Petersham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Petersham clear sky chart from 2005-11-05 to 2017-03-25.

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
24%
14384
18%
45281
7%
14352
36%
14352
18%
30061

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%
720
19%
2506
5%
716
30%
716
14%
1631
January16th to endNight
12%
768
19%
2688
3%
768
31%
768
12%
1756
February1st to 15thNight
9%
704
15%
2198
2%
701
25%
701
14%
1527
February16th to endNight
12%
631
20%
1893
5%
631
31%
631
15%
1293
March1st to 15thNight
14%
720
17%
2160
5%
720
29%
720
14%
1501
March16th to endNight
19%
744
22%
1954
6%
744
32%
744
19%
1335
April1st to 15thNight
15%
658
17%
1646
3%
658
24%
658
18%
1200
April16th to endNight
22%
566
18%
1556
4%
566
32%
566
20%
1053
May1st to 15thNight
20%
495
17%
1485
6%
495
31%
495
17%
1041
May16th to endNight
28%
344
12%
1370
9%
343
36%
343
15%
897
June1st to 15thNight
30%
330
13%
1320
9%
330
41%
330
13%
900
June16th to endNight
39%
330
11%
1320
11%
330
46%
330
21%
814
July1st to 15thNight
48%
310
13%
1240
15%
310
54%
310
22%
769
July16th to endNight
45%
348
13%
1306
13%
326
51%
326
22%
784
August1st to 15thNight
43%
432
17%
1413
11%
430
50%
430
26%
859
August16th to endNight
49%
695
18%
1745
9%
695
52%
695
32%
1152
September1st to 15thNight
37%
660
21%
1686
8%
660
44%
660
31%
1109
September16th to endNight
42%
657
25%
1815
16%
657
50%
657
32%
1117
October1st to 15thNight
30%
660
22%
1866
10%
660
42%
660
24%
1249
October16th to endNight
22%
704
15%
2271
5%
704
35%
704
18%
1518
November1st to 15thNight
24%
700
21%
2277
8%
700
42%
700
19%
1474
November16th to endNight
20%
720
19%
2358
7%
720
34%
720
16%
1562
December1st to 15thNight
17%
720
15%
2520
5%
720
30%
720
14%
1690
December16th to endNight
13%
768
17%
2688
4%
768
29%
768
12%
1830

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
16%7%25%5%3445%6%37%
45681
Night
16%7%25%5%3445%6%39%
45451
Cloud CoverDay
16%6%35%5%3445%6%37%
45691
Night
16%7%25%434446%39%
45456
TransparencyDay
45%5%11%21%13%1
45369
Night
44%5%11%21%14%1
45281
SeeingDay
21%15%29%20%9%3
5134
Night
21%15%33%21%6%
14384

