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Post Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Post Observatory clear sky chart from 2005-11-05 to 2019-07-11.

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
23%
17227
16%
54095
6%
17190
34%
17190
15%
36913

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
12%
840
17%
2926
4%
836
28%
836
10%
1932
January16th to endNight
9%
896
14%
3085
1%
896
24%
896
9%
2116
February1st to 15thNight
10%
824
14%
2676
2%
821
26%
821
10%
1872
February16th to endNight
14%
735
17%
2208
3%
735
31%
735
14%
1521
March1st to 15thNight
15%
840
17%
2429
5%
840
28%
840
14%
1732
March16th to endNight
20%
896
19%
2464
6%
896
31%
896
18%
1682
April1st to 15thNight
15%
775
18%
2067
4%
775
25%
775
15%
1505
April16th to endNight
23%
623
13%
1869
3%
623
31%
623
16%
1342
May1st to 15thNight
25%
630
14%
1890
7%
630
33%
630
15%
1370
May16th to endNight
25%
567
11%
1797
7%
566
32%
566
13%
1317
June1st to 15thNight
31%
420
11%
1470
9%
420
37%
420
13%
1070
June16th to endNight
34%
420
10%
1470
8%
420
39%
420
15%
1021
July1st to 15thNight
42%
392
12%
1486
13%
392
47%
392
17%
940
July16th to endNight
34%
447
10%
1582
9%
422
41%
422
14%
1113
August1st to 15thNight
42%
574
11%
1713
8%
570
45%
570
19%
1198
August16th to endNight
47%
621
18%
1863
9%
621
53%
621
25%
1186
September1st to 15thNight
43%
767
18%
2080
10%
767
49%
767
30%
1327
September16th to endNight
40%
777
24%
2145
12%
777
49%
777
29%
1321
October1st to 15thNight
32%
780
20%
2256
9%
780
42%
780
22%
1490
October16th to endNight
20%
832
17%
2629
6%
832
33%
832
18%
1737
November1st to 15thNight
25%
820
19%
2667
9%
820
38%
820
16%
1754
November16th to endNight
17%
892
16%
2911
4%
892
32%
892
12%
1948
December1st to 15thNight
14%
900
15%
3055
3%
900
28%
900
11%
2120
December16th to endNight
10%
959
15%
3357
1%
959
26%
959
10%
2299

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
19%7%25%424447%37%
55105
Night
19%8%35%424446%36%
54270
TransparencyDay
46%412%22%12%1
54798
Night
48%411%21%12%1
54095
SeeingDay
22%19%29%20%7%1
8829
Night
22%24%25%19%7%
17227

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%9%35%424347%38%
1672
16th to endDay
17%9%36%423347%37%
1841
February1st to 15thDay
17%10%26%4345%5%7%32%
1845
16th to endDay
13%9%47%426%45%6%34%
1834
March1st to 15thDay
20%9%26%5%34346%33%
2179
16th to endDay
17%7%25%424346%41%
2464
April1st to 15thDay
20%8%34424347%36%
2539
16th to endDay
21%8%35%423346%35%
2696
May1st to 15thDay
23%8%36%5%23346%32%
2722
16th to endDay
22%7%25%424447%33%
3032
June1st to 15thDay
23%7%35%5%23447%31%
3150
16th to endDay
23%6%25%5%24447%32%
3150
July1st to 15thDay
15%423425%46%8%42%
2953
16th to endDay
16%7%35%425%5%6%8%36%
2826
August1st to 15thDay
16%6%25%42435%8%40%
2488
16th to endDay
11%41442445%8%46%
2691
September1st to 15thDay
12%414425%447%48%
2207
16th to endDay
15%6%24322446%48%
2145
October1st to 15thDay
19%6%24423445%42%
2033
16th to endDay
20%6%24423435%41%
1947
November1st to 15thDay
22%6%15%324346%39%
1851
16th to endDay
20%7%35%324446%37%
1708
December1st to 15thDay
23%8%24334346%34%
1564
16th to endDay
23%10%35%313335%34%
1568
January1st to 15thNight
16%8%35%433448%37%
2926
16th to endNight
15%11%36%5%2445%9%31%
3085
February1st to 15thNight
16%8%26%5%35%446%35%
2675
16th to endNight
16%9%35%5%23446%37%
2208
March1st to 15thNight
20%10%45%5%2335%5%34%
2417
16th to endNight
16%7%35%5%23445%40%
2464
April1st to 15thNight
22%9%345%33445%32%
2067
16th to endNight
24%8%36%423336%32%
1869
May1st to 15thNight
24%7%26%5%34347%30%
1890
16th to endNight
24%8%36%434346%29%
1795
June1st to 15thNight
27%9%25%325%35%6%27%
1470
16th to endNight
23%8%35%43345%6%32%
1470
July1st to 15thNight
18%6%25%5%3446%8%35%
1565
16th to endNight
21%8%36%5%2445%6%29%
1681
August1st to 15thNight
18%7%25%425%47%8%32%
1725
16th to endNight
15%5%2443345%9%42%
1863
September1st to 15thNight
16%423424437%45%
2080
16th to endNight
14%424323346%48%
2145
October1st to 15thNight
21%6%13424445%41%
2256
16th to endNight
20%7%25%423435%39%
2629
November1st to 15thNight
20%6%24424445%40%
2667
16th to endNight
18%7%35%324445%37%
2911
December1st to 15thNight
23%8%24423337%34%
3055
16th to endNight
22%8%34423347%35%
3357

