Westminster
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Westminster clear sky chart from 2005-11-05 to 2017-09-18.

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
54%
15946
28%
46883
18%
15905
63%
15905
42%
25395

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
42%
897
32%
2506
16%
895
57%
895
48%
1310
January16th to endNight
38%
813
33%
2541
18%
813
56%
813
41%
1337
February1st to 15thNight
34%
699
25%
2247
11%
698
52%
698
37%
1230
February16th to endNight
36%
632
32%
1894
15%
632
54%
632
38%
1047
March1st to 15thNight
43%
720
28%
2118
13%
720
56%
720
44%
1144
March16th to endNight
43%
677
28%
2021
16%
677
55%
677
39%
1104
April1st to 15thNight
42%
539
24%
1798
15%
539
56%
539
36%
956
April16th to endNight
43%
540
23%
1800
17%
540
54%
540
29%
1059
May1st to 15thNight
53%
540
19%
1604
16%
540
57%
540
29%
1023
May16th to endNight
61%
563
18%
1498
13%
561
65%
561
37%
895
June1st to 15thNight
70%
540
21%
1440
17%
540
72%
540
49%
775
June16th to endNight
77%
540
27%
1440
21%
540
80%
540
57%
728
July1st to 15thNight
73%
510
17%
1360
15%
510
75%
510
37%
814
July16th to endNight
80%
564
26%
1438
24%
531
80%
531
45%
793
August1st to 15thNight
77%
539
20%
1669
20%
536
79%
536
40%
885
August16th to endNight
78%
573
29%
1910
33%
573
79%
573
41%
985
September1st to 15thNight
74%
540
28%
1800
30%
540
78%
540
40%
928
September16th to endNight
74%
501
36%
1804
36%
501
81%
501
50%
796
October1st to 15thNight
56%
647
29%
1967
20%
647
62%
647
42%
1048
October16th to endNight
57%
737
33%
2145
19%
737
67%
737
47%
1121
November1st to 15thNight
54%
875
33%
2275
17%
875
62%
875
53%
1219
November16th to endNight
52%
900
35%
2400
18%
900
63%
900
52%
1276
December1st to 15thNight
41%
900
26%
2520
10%
900
55%
900
44%
1377
December16th to endNight
42%
960
27%
2688
12%
960
55%
960
40%
1545

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
5%4144245%6%8%52%
47928
Night
442332335%7%58%
47058
Cloud CoverDay
5%4144245%6%8%52%
47926
Night
5%42332345%7%58%
47058
TransparencyDay
27%6%12%24%21%8%
47605
Night
25%5%11%25%23%8%
46883
SeeingDay
9%7%24%30%20%6%
5865
Night
6%329%40%17%2
15954

