Brookings
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brookings clear sky chart from 2005-11-05 to 2018-04-23.

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
38%
17029
18%
49475
10%
16982
44%
16982
27%
31959

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
28%
975
15%
2790
7%
970
35%
970
21%
1946
January16th to endNight
27%
1040
14%
2766
6%
1040
33%
1040
18%
2049
February1st to 15thNight
29%
954
12%
2483
4%
950
34%
950
21%
1813
February16th to endNight
31%
757
17%
2123
5%
757
40%
757
24%
1433
March1st to 15thNight
34%
647
17%
2207
10%
647
42%
647
23%
1429
March16th to endNight
31%
624
14%
2288
11%
624
38%
624
17%
1546
April1st to 15thNight
34%
584
17%
1952
12%
584
43%
584
22%
1259
April16th to endNight
32%
558
18%
1673
13%
558
40%
558
22%
1144
May1st to 15thNight
35%
540
16%
1620
10%
540
41%
540
28%
1010
May16th to endNight
38%
563
14%
1532
5%
561
41%
561
29%
1005
June1st to 15thNight
47%
540
16%
1270
6%
540
50%
540
38%
823
June16th to endNight
58%
540
20%
1260
8%
540
61%
540
55%
722
July1st to 15thNight
63%
510
19%
1209
7%
510
66%
510
55%
685
July16th to endNight
67%
564
20%
1520
13%
531
69%
531
50%
787
August1st to 15thNight
65%
539
21%
1610
18%
536
69%
536
42%
878
August16th to endNight
59%
573
25%
1719
23%
573
65%
573
43%
925
September1st to 15thNight
56%
540
22%
1887
20%
540
65%
540
36%
985
September16th to endNight
44%
537
26%
1980
19%
537
57%
537
30%
1060
October1st to 15thNight
35%
673
26%
2113
16%
673
43%
673
31%
1222
October16th to endNight
34%
831
22%
2367
10%
831
42%
831
29%
1502
November1st to 15thNight
36%
950
22%
2522
10%
950
43%
950
30%
1641
November16th to endNight
34%
975
22%
2730
12%
975
41%
975
28%
1741
December1st to 15thNight
25%
975
9%
2734
4%
975
31%
975
18%
2016
December16th to endNight
24%
1040
14%
3120
6%
1040
32%
1040
15%
2338

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
12%7%35%5%3445%7%40%
50042
Night
10%7%35%5%2445%7%43%
49644
Cloud CoverDay
12%7%35%5%3445%7%40%
50052
Night
10%7%35%5%34447%44%
49647
Cloud CoverDay
12%7%35%5%3445%7%40%
50053
Night
10%7%35%5%3445%7%43%
49648
Cloud CoverDay
12%7%36%5%3445%7%39%
50051
Night
10%7%36%5%3445%7%43%
49648
TransparencyDay
42%6%14%23%11%1
49729
Night
39%6%14%24%13%1
49475
SeeingDay
15%8%32%27%13%2
6779
Night
13%8%34%32%10%
17030

