Hildene Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hildene Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	16% 12% 4 10% 5% 4 4 3 5% 6% 29%	108816
	Night	16% 13% 5% 11% 6% 4 4 3 4 5% 28%	110523
Transparency	Day	52% 6% 11% 19% 10% 1	112358
	Night	54% 6% 11% 17% 10% 2	115145
Seeing	Day	24% 20% 29% 17% 7% 2	15239
	Night	22% 21% 27% 22% 8% 1	24381

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	9% 15% 8% 26% 8% 4 4 2 3 3 19%	3597
	11th to 20th	Day	9% 15% 7% 21% 11% 4 4 3 3 4 20%	3570
	21st to end	Day	12% 16% 8% 14% 7% 4 5% 3 4 3 24%	3975
February	1st to 10th	Day	14% 16% 5% 14% 9% 4 4 3 5% 5% 21%	1804
	11th to 20th	Day	10% 15% 7% 17% 7% 5% 4 3 4 4 24%	1948
	21st to end	Day	16% 16% 6% 11% 7% 3 4 3 3 5% 26%	1650
March	1st to 10th	Day	18% 18% 6% 11% 6% 4 4 3 3 4 23%	3216
	11th to 20th	Day	13% 19% 9% 10% 4 3 3 3 4 5% 27%	3369
	21st to end	Day	19% 11% 4 10% 6% 4 3 3 3 5% 32%	4238
April	1st to 10th	Day	18% 18% 4 8% 4 3 3 3 4 4 30%	2280
	11th to 20th	Day	15% 13% 4 8% 5% 4 4 4 4 5% 33%	2254
	21st to end	Day	20% 14% 4 8% 4 4 4 4 5% 5% 29%	2413
May	1st to 10th	Day	25% 12% 3 8% 5% 3 4 3 5% 5% 26%	4131
	11th to 20th	Day	19% 10% 4 7% 5% 5% 5% 4 6% 7% 29%	4398
	21st to end	Day	15% 7% 3 6% 5% 4 4 4 6% 8% 39%	4837
June	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 4 7% 7% 27%	2660
	11th to 20th	Day	19% 7% 3 6% 5% 5% 5% 4 6% 9% 30%	2660
	21st to end	Day	20% 9% 3 7% 6% 5% 5% 4 6% 7% 28%	2660
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 10% 34%	4440
	11th to 20th	Day	11% 7% 2 8% 5% 6% 6% 5% 7% 10% 32%	4440
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 33%	4814
August	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 8% 30%	4370
	11th to 20th	Day	15% 8% 4 8% 5% 5% 6% 4 6% 8% 30%	3826
	21st to end	Day	13% 7% 2 6% 4 5% 5% 5% 6% 9% 37%	4180
September	1st to 10th	Day	16% 7% 3 5% 4 3 5% 5% 6% 7% 40%	2280
	11th to 20th	Day	16% 7% 2 5% 4 4 4 3 6% 7% 45%	2164
	21st to end	Day	16% 8% 2 5% 5% 5% 5% 4 6% 7% 38%	2090
October	1st to 10th	Day	22% 11% 3 7% 4 4 4 3 4 4 32%	2027
	11th to 20th	Day	16% 12% 3 8% 4 4 5% 3 5% 6% 35%	1867
	21st to end	Day	22% 15% 4 7% 5% 3 4 3 5% 5% 27%	1881
November	1st to 10th	Day	13% 15% 3 6% 4 3 4 3 5% 7% 37%	1761
	11th to 20th	Day	16% 15% 5% 8% 5% 4 4 3 4 4 33%	1800
	21st to end	Day	19% 14% 5% 10% 6% 4 4 3 4 5% 26%	1670
December	1st to 10th	Day	13% 20% 6% 17% 6% 5% 5% 3 3 4 18%	2600
	11th to 20th	Day	16% 18% 9% 12% 6% 4 3 2 2 4 24%	2986
	21st to end	Day	14% 18% 5% 13% 5% 4 3 3 4 5% 25%	3960
January	1st to 10th	Night	11% 13% 7% 25% 7% 5% 3 3 3 4 19%	6349
	11th to 20th	Night	9% 16% 10% 22% 10% 6% 4 3 3 3 14%	6315
	21st to end	Night	13% 13% 5% 15% 8% 7% 5% 3 4 5% 21%	6978
February	1st to 10th	Night	12% 17% 6% 14% 8% 6% 4 3 4 5% 21%	2595
	11th to 20th	Night	10% 13% 6% 12% 10% 7% 6% 4 5% 4 23%	2338
