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 2017-12-02.

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%
15190
18%
47770
7%
15158
36%
15158
19%
31664

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
11%
768
19%
2688
3%
768
30%
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
30%
631
15%
1293
March1st to 15thNight
14%
720
17%
2160
5%
720
29%
720
14%
1501
March16th to endNight
19%
768
22%
2014
6%
768
32%
768
19%
1383
April1st to 15thNight
16%
718
16%
1796
3%
718
24%
718
18%
1317
April16th to endNight
20%
617
18%
1697
3%
617
30%
617
19%
1158
May1st to 15thNight
20%
540
16%
1620
5%
540
31%
540
16%
1150
May16th to endNight
28%
376
12%
1498
9%
375
37%
375
14%
990
June1st to 15thNight
30%
360
12%
1440
9%
360
41%
360
14%
962
June16th to endNight
36%
360
11%
1440
11%
360
45%
360
21%
896
July1st to 15thNight
47%
340
12%
1360
15%
340
53%
340
22%
843
July16th to endNight
46%
376
15%
1418
15%
354
51%
354
23%
850
August1st to 15thNight
45%
472
16%
1543
11%
470
51%
470
26%
941
August16th to endNight
50%
759
20%
1905
10%
759
54%
759
33%
1252
September1st to 15thNight
36%
720
20%
1839
8%
720
43%
720
31%
1201
September16th to endNight
43%
717
27%
1980
17%
717
50%
717
32%
1220
October1st to 15thNight
33%
720
21%
2036
11%
720
44%
720
25%
1347
October16th to endNight
22%
768
17%
2478
6%
768
36%
768
18%
1640
November1st to 15thNight
24%
760
20%
2472
8%
760
40%
760
19%
1613
November16th to endNight
19%
780
21%
2555
8%
780
35%
780
17%
1657
December1st to 15thNight
17%
728
15%
2546
5%
728
30%
728
14%
1706
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%
48685
Night
16%7%25%5%3445%6%39%
47940
Cloud CoverDay
16%6%35%5%3445%6%37%
48695
Night
16%7%25%434446%39%
47945
TransparencyDay
45%5%11%21%13%1
48373
Night
44%5%11%21%14%1
47770
SeeingDay
22%15%29%19%9%3
6082
Night
21%15%33%21%6%
15190

