Quarry Road Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Quarry Road Observatory clear sky chart from 2005-11-06 to 2024-01-22.

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
26%
22751
12%
74836
7%
22708
32%
22708
13%
55325

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 10thNight
13%
760
7%
2649
4%
757
17%
757
6%
2148
January11th to 20thNight
14%
760
7%
2660
3%
760
20%
760
6%
2172
January21st to endNight
17%
800
10%
2798
5%
800
24%
800
9%
2127
February1st to 10thNight
13%
720
8%
2362
2%
720
20%
720
7%
1911
February11th to 20thNight
14%
703
10%
2120
3%
700
23%
700
9%
1677
February21st to endNight
17%
592
13%
1776
5%
592
27%
592
11%
1365
March1st to 10thNight
19%
720
12%
2160
7%
720
26%
720
12%
1685
March11th to 20thNight
18%
720
13%
2104
5%
720
27%
720
12%
1607
March21st to endNight
21%
792
18%
1980
6%
792
29%
792
17%
1466
April1st to 10thNight
21%
720
14%
1800
4%
720
27%
720
15%
1395
April11th to 20thNight
23%
711
12%
1775
4%
711
27%
711
17%
1351
April21st to endNight
22%
577
15%
1657
5%
577
29%
577
15%
1234
May1st to 10thNight
24%
504
12%
1584
8%
504
29%
504
14%
1153
May11th to 20thNight
29%
354
17%
1416
14%
354
40%
354
16%
933
May21st to endNight
37%
388
13%
1551
12%
388
46%
388
18%
1007
June1st to 10thNight
33%
360
11%
1440
10%
360
42%
360
14%
995
June11th to 20thNight
41%
360
11%
1440
13%
360
48%
360
17%
942
June21st to endNight
42%
360
9%
1440
12%
360
47%
360
16%
970
July1st to 10thNight
50%
350
13%
1400
16%
350
56%
350
22%
857
July11th to 20thNight
51%
348
13%
1392
17%
348
55%
348
20%
861
July21st to endNight
53%
394
15%
1486
18%
372
56%
372
22%
903
August1st to 10thNight
55%
358
17%
1422
18%
356
60%
356
22%
853
August11th to 20thNight
47%
607
15%
1671
10%
607
49%
607
26%
1137
August21st to endNight
48%
792
17%
1980
8%
792
51%
792
28%
1348
September1st to 10thNight
44%
720
13%
1800
7%
720
46%
720
29%
1225
September11th to 20thNight
46%
720
21%
1903
13%
720
50%
720
32%
1204
September21st to endNight
40%
720
20%
1980
14%
720
44%
720
26%
1324
October1st to 10thNight
32%
720
14%
2048
9%
720
38%
720
17%
1513
October11th to 20thNight
29%
720
12%
2201
9%
720
35%
720
15%
1625
October21st to endNight
18%
792
10%
2574
4%
792
26%
792
10%
2020
November1st to 10thNight
20%
740
13%
2407
7%
740
28%
740
13%
1776
November11th to 20thNight
21%
760
16%
2470
10%
760
29%
760
13%
1810
November21st to endNight
18%
760
10%
2631
6%
760
24%
760
10%
1999
December1st to 10thNight
12%
760
5%
2660
2%
760
16%
760
5%
2241
December11th to 20thNight
17%
760
8%
3253
3%
759
22%
759
6%
2539
December21st to endNight
11%
829
5%
4846
1%
817
17%
817
4%
3952

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%10%47%6%35%5%5%7%32%
74845
Night
19%11%47%6%3445%6%30%
75080
TransparencyDay
52%7%12%18%9%1
74550
Night
55%7%12%17%8%1
74909
SeeingDay
25%19%29%18%7%2
13646
Night
24%16%28%23%8%1
22759

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 10thDay
15%16%8%9%8%36%5%6%5%19%
1517
11th to 20thDay
