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 2005-11-06 to 2024-01-13.

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
25%
22977
16%
74685
7%
22932
36%
22932
17%
50206

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 10thNight
12%
760
18%
2649
5%
757
29%
757
13%
1730
January11th to 20thNight
13%
732
12%
2560
4%
732
25%
732
11%
1833
January21st to endNight
13%
792
17%
2709
4%
792
29%
792
13%
1765
February1st to 10thNight
12%
720
13%
2324
3%
720
25%
720
12%
1638
February11th to 20thNight
10%
703
15%
2100
3%
700
25%
700
11%
1488
February21st to endNight
13%
592
19%
1776
6%
592
29%
592
12%
1254
March1st to 10thNight
13%
720
17%
2160
5%
720
29%
720
15%
1483
March11th to 20thNight
15%
720
16%
2097
4%
720
28%
720
14%
1460
March21st to endNight
17%
792
20%
1980
5%
792
28%
792
16%
1388
April1st to 10thNight
16%
720
16%
1800
3%
720
24%
720
16%
1356
April11th to 20thNight
20%
711
14%
1775
2%
711
27%
711
19%
1278
April21st to endNight
20%
640
17%
1720
4%
640
28%
640
19%
1218
May1st to 10thNight
22%
540
13%
1620
5%
540
29%
540
13%
1195
May11th to 20thNight
22%
384
18%
1446
10%
384
35%
384
16%
947
May21st to endNight
37%
388
13%
1551
12%
388
44%
388
17%
987
June1st to 10thNight
34%
360
15%
1440
11%
360
45%
360
17%
907
June11th to 20thNight
36%
360
12%
1440
12%
360
44%
360
16%
934
June21st to endNight
46%
360
13%
1440
14%
360
55%
360
21%
873
July1st to 10thNight
51%
350
14%
1400
18%
350
56%
350
22%
840
July11th to 20thNight
49%
348
13%
1392
14%
348
53%
348
20%
862
July21st to endNight
50%
394
16%
1486
17%
372
57%
372
22%
882
August1st to 10thNight
53%
386
17%
1450
13%
384
59%
384
26%
862
August11th to 20thNight
49%
710
13%
1774
7%
710
51%
710
27%
1256
August21st to endNight
47%
792
18%
1980
8%
792
51%
792
31%
1323
September1st to 10thNight
38%
720
18%
1800
6%
720
44%
720
28%
1219
September11th to 20thNight
45%
720
25%
1898
14%
720
52%
720
35%
1158
September21st to endNight
39%
720
23%
1980
15%
720
47%
720
27%
1299
October1st to 10thNight
34%
720
17%
2047
10%
720
40%
720
21%
1412
October11th to 20thNight
29%
720
19%
2160
9%
720
41%
720
21%
1410
October21st to endNight
22%
792
14%
2569
7%
792
33%
792
15%
1789
November1st to 10thNight
27%
740
21%
2407
9%
740
42%
740
21%
1498
November11th to 20thNight
21%
760
20%
2470
9%
760
36%
760
17%
1622
November21st to endNight
22%
760
17%
2555
6%
760
35%
760
17%
1644
December1st to 10thNight
15%
760
13%
2660
4%
760
26%
760
11%
1863
December11th to 20thNight
16%
760
12%
3234
2%
759
28%
759
9%
2192
December21st to endNight
11%
831
10%
4836
3%
817
27%
817
7%
3341

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%36%5%35%5%5%7%37%
74781
Night
16%8%36%5%35%5%5%7%38%
74958
TransparencyDay
45%8%13%20%12%2
74486
Night
45%8%14%20%12%2
74787
SeeingDay
24%19%30%18%8%2
13804
Night
22%18%30%22%7%1
22983

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 10thDay
13%9%36%6%36%5%6%7%36%
1517
11th to 20thDay
16%10%36%6%5%5%5%6%7%30%
1462
21st to endDay