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
17%6%35%5%33345%40%
2205
April1st to 15thDay
18%8%45%5%2344436%
2152
16th to endDay
17%6%25%424447%40%
2262
May1st to 15thDay
23%8%36%5%23445%31%
2341
16th to endDay
18%6%35%5%3445%7%36%
2618
June1st to 15thDay
22%6%34425%5%5%7%31%
2520
16th to endDay
15%5%25%6%35%5%7%9%32%
2520
July1st to 15thDay
12%5%36%6%46%5%6%7%35%
2520
16th to endDay
13%5%25%5%35%5%6%9%36%
2654
August1st to 15thDay
13%5%25%6%46%5%6%8%36%
2394
16th to endDay
10%5%25%435%5%6%8%41%
2296
September1st to 15thDay
14%5%25%5%3445%6%43%
2121
16th to endDay
14%415%323336%50%
1980
October1st to 15thDay
17%8%25%423445%40%
1923
16th to endDay
17%7%26%5%2335%440%
1746
November1st to 15thDay
16%8%26%414345%41%
1716
16th to endDay
18%6%34434335%43%
1732
December1st to 15thDay
19%6%346%34346%36%
1451
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%424445%41%
2014
April1st to 15thNight
19%6%36%6%24445%35%
1796
16th to endNight
21%6%34423335%40%
1697
May1st to 15thNight
18%8%36%5%24446%33%
1620
16th to endNight
17%7%25%6%3345%8%34%
1496
June1st to 15thNight
23%6%245%2345%6%34%
1440
16th to endNight
16%5%1443345%8%41%
1440
July1st to 15thNight
13%5%245%35%5%6%8%39%
1440
16th to endNight
11%415%5%245%7%9%40%
1514
August1st to 15thNight
10%415%6%25%5%5%7%45%
1553
16th to endNight
11%325%5%345%5%8%44%
1905
September1st to 15thNight
14%7%24323346%46%
1839
16th to endNight
13%5%23323346%51%
1980
October1st to 15thNight
17%6%25%5%23345%44%
2036
16th to endNight
19%7%25%5%23446%38%
2478
November1st to 15thNight
16%6%25%424345%42%
2472
16th to endNight
15%6%35%5%34345%40%
2555
December1st to 15thNight
20%7%35%42345%5%36%
2546
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
17%7%35%433445%40%
2205
April1st to 15thDay
17%9%36%5%2344436%
2152
16th to endDay
17%6%25%424347%39%
2262
May1st to 15thDay
23%8%37%424346%30%
2341
16th to endDay
18%6%345%3445%7%36%
2618
June1st to 15thDay
23%6%344345%5%7%31%
2520
16th to endDay
16%5%35%6%45%5%7%9%31%
2520
July1st to 15thDay
12%5%35%6%36%5%5%8%35%
2520
16th to endDay
13%5%25%5%35%5%6%8%37%
2654
August1st to 15thDay
13%5%246%46%5%6%8%36%
2394
16th to endDay
10%5%35%5%35%5%5%8%41%
2296
September1st to 15thDay
13%5%25%5%345%45%43%
2121
16th to endDay
14%424323345%50%
1980
October1st to 15thDay
17%7%26%423445%40%
1923
16th to endDay
17%7%25%5%34445%40%
1746
November1st to 15thDay
16%9%25%423445%41%
1716
16th to endDay
18%5%34434335%43%
1732
December1st to 15thDay
19%6%35%5%34346%37%
1451
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%
2013
April1st to 15thNight
19%7%26%6%25%445%35%
1796
16th to endNight
21%6%25%423335%40%
1697
May1st to 15thNight
18%8%36%5%34445%34%
1620
16th to endNight
17%6%35%6%3445%8%33%
1496
June1st to 15thNight
22%6%35%42445%6%34%
1440
16th to endNight
17%414435%46%8%41%
1440
July1st to 15thNight
12%5%25%5%3447%8%39%
1440
16th to endNight
11%425%5%345%6%9%40%
1514
August1st to 15thNight
9%4246%345%5%7%44%
1553
16th to endNight
11%325%5%25%45%8%44%
1905
September1st to 15thNight
13%7%24323447%46%
1839
16th to endNight
12%5%24323445%51%
1980
October1st to 15thNight
16%6%26%423345%43%
2036
16th to endNight
18%6%25%43445%5%39%
2478
November1st to 15thNight
16%6%35%5%24435%42%
2472
16th to endNight
16%6%35%5%33445%41%
2555
December1st to 15thNight
20%7%35%43335%6%36%
2546
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
46%49%18%16%4
2205
April1st to 15thDay
49%49%19%14%2
2152
16th to endDay
43%5%8%20%17%3
2262
May1st to 15thDay
54%47%19%12%1
2341
16th to endDay
46%6%12%19%12%1
2622
June1st to 15thDay
50%7%11%19%9%
2520
16th to endDay
45%7%13%19%12%
2520
July1st to 15thDay
44%8%10%22%13%1
2371
16th to endDay
41%6%15%20%13%1
2487
August1st to 15thDay
43%7%14%22%11%1
2380
16th to endDay
37%7%9%27%15%1
2296
September1st to 15thDay
40%5%10%22%17%2
2121
16th to endDay
35%47%24%25%2
1980
October1st to 15thDay
45%5%9%23%12%3
1923
16th to endDay
45%5%11%21%13%2
1746
November1st to 15thDay
45%412%22%13%1
1716
16th to endDay
43%411%23%15%1
1732
December1st to 15thDay
48%414%18%13%
1451
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%
2014
April1st to 15thNight
49%5%9%18%15%1
1796
16th to endNight
46%47%20%16%4
1697
May1st to 15thNight
50%5%8%18%14%2
1620
16th to endNight
47%7%13%19%10%1
1498
June1st to 15thNight
49%6%11%21%10%
1440
16th to endNight
40%8%13%23%14%
1440
July1st to 15thNight
39%8%12%26%12%
1360
16th to endNight
38%7%14%25%11%1
1418
August1st to 15thNight
36%7%14%23%15%1
1543
16th to endNight
36%6%11%27%16%
1905
September1st to 15thNight
38%5%12%24%18%
1839
16th to endNight
34%5%6%26%24%1
1980
October1st to 15thNight
42%411%23%14%3
2036
16th to endNight
45%413%24%11%
2478
November1st to 15thNight
44%5%10%20%17%2
2472
16th to endNight
45%413%21%13%1
2555
December1st to 15thNight
48%5%14%20%10%
2546
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
25%27%28%11%6%1
288
April1st to 15thDay
25%20%33%11%8%
270
16th to endDay
23%16%31%18%6%4
270
May1st to 15thDay
35%15%23%15%7%2
288
16th to endDay
20%14%32%22%7%2
331
June1st to 15thDay
25%14%27%21%9%1
315
16th to endDay
18%14%35%19%9%2
315
July1st to 15thDay
19%8%32%27%10%1
305
16th to endDay
15%7%28%30%15%3
326
August1st to 15thDay
17%7%27%28%14%5%
313
16th to endDay
13%8%25%25%18%6%
332
September1st to 15thDay
17%10%31%20%12%7%
315
16th to endDay
17%7%27%17%16%13%
314
October1st to 15thDay
22%9%26%23%11%5%
315
16th to endDay
23%19%31%18%6%
292
November1st to 15thDay
22%14%34%18%8%1
280
16th to endDay
25%20%27%18%6%1
262
December1st to 15thDay
22%30%28%12%6%
63
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
768
April1st to 15thNight
24%19%36%16%2
718
16th to endNight
26%17%32%18%5%
617
May1st to 15thNight
24%14%36%19%4
540
16th to endNight
18%9%37%26%6%
376
June1st to 15thNight
25%7%30%28%7%
360
16th to endNight
18%5%35%34%6%
360
July1st to 15thNight
15%230%40%10%
340
16th to endNight
11%332%35%16%
376
August1st to 15thNight
12%429%41%12%
472
16th to endNight
14%325%39%15%2
759
September1st to 15thNight
18%6%33%30%10%1
720
16th to endNight
16%6%28%27%17%4
717
October1st to 15thNight
21%10%30%25%10%1
720
16th to endNight
25%16%31%19%6%
768
November1st to 15thNight
21%18%31%19%7%
760
16th to endNight
20%19%36%16%6%
780
December1st to 15thNight
26%17%35%15%4
728
16th to endNight
26%23%34%14%1
768
Page generated at 2017-12-03 11:16:46. Version ac04

Copyright 2017 A. Danko


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