Amherst
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Amherst clear sky chart from 2005-11-05 to 2017-09-10.

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
28%
14864
18%
46758
8%
14832
39%
14832
20%
30960

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
19%
720
19%
2506
7%
716
34%
716
16%
1621
January16th to endNight
16%
768
18%
2688
5%
768
33%
768
15%
1773
February1st to 15thNight
14%
704
15%
2193
3%
701
29%
701
16%
1515
February16th to endNight
17%
631
20%
1893
7%
631
33%
631
16%
1292
March1st to 15thNight
17%
720
17%
2160
6%
720
30%
720
15%
1494
March16th to endNight
21%
768
22%
2010
7%
768
34%
768
22%
1367
April1st to 15thNight
20%
718
16%
1796
3%
718
28%
718
19%
1308
April16th to endNight
23%
629
18%
1709
5%
629
32%
629
21%
1172
May1st to 15thNight
21%
529
16%
1609
7%
529
30%
529
16%
1131
May16th to endNight
34%
375
12%
1497
11%
374
41%
374
16%
981
June1st to 15thNight
35%
360
13%
1440
11%
360
45%
360
16%
946
June16th to endNight
42%
360
12%
1440
12%
360
51%
360
21%
880
July1st to 15thNight
52%
340
13%
1360
17%
340
57%
340
22%
832
July16th to endNight
53%
376
15%
1418
17%
354
57%
354
22%
848
August1st to 15thNight
47%
473
17%
1544
12%
471
53%
471
27%
941
August16th to endNight
55%
764
19%
1910
10%
764
57%
764
32%
1258
September1st to 15thNight
41%
700
20%
1777
9%
700
46%
700
31%
1171
September16th to endNight
47%
657
26%
1815
17%
657
54%
657
32%
1117
October1st to 15thNight
35%
660
22%
1878
12%
660
46%
660
25%
1242
October16th to endNight
25%
704
15%
2260
6%
704
36%
704
20%
1511
November1st to 15thNight
28%
700
21%
2277
10%
700
43%
700
22%
1440
November16th to endNight
24%
720
18%
2370
8%
720
37%
720
17%
1581
December1st to 15thNight
21%
720
15%
2520
6%
720
34%
720
15%
1699
December16th to endNight
16%
768
17%
2688
5%
768
31%
768
14%
1840

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
17%6%25%43445%7%38%
47877
Night
17%6%25%42445%6%40%
46928
Cloud CoverDay
17%6%25%43445%7%38%
47887
Night
17%6%25%42445%6%40%
46933
TransparencyDay
45%5%12%21%13%1
47565
Night
45%5%11%21%14%1
46758
SeeingDay
22%12%28%21%10%3
5784
Night
22%12%31%25%7%
14864

