Z-Field Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Z-Field Observatory clear sky chart from 2005-11-05 to 2018-01-20.

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
51%
16140
29%
48454
20%
16096
59%
16096
38%
26875

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
39%
972
30%
2522
12%
970
50%
970
39%
1489
January16th to endNight
36%
985
34%
2561
13%
985
49%
985
41%
1494
February1st to 15thNight
33%
785
26%
2192
9%
781
44%
781
34%
1310
February16th to endNight
27%
603
30%
1865
11%
603
42%
603
32%
1045
March1st to 15thNight
33%
540
27%
1980
15%
540
48%
540
26%
1118
March16th to endNight
38%
576
24%
2112
18%
576
50%
576
27%
1181
April1st to 15thNight
33%
539
24%
1956
14%
539
49%
539
31%
1048
April16th to endNight
37%
540
26%
1728
18%
540
50%
540
33%
968
May1st to 15thNight
38%
540
24%
1620
14%
540
51%
540
33%
964
May16th to endNight
55%
563
26%
1684
23%
561
65%
561
38%
951
June1st to 15thNight
73%
540
24%
1620
25%
540
78%
540
50%
816
June16th to endNight
76%
540
26%
1620
28%
540
79%
540
46%
848
July1st to 15thNight
80%
510
23%
1530
27%
510
82%
510
44%
807
July16th to endNight
90%
564
31%
1594
39%
531
91%
531
55%
760
August1st to 15thNight
89%
539
37%
1610
43%
536
89%
536
59%
750
August16th to endNight
80%
573
29%
1866
35%
573
81%
573
37%
1034
September1st to 15thNight
71%
540
23%
1980
27%
540
72%
540
34%
1035
September16th to endNight
73%
537
34%
1980
37%
537
77%
537
40%
973
October1st to 15thNight
62%
633
33%
2073
27%
633
67%
633
41%
1063
October16th to endNight
52%
768
38%
2304
22%
768
63%
768
48%
1155
November1st to 15thNight
52%
763
30%
2283
19%
763
60%
763
40%
1257
November16th to endNight
42%
975
34%
2535
16%
975
52%
975
41%
1478
December1st to 15thNight
38%
975
29%
2535
14%
975
48%
975
35%
1590
December16th to endNight
35%
1040
26%
2704
12%
1040
44%
1040
33%
1741

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
11%424424447%51%
49061
Night
9%413323346%56%
48651
TransparencyDay
32%6%10%22%21%6%
48761
Night
28%49%22%23%10%
48456
SeeingDay
10%418%28%28%9%
5976
Night
11%5%27%33%19%2
16143

By Month

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
13%5%23312235%56%
1746
16th to endDay
14%412222335%55%
1767
February1st to 15thDay
16%43432223452%
1678
16th to endDay
17%423312235%52%
1607
March1st to 15thDay
18%6%14413235%46%
1980
16th to endDay
14%5%34423345%47%
2110
April1st to 15thDay
13%5%13324446%50%
1990
16th to endDay
14%42332445%6%48%
2229
May1st to 15thDay
14%5%35%434446%43%
2331
16th to endDay
9%5%35%5%36%6%5%7%40%
2430
June1st to 15thDay
5%3145%35%6%6%7%51%
2340
16th to endDay
6%313425%5%5%10%51%
2340
July1st to 15thDay
6%324436%6%6%9%45%
2340
16th to endDay
32144346%7%11%50%
2466
August1st to 15thDay
31134245%6%9%58%
2340
16th to endDay
6%415%5%35%7%7%10%41%
2331
September1st to 15thDay
11%41433445%7%50%
1980
16th to endDay
8%31342345%6%55%
1980
October1st to 15thDay
10%31332323459%
1873
16th to endDay
8%31321223466%
1920
November1st to 15thDay
9%413433435%55%
1901
16th to endDay
12%41231222460%
1755
December1st to 15thDay
16%6%1432323450%
1755
16th to endDay
17%7%24322336%46%
1872
January1st to 15thNight
11%413422335%57%
2522
16th to endNight
11%42321234458%
2561
February1st to 15thNight
12%5%2432423455%
2183
16th to endNight
13%412222335%56%
1865
March1st to 15thNight
16%5%24312346%49%
1980
16th to endNight
11%5%14323345%52%
2112
April1st to 15thNight
9%32431435%7%53%
1956
16th to endNight
11%413323446%54%
1728
May1st to 15thNight
9%5%24433347%51%
1619
16th to endNight
6%3133245%5%11%51%
1683
June1st to 15thNight
321332245%9%61%
1620
16th to endNight
311232345%10%59%
1620
July1st to 15thNight
43134235%7%12%52%
1620
16th to endNight
21121346%10%62%
1702
August1st to 15thNight
22221245%9%67%
1620
16th to endNight
5%314335%5%7%11%48%
1866
September1st to 15thNight
10%42432335%7%53%
1980
16th to endNight
6%413323347%59%
1980
October1st to 15thNight
9%413222336%60%
2073
16th to endNight
9%21221222368%
2304
November1st to 15thNight
7%42332333460%
2283
16th to endNight
11%413222345%58%
2535
December1st to 15thNight
14%5%14423245%52%
2535
16th to endNight
16%6%24312346%47%
2704