By Month

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
26%23313347%62%
1432
16th to endDay
15%23223348%63%
1693
February1st to 15thDay
15%36%3245%48%54%
1615
16th to endDay
25%3442335%7%56%
1580
March1st to 15thDay
3425%41435%10%53%
1840
16th to endDay
6%6%233245%5%8%51%
2202
April1st to 15thDay
7%5%1432345%7%53%
2150
16th to endDay
12%6%1432446%7%45%
2160
May1st to 15thDay
11%6%15%425%5%7%8%41%
2344
16th to endDay
9%415%5%35%5%7%9%43%
2616
June1st to 15thDay
5%313425%5%7%8%50%
2520
16th to endDay
2112335%7%8%9%53%
2520
July1st to 15thDay
6%315%435%7%9%9%45%
2520
16th to endDay
4324435%6%7%11%47%
2630
August1st to 15thDay
5%315%5%36%6%8%10%42%
2280
16th to endDay
42135%35%6%7%9%50%
2292
September1st to 15thDay
6%31332446%7%56%
2160
16th to endDay
4312313346%64%
1881
October1st to 15thDay
6%31332335%7%59%
1661
16th to endDay
5%412312326%66%
1716
November1st to 15thDay
231431236%8%60%
1583
16th to endDay
342322235%7%62%
1557
December1st to 15thDay
25%25%32335%8%55%
1440
16th to endDay
36%34323348%55%
1536
January1st to 15thNight
334322347%64%
2506
16th to endNight
25%25%423235%61%
2541
February1st to 15thNight
27%35%323347%55%
2237
16th to endNight
27%35%423335%57%
1894
March1st to 15thNight
35%2342345%7%58%
2118
16th to endNight
6%6%23313348%56%
2021
April1st to 15thNight
7%5%2231345%7%55%
1798
16th to endNight
13%6%24323336%48%
1800
May1st to 15thNight
13%5%16%42445%7%45%
1603
16th to endNight
7%414324447%54%
1497
June1st to 15thNight
4212215%45%9%59%
1440
16th to endNight
211221447%8%64%
1440
July1st to 15thNight
5%2235%25%5%6%11%48%
1440
16th to endNight
32133146%8%9%54%
1536
August1st to 15thNight
3313425%5%6%10%54%
1678
16th to endNight
423425%5%5%8%55%
1910
September1st to 15thNight
8%31321436%8%56%
1800
16th to endNight
4112312346%67%
1804
October1st to 15thNight
8%422322345%60%
1967
16th to endNight
5%413412336%61%
2145
November1st to 15thNight
231321235%6%65%
2275
16th to endNight
3423213346%64%
2400
December1st to 15thNight
26%3432235%8%57%
2520
16th to endNight
37%3331436%8%54%
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
26%13312337%61%
1432
16th to endDay
15%23223347%63%
1693
February1st to 15thDay
15%26%3345%48%54%
1615
16th to endDay
25%2432335%7%57%
1580
March1st to 15thDay
343442335%10%53%
1840
16th to endDay
6%6%233245%6%8%52%
2200
April1st to 15thDay
8%5%1332435%7%53%
2150
16th to endDay
12%6%233345%5%7%45%
2160
May1st to 15thDay
11%5%245%25%6%6%9%41%
2344
16th to endDay
10%415%4345%7%9%43%
2616
June1st to 15thDay
5%214425%6%7%8%50%
2520
16th to endDay
213325%7%8%9%53%
2520
July1st to 15thDay
6%214445%6%8%9%45%
2520
16th to endDay
4324435%6%7%10%47%
2630
August1st to 15thDay
5%2245%36%7%8%11%42%
2280
16th to endDay
4213435%6%7%9%50%
2292
September1st to 15thDay
6%31332346%7%56%
2160
16th to endDay
4312322346%64%
1881
October1st to 15thDay
5%34323446%59%
1661
16th to endDay
5%412213326%66%
1716
November1st to 15thDay
231422236%7%61%
1583
16th to endDay
332322235%7%62%
1557
December1st to 15thDay
16%25%32345%9%55%
1440
16th to endDay
46%2432335%8%55%
1536
January1st to 15thNight
1324323346%64%
2506
16th to endNight
25%34413335%61%
2541
February1st to 15thNight
27%35%324347%55%
2237
16th to endNight
28%35%423445%57%
1894
March1st to 15thNight
342442245%7%57%
2118
16th to endNight
6%6%23312348%56%
2021
April1st to 15thNight
8%5%1342335%6%56%
1798
16th to endNight
13%6%23422346%48%
1800
May1st to 15thNight
13%5%25%4245%5%7%44%
1603
16th to endNight
7%414324447%54%
1497
June1st to 15thNight
5%2122145%5%9%59%
1440
16th to endNight
21222347%7%65%
1440
July1st to 15thNight
5%313435%5%6%10%48%
1440
16th to endNight
4222325%6%7%9%54%
1536
August1st to 15thNight
321343447%9%54%
1678
16th to endNight
4213325%5%5%8%56%
1910
September1st to 15thNight
8%31322336%7%56%
1800
16th to endNight
4213222245%67%
1804
October1st to 15thNight
8%423313345%60%
1967
16th to endNight
5%413322337%61%
2145
November1st to 15thNight
3213313346%65%
2275
16th to endNight
3423212345%65%
2400
December1st to 15thNight