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
14%7%25%5%35%5%6%7%35%
1432
16th to endDay
13%7%26%5%245%5%6%38%
1536
February1st to 15thDay
15%7%37%7%36%5%5%6%31%
1660
16th to endDay
15%9%35%6%24345%38%
1580
March1st to 15thDay
18%9%36%5%24446%33%
1790
16th to endDay
16%6%35%5%33345%41%
2133
April1st to 15thDay
17%9%36%5%2444436%
1972
16th to endDay
16%7%25%424447%40%
2073
May1st to 15thDay
22%8%26%424445%32%
2145
16th to endDay
17%5%25%5%3445%8%37%
2394
June1st to 15thDay
23%7%34425%5%5%7%30%
2310
16th to endDay
15%5%25%6%35%5%7%9%32%
2310
July1st to 15thDay
11%6%36%6%46%5%6%8%35%
2310
16th to endDay
13%5%25%5%35%5%6%9%36%
2458
August1st to 15thDay
13%425%6%46%5%6%8%37%
2195
16th to endDay
10%5%25%5%35%5%6%8%40%
2104
September1st to 15thDay
14%6%25%5%3445%6%42%
1944
16th to endDay
15%414323336%50%
1815
October1st to 15thDay
18%8%26%423445%39%
1763
16th to endDay
17%7%26%5%2335%5%39%
1601
November1st to 15thDay
16%8%26%414345%41%
1581
16th to endDay
18%6%34434335%43%
1599
December1st to 15thDay
19%6%346%34346%36%
1440
16th to endDay
20%9%245%245%5%6%31%
1536
January1st to 15thNight
15%9%35%434446%38%
2506
16th to endNight
14%8%36%6%34446%37%
2688
February1st to 15thNight
13%9%36%6%35%5%5%6%35%
2197
16th to endNight
15%8%35%5%25%45%5%37%
1893
March1st to 15thNight
17%8%34445%446%36%
2147
16th to endNight
15%7%35%435%445%41%
1954
April1st to 15thNight
19%6%35%6%34446%35%
1646
16th to endNight
21%5%33423336%41%
1556
May1st to 15thNight
17%8%36%5%24446%35%
1485
16th to endNight
17%7%25%6%3435%8%35%
1368
June1st to 15thNight
23%6%345%235%5%5%33%
1320
16th to endNight
15%41443345%8%42%
1320
July1st to 15thNight
12%5%25%5%35%5%6%8%39%
1320
16th to endNight
12%415%5%245%7%9%40%
1402
August1st to 15thNight
10%4146%245%5%7%44%
1423
16th to endNight
11%325%5%345%5%8%43%
1745
September1st to 15thNight
14%7%24323447%45%
1686
16th to endNight
14%5%23322346%51%
1815
October1st to 15thNight
18%6%25%5%2334443%
1866
16th to endNight
18%7%25%434446%37%
2271
November1st to 15thNight
16%6%25%424346%43%
2277
16th to endNight
16%6%45%5%34345%39%
2358
December1st to 15thNight
20%7%35%42345%5%36%
2520
16th to endNight
19%8%245%2445%6%34%
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
14%8%15%6%35%5%6%6%36%
1432
16th to endDay
13%6%35%5%35%5%5%6%39%
1536
February1st to 15thDay
15%7%37%7%36%5%46%31%
1660
16th to endDay
15%9%36%5%24335%39%
1580
March1st to 15thDay
18%9%45%434446%33%
1800
16th to endDay
16%6%35%433445%41%
2133
April1st to 15thDay
17%9%35%5%23445%36%
1972
16th to endDay
16%6%25%424347%40%
2073
May1st to 15thDay
22%8%36%424346%32%
2145
16th to endDay
17%5%345%3445%7%37%
2394
June1st to 15thDay
24%6%444245%5%7%30%
2310
16th to endDay
15%425%6%45%5%7%9%32%
2310
July1st to 15thDay
12%5%35%7%36%5%6%8%34%
2310
16th to endDay
13%5%245%36%5%6%8%36%
2458
August1st to 15thDay
13%425%5%46%5%5%8%36%
2195
16th to endDay
10%5%35%5%35%5%5%8%39%
2104
September1st to 15thDay
13%6%25%5%34445%43%
1944
16th to endDay
14%424323335%50%
1815
October1st to 15thDay
17%7%26%423345%39%
1763
16th to endDay
16%8%25%5%34445%39%
1601
November1st to 15thDay
16%8%25%433445%42%
1581
16th to endDay
18%5%34434335%43%
1599
December1st to 15thDay
19%6%35%5%34346%37%
1440
16th to endDay
20%9%34435%5%5%5%32%
1536
January1st to 15thNight
16%8%35%43435%6%38%
2506
16th to endNight
14%8%36%5%34446%37%
2688
February1st to 15thNight
13%8%36%435%5%5%6%35%
2190
16th to endNight
16%8%35%435%5%45%37%
1893
March1st to 15thNight
18%8%35%435%446%35%
2160
16th to endNight
15%7%34425%436%41%
1953
April1st to 15thNight
19%7%26%6%25%445%35%
1646
16th to endNight
21%5%24423335%41%
1556
May1st to 15thNight
17%8%46%434445%35%
1485
16th to endNight
17%6%346%3445%8%35%
1368
June1st to 15thNight
22%7%35%42445%6%32%
1320
16th to endNight
16%414335%46%8%41%
1320
July1st to 15thNight
12%425%5%45%5%7%8%39%
1320
16th to endNight
12%425%5%345%6%9%40%
1402
August1st to 15thNight
9%425%7%245%5%7%44%
1423
16th to endNight
11%325%5%25%45%8%43%
1745
September1st to 15thNight
13%6%24323447%45%
1686
16th to endNight
13%424322445%51%
1815
October1st to 15thNight
17%6%26%423335%42%
1866
16th to endNight
