%33%21%
360
16th to endNight
11%123%47%15%
360
July1st to 15thNight
11%219%42%23%
340
16th to endNight
6%117%41%28%3
509
August1st to 15thNight
13%115%41%25%2
539
16th to endNight
13%217%35%27%3
761
September1st to 15thNight
14%421%32%25%2
720
16th to endNight
16%321%29%24%4
693
October1st to 15thNight
21%9%22%26%17%3
660
16th to endNight
18%15%29%25%10%
704
November1st to 15thNight
15%15%30%26%10%1
700
16th to endNight
19%20%30%21%8%
720
December1st to 15thNight
22%15%34%18%7%
720
16th to endNight
21%18%37%20%2
768
Page generated at 2017-09-25 09:02:18. Version ac04

Copyright 2017 A. Danko


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