11%8%46%5%5%6%5%6%8%35%
1645
February1st to 10thDay
18%8%38%7%45%5%5%7%30%
1634
11th to 20thDay
14%8%46%6%46%447%36%
1748
21st to endDay
18%11%47%5%35%445%33%
1480
March1st to 10thDay
13%9%47%5%45%46%7%36%
1797
11th to 20thDay
16%8%47%7%445%5%7%33%
1861
21st to endDay
20%9%35%5%34446%36%
2376
April1st to 10thDay
18%9%46%5%35%45%6%36%
2160
11th to 20thDay
16%7%36%6%345%46%40%
2134
21st to endDay
20%8%35%5%345%5%6%36%
2240
May1st to 10thDay
24%8%36%5%35%45%6%31%
2340
11th to 20thDay
19%6%25%43446%7%39%
2463
21st to endDay
13%7%36%5%35%5%6%8%39%
2725
June1st to 10thDay
18%6%35%6%46%5%7%9%33%
2520
11th to 20thDay
18%7%35%6%45%5%7%9%33%
2520
21st to endDay
15%7%36%6%45%5%6%8%35%
2520
July1st to 10thDay
13%5%36%5%45%6%6%8%39%
2520
11th to 20thDay
12%5%25%5%45%6%6%10%40%
2520
21st to endDay
12%5%36%6%45%7%7%8%38%
2744
August1st to 10thDay
12%6%36%6%5%5%6%7%9%36%
2455
11th to 20thDay
13%6%35%6%35%5%7%9%37%
2126
21st to endDay
13%6%25%5%345%7%9%40%
2376
September1st to 10thDay
13%7%36%5%36%6%6%7%40%
2160
11th to 20thDay
14%5%3343445%6%49%
2062
21st to endDay
17%6%244246%6%7%40%
1980
October1st to 10thDay
22%7%35%6%23445%39%
1910
11th to 20thDay
15%8%25%5%3445%7%42%
1800
21st to endDay
23%8%36%5%35%5%5%6%32%
1787
November1st to 10thDay
13%8%35%43335%7%46%
1671
11th to 20thDay
18%7%34435%5%5%6%40%
1710
21st to endDay
17%7%36%6%25%447%39%
1620
December1st to 10thDay
18%9%48%6%35%5%46%31%
1520
11th to 20thDay
19%9%36%42445%6%37%
1906
21st to endDay
19%11%5%8%7%45%45%6%25%
2772
January1st to 10thNight
14%8%36%6%345%6%7%37%
2649
11th to 20thNight
19%9%46%7%46%45%6%31%
2560
21st to endNight
12%9%36%6%35%47%7%37%
2709
February1st to 10thNight
18%9%5%6%5%35%6%5%6%33%
2324
11th to 20thNight
16%8%37%6%45%6%6%6%33%
2098
21st to endNight
18%8%47%6%35%45%5%35%
1776
March1st to 10thNight
15%8%36%5%45%5%6%6%37%
2150
11th to 20thNight
15%8%46%7%45%5%6%6%35%
2096
21st to endNight
17%7%37%5%2445%6%39%
1980
April1st to 10thNight
17%9%47%7%45%46%6%31%
1800
11th to 20thNight
18%8%36%5%33447%39%
1775
21st to endNight
19%8%36%5%2345%6%38%
1720
May1st to 10thNight
20%10%47%6%35%45%5%31%
1620
11th to 20thNight
17%7%36%5%345%6%7%37%
1445
21st to endNight
16%6%25%6%35%46%6%40%
1550
June1st to 10thNight
17%6%36%44445%8%40%
1440
11th to 20thNight
15%7%36%5%35%6%6%9%35%
1440
21st to endNight
13%5%25%5%35%5%6%8%43%
1440
July1st to 10thNight
14%425%435%47%8%44%
1440
11th to 20thNight
14%425%5%36%5%6%11%38%
1440
21st to endNight
11%5%35%5%35%6%6%7%44%
1574
August1st to 10thNight
9%5%25%5%347%6%10%44%
1460
11th to 20thNight
11%7%26%6%46%6%6%9%37%
1774
21st to endNight
15%425%434447%46%
1980
September1st to 10thNight
14%6%26%5%35%35%7%44%
1800
11th to 20thNight
14%5%23224446%53%
1898
21st to endNight
17%7%35%5%23446%45%
1980
October1st to 10thNight
20%7%36%5%34345%41%
2047
11th to 20thNight
14%6%25%5%345%5%7%43%
2160
21st to endNight
22%9%36%5%2445%5%35%
2569
November1st to 10thNight
14%7%25%42445%7%45%
2407
11th to 20thNight