18%13%5%10%8%36%436%24%
1520
21st to endDay
13%14%6%9%7%47%5%5%6%25%
1592
February1st to 10thDay
14%14%5%10%9%45%46%5%23%
1597
11th to 20thDay
13%14%6%11%7%45%446%25%
1727
21st to endDay
16%13%6%11%8%345%46%25%
1480
March1st to 10thDay
14%12%6%10%7%45%446%28%
1797
11th to 20thDay
18%14%6%8%5%2445%430%
1852
21st to endDay
18%13%5%8%5%34345%33%
2376
April1st to 10thDay
22%13%46%433446%31%
2160
11th to 20thDay
19%10%36%5%34446%36%
2134
21st to endDay
24%11%36%5%34446%29%
2301
May1st to 10thDay
24%9%37%5%345%45%32%
2389
11th to 20thDay
19%8%35%5%3445%7%38%
2478
21st to endDay
16%7%36%6%3445%8%37%
2725
June1st to 10thDay
21%7%36%6%45%5%5%7%29%
2520
11th to 20thDay
18%6%35%6%345%6%9%35%
2520
21st to endDay
19%8%36%6%45%6%6%7%30%
2520
July1st to 10thDay
14%6%25%6%35%5%6%9%39%
2520
11th to 20thDay
13%5%27%5%36%6%7%10%35%
2520
21st to endDay
15%6%246%36%6%6%9%36%
2744
August1st to 10thDay
13%6%26%5%46%5%7%10%37%
2505
11th to 20thDay
14%7%26%5%35%5%7%9%36%
2209
21st to endDay
14%5%36%5%45%6%5%8%40%
2376
September1st to 10thDay
15%5%245%345%6%8%44%
2160
11th to 20thDay
16%6%2433345%7%47%
2056
21st to endDay
17%6%36%5%245%5%7%41%
1980
October1st to 10thDay
24%9%37%5%34445%32%
1908
11th to 20thDay
18%9%46%6%3346%7%34%
1757
21st to endDay
28%11%35%6%44346%25%
1782
November1st to 10thDay
17%11%47%6%25%45%6%35%
1671
11th to 20thDay
19%13%47%5%45%35%5%28%
1710
21st to endDay
18%12%5%9%5%445%5%6%27%
1544
December1st to 10thDay
21%17%5%10%6%3445%6%19%
1520
11th to 20thDay
22%15%6%8%5%3435%6%23%
1906
21st to endDay
24%16%6%9%6%45%44417%
2772
January1st to 10thNight
16%17%6%11%7%45%445%19%
2649
11th to 20thNight
18%17%5%10%8%45%445%20%
2660
21st to endNight
12%14%7%11%7%46%45%5%26%
2798
February1st to 10thNight
15%15%6%11%8%5%5%445%21%
2362
11th to 20thNight
14%14%6%11%9%45%45%5%23%
2118
21st to endNight
15%14%6%10%7%3445%428%
1776
March1st to 10thNight
18%14%6%9%6%35%435%28%
2150
11th to 20thNight
17%14%6%9%6%34335%30%
2103
21st to endNight
18%12%5%7%424435%35%
1980
April1st to 10thNight
22%12%5%6%6%33445%31%
1800
11th to 20thNight
24%10%46%5%34346%31%
1775
21st to endNight
23%10%46%5%23446%32%
1657
May1st to 10thNight
24%10%45%5%3345%7%30%
1584
11th to 20thNight
19%8%45%5%35%46%6%36%
1415
21st to endNight
18%6%25%5%345%6%8%37%
1550
June1st to 10thNight
21%7%37%6%35%45%7%32%
1440
11th to 20thNight
21%6%345%245%48%38%
1440
21st to endNight
19%7%36%5%345%6%8%34%
1440
July1st to 10thNight
15%425%5%45%5%7%7%42%
1440
11th to 20thNight
13%6%345%35%6%7%10%40%
1440
21st to endNight
11%6%45%5%35%5%7%8%42%
1574
August1st to 10thNight
13%5%245%346%6%8%44%
1432
11th to 20thNight
13%6%35%6%45%46%9%38%
1671
21st to endNight
13%6%35%5%35%5%5%8%42%
1980
September1st to 10thNight
16%6%25%33435%7%45%
1800
11th to 20thNight
15%6%245%23346%50%
1903
21st to endNight
17%6%245%35%5%5%6%43%
1980
October1st to 10thNight
24%8%36%5%3435%5%33%
2048
11th to 20thNight
21%9%47%6%445%46%31%
2201
21st to endNight
26%12%46%6%35%435%25%
2574
