The Hotchkiss School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Hotchkiss School clear sky chart from 2005-11-05 to 2017-09-22.

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%
14945
16%
46897
6%
14914
34%
14914
16%
31698

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
13%
720
16%
2506
6%
716
27%
716
11%
1706
January16th to endNight
11%
768
15%
2646
3%
768
27%
768
10%
1795
February1st to 15thNight
11%
704
12%
2221
2%
701
23%
701
10%
1577
February16th to endNight
13%
631
18%
1893
5%
631
29%
631
12%
1328
March1st to 15thNight
11%
720
14%
2160
4%
720
24%
720
11%
1517
March16th to endNight
16%
768
21%
1997
5%
768
28%
768
15%
1363
April1st to 15thNight
13%
718
14%
1796
2%
718
21%
718
14%
1357
April16th to endNight
20%
666
16%
1746
2%
666
31%
666
17%
1221
May1st to 15thNight
19%
499
16%
1579
5%
499
31%
499
13%
1099
May16th to endNight
25%
374
12%
1497
8%
374
34%
374
13%
981
June1st to 15thNight
27%
360
11%
1440
7%
360
37%
360
14%
965
June16th to endNight
38%
360
12%
1440
12%
360
47%
360
19%
857
July1st to 15thNight
45%
340
12%
1360
14%
340
51%
340
20%
823
July16th to endNight
44%
376
14%
1418
14%
354
54%
354
18%
851
August1st to 15thNight
46%
500
16%
1571
8%
498
52%
498
26%
962
August16th to endNight
49%
764
19%
1910
8%
764
53%
764
31%
1269
September1st to 15thNight
36%
720
20%
1806
8%
720
42%
720
29%
1198
September16th to endNight
44%
685
25%
1891
14%
685
52%
685
29%
1169
October1st to 15thNight
29%
660
21%
1911
12%
660
39%
660
20%
1277
October16th to endNight
20%
704
13%
2230
5%
704
31%
704
15%
1541
November1st to 15thNight
22%
700
19%
2277
8%
700
36%
700
17%
1484
November16th to endNight
19%
720
16%
2394
7%
720
31%
720
13%
1633
December1st to 15thNight
16%
720
13%
2520
3%
720
27%
720
9%
1784
December16th to endNight
10%
768
13%
2688
2%
768
23%
768
8%
1941

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%35%5%35%5%5%7%36%
47993
Night
16%7%35%5%3445%6%38%
47067
Cloud CoverDay
16%6%25%5%35%45%7%36%
48003
Night
16%6%35%5%3445%6%38%
47072
TransparencyDay
46%7%13%19%12%1
47681
Night
45%6%13%20%12%1
46897
SeeingDay
19%23%25%18%8%3
5789
Night
18%28%25%19%6%
14946