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
16%6%35%4345%7%6%36%
1432
16th to endDay
14%8%25%42445%7%39%
1536
February1st to 15thDay
15%8%47%5%35%45%7%32%
1663
16th to endDay
15%8%36%5%33446%37%
1580
March1st to 15thDay
19%8%35%424447%34%
1790
16th to endDay
17%6%44433345%42%
2211
April1st to 15thDay
19%6%45%434346%37%
2152
16th to endDay
17%5%14425%45%6%42%
2250
May1st to 15thDay
23%6%36%424446%32%
2352
16th to endDay
18%5%25%5%345%47%37%
2618
June1st to 15thDay
23%5%3443445%8%33%
2520
16th to endDay
15%4246%36%5%7%9%35%
2520
July1st to 15thDay
12%436%6%36%5%7%8%36%
2520
16th to endDay
13%425%5%35%5%7%8%38%
2654
August1st to 15thDay
12%5%25%6%46%5%6%7%38%
2393
16th to endDay
11%3244345%6%7%44%
2292
September1st to 15thDay
15%4245%2445%5%44%
2071
16th to endDay
15%424322336%50%
1815
October1st to 15thDay
20%5%245%34337%39%
1749
16th to endDay
17%6%35%5%34446%38%
1612
November1st to 15thDay
17%6%3333345%6%42%
1581
16th to endDay
20%6%24423446%42%
1590
December1st to 15thDay
20%6%45%424446%38%
1440
16th to endDay
21%8%34424447%33%
1536
January1st to 15thNight
17%8%35%424445%40%
2506
16th to endNight
14%8%35%5%345%5%5%38%
2688
February1st to 15thNight
12%8%36%6%36%45%6%36%
2192
16th to endNight
15%7%26%424445%40%
1893
March1st to 15thNight
19%7%24435%446%37%
2147
16th to endNight
15%6%34423446%43%
2010
April1st to 15thNight
18%7%36%434335%38%
1796
16th to endNight
22%5%24323345%41%
1709
May1st to 15thNight
20%7%25%5%2345%5%36%
1609
16th to endNight
19%425%4335%6%9%35%
1495
June1st to 15thNight
22%5%34433446%37%
1440
16th to endNight
16%42442435%7%42%
1440
July1st to 15thNight
14%424435%46%8%42%
1440
16th to endNight
11%414435%5%7%8%43%
1514
August1st to 15thNight
10%425%5%3335%8%46%
1554
16th to endNight
13%41443445%8%45%
1910
September1st to 15thNight
15%424334445%47%
1777
16th to endNight
15%414223336%52%
1815
October1st to 15thNight
20%42442324445%
1878
16th to endNight
19%7%2442345%5%39%
2260
November1st to 15thNight
17%6%24424345%45%
2277
16th to endNight
19%7%245%24445%39%
2370
December1st to 15thNight
21%6%25%42345%6%37%
2520
16th to endNight
21%7%3442446%5%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
16%7%24425%5%6%7%36%
1432
16th to endDay
15%6%35%5%2446%5%40%
1536
February1st to 15thDay
16%8%38%43445%8%31%
1663
16th to endDay
16%8%46%5%24345%37%
1580
March1st to 15thDay
20%9%26%424346%34%
1800
16th to endDay
18%6%35%5%34335%41%
2211
April1st to 15thDay
19%7%35%424446%37%
2152
16th to endDay
17%6%23424447%42%
2250
May1st to 15thDay
23%7%36%424445%32%
2352
16th to endDay
17%6%25%5%25%45%7%37%
2618
June1st to 15thDay
22%5%35%5%2345%8%33%
2520
16th to endDay
15%5%246%35%5%6%8%34%
2520
July1st to 15thDay
13%5%25%5%35%6%6%9%36%
2520
16th to endDay
13%425%5%35%6%7%7%37%
2654
August1st to 15thDay
12%5%146%45%5%5%8%38%
2393
16th to endDay
11%414435%46%8%44%
2292
September1st to 15thDay
15%5%25%5%34446%44%
2071
16th to endDay
15%5%14323345%50%
1815
October1st to 15thDay
20%6%24433347%40%
1749
16th to endDay
17%6%35%5%235%46%39%
1612
November1st to 15thDay
18%7%24333446%41%
1581
16th to endDay
20%6%23324435%42%
1590
December1st to 15thDay
20%7%35%434435%38%
1440
16th to endDay
23%7%34325%448%32%
1536
January1st to 15thNight
17%8%34423346%39%
2506
16th to endNight
16%9%25%5%3445%5%37%
2688
February1st to 15thNight
13%8%36%6%35%5%5%6%36%
2185
16th to endNight
16%8%35%43444439%
1893
March1st to 15thNight
20%7%345%24446%36%
2160
16th to endNight
16%7%34423346%43%
2009
April1st to 15thNight
19%8%35%5%24345%37%
1796
16th to endNight
22%5%24423345%41%
1709
May1st to 15thNight
20%6%35%6%33446%35%
1609
16th to endNight
19%5%235%335%6%8%36%
1495
June1st to 15thNight
22%425%4235%5%5%37%
1440
16th to endNight
16%6%2433445%8%41%
1440
July1st to 15thNight
14%5%14435%5%6%7%41%
1440
16th to endNight
11%335%425%47%9%41%
1514
August1st to 15thNight
10%5%25%6%24448%46%
1554
16th to endNight
12%41443445%8%45%
1910
September1st to 15thNight
14%5%2442345%5%46%
1777
16th to endNight
15%5%23323246%51%
1815
October1st to 15thNight
19%6%24423235%44%
1878
16th to endNight
20%6%25%42345%5%38%
2260