November1st to 10thNight
19%11%47%6%3444432%
2407
11th to 20thNight
21%11%47%5%33446%31%
2470
21st to endNight
19%14%6%8%6%3446%6%25%
2631
December1st to 10thNight
21%18%8%10%6%25%335%17%
2660
11th to 20thNight
23%13%5%8%6%345%45%24%
3288
21st to endNight
25%16%6%11%7%3433418%
4884

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 10thDay
66%5%11%13%5%
1518
11th to 20thDay
63%7%11%11%7%1
1520
21st to endDay
59%6%14%12%8%1
1592
February1st to 10thDay
63%6%10%14%5%2
1597
11th to 20thDay
62%7%9%15%7%1
1736
21st to endDay
60%7%10%10%11%1
1480
March1st to 10thDay
58%8%11%13%9%2
1800
11th to 20thDay
58%5%12%12%12%2
1852
21st to endDay
55%5%9%16%12%3
2376
April1st to 10thDay
55%5%9%19%9%2
2160
11th to 20thDay
49%7%10%18%15%1
2134
21st to endDay
57%6%10%15%10%2
2301
May1st to 10thDay
55%6%7%18%12%2
2389
11th to 20thDay
47%6%10%19%16%2
2482
21st to endDay
46%9%14%20%10%1
2725
June1st to 10thDay
53%8%14%16%8%1
2520
11th to 20thDay
47%8%12%19%12%2
2520
21st to endDay
53%8%12%17%9%1
2520
July1st to 10thDay
41%8%14%23%13%
2450
11th to 20thDay
43%10%15%24%8%
2445
21st to endDay
43%11%16%21%9%
2591
August1st to 10thDay
42%10%17%23%8%
2491
11th to 20thDay
44%8%17%22%8%
2209
21st to endDay
41%10%13%24%11%1
2376
September1st to 10thDay
38%7%15%27%12%1
2160
11th to 20thDay
37%6%11%28%16%1
2056
21st to endDay
41%8%15%23%11%2
1980
October1st to 10thDay
56%6%9%19%10%1
1908
11th to 20thDay
49%8%16%20%6%
1757
21st to endDay
62%7%10%13%8%1
1782
November1st to 10thDay
51%7%14%19%9%
1671
11th to 20thDay
57%6%12%13%9%2
1710
21st to endDay
57%9%15%15%4
1544
December1st to 10thDay
66%8%12%10%4
1520
11th to 20thDay
62%6%13%14%41
1906
21st to endDay
68%7%10%11%4
2772
January1st to 10thNight
67%7%11%11%4
2649
11th to 20thNight
68%7%10%10%5%
2660
21st to endNight
59%7%11%13%9%1
2798
February1st to 10thNight
67%6%10%10%6%1
2362
11th to 20thNight
63%5%9%14%8%1
2120
21st to endNight
59%7%10%16%7%2
1776
March1st to 10thNight
61%6%9%14%9%1
2160
11th to 20thNight
58%6%9%16%9%2
2104
21st to endNight
52%6%10%17%12%3
1980
April1st to 10thNight
57%5%8%17%11%2
1800
11th to 20thNight
54%7%10%17%11%1
1775
21st to endNight
54%7%9%17%10%4
1657
May1st to 10thNight
54%6%8%16%14%2
1584
11th to 20thNight
48%6%10%18%16%2
1416
21st to endNight
44%8%15%19%13%1
1551
June1st to 10thNight
50%8%13%17%11%1
1440
11th to 20thNight
45%8%14%20%11%2
1440
21st to endNight
48%9%13%22%9%1
1440
July1st to 10thNight
39%9%17%22%13%1
1400
11th to 20thNight
38%11%16%25%10%
1392
21st to endNight
38%9%16%25%12%1
1486
August1st to 10thNight
37%9%15%28%10%
1422
11th to 20thNight
42%7%16%24%10%
1671
21st to endNight
40%8%16%24%12%
1980
September1st to 10thNight
39%8%15%27%10%1
1800
11th to 20thNight
36%5%14%29%15%
1903
21st to endNight
41%7%13%24%13%2
1980
October1st to 10thNight
53%7%11%19%10%1
2048
11th to 20thNight
54%7%14%18%6%
2201
21st to endNight
62%7%10%13%7%1
2574
November1st to 10thNight
55%7%11%16%8%2
2407
11th to 20thNight
55%7%12%16%8%2
2470
21st to endNight
