Orchard Hill Observatory
Clear Sky Chart
History

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

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	17% 10% 4 8% 5% 5% 5% 4 5% 7% 30%	126891
	Night	18% 13% 4 9% 6% 5% 4 3 5% 6% 28%	123145
Transparency	Day	54% 7% 11% 19% 8% 1	130360
	Night	59% 6% 10% 16% 8% 1	127763
Seeing	Day	25% 17% 29% 20% 8% 2	15919
	Night	26% 14% 27% 24% 8% 1	24832

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	17% 20% 8% 13% 6% 5% 3 3 3 4 16%	3597
	11th to 20th	Day	13% 17% 8% 17% 7% 6% 5% 3 4 4 16%	3570
	21st to end	Day	14% 15% 6% 11% 6% 7% 6% 3 5% 6% 20%	3960
February	1st to 10th	Day	16% 14% 5% 14% 8% 5% 4 4 4 5% 20%	1755
	11th to 20th	Day	14% 17% 5% 12% 7% 5% 4 4 4 6% 23%	1894
	21st to end	Day	16% 16% 4 8% 7% 5% 4 3 6% 6% 24%	1650
March	1st to 10th	Day	19% 14% 5% 11% 6% 5% 5% 3 4 5% 23%	4476
	11th to 20th	Day	15% 15% 4 9% 5% 4 4 4 5% 6% 29%	4671
	21st to end	Day	19% 12% 4 8% 4 4 4 3 4 5% 32%	5970
April	1st to 10th	Day	22% 13% 3 8% 5% 5% 4 3 4 5% 30%	2400
	11th to 20th	Day	21% 9% 3 8% 4 4 4 3 5% 5% 35%	2374
	21st to end	Day	22% 9% 3 7% 5% 4 4 3 4 6% 32%	2569
May	1st to 10th	Day	27% 11% 4 8% 5% 4 5% 4 5% 5% 23%	6134
	11th to 20th	Day	19% 7% 3 8% 6% 5% 5% 3 5% 7% 31%	6318
	21st to end	Day	20% 9% 3 6% 6% 4 5% 4 6% 7% 30%	6949
June	1st to 10th	Day	22% 8% 2 7% 5% 4 6% 4 6% 7% 30%	2800
	11th to 20th	Day	15% 7% 2 6% 4 6% 5% 4 6% 9% 36%	2800
	21st to end	Day	20% 8% 3 7% 6% 5% 5% 4 6% 7% 32%	2800
July	1st to 10th	Day	12% 6% 3 8% 6% 6% 6% 5% 7% 9% 32%	6360
	11th to 20th	Day	10% 6% 2 6% 4 5% 6% 5% 7% 9% 38%	6360
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 10% 42%	6926
August	1st to 10th	Day	15% 7% 3 8% 5% 5% 6% 5% 7% 8% 33%	4563
	11th to 20th	Day	14% 6% 2 8% 4 4 5% 5% 6% 8% 38%	4063
	21st to end	Day	15% 5% 2 6% 4 4 5% 3 6% 8% 41%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 6% 7% 45%	2400
	11th to 20th	Day	15% 4 1 5% 4 4 4 3 4 7% 48%	2295
	21st to end	Day	15% 6% 3 6% 4 5% 4 4 5% 8% 41%	2200
October	1st to 10th	Day	20% 7% 3 7% 5% 4 5% 4 5% 7% 33%	2100
	11th to 20th	Day	17% 9% 4 7% 5% 5% 6% 5% 5% 7% 31%	1945
	21st to end	Day	24% 12% 3 8% 6% 5% 4 3 4 5% 27%	1980
November	1st to 10th	Day	18% 11% 2 8% 6% 4 5% 3 6% 5% 31%	1779
	11th to 20th	Day	18% 11% 4 10% 5% 5% 5% 4 6% 5% 28%	1775
	21st to end	Day	21% 15% 4 9% 6% 5% 5% 5% 5% 5% 21%	1600
December	1st to 10th	Day	18% 18% 7% 15% 6% 5% 4 3 4 4 15%	2600
	11th to 20th	Day	20% 13% 6% 10% 5% 4 4 3 5% 5% 24%	2986
	21st to end	Day	22% 15% 5% 12% 7% 5% 4 4 4 5% 17%	3960
January	1st to 10th	Night	16% 24% 8% 14% 5% 4 3 3 3 4 16%	6349
	11th to 20th	Night	11% 19% 8% 16% 8% 5% 5% 4 5% 4 15%	6315
	21st to end	Night	15% 17% 5% 12% 8% 5% 5% 3 4 5% 20%	6996
February	1st to 10th	Night	16% 17% 6% 13% 8% 6% 4 3 5% 4 18%	2644
	11th to 20th	Night	14% 13% 5% 11% 8% 6% 5% 4 5% 6% 23%	2392