17%7%36%5%35%5%5%6%39%
2470
21st to endNight
13%8%36%6%45%46%6%40%
2555
December1st to 10thNight
19%9%37%5%45%45%6%32%
2660
11th to 20thNight
19%8%37%5%45%5%5%5%33%
3288
21st to endNight
22%10%5%7%5%345%47%29%
4884

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 10thDay
44%7%14%23%10%2
1518
11th to 20thDay
52%7%15%17%7%1
1462
21st to endDay
45%8%14%20%11%2
1645
February1st to 10thDay
54%7%13%15%10%1
1634
11th to 20thDay
48%6%11%21%12%2
1757
21st to endDay
53%5%12%17%12%2
1480
March1st to 10thDay
46%7%14%19%12%3
1800
11th to 20thDay
49%7%11%19%12%2
1861
21st to endDay
50%5%9%17%15%4
2376
April1st to 10thDay
49%7%10%19%12%3
2160
11th to 20thDay
44%7%10%21%16%2
2134
21st to endDay
48%6%11%19%13%4
2240
May1st to 10thDay
53%8%9%18%11%2
2340
11th to 20thDay
43%7%11%22%15%2
2467
21st to endDay
42%10%14%21%12%2
2725
June1st to 10thDay
46%9%15%19%10%1
2520
11th to 20thDay
45%8%15%20%11%1
2520
21st to endDay
45%7%16%20%11%1
2520
July1st to 10thDay
42%9%12%20%15%1
2450
11th to 20thDay
37%9%18%23%12%1
2445
21st to endDay
41%9%15%19%14%2
2591
August1st to 10thDay
40%12%15%21%12%1
2441
11th to 20thDay
41%11%17%22%9%
2126
21st to endDay
38%8%14%24%15%1
2376
September1st to 10thDay
42%8%12%23%14%1
2160
11th to 20thDay
36%6%11%23%22%2
2062
21st to endDay
39%7%13%22%16%2
1980
October1st to 10thDay
48%6%9%21%12%3
1910
11th to 20thDay
41%7%15%24%12%2
1800
21st to endDay
52%7%12%16%11%2
1787
November1st to 10thDay
38%6%15%25%14%3
1671
11th to 20thDay
43%7%12%21%15%2
1710
21st to endDay
45%5%16%24%9%2
1620
December1st to 10thDay
52%8%15%16%10%
1520
11th to 20thDay
48%7%15%20%9%1
1906
21st to endDay
56%9%12%15%8%
2772
January1st to 10thNight
45%6%14%21%12%2
2649
11th to 20thNight
53%7%16%15%7%2
2560
21st to endNight
44%7%14%20%13%2
2709
February1st to 10thNight
49%8%14%18%9%1
2324
11th to 20thNight
48%8%12%17%12%2
2100
21st to endNight
48%7%11%20%10%3
1776
March1st to 10thNight
45%8%14%20%11%2
2160
11th to 20thNight
46%8%13%20%11%2
2097
21st to endNight
45%6%11%18%16%4
1980
April1st to 10thNight
50%8%10%19%11%2
1800
11th to 20thNight
46%7%11%19%16%1
1775
21st to endNight
46%7%10%21%13%3
1720
May1st to 10thNight
53%8%10%16%11%3
1620
11th to 20thNight
46%7%10%20%14%2
1446
21st to endNight
41%10%17%18%12%1
1551
June1st to 10thNight
43%8%13%23%11%1
1440
11th to 20thNight
42%11%16%21%10%
1440
21st to endNight
37%9%15%25%13%1
1440
July1st to 10thNight
36%10%14%24%15%2
1400
11th to 20thNight
38%12%16%26%9%
1392
21st to endNight
35%10%17%21%14%2
1486
August1st to 10thNight
33%10%17%25%12%1
1450
11th to 20thNight
40%11%18%22%9%
1774
21st to endNight
37%8%13%25%16%1
1980
September1st to 10thNight
40%8%12%24%15%1
1800
11th to 20thNight
32%6%13%24%24%1
1898
21st to endNight
40%6%10%24%19%1
1980
October1st to 10thNight
47%6%12%21%13%2
2047
11th to 20thNight
39%9%15%24%12%1
2160
21st to endNight
49%7%13%18%11%1
2569
November1st to 10thNight
39%6%12%20%18%3
2407
11th to 20thNight
45%6%14%20%13%3
2470
21st to endNight