59%7%15%14%5%
2631
December1st to 10thNight
71%7%10%10%2
2660
11th to 20thNight
61%9%11%11%7%
3288
21st to endNight
71%6%8%13%2
4884

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 10thDay
26%28%28%16%2
180
11th to 20thDay
28%28%30%12%11
180
21st to endDay
21%31%33%11%3
178
February1st to 10thDay
32%40%23%41
230
11th to 20thDay
28%31%28%10%2
395
21st to endDay
31%25%31%10%2
346
March1st to 10thDay
27%21%35%14%2
420
11th to 20thDay
28%30%26%13%21
420
21st to endDay
32%23%26%11%5%2
462
April1st to 10thDay
35%20%25%14%5%1
420
11th to 20thDay
34%22%24%13%6%1
412
21st to endDay
32%22%26%13%5%2
420
May1st to 10thDay
30%22%24%14%8%1
423
11th to 20thDay
23%17%36%18%5%1
447
21st to endDay
20%18%32%22%7%1
485
June1st to 10thDay
24%17%36%16%7%
450
11th to 20thDay
20%18%34%19%8%1
450
21st to endDay
23%17%32%21%7%1
450
July1st to 10thDay
17%12%31%27%11%2
445
11th to 20thDay
17%10%34%27%11%1
444
21st to endDay
16%12%38%24%7%1
488
August1st to 10thDay
14%12%32%29%12%2
446
11th to 20thDay
20%10%28%23%14%5%
439
21st to endDay
15%16%28%25%11%4
495
September1st to 10thDay
18%12%28%26%10%6%
450
11th to 20thDay
18%15%23%21%16%7%
450
21st to endDay
18%17%30%18%12%5%
450
October1st to 10thDay
29%15%24%16%12%4
450
11th to 20thDay
25%18%28%21%7%
428
21st to endDay
34%20%27%17%3
396
November1st to 10thDay
27%21%27%19%6%
365
11th to 20thDay
30%25%24%16%41
370
21st to endDay
30%23%29%17%1
204
December1st to 10thDay
29%26%29%13%3
180
11th to 20thDay
33%28%30%8%1
180
21st to endDay
33%26%32%8%2
198
January1st to 10thNight
24%24%34%16%2
760
11th to 20thNight
28%23%30%16%3
760
21st to endNight
17%23%35%22%3
800
February1st to 10thNight
24%22%35%18%1
720
11th to 20thNight
22%25%34%16%2
703
21st to endNight
24%23%32%19%2
592
March1st to 10thNight
25%20%32%21%2
720
11th to 20thNight
26%23%28%19%5%
720
21st to endNight
26%21%27%19%6%1
792
April1st to 10thNight
30%18%27%21%4
720
11th to 20thNight
29%19%26%18%7%1
711
21st to endNight
27%17%29%21%5%1
577
May1st to 10thNight
30%14%27%20%8%
504
11th to 20thNight
22%9%36%25%6%1
354
21st to endNight
20%9%31%31%9%1
389
June1st to 10thNight
24%9%30%26%11%
360
11th to 20thNight
22%6%26%33%12%1
360
21st to endNight
21%427%38%8%1
360
July1st to 10thNight
14%327%37%17%1
350
11th to 20thNight
14%423%40%17%2
348
21st to endNight
13%6%22%46%14%1
394
August1st to 10thNight
15%5%19%43%18%1
358
11th to 20thNight
17%5%22%34%19%2
607
21st to endNight
17%8%21%34%19%2
792
September1st to 10thNight
19%8%25%31%15%2
720
11th to 20thNight
19%8%23%28%18%4
720
21st to endNight
20%11%26%24%17%2
720
October1st to 10thNight
30%11%21%22%15%1
720
11th to 20thNight
25%17%24%25%9%1
720
21st to endNight
33%16%27%19%5%
792
November1st to 10thNight
26%18%30%18%7%
740
11th to 20thNight
26%21%29%17%6%1
760
21st to endNight
27%24%28%16%5%1
760
December1st to 10thNight
31%24%27%16%2
760
11th to 20thNight
26%23%28%20%3
760
21st to endNight
33%19%31%15%2
836
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