Over The Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Over The Hill Observatory clear sky chart from 2005-11-06 to 2025-07-06.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	119908
	Night	13% 6% 2 5% 4 4 4 4 5% 7% 45%	118845
Transparency	Day	38% 7% 13% 23% 16% 4	123551
	Night	36% 6% 12% 23% 18% 4	123340
Seeing	Day	19% 13% 27% 24% 13% 4	15794
	Night	17% 10% 29% 31% 13% 1	25437

By Month

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 5% 5% 50%	3919
	11th to 20th	Day	11% 9% 3 6% 4 4 3 3 4 5% 48%	4060
	21st to end	Day	13% 7% 3 7% 5% 4 3 3 5% 6% 44%	4466
February	1st to 10th	Day	12% 8% 3 6% 5% 5% 4 5% 6% 6% 39%	1800
	11th to 20th	Day	17% 8% 2 6% 4 4 4 4 5% 7% 40%	1758
	21st to end	Day	16% 8% 3 7% 4 3 3 4 4 6% 41%	1681
March	1st to 10th	Day	16% 6% 2 6% 4 4 5% 4 4 6% 43%	4935
	11th to 20th	Day	10% 6% 2 5% 4 3 4 3 6% 6% 51%	4980
	21st to end	Day	15% 7% 2 6% 4 4 5% 4 5% 7% 40%	5465
April	1st to 10th	Day	12% 7% 2 6% 6% 5% 4 4 6% 7% 43%	2390
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 5% 48%	2374
	21st to end	Day	16% 6% 2 7% 5% 4 4 4 5% 7% 40%	2421
May	1st to 10th	Day	17% 6% 2 7% 5% 6% 6% 5% 7% 10% 30%	5862
	11th to 20th	Day	21% 7% 3 6% 4 4 5% 4 7% 9% 30%	5924
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 7% 8% 39%	6934
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 9% 42%	2800
	11th to 20th	Day	10% 6% 1 7% 5% 4 6% 5% 7% 9% 38%	2800
	21st to end	Day	9% 4 2 5% 5% 5% 6% 5% 7% 10% 44%	2800
July	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 6% 8% 11% 43%	4493
	11th to 20th	Day	8% 5% 2 6% 4 6% 6% 6% 9% 11% 38%	4120
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 4 8% 8% 35%	4466
August	1st to 10th	Day	13% 5% 2 8% 6% 5% 6% 5% 8% 10% 32%	4104
	11th to 20th	Day	9% 4 2 6% 4 5% 6% 5% 8% 9% 40%	3953
	21st to end	Day	11% 4 2 5% 4 4 5% 4 6% 8% 48%	4180
September	1st to 10th	Day	13% 6% 3 6% 4 4 5% 4 5% 6% 45%	2280
	11th to 20th	Day	13% 6% 2 5% 3 3 5% 4 5% 8% 47%	2241
	21st to end	Day	22% 7% 2 6% 3 4 4 4 5% 6% 37%	2004
October	1st to 10th	Day	15% 6% 2 6% 3 4 4 4 5% 7% 45%	1900
	11th to 20th	Day	15% 5% 1 5% 3 4 4 3 4 6% 50%	1900
	21st to end	Day	14% 5% 2 4 3 3 4 3 4 6% 52%	2090
November	1st to 10th	Day	11% 6% 2 4 4 3 3 3 5% 6% 53%	1937
	11th to 20th	Day	14% 6% 1 4 4 3 4 3 6% 6% 50%	1721
	21st to end	Day	14% 4 2 4 4 4 4 4 5% 7% 48%	1600
December	1st to 10th	Day	11% 5% 2 5% 3 4 4 4 5% 7% 52%	2600
	11th to 20th	Day	17% 8% 2 5% 3 3 4 3 5% 6% 45%	2990
	21st to end	Day	17% 7% 3 5% 4 4 4 4 5% 8% 38%	3960
January	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 5% 7% 53%	6009
	11th to 20th	Night	13% 7% 2 4 2 2 3 3 5% 5% 53%	5900
	21st to end	Night	14% 7% 2 5% 3 3 3 2 4 5% 51%	6490
February	1st to 10th	Night	13% 7% 3 6% 5% 4 5% 4 6% 5% 42%	2600
	11th to 20th	Night	18% 7% 2 5% 4 3 5% 3 5% 5% 42%	2528
	21st to end	Night	14% 8% 3 5% 4 4 4 3 5% 7% 43%	1945