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
6%9%5%10%9%47%5%6%7%26%
1475
16th to endDay
7%10%6%11%8%45%5%47%27%
1809
February1st to 15thDay
10%11%5%8%7%35%447%30%
1710
16th to endDay
8%8%49%7%46%6%5%6%32%
1649
March1st to 15thDay
14%10%6%8%5%2345%6%33%
2072
16th to endDay
15%12%48%6%34446%29%
2288
April1st to 15thDay
17%7%35%42345%7%38%
2323
16th to endDay
22%9%35%323435%34%
2401
May1st to 15thDay
19%7%246%34347%36%
2331
16th to endDay
19%6%25%5%35%35%8%35%
2581
June1st to 15thDay
17%5%245%25%45%7%39%
2688
16th to endDay
9%2143245%6%9%49%
2700
July1st to 15thDay
5%21332446%10%55%
2678
16th to endDay
6%2143245%6%8%54%
2541
August1st to 15thDay
9%425%5%35%5%7%9%41%
2340
16th to endDay
10%313424348%53%
2483
September1st to 15thDay
12%5%2332435%6%51%
2071
16th to endDay
12%5%24323435%53%
1980
October1st to 15thDay
15%6%34323236%48%
1834
16th to endDay
15%6%24424345%45%
1854
November1st to 15thDay
9%8%35%5%2345%7%44%
1665
16th to endDay
12%10%25%435%45%8%36%
1560
December1st to 15thDay
10%12%49%8%45%45%6%27%
1553
16th to endDay
15%9%49%9%445%47%24%
1456
January1st to 15thNight
6%8%48%8%46%5%5%9%32%
2790
16th to endNight
8%10%5%8%7%45%6%5%7%30%
2766
February1st to 15thNight
8%10%5%8%7%35%5%46%34%
2480
16th to endNight
7%8%37%7%345%6%6%39%
2123
March1st to 15thNight
11%9%46%425%45%7%40%
2195
16th to endNight
13%8%5%7%5%24346%37%
2288
April1st to 15thNight
14%7%245%24446%42%
1952
16th to endNight
19%8%35%423445%38%
1673
May1st to 15thNight
17%5%12313336%49%
1620
16th to endNight
12%5%25%4345%5%7%43%
1530
June1st to 15thNight
10%5%2442425%8%49%
1270
16th to endNight
5%2331335%9%62%
1260
July1st to 15thNight
331231335%8%63%
1280
16th to endNight
211232346%10%60%
1625
August1st to 15thNight
43233245%5%8%56%
1620
16th to endNight
8%212313346%61%
1719
September1st to 15thNight
8%413323345%59%
1887
16th to endNight
11%414323346%54%
1980
October1st to 15thNight
15%424212246%51%
2113
16th to endNight
14%8%24223246%46%
2367
November1st to 15thNight
8%7%26%5%24446%46%
2522
16th to endNight
10%7%26%43345%7%44%
2730
December1st to 15thNight
9%10%49%8%46%5%5%7%29%
2734
16th to endNight
15%12%49%6%4445%6%27%
3120

Percentage Cloud Cover NW (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
5%9%412%8%5%6%6%6%8%26%
1476
16th to endDay
7%10%7%10%7%45%5%46%28%
1809
February1st to 15thDay
9%12%49%7%45%447%29%
1710
16th to endDay
7%9%48%7%46%5%6%6%32%
1649
March1st to 15thDay
14%9%5%8%43445%6%33%
2083
16th to endDay
15%11%5%7%7%34346%29%
2286
April1st to 15thDay
17%7%35%42435%7%38%
2323
16th to endDay
22%9%25%423336%34%
2401
May1st to 15thDay
18%7%345%33447%36%
2331
16th to endDay
18%6%245%3445%8%35%
2581
June1st to 15thDay
16%5%24425%45%7%40%
2688
16th to endDay
8%2134245%6%9%50%
2700
July1st to 15thDay
5%21232446%10%55%
2678
16th to endDay
6%31332446%9%54%
2541
August1st to 15thDay
8%425%5%35%6%6%9%41%
2340
16th to endDay
9%42432335%8%53%
2483
September1st to 15thDay
12%5%1332335%5%51%
2071
16th to endDay
11%5%14422345%53%
1980
October1st to 15thDay
14%6%23422335%49%
1834
16th to endDay
15%6%24433346%45%
1854
November1st to 15thDay
8%8%35%5%2445%7%44%
1665
16th to endDay
12%9%35%5%2436%9%36%
1560
December1st to 15thDay
9%12%49%9%45%446%28%
1553
16th to endDay
15%9%48%9%5%5%5%5%6%24%
1456
January1st to 15thNight
6%8%38%7%46%5%5%8%33%
2790
16th to endNight
8%9%5%8%7%45%6%5%7%31%
2766
February1st to 15thNight
8%10%47%7%35%446%35%
2473
16th to endNight
7%8%36%7%45%46%6%39%
2123
March1st to 15thNight
11%8%37%424447%40%
2207
16th to endNight
12%9%47%5%34446%37%
2286
April1st to 15thNight
14%8%25%434446%43%
1952
16th to endNight
18%9%35%323446%38%
1673
May1st to 15thNight
16%5%13213446%49%
1619
16th to endNight
11%5%245%345%5%8%43%
1531
June1st to 15thNight
11%42332445%8%49%
1270
16th to endNight
5%2331325%9%62%
1260
July1st to 15thNight
4212223348%63%
1280
16th to endNight
211232347%9%61%
1625
August1st to 15thNight
4322315%5%5%8%56%
1620
16th to endNight
7%213223446%61%
1719
September1st to 15thNight
7%412324345%59%
1887
16th to endNight
10%423323446%54%
1980
October1st to 15thNight
15%433312246%51%
2113
16th to endNight
14%7%24323237%47%
2367
November1st to 15thNight
8%7%35%434446%46%
2522
16th to endNight
8%7%35%424447%45%
2730
December1st to 15thNight
9%10%38%7%46%5%5%7%30%
2734
16th to endNight
14%11%5%8%6%445%46%28%
3120