	21st to end	Night	16% 16% 4 10% 5% 4 4 3 4 5% 28%	1980
March	1st to 10th	Night	20% 13% 5% 9% 6% 5% 4 3 4 5% 26%	5364
	11th to 20th	Night	16% 14% 4 7% 5% 5% 5% 4 5% 6% 29%	5270
	21st to end	Night	16% 14% 4 11% 6% 5% 5% 3 4 4 29%	4962
April	1st to 10th	Night	22% 12% 3 8% 5% 4 5% 3 5% 6% 28%	2000
	11th to 20th	Night	23% 11% 3 7% 6% 4 4 3 3 5% 30%	1975
	21st to end	Night	21% 9% 3 7% 4 4 4 3 4 5% 34%	1830
May	1st to 10th	Night	24% 11% 4 9% 5% 4 5% 3 5% 6% 23%	3836
	11th to 20th	Night	22% 9% 3 8% 5% 4 3 3 5% 7% 30%	3575
	21st to end	Night	17% 10% 4 8% 5% 5% 4 4 5% 7% 31%	3926
June	1st to 10th	Night	23% 8% 3 6% 5% 4 4 4 6% 7% 30%	1600
	11th to 20th	Night	18% 7% 2 7% 5% 4 4 4 7% 7% 37%	1600
	21st to end	Night	18% 7% 2 7% 4 6% 5% 4 6% 7% 33%	1600
July	1st to 10th	Night	12% 6% 2 7% 6% 6% 6% 5% 7% 10% 35%	3600
	11th to 20th	Night	13% 7% 3 6% 5% 5% 5% 4 5% 8% 40%	3600
	21st to end	Night	11% 6% 3 7% 6% 5% 4 5% 6% 8% 39%	3926
August	1st to 10th	Night	16% 8% 3 8% 5% 4 4 4 5% 8% 36%	2586
	11th to 20th	Night	14% 7% 3 8% 5% 5% 5% 4 5% 6% 38%	2947
	21st to end	Night	16% 6% 2 6% 4 4 4 4 6% 7% 42%	3586
September	1st to 10th	Night	13% 5% 2 6% 5% 3 4 3 5% 8% 45%	2000
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 6% 6% 49%	2105
	21st to end	Night	18% 6% 2 6% 4 4 4 4 4 6% 43%	2200
October	1st to 10th	Night	19% 7% 3 7% 5% 4 4 3 5% 6% 37%	2298
	11th to 20th	Night	20% 9% 3 7% 4 5% 5% 4 5% 5% 33%	2453
	21st to end	Night	24% 10% 3 7% 6% 5% 4 4 5% 6% 27%	2860
November	1st to 10th	Night	20% 10% 3 7% 5% 5% 4 4 5% 6% 30%	2564
	11th to 20th	Night	19% 11% 4 8% 6% 5% 4 3 5% 5% 32%	2622
	21st to end	Night	22% 13% 4 9% 5% 5% 5% 4 5% 5% 23%	2800
December	1st to 10th	Night	18% 24% 7% 12% 6% 5% 5% 3 3 3 13%	4580
	11th to 20th	Night	23% 14% 5% 10% 5% 5% 5% 3 5% 4 20%	5208
	21st to end	Night	23% 16% 5% 11% 5% 5% 4 4 7% 4 17%	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	74% 7% 9% 9% 1	3598
	11th to 20th	Day	69% 9% 8% 9% 4 1	3600
	21st to end	Day	66% 8% 9% 10% 6% 1	3960
February	1st to 10th	Day	68% 6% 7% 13% 5% 1	1755
	11th to 20th	Day	66% 6% 10% 13% 5%	1903
	21st to end	Day	64% 5% 9% 13% 7% 1	1650
March	1st to 10th	Day	64% 7% 8% 11% 7% 4	4479
	11th to 20th	Day	55% 8% 13% 13% 9% 1	4671
	21st to end	Day	57% 6% 6% 15% 15% 1	5970
April	1st to 10th	Day	58% 5% 7% 19% 9% 2	2400
	11th to 20th	Day	53% 6% 7% 18% 14% 2	2374
	21st to end	Day	56% 5% 9% 17% 11% 2	2569
May	1st to 10th	Day	63% 8% 8% 15% 6% 1	6145
	11th to 20th	Day	52% 9% 14% 16% 8% 1	6322
	21st to end	Day	49% 9% 12% 21% 8% 1	6949
June	1st to 10th	Day	53% 7% 12% 18% 10% 1	2800
	11th to 20th	Day	43% 9% 12% 23% 12% 2	2800
	21st to end	Day	52% 7% 11% 20% 8% 1	2800
July	1st to 10th	Day	44% 8% 15% 23% 9%	6290
	11th to 20th	Day	40% 9% 14% 27% 10%	6285
	21st to end	Day	39% 7% 12% 34% 7%	6728