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
32%37%21%28%6%
1747
16th to endDay
32%37%20%28%8%
1767
February1st to 15thDay
37%35%22%23%7%
1679
16th to endDay
35%36%22%21%10%
1607
March1st to 15thDay
41%36%18%20%8%
1980
16th to endDay
40%5%9%17%21%5%
2112
April1st to 15thDay
35%49%22%17%10%
1990
16th to endDay
35%7%9%18%24%5%
2229
May1st to 15thDay
42%5%8%16%19%7%
2331
16th to endDay
39%9%11%21%16%2
2434
June1st to 15thDay
30%8%11%27%19%2
2340
16th to endDay
28%7%12%26%20%3
2340
July1st to 15thDay
30%8%14%31%14%1
2201
16th to endDay
22%10%19%30%15%1
2311
August1st to 15thDay
19%8%15%31%23%2
2326
16th to endDay
33%9%16%25%11%2
2331
September1st to 15thDay
32%6%12%22%18%7%
1980
16th to endDay
26%6%10%21%26%8%
1980
October1st to 15thDay
28%410%21%19%14%
1873
16th to endDay
23%38%17%31%16%
1920
November1st to 15thDay
31%48%24%23%8%
1901
16th to endDay
29%35%18%32%10%
1755
December1st to 15thDay
39%36%19%25%6%
1755
16th to endDay
40%37%22%19%6%
1872
January1st to 15thNight
29%37%18%29%10%
2522
16th to endNight
29%26%19%29%12%
2561
February1st to 15thNight
34%37%18%23%12%
2193
16th to endNight
32%38%21%21%13%
1865
March1st to 15thNight
36%38%17%21%11%
1980
16th to endNight
34%410%19%24%6%
2112
April1st to 15thNight
29%5%11%21%18%13%
1956
16th to endNight
31%410%20%23%9%
1728
May1st to 15thNight
33%5%11%18%21%8%
1620
16th to endNight
26%7%17%26%18%3
1685
June1st to 15thNight
20%413%34%22%5%
1620
16th to endNight
21%6%11%29%24%6%
1620
July1st to 15thNight
22%6%14%35%17%3
1530
16th to endNight
13%7%18%35%22%2
1594
August1st to 15thNight
13%413%32%28%6%
1610
16th to endNight
27%6%13%30%16%4
1866
September1st to 15thNight
30%410%22%22%10%
1980
16th to endNight
24%48%23%26%11%
1980
October1st to 15thNight
25%39%22%20%16%
2073
16th to endNight
22%2417%30%22%
2304
November1st to 15thNight
28%38%24%23%12%
2283
16th to endNight
29%36%15%32%12%
2535
December1st to 15thNight
36%38%19%22%10%
2535
16th to endNight
38%37%20%21%8%
2704

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
13%8%37%30%9%
135
16th to endDay
16%7%39%21%11%3
109
February1st to 15thDay
14%7%45%23%8%
90
16th to endDay
13%11%44%23%7%
81
March1st to 15thDay
22%14%35%20%6%1
90
16th to endDay
9%7%42%25%12%3
96
April1st to 15thDay
14%8%30%33%13%
109
16th to endDay
17%8%35%26%9%1
339
May1st to 15thDay
17%9%33%27%9%3
468
16th to endDay
13%5%31%28%19%2
517
June1st to 15thDay
6%112%38%31%9%
495
16th to endDay
8%111%29%36%12%
495
July1st to 15thDay
6%6%34%39%11%
475
16th to endDay
3318%51%22%
515
August1st to 15thDay
5%228%48%15%
493
16th to endDay
11%15%28%38%15%
371
September1st to 15thDay
2215%43%36%
135
16th to endDay
6%5%22%43%22%
135
October1st to 15thDay
113%31%40%13%
135
16th to endDay
10%5%15%34%28%5%
144
November1st to 15thDay
19%224%31%14%7%
135
16th to endDay
18%11%26%28%12%2
135
December1st to 15thDay
22%230%30%11%2
135
16th to endDay
22%12%35%23%5%
144
January1st to 15thNight
13%10%32%35%7%
975
16th to endNight
13%8%39%30%6%
985
February1st to 15thNight
14%9%39%29%6%
785
16th to endNight
15%11%44%25%2
603
March1st to 15thNight
17%9%36%30%6%
540
16th to endNight
12%9%36%32%8%
576
April1st to 15thNight
9%9%43%29%7%
539
16th to endNight
12%7%40%27%11%
540
May1st to 15thNight
11%440%31%9%2
540
16th to endNight
5%231%39%19%2
563
June1st to 15thNight
3115%44%32%2
540
16th to endNight
3112%45%32%5%
540
July1st to 15thNight
47%41%41%4
510
16th to endNight
3235%53%6%
564
August1st to 15thNight
2334%52%7%
539
16th to endNight
6%5%35%45%6%
573
September1st to 15thNight
13%9%31%42%3
540
16th to endNight
8%9%39%37%5%
537
October1st to 15thNight
10%120%37%25%3
633
16th to endNight
10%5%29%32%19%2
768
November1st to 15thNight
8%5%30%36%18%1
763
16th to endNight
14%8%32%29%12%
975
December1st to 15thNight
17%7%32%33%8%
975
16th to endNight
19%7%33%31%6%
1040
Page generated at 2018-01-21 11:19:26. Version ac04

Copyright 2018 A. Danko


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