	21st to end	Night	13% 16% 6% 13% 6% 4 4 4 5% 4 26%	1980
March	1st to 10th	Night	21% 11% 5% 10% 7% 5% 4 4 4 5% 24%	3844
	11th to 20th	Night	12% 18% 8% 9% 6% 5% 4 3 4 5% 28%	3806
	21st to end	Night	19% 17% 4 11% 4 3 3 2 3 4 30%	3533
April	1st to 10th	Night	18% 13% 6% 8% 5% 4 4 3 5% 6% 29%	1900
	11th to 20th	Night	18% 14% 4 8% 4 4 4 4 5% 5% 32%	1875
	21st to end	Night	20% 14% 4 7% 4 4 4 3 4 6% 31%	1767
May	1st to 10th	Night	26% 12% 2 7% 5% 3 4 4 5% 5% 26%	2834
	11th to 20th	Night	17% 11% 4 10% 6% 5% 4 4 5% 6% 28%	2498
	21st to end	Night	17% 7% 2 6% 5% 4 5% 3 6% 8% 36%	2738
June	1st to 10th	Night	19% 10% 2 6% 4 5% 4 3 7% 6% 33%	1520
	11th to 20th	Night	20% 7% 2 5% 4 3 4 3 5% 8% 38%	1520
	21st to end	Night	17% 7% 3 6% 4 5% 5% 4 6% 7% 36%	1520
July	1st to 10th	Night	13% 9% 3 7% 6% 4 5% 5% 6% 7% 35%	2520
	11th to 20th	Night	19% 9% 3 7% 4 4 5% 4 5% 6% 34%	2520
	21st to end	Night	15% 9% 3 7% 5% 5% 4 4 5% 6% 36%	2739
August	1st to 10th	Night	27% 8% 3 5% 3 3 3 3 5% 6% 33%	2528
	11th to 20th	Night	16% 9% 3 7% 4 5% 5% 4 5% 7% 35%	2997
	21st to end	Night	11% 9% 2 4 4 3 4 4 6% 9% 44%	3476
September	1st to 10th	Night	16% 8% 2 6% 4 3 3 3 4 6% 44%	1900
	11th to 20th	Night	17% 5% 2 5% 3 2 3 4 5% 5% 48%	2015
	21st to end	Night	17% 7% 2 6% 5% 4 4 3 4 5% 42%	2090
October	1st to 10th	Night	22% 10% 3 7% 4 4 4 3 4 5% 33%	2152
	11th to 20th	Night	16% 12% 3 8% 4 4 5% 4 4 5% 35%	2313
	21st to end	Night	21% 14% 3 8% 6% 4 4 4 4 6% 26%	2717
November	1st to 10th	Night	14% 13% 4 9% 4 4 3 3 4 5% 36%	2537
	11th to 20th	Night	15% 14% 5% 7% 5% 4 4 3 4 6% 33%	2600
	21st to end	Night	13% 16% 5% 10% 6% 4 4 3 5% 6% 28%	2725
December	1st to 10th	Night	15% 20% 9% 19% 5% 4 3 3 3 4 15%	4580
	11th to 20th	Night	19% 18% 6% 14% 6% 3 3 2 3 3 23%	5208
	21st to end	Night	16% 18% 7% 12% 5% 4 4 3 4 4 23%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	68% 5% 7% 16% 5%	3598
	11th to 20th	Day	66% 6% 5% 12% 9% 2	3600
	21st to end	Day	63% 4 6% 16% 9% 2	3975
February	1st to 10th	Day	62% 6% 9% 14% 8% 1	1804
	11th to 20th	Day	60% 6% 8% 15% 9% 1	1957
	21st to end	Day	59% 6% 8% 15% 11% 1	1650
March	1st to 10th	Day	63% 5% 10% 11% 9% 2	3219
	11th to 20th	Day	58% 5% 13% 13% 9% 2	3369
	21st to end	Day	54% 5% 5% 15% 18% 3	4238
April	1st to 10th	Day	56% 5% 7% 19% 10% 2	2280
	11th to 20th	Day	52% 6% 10% 19% 12% 1	2254
	21st to end	Day	56% 6% 8% 16% 11% 3	2413
May	1st to 10th	Day	59% 5% 9% 17% 8% 1	4145
	11th to 20th	Day	52% 8% 15% 17% 8% 1	4402
	21st to end	Day	41% 8% 14% 26% 11% 1	4837
June	1st to 10th	Day	52% 8% 14% 17% 9%	2660
	11th to 20th	Day	49% 6% 12% 19% 12% 1	2660
	21st to end	Day	53% 7% 12% 20% 7%	2660
July	1st to 10th	Day	43% 7% 13% 23% 12%	4370
	11th to 20th	Day	43% 7% 17% 24% 9%	4365
	21st to end	Day	48% 5% 14% 22% 10%	4661
August	1st to 10th	Day	49% 7% 14% 20% 9%	4356
	11th to 20th	Day	48% 8% 16% 23% 5%	3865