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%6%36%6%36%5%6%6%34%
1432
16th to endDay
13%6%36%5%46%5%6%7%33%
1576
February1st to 15thDay
15%7%47%8%35%5%5%6%29%
1639
16th to endDay
14%9%36%6%45%336%35%
1580
March1st to 15thDay
16%9%46%6%35%36%7%31%
1790
16th to endDay
17%8%45%5%33447%36%
2226
April1st to 15thDay
18%8%35%5%25%435%36%
2152
16th to endDay
15%6%25%5%23446%41%
2214
May1st to 15thDay
22%7%36%5%34446%32%
2382
16th to endDay
15%7%35%5%35%5%6%7%36%
2618
June1st to 15thDay
22%6%245%36%5%6%7%30%
2520
16th to endDay
14%6%25%6%345%6%9%32%
2520
July1st to 15thDay
12%5%36%6%35%5%5%8%36%
2520
16th to endDay
12%4245%45%6%6%8%37%
2654
August1st to 15thDay
14%5%35%5%45%5%6%8%35%
2364
16th to endDay
11%5%25%4245%6%8%41%
2292
September1st to 15thDay
13%6%35%5%26%5%5%7%39%
2154
16th to endDay
14%42442446%7%44%
1881
October1st to 15thDay
18%8%25%5%23435%39%
1717
16th to endDay
18%7%35%5%35%446%35%
1642
November1st to 15thDay
16%8%2432345%7%40%
1581
16th to endDay
18%6%25%425%446%39%
1563
December1st to 15thDay
17%7%38%5%345%46%33%
1440
16th to endDay
18%10%47%6%35%5%5%5%27%
1536
January1st to 15thNight
16%7%35%6%35%45%6%34%
2506
16th to endNight
14%10%36%5%36%45%6%35%
2646
February1st to 15thNight
14%7%47%5%36%6%5%6%31%
2220
16th to endNight
15%7%46%6%35%5%5%5%36%
1893
March1st to 15thNight
16%8%35%5%3446%6%34%
2147
16th to endNight
16%5%36%42445%6%39%
1997
April1st to 15thNight
17%8%36%6%3445%5%33%
1796
16th to endNight
17%7%35%422446%40%
1746
May1st to 15thNight
18%8%36%5%24445%36%
1579
16th to endNight
17%6%25%5%2446%6%37%
1495
June1st to 15thNight
19%6%26%43445%7%35%
1440
16th to endNight
12%425%4235%7%9%42%
1440
July1st to 15thNight
12%4245%35%5%6%8%42%
1440
16th to endNight
11%435%436%5%6%8%40%
1514
August1st to 15thNight
11%4144246%6%9%44%
1581
16th to endNight
12%425%43445%7%45%
1910
September1st to 15thNight
14%42433435%6%46%
1806
16th to endNight
13%5%24323346%50%
1891
October1st to 15thNight
16%6%35%5%34236%43%
1911
16th to endNight
19%7%26%424445%37%
2230
November1st to 15thNight
16%7%35%5%23346%41%
2277
16th to endNight
15%8%46%5%35%45%5%35%
2394
December1st to 15thNight
20%7%26%5%45%45%5%32%
2520
16th to endNight
19%8%36%6%345%5%7%28%
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%6%36%6%35%5%5%6%35%
1432
16th to endDay
14%6%36%437%5%5%7%34%
1576
February1st to 15thDay
14%7%48%8%35%5%47%30%
1639
16th to endDay
14%9%37%5%44446%35%
1580
March1st to 15thDay
17%9%36%6%25%5%47%32%
1800
16th to endDay
17%8%35%5%33446%37%
2226
April1st to 15thDay
18%8%35%5%24435%36%
2152
16th to endDay
16%6%25%423446%41%
2214
May1st to 15thDay
23%7%35%5%3445%6%32%
2382
16th to endDay
16%6%25%5%25%5%5%7%36%
2618
June1st to 15thDay
22%6%245%35%5%5%7%30%
2520
16th to endDay
15%6%25%5%45%5%6%9%33%
2520
July1st to 15thDay
13%5%26%5%36%5%5%7%37%
2520
16th to endDay
12%5%245%36%6%6%8%37%
2654
August1st to 15thDay
14%5%246%36%5%7%8%35%
2364
16th to endDay
12%5%145%245%6%7%41%
2292
September1st to 15thDay
14%5%25%5%35%5%5%6%40%
2154
16th to endDay
14%42442345%7%45%
1881
October1st to 15thDay
19%7%25%424336%40%
1717
16th to endDay
18%6%35%5%25%445%36%
1642
November1st to 15thDay
17%7%2432435%6%41%
1581
16th to endDay
18%5%34425%446%40%
1563
December1st to 15thDay
17%7%36%5%25%45%6%33%
1440
16th to endDay
18%10%36%6%35%5%5%5%28%
1536
January1st to 15thNight
16%7%35%6%3445%6%35%
2506
16th to endNight
15%9%45%435%45%6%35%
2646
February1st to 15thNight
14%8%47%5%35%6%5%6%33%
2213
16th to endNight
15%6%36%5%3435%6%36%
1893
March1st to 15thNight
16%8%35%43445%6%35%
2160
16th to endNight
16%6%35%42445%6%40%
1996
April1st to 15thNight
17%9%36%6%4435%5%34%
1796
16th to endNight
18%6%25%42235%6%41%
1746
May1st to 15thNight
18%8%27%5%24345%36%
1579
16th to endNight
17%6%25%5%345%6%7%36%
1495
June1st to 15thNight
20%5%35%425%447%36%
1440
16th to endNight
13%4244235%6%8%43%
1440
July1st to 15thNight
13%423435%5%6%8%42%
1440
16th to endNight
11%435%435%47%8%40%
1514
August1st to 15thNight
11%4144346%5%8%45%
1581
16th to endNight
13%424425%45%6%45%
1910
September1st to 15thNight
15%424424347%47%
1806
16th to endNight
13%5%24313445%51%
1891
October1st to 15thNight
16%6%35%5%24235%44%
1911