August	1st to 10th	Day	48% 8% 15% 22% 7%	4549
	11th to 20th	Day	46% 7% 17% 23% 6%	4103
	21st to end	Day	40% 7% 17% 21% 14% 1	4312
September	1st to 10th	Day	38% 7% 12% 30% 12% 1	2400
	11th to 20th	Day	37% 6% 11% 32% 14% 1	2295
	21st to end	Day	42% 8% 16% 23% 10% 1	2200
October	1st to 10th	Day	45% 6% 12% 27% 9%	3425
	11th to 20th	Day	49% 7% 14% 21% 9%	3148
	21st to end	Day	51% 6% 14% 23% 6% 1	3180
November	1st to 10th	Day	57% 7% 11% 17% 7%	1779
	11th to 20th	Day	61% 7% 10% 13% 8% 1	1775
	21st to end	Day	66% 8% 13% 10% 3	1600
December	1st to 10th	Day	78% 6% 8% 7% 2	2600
	11th to 20th	Day	64% 8% 7% 16% 5%	2986
	21st to end	Day	66% 9% 8% 10% 6%	3960
January	1st to 10th	Night	77% 6% 9% 7% 1	6349
	11th to 20th	Night	73% 8% 9% 8% 2	6360
	21st to end	Night	71% 5% 9% 7% 8% 1	6996
February	1st to 10th	Night	69% 6% 8% 11% 5%	2644
	11th to 20th	Night	66% 7% 8% 12% 6%	2394
	21st to end	Night	63% 5% 9% 14% 8% 1	1980
March	1st to 10th	Night	64% 6% 7% 12% 10% 1	5374
	11th to 20th	Night	56% 7% 13% 15% 7% 1	5271
	21st to end	Night	62% 5% 5% 16% 10% 1	4962
April	1st to 10th	Night	59% 5% 9% 16% 9% 2	2000
	11th to 20th	Night	58% 5% 9% 16% 12% 1	1975
	21st to end	Night	54% 6% 8% 16% 13% 3	1830
May	1st to 10th	Night	63% 8% 8% 14% 6% 1	3842
	11th to 20th	Night	53% 8% 10% 20% 8% 1	3576
	21st to end	Night	52% 8% 10% 18% 11% 1	3927
June	1st to 10th	Night	53% 7% 11% 19% 10% 1	1600
	11th to 20th	Night	45% 7% 14% 21% 11% 2	1600
	21st to end	Night	49% 9% 13% 19% 10% 1	1600
July	1st to 10th	Night	44% 7% 18% 22% 9%	3560
	11th to 20th	Night	43% 8% 12% 27% 10%	3552
	21st to end	Night	43% 7% 13% 26% 10%	3809
August	1st to 10th	Night	49% 7% 14% 23% 8%	2576
	11th to 20th	Night	48% 7% 15% 20% 9%	2973
	21st to end	Night	41% 7% 14% 23% 14% 1	3586
September	1st to 10th	Night	40% 8% 13% 28% 11% 1	2000
	11th to 20th	Night	37% 6% 14% 30% 12%	2105
	21st to end	Night	42% 8% 11% 24% 14% 1	2200
October	1st to 10th	Night	53% 5% 10% 23% 8%	3753
	11th to 20th	Night	51% 5% 11% 22% 11%	3989
	21st to end	Night	52% 6% 13% 20% 9%	4610
November	1st to 10th	Night	56% 6% 13% 17% 8% 1	2564
	11th to 20th	Night	58% 7% 10% 15% 8% 2	2622
	21st to end	Night	66% 7% 11% 13% 3	2800
December	1st to 10th	Night	79% 5% 8% 6% 1	4580
	11th to 20th	Night	72% 5% 7% 11% 6%	5208
	21st to end	Night	71% 5% 7% 11% 6% 1	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	27% 25% 28% 15% 5% 1	200
	11th to 20th	Day	30% 28% 30% 10% 3 1	200
	21st to end	Day	27% 20% 34% 15% 4	220
February	1st to 10th	Day	33% 30% 28% 8% 1	277
	11th to 20th	Day	28% 28% 31% 10% 2	442
	21st to end	Day	29% 21% 33% 15% 2	414
March	1st to 10th	Day	34% 19% 28% 18% 2	500
	11th to 20th	Day	27% 27% 28% 15% 3 1	500
	21st to end	Day	34% 22% 25% 13% 5% 1	550
April	1st to 10th	Day	34% 20% 24% 18% 3 1	500
	11th to 20th	Day	33% 21% 23% 15% 6% 2	492