	21st to end	Day	40% 7% 15% 28% 10% 1	4180
September	1st to 10th	Day	40% 7% 11% 29% 12% 1	2280
	11th to 20th	Day	39% 6% 10% 25% 18% 1	2164
	21st to end	Day	44% 7% 11% 24% 13% 2	2090
October	1st to 10th	Day	51% 6% 7% 20% 15% 1	3365
	11th to 20th	Day	45% 4 12% 22% 12% 5%	3083
	21st to end	Day	43% 6% 12% 27% 11% 2	3081
November	1st to 10th	Day	44% 7% 13% 22% 13% 1	1761
	11th to 20th	Day	52% 6% 9% 16% 16% 2	1800
	21st to end	Day	58% 6% 14% 16% 6% 1	1670
December	1st to 10th	Day	66% 6% 8% 15% 4 1	2600
	11th to 20th	Day	63% 4 8% 14% 10% 1	2986
	21st to end	Day	60% 5% 7% 11% 13% 4	3960
January	1st to 10th	Night	66% 5% 8% 14% 6% 1	6349
	11th to 20th	Night	69% 8% 7% 9% 6% 1	6360
	21st to end	Night	60% 6% 9% 11% 12% 2	6978
February	1st to 10th	Night	62% 6% 10% 14% 6% 2	2595
	11th to 20th	Night	58% 7% 10% 15% 10% 1	2340
	21st to end	Night	57% 6% 10% 17% 7% 2	1980
March	1st to 10th	Night	59% 8% 8% 16% 8% 2	3854
	11th to 20th	Night	55% 6% 12% 17% 8% 2	3807
	21st to end	Night	57% 6% 6% 11% 17% 3	3533
April	1st to 10th	Night	54% 6% 10% 19% 11% 1	1900
	11th to 20th	Night	51% 6% 10% 19% 12% 1	1875
	21st to end	Night	54% 7% 8% 16% 12% 3	1767
May	1st to 10th	Night	58% 6% 11% 16% 8% 1	2840
	11th to 20th	Night	56% 8% 13% 15% 7% 1	2499
	21st to end	Night	44% 8% 13% 20% 13% 1	2739
June	1st to 10th	Night	49% 6% 14% 20% 10%	1520
	11th to 20th	Night	44% 7% 17% 21% 11%	1520
	21st to end	Night	45% 7% 14% 22% 10% 1	1520
July	1st to 10th	Night	44% 8% 15% 20% 13% 1	2480
	11th to 20th	Night	46% 7% 13% 24% 9%	2472
	21st to end	Night	46% 7% 11% 26% 9% 1	2651
August	1st to 10th	Night	51% 6% 15% 20% 8%	2518
	11th to 20th	Night	45% 8% 18% 22% 8%	3024
	21st to end	Night	34% 6% 18% 31% 10% 1	3476
September	1st to 10th	Night	40% 5% 13% 27% 14% 1	1900
	11th to 20th	Night	37% 5% 10% 29% 18% 1	2015
	21st to end	Night	44% 5% 10% 25% 13% 2	2090
October	1st to 10th	Night	51% 7% 12% 20% 9% 1	3594
	11th to 20th	Night	42% 6% 12% 21% 15% 5%	3836
	21st to end	Night	46% 5% 11% 27% 9% 1	4467
November	1st to 10th	Night	47% 6% 11% 20% 14% 3	2537
	11th to 20th	Night	51% 7% 9% 17% 13% 3	2600
	21st to end	Night	53% 8% 14% 18% 7% 1	2725
December	1st to 10th	Night	69% 6% 8% 13% 3	4580
	11th to 20th	Night	65% 6% 8% 9% 12% 1	5208
	21st to end	Night	63% 6% 7% 9% 12% 4	6996

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	32% 27% 27% 11% 4	200
	11th to 20th	Day	33% 29% 28% 9% 2 1	200
	21st to end	Day	26% 32% 29% 9% 3	222
February	1st to 10th	Day	37% 38% 20% 5% 1	304
	11th to 20th	Day	30% 33% 26% 9% 1	496
	21st to end	Day	31% 28% 27% 11% 2	414
March	1st to 10th	Day	31% 27% 28% 13% 2	500
	11th to 20th	Day	31% 28% 27% 10% 3 1	500
	21st to end	Day	33% 26% 24% 10% 5% 2	530
April	1st to 10th	Day	34% 23% 25% 12% 5% 1	460
	11th to 20th	Day	32% 24% 22% 14% 6% 1	452
	21st to end	Day	29% 20% 30% 13% 4 2	460