43%6%19%24%8%1
2555
December1st to 10thNight
51%7%15%18%9%
2660
11th to 20thNight
49%8%16%14%11%1
3288
21st to endNight
53%8%13%17%9%
4884

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 10thDay
30%28%29%11%2
180
11th to 20thDay
34%25%28%10%2
166
21st to endDay
26%30%31%9%31
204
February1st to 10thDay
39%33%22%41
254
11th to 20thDay
34%28%28%9%1
416
21st to endDay
33%25%30%11%2
346
March1st to 10thDay
30%25%32%11%2
420
11th to 20thDay
30%27%30%10%2
420
21st to endDay
34%24%28%9%32
462
April1st to 10thDay
31%25%27%11%6%
420
11th to 20thDay
30%26%25%14%41
412
21st to endDay
31%21%31%11%31
420
May1st to 10thDay
33%23%25%12%6%2
423
11th to 20thDay
23%21%34%18%31
447
21st to endDay
19%14%31%22%12%2
485
June1st to 10thDay
19%16%33%20%11%2
450
11th to 20thDay
19%16%32%23%8%2
450
21st to endDay
16%18%31%24%9%2
450
July1st to 10thDay
13%10%33%28%13%2
445
11th to 20thDay
13%8%30%34%12%2
444
21st to endDay
12%12%38%26%11%2
488
August1st to 10thDay
13%10%31%31%12%4
446
11th to 20thDay
14%9%33%25%13%6%
439
21st to endDay
13%14%31%24%14%5%
495
September1st to 10thDay
19%13%22%26%15%5%
450
11th to 20thDay
15%16%28%19%14%8%
450
21st to endDay
21%12%33%19%11%4
450
October1st to 10thDay
28%13%26%18%12%3
450
11th to 20thDay
21%19%30%22%6%1
450
21st to endDay
32%17%28%16%7%1
399
November1st to 10thDay
24%17%27%22%9%
365
11th to 20thDay
32%22%26%15%41
370
21st to endDay
26%23%30%17%3
280
December1st to 10thDay
27%31%28%10%5%
180
11th to 20thDay
38%26%25%9%2
180
21st to endDay
30%25%30%13%2
198
January1st to 10thNight
23%27%34%14%2
760
11th to 20thNight
28%22%33%15%2
732
21st to endNight
21%27%34%15%2
792
February1st to 10thNight
27%22%33%17%1
720
11th to 20thNight
26%27%35%12%1
703
21st to endNight
30%25%29%14%2
592
March1st to 10thNight
26%24%35%13%2
720
11th to 20thNight
25%27%32%14%3
720
21st to endNight
25%25%28%16%41
792
April1st to 10thNight
28%22%31%16%3
720
11th to 20thNight
23%22%32%18%5%
711
21st to endNight
26%20%32%18%4
640
May1st to 10thNight
27%17%31%19%5%
540
11th to 20thNight
20%15%40%21%3
384
21st to endNight
20%11%27%32%11%
389
June1st to 10thNight
19%9%33%30%9%
360
11th to 20thNight
18%9%31%29%12%
360
21st to endNight
15%7%27%40%11%1
360
July1st to 10thNight
14%6%22%39%19%
350
11th to 20thNight
14%426%41%14%1
348
21st to endNight
11%7%26%40%15%
394
August1st to 10thNight
10%329%44%14%1
386
11th to 20thNight
14%5%24%37%20%1
710
21st to endNight
15%8%22%36%17%2
792
September1st to 10thNight
17%7%31%31%12%2
720
11th to 20thNight
17%8%27%27%17%4
720
21st to endNight
21%9%26%30%12%2
720
October1st to 10thNight
23%13%26%25%11%1
720
11th to 20thNight
18%18%30%25%8%1
720
21st to endNight
28%17%30%20%5%
792
November1st to 10thNight
19%19%30%23%7%1
740
11th to 20thNight
24%22%29%18%5%1
760
21st to endNight
21%21%32%20%5%1
760
December1st to 10thNight
28%22%31%15%4
760
11th to 20thNight
27%24%30%18%2
760
21st to endNight
29%22%34%13%2
836
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