Washington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washington clear sky chart from 2005-11-05 to 2017-04-25.

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
37%
14840
21%
45628
11%
14801
48%
14801
25%
28479

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%
717
23%
2420
10%
716
41%
716
20%
1493
January16th to endNight
22%
768
23%
2496
8%
768
41%
768
22%
1540
February1st to 15thNight
26%
703
20%
2287
7%
701
43%
701
21%
1395
February16th to endNight
19%
631
24%
1910
6%
631
41%
631
23%
1194
March1st to 15thNight
26%
720
22%
2010
9%
720
39%
720
25%
1278
March16th to endNight
23%
768
21%
2085
8%
768
34%
768
24%
1392
April1st to 15thNight
26%
718
20%
1796
5%
718
35%
718
27%
1180
April16th to endNight
37%
700
20%
1750
7%
700
43%
700
29%
1164
May1st to 15thNight
37%
518
18%
1508
9%
518
44%
518
22%
1018
May16th to endNight
54%
342
17%
1369
15%
342
60%
342
23%
790
June1st to 15thNight
50%
330
20%
1320
19%
330
60%
330
23%
777
June16th to endNight
54%
330
16%
1320
16%
330
61%
330
25%
760
July1st to 15thNight
56%
310
14%
1240
16%
310
60%
310
19%
778
July16th to endNight
69%
505
16%
1452
13%
472
75%
472
37%
827
August1st to 15thNight
64%
494
19%
1474
15%
491
65%
491
32%
900
August16th to endNight
58%
697
18%
1747
11%
697
59%
697
32%
1167
September1st to 15thNight
57%
660
26%
1650
15%
660
62%
660
34%
1088
September16th to endNight
52%
657
21%
1807
17%
657
55%
657
28%
1210
October1st to 15thNight
45%
660
25%
1980
21%
660
53%
660
27%
1269
October16th to endNight
36%
704
29%
2112
16%
704
52%
704
32%
1219
November1st to 15thNight
37%
700
25%
2227
15%
700
52%
700
28%
1317
November16th to endNight
33%
720
22%
2460
12%
720
47%
720
23%
1457
December1st to 15thNight
28%
720
19%
2520
8%
720
42%
720
19%
1578
December16th to endNight
23%
768
21%
2688
8%
768
40%
768
18%
1688

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
13%5%25%435%5%5%7%41%
45999
Night
14%5%2442445%7%43%
45810
Cloud CoverDay
14%5%2443445%7%41%
46009
Night
14%5%2442445%7%44%
45814
TransparencyDay
39%6%11%23%16%2
45699
Night
38%5%11%23%17%3
45628
SeeingDay
17%9%24%25%15%6%
5120
Night
17%10%28%29%13%1
14845