May	1st to 10th	Day	33% 21% 26% 12% 6% 2	463
	11th to 20th	Day	21% 19% 35% 19% 5% 2	487
	21st to end	Day	20% 17% 30% 23% 8% 2	529
June	1st to 10th	Day	19% 19% 33% 18% 9% 1	490
	11th to 20th	Day	20% 13% 33% 24% 9% 1	490
	21st to end	Day	22% 18% 33% 18% 8% 2	490
July	1st to 10th	Day	15% 12% 33% 25% 13% 2	485
	11th to 20th	Day	15% 7% 39% 27% 10% 2	484
	21st to end	Day	13% 14% 39% 23% 9% 1	532
August	1st to 10th	Day	13% 12% 31% 27% 12% 5%	486
	11th to 20th	Day	16% 13% 31% 22% 13% 6%	479
	21st to end	Day	14% 14% 27% 28% 11% 5%	539
September	1st to 10th	Day	17% 13% 27% 24% 13% 7%	490
	11th to 20th	Day	14% 16% 25% 20% 15% 10%	490
	21st to end	Day	16% 16% 32% 19% 11% 7%	490
October	1st to 10th	Day	27% 16% 25% 16% 12% 3	490
	11th to 20th	Day	21% 22% 29% 19% 9% 1	473
	21st to end	Day	31% 19% 28% 18% 3 1	429
November	1st to 10th	Day	26% 23% 25% 18% 8% 1	395
	11th to 20th	Day	25% 28% 28% 14% 5% 1	400
	21st to end	Day	32% 25% 31% 9% 3	270
December	1st to 10th	Day	31% 31% 22% 13% 4	200
	11th to 20th	Day	37% 31% 23% 8% 2	200
	21st to end	Day	33% 24% 28% 12% 2	220
January	1st to 10th	Night	22% 32% 31% 14% 2	800
	11th to 20th	Night	26% 27% 28% 17% 2 1	800
	21st to end	Night	19% 30% 31% 17% 3	880
February	1st to 10th	Night	23% 30% 32% 14% 1	800
	11th to 20th	Night	23% 32% 30% 13% 2	783
	21st to end	Night	25% 30% 30% 13% 2	660
March	1st to 10th	Night	26% 24% 31% 17% 3	800
	11th to 20th	Night	25% 26% 29% 17% 3	800
	21st to end	Night	27% 25% 24% 17% 6% 1	863
April	1st to 10th	Night	25% 24% 28% 20% 4	760
	11th to 20th	Night	25% 22% 28% 18% 7%	751
	21st to end	Night	29% 20% 25% 21% 6%	627
May	1st to 10th	Night	27% 17% 25% 23% 8%	570
	11th to 20th	Night	19% 18% 28% 28% 6%	377
	21st to end	Night	17% 12% 27% 31% 12% 1	411
June	1st to 10th	Night	18% 14% 27% 28% 12% 1	380
	11th to 20th	Night	20% 11% 22% 35% 12%	380
	21st to end	Night	17% 10% 28% 34% 11% 1	380
July	1st to 10th	Night	14% 6% 25% 38% 16% 1	370
	11th to 20th	Night	16% 5% 24% 38% 16% 2	368
	21st to end	Night	13% 6% 30% 35% 15% 1	416
August	1st to 10th	Night	14% 7% 20% 39% 19% 1	387
	11th to 20th	Night	16% 8% 22% 32% 19% 4	686
	21st to end	Night	17% 8% 23% 30% 20% 2	836
September	1st to 10th	Night	16% 11% 26% 30% 15% 3	760
	11th to 20th	Night	17% 9% 21% 28% 19% 6%	760
	21st to end	Night	20% 13% 24% 26% 14% 3	760
October	1st to 10th	Night	25% 14% 21% 24% 13% 3	760
	11th to 20th	Night	20% 17% 27% 24% 10% 1	760
	21st to end	Night	28% 22% 26% 19% 5% 1	836
November	1st to 10th	Night	22% 22% 28% 21% 6% 1	780
	11th to 20th	Night	23% 26% 27% 16% 7% 2	800
	21st to end	Night	24% 26% 29% 14% 6% 1	800
December	1st to 10th	Night	28% 29% 26% 14% 3	800
	11th to 20th	Night	29% 28% 24% 15% 4	800
	21st to end	Night	27% 31% 25% 14% 3	880

Page generated at 2025-03-24 09:04:32. Version ac07

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