November1st to 15thNight
17%5%25%42334445%
2277
16th to endNight
19%7%24424445%38%
2370
December1st to 15thNight
22%6%35%423346%36%
2520
16th to endNight
22%8%25%42446%5%33%
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%5%14%24%9%2
1433
16th to endDay
43%5%14%23%12%
1536
February1st to 15thDay
52%5%12%18%11%
1666
16th to endDay
48%39%21%13%2
1580
March1st to 15thDay
50%312%19%10%2
1800
16th to endDay
45%39%19%16%4
2211
April1st to 15thDay
49%310%19%14%2
2152
16th to endDay
42%5%8%21%19%3
2250
May1st to 15thDay
54%46%19%12%1
2352
16th to endDay
46%6%12%19%13%1
2622
June1st to 15thDay
50%5%12%20%9%
2520
16th to endDay
45%7%13%20%12%
2520
July1st to 15thDay
43%7%11%21%13%1
2371
16th to endDay
40%7%15%20%12%2
2487
August1st to 15thDay
43%6%13%23%11%1
2379
16th to endDay
36%5%10%27%16%1
2292
September1st to 15thDay
41%5%11%23%17%1
2071
16th to endDay
36%38%24%25%1
1815
October1st to 15thDay
45%49%24%12%3
1749
16th to endDay
45%414%19%13%2
1612
November1st to 15thDay
44%5%13%21%12%1
1581
16th to endDay
44%312%22%15%1
1590
December1st to 15thDay
48%415%18%12%
1440
16th to endDay
52%415%17%10%
1536
January1st to 15thNight
46%313%21%12%2
2506
16th to endNight
47%411%20%13%2
2688
February1st to 15thNight
48%5%13%19%11%1
2193
16th to endNight
48%49%21%13%2
1893
March1st to 15thNight
49%310%20%13%2
2160
16th to endNight
43%49%19%17%5%
2010
April1st to 15thNight
51%38%18%16%1
1796
16th to endNight
46%37%20%16%4
1709
May1st to 15thNight
50%5%8%17%14%3
1609
16th to endNight
45%8%14%19%10%1
1497
June1st to 15thNight
48%5%12%22%9%
1440
16th to endNight
40%6%13%24%14%
1440
July1st to 15thNight
39%7%13%26%13%
1360
16th to endNight
38%8%13%25%12%1
1418
August1st to 15thNight
37%6%13%25%15%1
1544
16th to endNight
36%6%10%27%17%
1910
September1st to 15thNight
38%5%11%24%18%
1777
16th to endNight
35%47%25%25%2
1815
October1st to 15thNight
43%311%22%15%3
1878
16th to endNight
46%5%13%22%11%
2260
November1st to 15thNight
42%412%20%17%2
2277
16th to endNight
47%412%20%12%1
2370
December1st to 15thNight
49%5%13%20%10%
2520
16th to endNight
50%5%12%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
21%28%26%18%5%
60
16th to endDay
29%15%37%17%
64
February1st to 15thDay
27%20%31%18%1
181
16th to endDay
21%22%33%20%2
239
March1st to 15thDay
32%17%30%15%4
270
16th to endDay
23%20%34%12%7%2
288
April1st to 15thDay
27%13%32%17%8%
270
16th to endDay
23%14%28%21%6%4
270
May1st to 15thDay
34%14%23%17%9%1
288
16th to endDay
22%9%33%21%11%2
331
June1st to 15thDay
24%12%26%22%11%2
315
16th to endDay
17%14%27%25%11%3
315
July1st to 15thDay
19%8%28%27%12%3
305
16th to endDay
15%5%27%28%18%3
326
August1st to 15thDay
13%6%26%28%18%6%
313
16th to endDay
15%6%24%24%20%9%
332
September1st to 15thDay
21%7%23%24%12%8%
294
16th to endDay
18%7%24%20%14%14%
254
October1st to 15thDay
22%8%24%23%13%7%
255
16th to endDay
21%16%30%23%7%
245
November1st to 15thDay
25%5%31%25%8%3
235
16th to endDay
27%11%31%20%7%2
210
December1st to 15thDay
26%20%36%8%8%
60
16th to endDay
39%18%31%10%
64
January1st to 15thNight
23%17%35%20%3
720
16th to endNight
23%16%38%19%2
768
February1st to 15thNight
20%17%42%17%1
704
16th to endNight
24%19%33%19%3
631
March1st to 15thNight
27%19%32%18%1
720
16th to endNight
22%17%34%20%5%
768
April1st to 15thNight
25%16%34%19%3
718
16th to endNight
27%15%29%20%5%
629
May1st to 15thNight
24%13%35%20%5%
529
16th to endNight
20%7%33%27%9%
375
June1st to 15thNight
25%6%25%31%10%
360
16th to endNight
19%330%37%8%
360
July1st to 15thNight
14%225%43%13%
340
16th to endNight
11%425%40%17%
376
August1st to 15thNight
13%326%42%13%1
473
16th to endNight
14%321%37%20%2
764
September1st to 15thNight
19%428%33%13%1
700
16th to endNight
19%5%23%29%17%4
657
October1st to 15thNight
23%9%26%28%10%1
660
16th to endNight
25%12%32%24%6%
704
November1st to 15thNight
21%14%30%24%8%1
700
16th to endNight
22%14%34%20%7%
720
December1st to 15thNight
27%12%35%19%5%
720
16th to endNight
27%16%36%17%2
768
Page generated at 2017-09-11 19:35:33. Version ac04

Copyright 2017 A. Danko


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