16th to endNight
20%7%25%5%24446%37%
2230
November1st to 15thNight
16%7%25%424346%42%
2277
16th to endNight
16%7%45%5%3445%5%35%
2394
December1st to 15thNight
20%8%26%5%35%5%45%32%
2520
16th to endNight
19%8%36%5%345%5%7%29%
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
46%7%14%20%8%1
1433
16th to endDay
47%7%14%20%9%
1576
February1st to 15thDay
53%6%13%15%9%
1642
16th to endDay
50%412%20%11%1
1580
March1st to 15thDay
50%7%13%19%7%1
1800
16th to endDay
48%5%10%18%15%3
2226
April1st to 15thDay
50%5%9%19%12%2
2152
16th to endDay
43%5%9%21%16%3
2214
May1st to 15thDay
52%6%7%18%11%2
2382
16th to endDay
45%8%13%19%12%1
2622
June1st to 15thDay
50%8%14%17%8%
2520
16th to endDay
45%7%16%18%11%
2520
July1st to 15thDay
44%8%11%20%13%1
2371
16th to endDay
41%8%17%19%12%1
2487
August1st to 15thDay
43%9%14%20%11%
2350
16th to endDay
37%8%13%25%14%
2292
September1st to 15thDay
42%7%11%20%15%1
2154
16th to endDay
37%5%12%22%21%1
1881
October1st to 15thDay
46%5%10%21%12%2
1717
16th to endDay
49%6%13%18%10%1
1642
November1st to 15thDay
42%5%15%19%13%2
1581
16th to endDay
45%5%13%22%11%1
1563
December1st to 15thDay
50%6%15%18%7%
1440
16th to endDay
56%6%14%15%6%
1536
January1st to 15thNight
49%5%14%19%9%1
2506
16th to endNight
48%6%13%20%10%1
2646
February1st to 15thNight
48%8%15%16%9%
2221
16th to endNight
47%7%11%19%12%2
1893
March1st to 15thNight
48%6%13%19%9%1
2160
16th to endNight
43%5%12%19%15%4
1997
April1st to 15thNight
51%6%9%19%12%1
1796
16th to endNight
44%6%8%21%16%2
1746
May1st to 15thNight
50%5%9%18%13%2
1579
16th to endNight
44%9%17%16%10%1
1497
June1st to 15thNight
47%8%13%19%9%1
1440
16th to endNight
36%8%16%25%11%
1440
July1st to 15thNight
36%9%14%26%12%
1360
16th to endNight
38%8%16%22%12%1
1418
August1st to 15thNight
34%9%17%25%12%1
1571
16th to endNight
35%8%12%26%15%
1910
September1st to 15thNight
38%6%12%23%17%
1806
16th to endNight
34%5%9%25%24%
1891
October1st to 15thNight
44%411%19%17%2
1911
16th to endNight
47%7%15%20%9%
2230
November1st to 15thNight
44%412%19%15%3
2277
16th to endNight
48%5%15%18%9%1
2394
December1st to 15thNight
52%6%15%17%8%
2520
16th to endNight
53%7%13%17%7%
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
20%40%26%10%3
60
16th to endDay
23%26%40%8%
71
February1st to 15thDay
23%42%22%10%1
155
16th to endDay
20%34%31%10%2
239
March1st to 15thDay
30%30%25%12%1
270
16th to endDay
24%34%25%10%31
288
April1st to 15thDay
28%26%24%14%5%
270
16th to endDay
18%28%28%15%6%2
270
May1st to 15thDay
34%26%18%12%5%3
288
16th to endDay
17%23%28%21%8%
331
June1st to 15thDay
22%21%24%19%10%1
315
16th to endDay
15%23%28%20%10%
315
July1st to 15thDay
14%13%32%26%11%1
305
16th to endDay
11%15%29%26%13%3
326
August1st to 15thDay
16%13%23%26%12%6%
313
16th to endDay
11%12%24%24%17%10%
332
September1st to 15thDay
13%19%20%23%13%8%
315
16th to endDay
15%13%26%16%15%12%
278
October1st to 15thDay
22%15%22%23%11%4
255
16th to endDay
19%25%29%20%4
251
November1st to 15thDay
22%23%22%22%8%
235
16th to endDay
19%32%26%12%7%1
183
December1st to 15thDay
15%43%23%15%3
60
16th to endDay
35%28%26%9%
64
January1st to 15thNight
17%38%28%13%2
720
16th to endNight
19%38%27%13%1
768
February1st to 15thNight
18%36%30%14%
704
16th to endNight
19%38%27%12%1
631
March1st to 15thNight
24%36%24%11%1
720
16th to endNight
19%38%23%13%5%
768
April1st to 15thNight
22%34%27%13%2
718
16th to endNight
20%31%24%16%6%
666
May1st to 15thNight
20%29%28%17%4
499
16th to endNight
19%24%26%21%7%
375
June1st to 15thNight
17%23%27%22%8%
360
16th to endNight
13%20%24%32%9%
360
July1st to 15thNight
12%15%22%32%17%
340
16th to endNight
11%19%19%33%15%
376
August1st to 15thNight
13%10%24%36%14%1
500
16th to endNight
12%11%19%32%20%2
764
September1st to 15thNight
15%14%28%27%12%1
720
16th to endNight
16%15%20%26%16%4
685
October1st to 15thNight
19%22%24%24%8%1
660
16th to endNight
21%25%29%19%3
704
November1st to 15thNight
18%33%23%18%6%1
700
16th to endNight
17%34%24%15%6%1
720
December1st to 15thNight
25%32%23%14%4
720
16th to endNight
26%37%23%10%1
768
Page generated at 2017-09-23 06:43:40. Version ac04

Copyright 2017 A. Danko


The Hotchkiss School Clear Sky Chart | Charts Home