March	1st to 10th	Night	16% 5% 2 6% 4 4 4 3 5% 5% 47%	4918
	11th to 20th	Night	9% 5% 2 4 4 4 4 3 6% 7% 52%	4980
	21st to end	Night	10% 7% 2 6% 4 4 4 4 6% 7% 46%	5467
April	1st to 10th	Night	13% 7% 3 7% 4 4 4 4 4 6% 42%	2010
	11th to 20th	Night	11% 6% 3 5% 4 3 4 4 5% 7% 47%	1975
	21st to end	Night	16% 4 2 5% 3 5% 4 5% 6% 6% 44%	1995
May	1st to 10th	Night	19% 7% 3 8% 5% 4 5% 4 6% 8% 30%	4033
	11th to 20th	Night	25% 7% 2 5% 4 5% 4 4 5% 7% 32%	3953
	21st to end	Night	16% 5% 2 6% 5% 5% 5% 4 6% 9% 37%	3920
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 7% 8% 45%	1600
	11th to 20th	Night	13% 5% 2 5% 5% 5% 5% 4 7% 7% 43%	1600
	21st to end	Night	8% 3 2 6% 5% 5% 5% 5% 7% 10% 44%	1600
July	1st to 10th	Night	7% 4 2 5% 5% 6% 6% 5% 8% 12% 40%	2560
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 11% 39%	2838
	21st to end	Night	9% 7% 2 7% 5% 6% 5% 5% 8% 9% 37%	3087
August	1st to 10th	Night	16% 5% 3 6% 5% 6% 5% 5% 7% 10% 33%	2825
	11th to 20th	Night	9% 7% 2 7% 6% 6% 5% 4 6% 9% 40%	2844
	21st to end	Night	13% 4 1 4 4 3 4 3 6% 8% 49%	3476
September	1st to 10th	Night	14% 5% 2 6% 4 3 4 4 5% 8% 45%	1900
	11th to 20th	Night	13% 5% 2 6% 3 3 4 3 5% 7% 49%	1935
	21st to end	Night	23% 6% 2 5% 4 3 3 3 4 5% 42%	2175
October	1st to 10th	Night	18% 6% 2 6% 3 4 4 3 4 5% 47%	2280
	11th to 20th	Night	14% 4 2 5% 4 3 3 3 5% 6% 52%	2280
	21st to end	Night	14% 5% 2 4 3 4 3 3 4 6% 52%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 54%	2358
	11th to 20th	Night	13% 7% 2 5% 4 3 4 3 4 6% 50%	2675
	21st to end	Night	13% 5% 2 4 4 4 4 4 5% 7% 48%	2800
December	1st to 10th	Night	9% 5% 1 5% 4 4 3 3 5% 7% 54%	4580
	11th to 20th	Night	17% 7% 2 5% 4 4 4 3 5% 6% 42%	5205
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 7% 8% 40%	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	35% 5% 7% 25% 20% 8%	3920
	11th to 20th	Day	38% 4 9% 18% 19% 12%	4060
	21st to end	Day	41% 3 10% 25% 17% 4	4466
February	1st to 10th	Day	43% 6% 12% 24% 13% 3	1800
	11th to 20th	Day	43% 5% 10% 22% 17% 3	1767
	21st to end	Day	43% 5% 13% 18% 16% 5%	1681
March	1st to 10th	Day	40% 4 10% 19% 17% 9%	4935
	11th to 20th	Day	31% 5% 9% 26% 21% 7%	4980
	21st to end	Day	42% 5% 12% 16% 17% 7%	5465
April	1st to 10th	Day	39% 6% 13% 18% 17% 7%	2390
	11th to 20th	Day	36% 5% 12% 21% 20% 6%	2374
	21st to end	Day	43% 6% 11% 21% 14% 4	2421
May	1st to 10th	Day	46% 9% 15% 18% 10% 2	5897
	11th to 20th	Day	47% 9% 12% 22% 10% 1	5928
	21st to end	Day	39% 8% 15% 19% 17% 2	6949
June	1st to 10th	Day	36% 8% 15% 23% 17% 1	2800
	11th to 20th	Day	39% 9% 14% 21% 14% 2	2800
	21st to end	Day	33% 9% 16% 24% 16% 2	2800
July	1st to 10th	Day	32% 12% 20% 21% 11% 5%	4423
	11th to 20th	Day	35% 9% 17% 28% 10% 1	4051
	21st to end	Day	44% 8% 10% 23% 15% 1	4324
August	1st to 10th	Day	43% 10% 17% 23% 6%	4091
	11th to 20th	Day	35% 9% 15% 27% 13% 1	3992