18%7%25%43445%5%38%
2271
November1st to 15thNight
16%6%25%5%24436%42%
2277
16th to endNight
17%6%35%5%33445%39%
2358
December1st to 15thNight
20%7%35%43335%5%36%
2520
16th to endNight
19%8%245%2445%6%34%
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
44%6%12%24%9%2
1433
16th to endDay
43%5%14%22%12%1
1536
February1st to 15thDay
52%6%10%16%12%
1663
16th to endDay
48%49%20%13%2
1580
March1st to 15thDay
50%411%20%11%2
1800
16th to endDay
45%49%18%17%4
2133
April1st to 15thDay
49%49%19%14%2
1972
16th to endDay
43%5%8%21%18%3
2073
May1st to 15thDay
52%47%19%13%2
2145
16th to endDay
45%7%12%20%12%1
2398
June1st to 15thDay
52%7%12%19%8%
2310
16th to endDay
44%7%13%19%12%
2310
July1st to 15thDay
44%8%9%23%12%1
2161
16th to endDay
41%6%16%20%12%2
2291
August1st to 15thDay
42%7%13%22%12%1
2181
16th to endDay
38%7%9%27%14%2
2104
September1st to 15thDay
41%5%10%23%18%1
1944
16th to endDay
36%47%24%25%2
1815
October1st to 15thDay
45%5%9%22%12%3
1763
16th to endDay
45%5%12%20%13%2
1601
November1st to 15thDay
45%412%22%13%1
1581
16th to endDay
43%411%22%15%1
1599
December1st to 15thDay
48%415%18%13%
1440
16th to endDay
52%5%14%16%10%
1536
January1st to 15thNight
46%412%21%12%2
2506
16th to endNight
46%410%21%13%2
2688
February1st to 15thNight
47%7%13%18%11%1
2198
16th to endNight
46%5%10%21%13%2
1893
March1st to 15thNight
48%5%10%20%12%2
2160
16th to endNight
43%5%9%18%17%5%
1954
April1st to 15thNight
49%5%9%18%15%1
1646
16th to endNight
45%47%20%17%4
1556
May1st to 15thNight
49%5%8%18%14%2
1485
16th to endNight
47%7%13%19%11%1
1370
June1st to 15thNight
50%6%11%20%9%
1320
16th to endNight
39%8%13%24%13%
1320
July1st to 15thNight
38%9%13%25%12%
1240
16th to endNight
37%8%14%25%11%1
1306
August1st to 15thNight
36%7%14%23%16%1
1413
16th to endNight
36%7%12%28%15%
1745
September1st to 15thNight
38%5%13%23%18%
1686
16th to endNight
34%5%6%26%25%1
1815
October1st to 15thNight
43%410%22%14%3
1866
16th to endNight
46%5%13%22%11%
2271
November1st to 15thNight
43%5%10%20%17%2
2277
16th to endNight
47%413%20%12%1
2358
December1st to 15thNight
48%5%14%20%10%
2520
16th to endNight
49%5%11%20%11%
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
23%28%31%13%3
60
16th to endDay
29%20%39%9%1
64
February1st to 15thDay
26%28%30%13%1
191
16th to endDay
22%25%33%15%2
239
March1st to 15thDay
30%23%31%11%3
270
16th to endDay
22%27%28%11%7%1
264
April1st to 15thDay
25%20%30%13%9%
210
16th to endDay
20%15%30%20%6%5%
210
May1st to 15thDay
33%14%23%17%8%2
228
16th to endDay
18%12%32%25%9%2
267
June1st to 15thDay
26%16%21%23%10%1
255
16th to endDay
17%8%35%23%11%3
255
July1st to 15thDay
17%7%31%29%12%1
245
16th to endDay
14%7%27%32%14%2
270
August1st to 15thDay
16%5%29%29%14%4
253
16th to endDay
14%7%22%25%21%8%
268
September1st to 15thDay
17%7%30%22%12%9%
255
16th to endDay
16%9%27%18%15%11%
254
October1st to 15thDay
23%11%24%23%10%5%
255
16th to endDay
21%21%32%18%5%
243
November1st to 15thDay
23%14%33%17%9%1
235
16th to endDay
25%19%27%18%7%1
219
December1st to 15thDay
23%31%28%11%5%
60
16th to endDay
42%25%25%7%
64
January1st to 15thNight
21%24%38%13%2
720
16th to endNight
23%23%35%14%1
768
February1st to 15thNight
21%22%43%12%
704
16th to endNight
25%23%35%13%1
631
March1st to 15thNight
26%22%34%14%1
720
16th to endNight
22%21%32%18%4
744
April1st to 15thNight
25%20%36%15%2
658
16th to endNight
25%17%31%19%5%
566
May1st to 15thNight
23%14%36%20%4
495
16th to endNight
18%9%37%27%6%
344
June1st to 15thNight
25%7%30%27%7%
330
16th to endNight
17%5%33%36%7%
330
July1st to 15thNight
15%230%40%11%
310
16th to endNight
12%332%35%15%
348
August1st to 15thNight
11%429%41%12%
432
16th to endNight
14%326%37%15%2
695
September1st to 15thNight
18%6%32%30%10%1
660
16th to endNight
16%7%28%28%16%3
657
October1st to 15thNight
22%11%30%24%9%1
660
16th to endNight
25%16%32%20%5%
704
November1st to 15thNight
21%18%32%20%7%
700
16th to endNight
21%19%34%16%6%
720
December1st to 15thNight
26%18%35%15%4
720
16th to endNight
26%23%34%14%1
768
Page generated at 2017-03-26 13:16:43. Version ac04

Copyright 2017 A. Danko


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