Percentage Cloud Cover SW (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
5%8%5%11%10%46%5%6%8%26%
1475
16th to endDay
7%10%6%11%7%46%5%47%27%
1809
February1st to 15thDay
10%11%5%9%7%45%5%47%29%
1710
16th to endDay
7%9%48%7%46%5%6%6%32%
1649
March1st to 15thDay
13%10%5%8%5%2345%6%33%
2083
16th to endDay
15%12%47%6%3435%6%29%
2288
April1st to 15thDay
16%7%35%42345%7%39%
2323
16th to endDay
22%10%35%433335%34%
2401
May1st to 15thDay
18%7%346%24347%36%
2331
16th to endDay
18%6%25%5%25%45%8%35%
2581
June1st to 15thDay
16%5%245%25%45%8%39%
2688
16th to endDay
8%2144245%6%9%49%
2700
July1st to 15thDay
5%21242445%10%55%
2678
16th to endDay
6%2133235%6%9%54%
2541
August1st to 15thDay
9%326%5%35%6%7%9%41%
2340
16th to endDay
9%31442345%8%52%
2483
September1st to 15thDay
12%5%2332335%6%51%
2071
16th to endDay
11%5%24323345%53%
1980
October1st to 15thDay
15%6%24423336%48%
1834
16th to endDay
15%6%34423346%44%
1854
November1st to 15thDay
8%8%35%5%2346%6%44%
1665
16th to endDay
12%10%25%5%2446%9%36%
1560
December1st to 15thDay
10%12%49%8%45%45%6%27%
1553
16th to endDay
14%9%5%9%9%5%45%5%6%24%
1456
January1st to 15thNight
6%8%48%8%45%6%5%9%33%
2790
16th to endNight
8%9%5%8%7%45%6%5%6%31%
2766
February1st to 15thNight
8%10%5%8%7%35%5%46%34%
2473
16th to endNight
7%8%47%6%4446%6%39%
2123
March1st to 15thNight
12%8%36%424447%40%
2207
16th to endNight
12%9%5%7%5%24446%37%
2288
April1st to 15thNight
14%8%24434446%42%
1952
16th to endNight
18%9%35%423445%38%
1673
May1st to 15thNight
16%5%23223436%49%
1619
16th to endNight
12%5%25%4345%5%7%43%
1530
June1st to 15thNight
10%5%2332435%8%49%
1270
16th to endNight
5%21231335%8%63%
1260
July1st to 15thNight
421231345%8%62%
1280
16th to endNight
21332346%10%60%
1625
August1st to 15thNight
43133245%48%56%
1620
16th to endNight
7%313322446%61%
1719
September1st to 15thNight
7%413223445%59%
1887
16th to endNight
11%423323346%54%
1980
October1st to 15thNight
15%424212246%51%
2113
16th to endNight
14%8%24323336%46%
2367
November1st to 15thNight
8%7%35%5%2445%6%46%
2522
16th to endNight
9%7%35%5%2445%7%44%
2730
December1st to 15thNight
9%10%48%8%46%5%5%7%29%
2734
16th to endNight
14%12%48%6%44446%27%
3120