	21st to end	Day	33% 8% 15% 29% 14% 1	4180
September	1st to 10th	Day	38% 7% 13% 17% 20% 4	2280
	11th to 20th	Day	35% 6% 10% 27% 19% 3	2241
	21st to end	Day	47% 6% 9% 19% 16% 2	2004
October	1st to 10th	Day	35% 5% 8% 19% 28% 6%	3160
	11th to 20th	Day	24% 4 10% 27% 23% 12%	3140
	21st to end	Day	21% 3 10% 31% 25% 10%	3428
November	1st to 10th	Day	32% 4 10% 25% 24% 5%	1937
	11th to 20th	Day	35% 5% 11% 20% 22% 6%	1721
	21st to end	Day	35% 5% 11% 27% 20% 3	1600
December	1st to 10th	Day	31% 6% 11% 27% 20% 5%	2600
	11th to 20th	Day	40% 4 14% 26% 12% 4	2986
	21st to end	Day	42% 7% 16% 20% 14% 1	3960
January	1st to 10th	Night	31% 3 10% 23% 26% 6%	6009
	11th to 20th	Night	34% 3 8% 23% 23% 9%	5900
	21st to end	Night	37% 3 9% 27% 21% 3	6490
February	1st to 10th	Night	41% 6% 13% 22% 15% 2	2600
	11th to 20th	Night	42% 4 11% 21% 16% 6%	2530
	21st to end	Night	42% 5% 12% 21% 16% 6%	1945
March	1st to 10th	Night	37% 5% 10% 22% 17% 9%	4918
	11th to 20th	Night	30% 6% 10% 24% 25% 6%	4980
	21st to end	Night	36% 6% 12% 21% 20% 6%	5467
April	1st to 10th	Night	41% 6% 11% 18% 20% 4	2010
	11th to 20th	Night	35% 5% 14% 21% 19% 5%	1975
	21st to end	Night	37% 7% 12% 26% 14% 4	1995
May	1st to 10th	Night	49% 8% 15% 16% 8% 4	4060
	11th to 20th	Night	48% 8% 12% 21% 10% 1	3954
	21st to end	Night	42% 9% 15% 23% 9% 3	3927
June	1st to 10th	Night	34% 9% 14% 26% 16% 2	1600
	11th to 20th	Night	37% 9% 15% 24% 13% 2	1600
	21st to end	Night	33% 9% 16% 22% 18% 3	1600
July	1st to 10th	Night	34% 11% 22% 16% 13% 3	2520
	11th to 20th	Night	35% 11% 20% 22% 11% 2	2785
	21st to end	Night	40% 9% 16% 20% 14% 1	2988
August	1st to 10th	Night	44% 8% 15% 25% 7% 1	2814
	11th to 20th	Night	39% 8% 12% 25% 15% 1	2871
	21st to end	Night	33% 6% 15% 29% 16% 1	3476
September	1st to 10th	Night	37% 7% 13% 20% 18% 5%	1900
	11th to 20th	Night	34% 6% 10% 26% 20% 3	1935
	21st to end	Night	45% 5% 10% 21% 16% 3	2175
October	1st to 10th	Night	40% 4 10% 19% 21% 5%	3800
	11th to 20th	Night	23% 4 11% 23% 31% 9%	3779
	21st to end	Night	22% 3 10% 32% 25% 8%	4120
November	1st to 10th	Night	32% 5% 10% 24% 23% 6%	2358
	11th to 20th	Night	36% 5% 7% 24% 21% 7%	2675
	21st to end	Night	34% 5% 12% 24% 20% 4	2800
December	1st to 10th	Night	30% 4 12% 29% 22% 3	4580
	11th to 20th	Night	42% 5% 13% 19% 17% 5%	5208
	21st to end	Night	39% 7% 16% 21% 17% 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	31% 26% 29% 12% 3	270
	11th to 20th	Day	34% 23% 27% 14% 2	300
	21st to end	Day	24% 28% 36% 9% 2 1	330
February	1st to 10th	Day	25% 29% 32% 12% 2	300
	11th to 20th	Day	41% 24% 30% 4	305
	21st to end	Day	29% 19% 32% 18% 2	414
March	1st to 10th	Day	23% 22% 32% 20% 3	500
	11th to 20th	Day	24% 24% 29% 16% 5% 1	500
	21st to end	Day	25% 20% 33% 17% 4 1	550
April	1st to 10th	Day	24% 26% 25% 17% 7% 1	500
	11th to 20th	Day	20% 18% 29% 19% 10% 3	492
	21st to end	Day	25% 13% 33% 18% 10% 1	500