26%3432335%8%57%
2520
16th to endNight
37%3432345%7%54%
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
22%313%32%23%4
1433
16th to endDay
20%39%35%22%7%
1693
February1st to 15thDay
28%5%14%32%15%4
1616
16th to endDay
25%5%8%27%27%5%
1580
March1st to 15thDay
26%5%10%26%23%7%
1840
16th to endDay
29%6%14%22%21%6%
2202
April1st to 15thDay
28%5%12%25%20%7%
2150
16th to endDay
35%7%13%21%18%4
2160
May1st to 15thDay
38%7%14%21%14%3
2344
16th to endDay
35%7%15%23%12%6%
2620
June1st to 15thDay
27%8%14%20%19%10%
2520
16th to endDay
20%7%12%21%28%10%
2520
July1st to 15thDay
31%8%12%19%16%11%
2371
16th to endDay
28%8%15%23%16%7%
2465
August1st to 15thDay
32%8%14%18%16%9%
2265
16th to endDay
26%6%11%23%23%7%
2292
September1st to 15thDay
25%5%9%22%21%15%
2160
16th to endDay
20%47%23%36%8%
1881
October1st to 15thDay
23%5%13%19%27%9%
1661
16th to endDay
20%38%26%29%10%
1716
November1st to 15thDay
20%310%27%25%12%
1583
16th to endDay
20%38%25%31%10%
1557
December1st to 15thDay
26%5%11%32%20%3
1440
16th to endDay
27%5%14%24%20%6%
1536
January1st to 15thNight
20%5%10%31%26%5%
2506
16th to endNight
26%38%28%25%8%
2541
February1st to 15thNight
28%5%10%31%19%4
2247
16th to endNight
27%5%5%27%24%9%
1894
March1st to 15thNight
24%49%25%27%7%
2118
16th to endNight
27%410%25%25%5%
2021
April1st to 15thNight
26%6%11%24%21%8%
1798
16th to endNight
35%5%10%24%18%5%
1800
May1st to 15thNight
37%7%14%21%14%4
1604
16th to endNight
28%8%19%23%13%6%
1498
June1st to 15thNight
20%6%17%27%15%11%
1440
16th to endNight
14%6%12%22%30%12%
1440
July1st to 15thNight
27%9%14%21%16%10%
1360
16th to endNight
21%8%15%28%17%7%
1438
August1st to 15thNight
24%7%16%24%17%8%
1669
16th to endNight
24%6%11%23%26%8%
1910
September1st to 15thNight
25%5%12%22%19%14%
1800
16th to endNight
18%38%26%34%8%
1804
October1st to 15thNight
24%5%12%18%27%10%
1967
16th to endNight
23%39%24%28%10%
2145
November1st to 15thNight
19%48%25%28%13%
2275
16th to endNight
21%48%24%31%10%
2400
December1st to 15thNight
25%414%30%20%5%
2520
16th to endNight
27%6%15%26%20%3
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
6%8%26%35%16%5%
90
16th to endDay
7%332%38%15%3
96
February1st to 15thDay
6%8%43%37%3
90
16th to endDay
4438%37%13%1
81
March1st to 15thDay
414%29%36%13%1
130
16th to endDay
14%25%30%23%4
288
April1st to 15thDay
13%18%34%20%10%3
269
16th to endDay
21%18%34%17%7%1
270
May1st to 15thDay
15%7%26%31%15%2
426
16th to endDay
15%7%22%29%17%8%
517
June1st to 15thDay
7%423%31%24%8%
495
16th to endDay
3423%32%26%9%
495
July1st to 15thDay
10%216%26%33%10%
475
16th to endDay
5%314%27%37%11%
489
August1st to 15thDay
8%5%25%31%20%8%
315
16th to endDay
8%320%36%23%7%
332
September1st to 15thDay
7%427%33%20%7%
315
16th to endDay
3322%34%25%11%
140
October1st to 15thDay
3316%38%26%11%
90
16th to endDay
8%426%30%23%7%
96
November1st to 15thDay
1417%35%28%12%
90
16th to endDay
11%11%28%33%15%
90
December1st to 15thDay
7%7%44%31%7%1
90
16th to endDay
45%27%53%8%2
96
January1st to 15thNight
2345%35%10%2
900
16th to endNight
5%445%35%8%1
813
February1st to 15thNight
5%246%37%6%
704
16th to endNight
6%444%38%5%
632
March1st to 15thNight
4242%37%11%
720
16th to endNight
8%6%35%41%7%
677
April1st to 15thNight
10%6%35%38%7%
539
16th to endNight
16%5%29%35%12%
540
May1st to 15thNight
16%318%44%15%1
540
16th to endNight
9%220%41%23%2
563
June1st to 15thNight
5%216%49%22%2
540
16th to endNight
314%47%32%1
540
July1st to 15thNight
6%17%38%40%3
510
16th to endNight
45%39%46%3
564
August1st to 15thNight
5%8%42%39%2
539
16th to endNight
5%8%43%38%4
573
September1st to 15thNight
8%110%44%28%6%
540
16th to endNight
414%45%29%5%
501
October1st to 15thNight
10%324%39%17%4
647
16th to endNight
8%325%42%18%2
737
November1st to 15thNight
4332%42%14%2
875
16th to endNight
5%334%45%11%1
900
December1st to 15thNight
5%441%40%7%
900
16th to endNight
6%439%39%8%1
960
Page generated at 2017-09-19 05:32:27. Version ac04

Copyright 2017 A. Danko


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