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
6%8%5%11%9%46%5%7%8%25%
1475
16th to endDay
7%11%6%12%7%45%5%46%27%
1809
February1st to 15thDay
10%10%49%8%445%47%29%
1710
16th to endDay
7%8%48%7%5%7%5%5%7%30%
1649
March1st to 15thDay
14%10%6%8%5%2345%6%32%
2083
16th to endDay
15%12%5%8%6%34445%28%
2286
April1st to 15thDay
16%7%35%424447%38%
2323
16th to endDay
22%9%36%423336%33%
2401
May1st to 15thDay
19%7%25%6%34347%35%
2331
16th to endDay
18%6%25%424448%35%
2581
June1st to 15thDay
16%5%25%5%35%45%8%38%
2688
16th to endDay
8%3144245%6%9%49%
2700
July1st to 15thDay
5%21333446%10%54%
2678
16th to endDay
5%3133245%6%10%52%
2541
August1st to 15thDay
8%5%25%5%45%6%7%10%40%
2340
16th to endDay
10%313324448%52%
2483
September1st to 15thDay
11%5%2442445%6%50%
2071
16th to endDay
12%5%234235%35%52%
1980
October1st to 15thDay
15%6%24323236%47%
1834
16th to endDay
15%7%24424346%44%
1854
November1st to 15thDay
9%8%35%5%25%35%6%44%
1665
16th to endDay
12%10%25%5%35%45%9%35%
1560
December1st to 15thDay
10%12%310%8%46%5%5%6%26%
1553
16th to endDay
15%10%39%10%45%5%46%24%
1456
January1st to 15thNight
6%8%48%8%46%5%6%9%32%
2790
16th to endNight
8%10%5%8%6%45%6%6%7%29%
2766
February1st to 15thNight
8%10%48%7%45%446%34%
2473
16th to endNight
7%8%37%7%45%46%7%38%
2123
March1st to 15thNight
11%9%46%424447%39%
2207
16th to endNight
12%10%47%434446%36%
2288
April1st to 15thNight
14%7%25%4245%36%42%
1952
16th to endNight
17%9%35%433435%38%
1673
May1st to 15thNight
16%6%23314436%48%
1619
16th to endNight
11%5%25%435%45%7%43%
1530
June1st to 15thNight
9%5%243335%6%7%49%
1270
16th to endNight
5%2222335%8%62%
1260
July1st to 15thNight
331231435%8%62%
1280
16th to endNight
211232345%11%60%
1625
August1st to 15thNight
432432446%8%56%
1620
16th to endNight
8%213313446%61%
1719
September1st to 15thNight
7%413324346%58%
1887
16th to endNight
10%424323346%54%
1980
October1st to 15thNight
14%5%24322245%51%
2113
16th to endNight
14%8%24323336%45%
2367
November1st to 15thNight
8%7%26%5%3445%6%45%
2522
16th to endNight
9%7%36%5%24447%44%
2730
December1st to 15thNight
9%10%48%8%46%5%5%7%29%
2734
16th to endNight
15%12%49%7%4445%6%26%
3120