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
17%5%344145%46%42%
1516
16th to endDay
14%7%24424347%41%
1728
February1st to 15thDay
14%6%25%6%245%5%7%40%
1575
16th to endDay
13%7%25%5%25%446%41%
1561
March1st to 15thDay
15%5%25%5%24345%45%
1939
16th to endDay
16%6%35%5%25%336%41%
2139
April1st to 15thDay
13%5%25%42435%6%45%
2152
16th to endDay
17%425%42335%5%44%
2099
May1st to 15thDay
15%6%25%5%345%7%9%35%
2119
16th to endDay
13%5%245%35%5%5%8%39%
2394
June1st to 15thDay
12%4245%25%6%6%9%39%
2310
16th to endDay
9%4245%35%5%6%10%41%
2310
July1st to 15thDay
9%5%25%6%36%6%6%10%37%
2310
16th to endDay
6%415%5%47%7%8%11%37%
2299
August1st to 15thDay
10%6%26%6%36%6%7%9%33%
2134
16th to endDay
11%415%5%46%5%7%8%39%
2100
September1st to 15thDay
12%5%2432446%7%44%
1980
16th to endDay
15%6%25%4245%6%7%39%
1820
October1st to 15thDay
18%424323446%45%
1650
16th to endDay
14%425%314445%48%
1760
November1st to 15thDay
14%5%2222345%7%47%
1631
16th to endDay
16%5%24324446%45%
1497
December1st to 15thDay
15%4144245%5%7%43%
1440
16th to endDay
18%6%25%324446%40%
1536
January1st to 15thNight
17%5%24414348%42%
2420
16th to endNight
16%6%24324446%43%
2496
February1st to 15thNight
14%6%1442445%5%45%
2285
16th to endNight
14%6%25%324346%45%
1910
March1st to 15thNight
16%425%32435%7%44%
1998
16th to endNight
15%6%35%5%2345%6%41%
2085
April1st to 15thNight
14%5%244245%5%6%44%
1796
16th to endNight
16%424325%45%8%42%
1750
May1st to 15thNight
17%7%34424447%38%
1508
16th to endNight
11%4234245%6%10%42%
1367
June1st to 15thNight
10%424426%5%7%8%42%
1320
16th to endNight
8%5%15%6%245%6%10%42%
1320
July1st to 15thNight
8%436%6%36%6%6%8%37%
1320
16th to endNight
5%313335%7%9%12%43%
1559
August1st to 15thNight
8%325%5%2447%9%44%
1485
16th to endNight
10%5%245%3445%9%42%
1747
September1st to 15thNight
11%5%24435%46%7%45%
1650
16th to endNight
13%6%345%345%5%7%40%
1807
October1st to 15thNight
18%5%24423345%45%
1980
16th to endNight
14%323323346%52%
2112
November1st to 15thNight
14%424422335%51%
2227
16th to endNight
16%4144235%5%7%45%
2460
December1st to 15thNight
16%5%25%5%2445%6%39%
2520
16th to endNight
17%5%245%2445%6%41%
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
17%5%24424447%42%
1516
16th to endDay
14%7%25%42435%6%42%
1728
February1st to 15thDay
15%7%245%3445%6%40%
1575
16th to endDay
13%7%25%434436%42%
1561
March1st to 15thDay
15%6%15%5%24346%45%
1950
16th to endDay
16%6%25%5%34346%40%
2138
April1st to 15thDay
13%435%33445%7%45%
2152
16th to endDay
18%4245%23345%45%
2099
May1st to 15thDay
16%5%24435%47%8%36%
2119
16th to endDay
13%424445%5%6%7%40%
2394
June1st to 15thDay
13%3245%345%6%9%40%
2310
16th to endDay
9%3245%35%6%6%9%42%
2310
July1st to 15thDay
9%5%26%6%36%6%7%9%38%
2310
16th to endDay
6%325%5%37%6%8%12%38%
2299
August1st to 15thDay
11%5%25%6%46%6%6%9%34%
2134
16th to endDay
11%4246%36%6%6%8%39%
2100
September1st to 15thDay
13%42433445%9%45%
1980
16th to endDay
15%5%244246%47%39%
1820
October1st to 15thDay
17%5%1342335%5%46%
1650
16th to endDay
14%424323445%48%
1760
November1st to 15thDay
15%42232435%7%48%
1631
16th to endDay
16%5%1432345%7%45%
1497
December1st to 15thDay
15%4144245%5%6%44%
1440
16th to endDay
19%6%15%434446%40%
1536
January1st to 15thNight
17%5%25%323447%43%
2420
16th to endNight
16%6%24424446%44%
2496
February1st to 15thNight
14%6%23425%346%45%
2278
16th to endNight
15%5%34324337%45%
1910
March1st to 15thNight
16%425%323347%44%
2010
16th to endNight
16%5%26%5%2435%6%41%
2085
April1st to 15thNight
14%425%4335%5%6%44%
1796
16th to endNight
17%42432445%7%43%
1750
May1st to 15thNight
18%5%35%42445%6%39%
1507
16th to endNight
11%5%234245%7%9%43%
1367
June1st to 15thNight
11%424425%6%7%8%43%
1320
16th to endNight
9%425%5%3447%9%43%
1320
July1st to 15thNight
8%436%6%36%6%7%9%38%
1320
16th to endNight
5%323435%6%9%11%44%
1559
August1st to 15thNight
8%325%5%336%6%9%44%
1485
16th to endNight
10%5%245%35%46%8%42%
1747
September1st to 15thNight
11%5%24425%46%7%45%
1650
16th to endNight
13%7%25%43445%8%40%
1807
October1st to 15thNight
18%5%2442324446%
1980
16th to endNight
13%313313346%52%
2112
November1st to 15thNight