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
47%5%13%20%11%1
1673
16th to endDay
49%313%22%9%1
1841
February1st to 15thDay
52%414%18%8%
1848
16th to endDay
49%412%20%11%2
1834
March1st to 15thDay
52%312%19%10%1
2190
16th to endDay
43%410%18%19%4
2464
April1st to 15thDay
47%38%21%15%3
2539
16th to endDay
49%410%18%13%3
2696
May1st to 15thDay
52%48%20%12%2
2722
16th to endDay
50%6%11%21%9%1
3036
June1st to 15thDay
52%5%11%19%10%1
3150
16th to endDay
51%411%18%12%1
3150
July1st to 15thDay
38%6%15%25%13%
2803
16th to endDay
44%6%16%22%9%1
2663
August1st to 15thDay
42%5%15%26%9%
2475
16th to endDay
33%5%14%27%18%
2691
September1st to 15thDay
35%5%13%25%18%2
2207
16th to endDay
36%5%8%29%18%1
2145
October1st to 15thDay
42%311%27%13%1
2033
16th to endDay
44%412%24%12%
1947
November1st to 15thDay
46%412%22%12%1
1851
16th to endDay
49%414%20%11%
1708
December1st to 15thDay
52%413%22%7%
1564
16th to endDay
54%49%20%10%
1568
January1st to 15thNight
47%414%23%9%
2926
16th to endNight
51%412%21%8%
3085
February1st to 15thNight
51%413%19%10%1
2676
16th to endNight
49%410%20%12%2
2208
March1st to 15thNight
53%39%20%11%2
2429
16th to endNight
46%39%17%20%2
2464
April1st to 15thNight
53%37%16%15%3
2067
16th to endNight
54%39%16%12%3
1869
May1st to 15thNight
52%5%8%17%14%1
1890
16th to endNight
56%5%8%19%9%1
1797
June1st to 15thNight
56%5%10%17%7%2
1470
16th to endNight
51%5%10%21%10%
1470
July1st to 15thNight
44%5%13%23%12%
1486
16th to endNight
53%5%12%18%7%1
1582
August1st to 15thNight
48%7%13%23%8%
1713
16th to endNight
39%6%12%24%16%
1863
September1st to 15thNight
38%413%26%16%1
2080
16th to endNight
36%48%30%19%
2145
October1st to 15thNight
44%310%24%14%1
2256
16th to endNight
46%310%26%12%
2629
November1st to 15thNight
46%412%21%13%1
2667
16th to endNight
47%412%21%11%1
2911
December1st to 15thNight
52%312%21%9%
3055
16th to endNight
51%412%20%10%
3357

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
20%35%33%9%1
120
16th to endDay
23%32%34%8%1
177
February1st to 15thDay
25%34%27%11%1
326
16th to endDay
22%26%34%13%2
341
March1st to 15thDay
26%27%30%13%2
390
16th to endDay
21%28%30%13%4
416
April1st to 15thDay
25%25%30%12%31
390
16th to endDay
28%18%32%13%6%1
379
May1st to 15thDay
26%21%30%16%41
408
16th to endDay
25%17%32%17%7%
542
June1st to 15thDay
22%15%30%23%7%
645
16th to endDay
28%13%26%22%8%
645
July1st to 15thDay
17%9%29%28%14%
484
16th to endDay
20%10%27%27%12%2
390
August1st to 15thDay
18%9%25%30%15%1
358
16th to endDay
15%8%24%32%16%3
396
September1st to 15thDay
16%7%30%31%11%2
375
16th to endDay
19%10%29%23%12%4
374
October1st to 15thDay
21%15%25%25%10%2
340
16th to endDay
23%22%34%14%3
336
November1st to 15thDay
22%28%21%20%5%1
325
16th to endDay
25%28%25%15%41
330
December1st to 15thDay
25%22%30%16%3
214
16th to endDay
30%28%29%10%
128
January1st to 15thNight
18%39%26%13%2
840
16th to endNight
20%37%27%11%2
896
February1st to 15thNight
20%37%28%11%2
824
16th to endNight
22%31%28%13%3
735
March1st to 15thNight
24%28%27%15%3
840
16th to endNight
17%29%29%17%6%
896
April1st to 15thNight
25%24%31%14%3
775
16th to endNight
26%20%24%21%6%
623
May1st to 15thNight
23%18%27%21%7%1
630
16th to endNight
25%18%23%22%8%
567
June1st to 15thNight
28%17%18%26%8%
420
16th to endNight
26%15%17%30%9%
420
July1st to 15thNight
20%13%17%34%13%1
392
16th to endNight
22%16%18%29%11%1
447
August1st to 15thNight
24%10%15%32%15%1
574
16th to endNight
16%8%19%31%21%2
621
September1st to 15thNight
16%9%23%32%15%2
767
16th to endNight
18%12%24%25%15%3
777
October1st to 15thNight
23%16%23%24%10%1
780
16th to endNight
22%23%30%15%6%
832
November1st to 15thNight
20%26%22%21%7%1
820
16th to endNight
21%32%25%14%6%
892
December1st to 15thNight
26%31%25%13%3
900
16th to endNight
25%33%26%11%2
959
Page generated at 2019-07-12 07:32:36. Version ac04

Copyright 2019 A. Danko


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