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
53%6%13%17%8%
1476
16th to endDay
55%5%15%17%4
1809
February1st to 15thDay
54%5%15%16%7%
1711
16th to endDay
50%7%14%15%9%1
1649
March1st to 15thDay
51%5%13%23%6%
2083
16th to endDay
55%7%14%18%4
2288
April1st to 15thDay
45%6%14%23%9%
2323
16th to endDay
52%5%12%20%9%
2401
May1st to 15thDay
48%5%12%22%10%
2331
16th to endDay
47%6%15%20%10%
2585
June1st to 15thDay
43%7%15%24%9%
2688
16th to endDay
30%7%17%28%14%1
2700
July1st to 15thDay
24%7%17%30%15%2
2520
16th to endDay
25%6%18%27%18%2
2383
August1st to 15thDay
35%9%16%23%12%2
2326
16th to endDay
29%6%15%30%13%3
2483
September1st to 15thDay
33%5%15%28%15%
2071
16th to endDay
34%5%12%32%15%
1980
October1st to 15thDay
38%314%24%17%1
1834
16th to endDay
40%5%15%24%13%
1854
November1st to 15thDay
39%5%13%23%16%1
1665
16th to endDay
43%7%16%20%10%
1560
December1st to 15thDay
57%6%13%14%7%1
1553
16th to endDay
59%6%11%13%7%1
1456
January1st to 15thNight
47%6%14%19%12%
2790
16th to endNight
51%7%14%18%7%
2766
February1st to 15thNight
49%6%12%20%10%1
2483
16th to endNight
42%6%14%20%12%2
2123
March1st to 15thNight
42%6%15%23%10%1
2207
16th to endNight
48%6%14%21%8%
2288
April1st to 15thNight
41%7%12%26%11%
1952
16th to endNight
46%5%10%23%12%1
1673
May1st to 15thNight
36%6%15%24%15%1
1620
16th to endNight
38%7%15%22%14%1
1532
June1st to 15thNight
33%5%17%28%12%1
1270
16th to endNight
20%5%17%30%22%4
1260
July1st to 15thNight
20%5%17%33%18%5%
1209
16th to endNight
17%8%18%30%20%4
1520
August1st to 15thNight
23%8%16%32%16%2
1610
16th to endNight
24%5%15%30%17%5%
1719
September1st to 15thNight
26%6%14%30%20%1
1887
16th to endNight
30%5%11%32%19%1
1980
October1st to 15thNight
34%312%27%19%2
2113
16th to endNight
39%414%25%14%1
2367
November1st to 15thNight
38%5%13%22%18%2
2522
16th to endNight
38%5%17%24%13%1
2730
December1st to 15thNight
51%7%16%16%7%
2734
16th to endNight
57%411%16%8%
3120

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
11%8%40%31%7%
135
16th to endDay
19%9%35%28%5%2
144
February1st to 15thDay
14%9%50%23%2
135
16th to endDay
14%19%35%25%5%
120
March1st to 15thDay
20%14%34%23%6%
135
16th to endDay
28%9%27%20%11%3
144
April1st to 15thDay
23%13%36%18%7%1
322
16th to endDay
27%13%27%19%9%2
474
May1st to 15thDay
21%11%30%26%9%1
468
16th to endDay
23%9%29%26%9%1
517
June1st to 15thDay
14%7%31%27%16%3
495
16th to endDay
7%7%37%29%15%2
495
July1st to 15thDay
5%330%36%19%4
475
16th to endDay
5%229%40%18%2
515
August1st to 15thDay
8%326%32%25%3
493
16th to endDay
11%226%32%22%4
523
September1st to 15thDay
12%6%29%30%16%4
226
16th to endDay
16%8%34%27%10%2
135
October1st to 15thDay
16%8%40%22%9%2
135
16th to endDay
15%15%35%24%7%2
144
November1st to 15thDay
12%8%35%28%13%1
135
16th to endDay
29%14%31%19%4
135
December1st to 15thDay
17%20%41%12%7%
135
16th to endDay
21%13%40%19%3
144
January1st to 15thNight
10%10%42%28%7%1
975
16th to endNight
13%11%38%31%4
1040
February1st to 15thNight
14%10%37%28%7%1
955
16th to endNight
12%11%38%28%8%1
757
March1st to 15thNight
16%7%35%33%6%
647
16th to endNight
16%10%34%28%9%
624
April1st to 15thNight
18%11%32%30%6%
584
16th to endNight
22%9%32%24%8%2
558
May1st to 15thNight
17%9%34%26%10%
540
16th to endNight
14%5%36%34%8%
563
June1st to 15thNight
12%5%28%39%13%
540
16th to endNight
7%327%46%14%
540
July1st to 15thNight
5%126%48%17%1
510
16th to endNight
3125%52%17%
564
August1st to 15thNight
4222%47%21%
539
16th to endNight
9%225%38%21%2
573
September1st to 15thNight
9%327%39%16%3
540
16th to endNight
11%5%36%29%14%2
537
October1st to 15thNight
17%10%34%27%8%1
673
16th to endNight
19%8%33%29%9%
831
November1st to 15thNight
12%11%35%31%8%1
950
16th to endNight
13%11%37%28%8%
975
December1st to 15thNight
15%11%37%29%4
975
16th to endNight
21%9%34%26%6%
1040
Page generated at 2018-04-24 09:03:04. Version ac04

Copyright 2018 A. Danko


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