	21st to end	Day	32% 19% 29% 14% 5% 1	500
May	1st to 10th	Day	32% 18% 26% 17% 7% 1	503
	11th to 20th	Day	23% 15% 31% 21% 8% 1	527
	21st to end	Day	21% 17% 30% 23% 8% 1	573
June	1st to 10th	Day	27% 13% 34% 19% 7% 1	530
	11th to 20th	Day	19% 18% 30% 22% 10% 2	530
	21st to end	Day	24% 13% 35% 19% 8% 1	530
July	1st to 10th	Day	16% 9% 34% 28% 11% 2	525
	11th to 20th	Day	16% 9% 35% 28% 11% 2	524
	21st to end	Day	18% 8% 34% 31% 8% 1	576
August	1st to 10th	Day	14% 11% 29% 31% 13% 2	526
	11th to 20th	Day	16% 9% 28% 29% 14% 4	519
	21st to end	Day	16% 11% 31% 25% 11% 5%	583
September	1st to 10th	Day	17% 14% 30% 22% 12% 6%	530
	11th to 20th	Day	15% 12% 21% 25% 20% 7%	530
	21st to end	Day	15% 13% 30% 23% 13% 5%	530
October	1st to 10th	Day	24% 15% 27% 17% 14% 3	530
	11th to 20th	Day	28% 19% 24% 20% 7% 1	500
	21st to end	Day	39% 15% 26% 17% 4	462
November	1st to 10th	Day	31% 20% 28% 14% 5%	401
	11th to 20th	Day	29% 28% 22% 13% 7% 1	375
	21st to end	Day	36% 23% 26% 14% 2	200
December	1st to 10th	Day	27% 26% 29% 17% 2	200
	11th to 20th	Day	28% 28% 34% 10% 1	200
	21st to end	Day	29% 25% 30% 13% 2 1	220
January	1st to 10th	Night	28% 23% 31% 16% 3	800
	11th to 20th	Night	29% 20% 28% 18% 5%	800
	21st to end	Night	23% 16% 33% 25% 4 1	880
February	1st to 10th	Night	25% 18% 33% 22% 2	800
	11th to 20th	Night	24% 19% 33% 20% 3	783
	21st to end	Night	25% 19% 31% 22% 4	660
March	1st to 10th	Night	29% 18% 27% 23% 3 1	800
	11th to 20th	Night	30% 20% 26% 20% 5%	800
	21st to end	Night	29% 18% 24% 21% 7%	880
April	1st to 10th	Night	32% 14% 27% 23% 4	800
	11th to 20th	Night	31% 17% 23% 22% 7%	791
	21st to end	Night	31% 14% 27% 21% 6%	630
May	1st to 10th	Night	29% 11% 28% 25% 8%	512
	11th to 20th	Night	23% 11% 30% 26% 9% 1	394
	21st to end	Night	19% 9% 29% 33% 9% 1	433
June	1st to 10th	Night	28% 9% 25% 30% 9% 1	400
	11th to 20th	Night	18% 10% 28% 29% 14% 1	400
	21st to end	Night	21% 7% 26% 37% 9% 1	400
July	1st to 10th	Night	14% 5% 29% 37% 15%	390
	11th to 20th	Night	11% 4 24% 44% 16% 1	388
	21st to end	Night	17% 4 22% 43% 14%	438
August	1st to 10th	Night	16% 3 19% 44% 18% 1	398
	11th to 20th	Night	14% 5% 21% 36% 20% 3	626
	21st to end	Night	16% 7% 23% 33% 19% 1	880
September	1st to 10th	Night	17% 7% 26% 33% 16% 2	800
	11th to 20th	Night	18% 7% 18% 30% 23% 4	800
	21st to end	Night	20% 9% 26% 24% 18% 2	800
October	1st to 10th	Night	27% 11% 25% 22% 14% 1	800
	11th to 20th	Night	28% 12% 27% 24% 8% 1	800
	21st to end	Night	32% 14% 28% 21% 4	880
November	1st to 10th	Night	30% 17% 28% 19% 6% 1	789
	11th to 20th	Night	29% 18% 26% 18% 7% 1	800
	21st to end	Night	34% 19% 26% 16% 5% 1	800
December	1st to 10th	Night	33% 20% 28% 15% 4	800
	11th to 20th	Night	30% 20% 28% 17% 5% 1	800
	21st to end	Night	31% 21% 27% 17% 3	880

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