14%424423335%51%
2227
16th to endNight
15%4134235%5%6%45%
2460
December1st to 15thNight
16%6%25%43446%5%40%
2520
16th to endNight
17%5%24424446%41%
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
41%412%22%13%3
1517
16th to endDay
41%411%25%13%3
1728
February1st to 15thDay
42%410%25%14%2
1577
16th to endDay
42%411%26%14%1
1561
March1st to 15thDay
41%410%24%16%3
1950
16th to endDay
44%410%19%17%3
2139
April1st to 15thDay
38%49%24%18%4
2152
16th to endDay
40%49%17%21%6%
2099
May1st to 15thDay
43%8%11%21%14%1
2119
16th to endDay
39%7%13%24%12%2
2398
June1st to 15thDay
38%9%11%25%13%1
2310
16th to endDay
36%8%14%21%17%2
2310
July1st to 15thDay
39%8%13%20%15%3
2161
16th to endDay
35%9%14%23%14%2
2143
August1st to 15thDay
42%7%11%20%16%
2121
16th to endDay
40%7%10%22%16%2
2100
September1st to 15thDay
36%5%13%20%21%3
1980
16th to endDay
41%6%8%22%19%1
1820
October1st to 15thDay
39%5%9%25%17%3
1650
16th to endDay
36%38%27%20%2
1760
November1st to 15thDay
35%411%24%18%5%
1631
16th to endDay
38%49%26%18%2
1497
December1st to 15thDay
38%5%14%25%12%3
1440
16th to endDay
44%410%25%13%1
1536
January1st to 15thNight
41%410%20%18%4
2420
16th to endNight
40%410%26%17%1
2496
February1st to 15thNight
39%411%24%17%3
2287
16th to endNight
39%49%25%19%2
1910
March1st to 15thNight
38%411%26%14%2
2010
16th to endNight
42%5%10%21%16%3
2085
April1st to 15thNight
38%5%9%20%21%3
1796
16th to endNight
38%5%9%20%18%7%
1750
May1st to 15thNight
44%6%11%21%13%2
1508
16th to endNight
33%9%17%20%15%2
1369
June1st to 15thNight
36%8%12%25%16%1
1320
16th to endNight
35%7%15%21%18%2
1320
July1st to 15thNight
41%8%14%18%12%4
1240
16th to endNight
28%7%17%28%16%1
1452
August1st to 15thNight
34%7%13%24%18%1
1474
16th to endNight
37%7%11%24%16%3
1747
September1st to 15thNight
36%6%10%21%20%4
1650
16th to endNight
40%6%8%20%19%3
1807
October1st to 15thNight
41%47%24%18%3
1980
16th to endNight
32%49%28%22%3
2112
November1st to 15thNight
36%38%22%21%7%
2227
16th to endNight
36%5%10%23%19%4
2460
December1st to 15thNight
43%412%22%15%2
2520
16th to endNight
43%5%12%23%13%2
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
24%24%37%10%11
74
16th to endDay
21%28%31%14%4
96
February1st to 15thDay
27%21%37%10%3
90
16th to endDay
19%17%40%20%2
220
March1st to 15thDay
18%17%32%23%6%
270
16th to endDay
20%13%37%20%5%2
288
April1st to 15thDay
16%16%31%22%10%2
270
16th to endDay
20%11%26%24%11%6%
250
May1st to 15thDay
26%13%21%20%14%3
228
16th to endDay
16%5%21%30%21%5%
267
June1st to 15thDay
15%7%21%30%16%9%
255
16th to endDay
11%423%35%19%5%
255
July1st to 15thDay
15%118%33%24%7%
245
16th to endDay
7%114%34%31%10%
270
August1st to 15thDay
13%214%32%26%10%
253
16th to endDay
9%112%32%26%17%
268
September1st to 15thDay
12%416%26%19%20%
255
16th to endDay
23%218%27%14%13%
254
October1st to 15thDay
23%5%20%22%22%5%
255
16th to endDay
14%12%30%26%14%1
272
November1st to 15thDay
17%11%27%23%14%5%
244
16th to endDay
17%14%33%22%10%1
117
December1st to 15thDay
16%31%20%15%13%3
60
16th to endDay
37%14%29%15%3
64
January1st to 15thNight
20%16%34%23%5%
720
16th to endNight
19%16%37%24%1
768
February1st to 15thNight
20%15%33%28%2
704
16th to endNight
19%20%36%21%1
631
March1st to 15thNight
20%13%36%25%3
720
16th to endNight
18%14%40%20%5%1
768
April1st to 15thNight
16%16%36%24%4
718
16th to endNight
19%9%28%30%11%
700
May1st to 15thNight
21%7%28%30%12%
518
16th to endNight
12%5%19%40%20%1
343
June1st to 15thNight
11%228%36%20%1
330
16th to endNight
10%321%45%18%
330
July1st to 15thNight
10%221%39%25%
310
16th to endNight
6%115%41%31%3
505
August1st to 15thNight
11%111%43%29%1
494
16th to endNight
13%215%35%28%4
697
September1st to 15thNight
13%218%31%29%3
660
16th to endNight
15%318%33%25%3
657
October1st to 15thNight
20%6%20%27%21%3
660
16th to endNight
16%11%32%25%13%
704
November1st to 15thNight
15%10%30%28%12%2
700
16th to endNight
18%15%29%25%9%
720
December1st to 15thNight
20%11%33%25%8%
720
16th to endNight
22%13%35%25%3
768
Page generated at 2017-04-26 17:21:37. Version ac04

Copyright 2017 A. Danko


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