May	1st to 10th	Day	24% 15% 29% 20% 10% 3	503
	11th to 20th	Day	20% 12% 31% 23% 10% 3	527
	21st to end	Day	20% 7% 28% 24% 16% 6%	573
June	1st to 10th	Day	12% 9% 26% 29% 18% 5%	530
	11th to 20th	Day	17% 8% 26% 28% 14% 7%	530
	21st to end	Day	10% 6% 26% 33% 19% 6%	530
July	1st to 10th	Day	10% 4 19% 33% 25% 9%	505
	11th to 20th	Day	6% 2 21% 33% 29% 9%	484
	21st to end	Day	11% 4 21% 33% 24% 7%	532
August	1st to 10th	Day	9% 6% 21% 30% 22% 12%	486
	11th to 20th	Day	11% 5% 20% 32% 21% 12%	479
	21st to end	Day	9% 3 19% 33% 23% 13%	539
September	1st to 10th	Day	18% 6% 16% 29% 18% 13%	490
	11th to 20th	Day	16% 4 20% 27% 23% 10%	490
	21st to end	Day	28% 9% 21% 24% 14% 3	490
October	1st to 10th	Day	20% 6% 23% 29% 16% 6%	490
	11th to 20th	Day	17% 14% 26% 26% 16% 2	490
	21st to end	Day	17% 15% 33% 22% 11% 3	539
November	1st to 10th	Day	15% 12% 32% 28% 10% 3	485
	11th to 20th	Day	22% 13% 32% 23% 9% 1	321
	21st to end	Day	22% 21% 33% 21% 5%	200
December	1st to 10th	Day	19% 22% 40% 14% 5% 1	200
	11th to 20th	Day	22% 17% 36% 19% 8%	200
	21st to end	Day	27% 17% 32% 19% 5%	220
January	1st to 10th	Night	19% 18% 39% 21% 3	800
	11th to 20th	Night	20% 15% 34% 28% 4	800
	21st to end	Night	17% 14% 43% 24% 2	880
February	1st to 10th	Night	17% 14% 41% 26% 3	800
	11th to 20th	Night	25% 20% 36% 17% 2	783
	21st to end	Night	22% 14% 41% 20% 3	660
March	1st to 10th	Night	18% 17% 38% 24% 3	800
	11th to 20th	Night	21% 16% 35% 22% 5% 1	800
	21st to end	Night	18% 15% 39% 22% 5%	880
April	1st to 10th	Night	17% 19% 35% 24% 5%	800
	11th to 20th	Night	15% 13% 37% 26% 8% 1	791
	21st to end	Night	18% 11% 31% 32% 8%	795
May	1st to 10th	Night	21% 8% 26% 29% 15% 1	600
	11th to 20th	Night	21% 4 28% 32% 13% 1	564
	21st to end	Night	16% 3 20% 40% 20% 1	433
June	1st to 10th	Night	10% 3 22% 45% 20% 1	400
	11th to 20th	Night	14% 4 15% 43% 22% 2	400
	21st to end	Night	8% 2 17% 49% 23% 2	400
July	1st to 10th	Night	12% 3 15% 37% 28% 4	380
	11th to 20th	Night	6% 1 11% 37% 40% 4	551
	21st to end	Night	9% 3 13% 42% 30% 4	624
August	1st to 10th	Night	11% 3 9% 43% 31% 3	567
	11th to 20th	Night	12% 3 13% 39% 30% 3	597
	21st to end	Night	13% 2 14% 38% 29% 4	836
September	1st to 10th	Night	16% 3 14% 35% 28% 4	760
	11th to 20th	Night	14% 4 14% 37% 27% 4	760
	21st to end	Night	26% 4 18% 30% 21% 1	760
October	1st to 10th	Night	20% 6% 20% 31% 20% 4	760
	11th to 20th	Night	15% 8% 27% 34% 14% 1	760
	21st to end	Night	17% 9% 29% 33% 11% 1	836
November	1st to 10th	Night	14% 8% 31% 34% 12% 1	780
	11th to 20th	Night	17% 10% 34% 30% 8% 1	800
	21st to end	Night	15% 10% 37% 29% 9%	800
December	1st to 10th	Night	19% 13% 36% 26% 8%	800
	11th to 20th	Night	18% 10% 37% 31% 4	800
	21st to end	Night	18% 11% 33% 34% 3	880

Page generated at 2025-07-03 01:30:16. Version ac07

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