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 2026-10-29.

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% 41%	149297
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	142732
Transparency	Day	38% 7% 13% 23% 16% 4	152899
	Night	37% 6% 12% 22% 18% 4	147202
Seeing	Day	19% 13% 27% 23% 12% 4	17563
	Night	17% 10% 29% 31% 13% 1	27147

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% 4 2 5% 5% 4 5% 4 5% 6% 48%	5081
	11th to 20th	Day	10% 9% 3 6% 4 4 3 3 4 5% 51%	5280
	21st to end	Day	12% 6% 3 7% 5% 4 4 4 5% 7% 43%	5780
February	1st to 10th	Day	12% 8% 3 6% 5% 5% 4 5% 6% 6% 41%	1890
	11th to 20th	Day	16% 8% 2 6% 4 4 4 4 5% 7% 40%	1848
	21st to end	Day	16% 8% 3 7% 4 3 3 4 4 6% 41%	1763
March	1st to 10th	Day	16% 6% 3 6% 4 4 5% 4 4 6% 42%	5045
	11th to 20th	Day	10% 6% 2 5% 4 3 4 3 6% 6% 51%	5090
	21st to end	Day	14% 7% 2 6% 4 4 5% 4 5% 7% 41%	5586
April	1st to 10th	Day	12% 7% 2 6% 5% 5% 4 4 6% 7% 44%	2509
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 5% 48%	2494
	21st to end	Day	17% 6% 2 7% 5% 4 4 3 5% 7% 40%	2542
May	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 9% 33%	7616
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 4 7% 9% 36%	7703
	21st to end	Day	21% 5% 2 6% 4 4 4 4 6% 7% 38%	9045
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 7% 9% 42%	2940
	11th to 20th	Day	10% 6% 1 7% 5% 5% 6% 5% 7% 9% 38%	2940
	21st to end	Day	9% 4 2 5% 5% 5% 6% 5% 7% 10% 43%	2940
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	8263
	11th to 20th	Day	8% 5% 2 7% 5% 6% 7% 6% 9% 12% 32%	7680
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 5% 8% 10% 39%	8340
August	1st to 10th	Day	13% 5% 2 9% 6% 6% 7% 6% 9% 10% 27%	6014
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 5% 9% 10% 35%	5816
	21st to end	Day	10% 3 1 4 4 4 5% 4 7% 9% 48%	6080
September	1st to 10th	Day	13% 5% 3 6% 4 4 5% 4 5% 6% 46%	2520
	11th to 20th	Day	13% 6% 2 5% 3 3 5% 4 5% 8% 46%	2477
	21st to end	Day	22% 8% 2 6% 3 4 4 4 5% 6% 36%	2214
October	1st to 10th	Day	15% 6% 2 5% 3 4 4 4 5% 7% 47%	2100
	11th to 20th	Day	15% 4 1 5% 3 4 4 3 4 5% 51%	2100
	21st to end	Day	14% 5% 2 4 3 3 4 3 4 6% 53%	2280
November	1st to 10th	Day	10% 6% 1 4 4 4 3 3 6% 6% 53%	2036
	11th to 20th	Day	13% 6% 1 4 4 3 4 3 6% 6% 50%	1807
	21st to end	Day	14% 5% 2 4 4 5% 4 4 5% 7% 48%	1680
December	1st to 10th	Day	11% 5% 2 5% 3 4 4 4 5% 7% 51%	2680
	11th to 20th	Day	16% 8% 2 5% 3 4 4 3 6% 6% 45%	3070
	21st to end	Day	17% 7% 3 5% 4 4 4 4 5% 8% 38%	4048
January	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 6% 7% 48%	7795
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 4 5% 53%	7680
	21st to end	Night	12% 7% 2 6% 3 3 4 3 4 5% 51%	8401
February	1st to 10th	Night	13% 7% 2 6% 5% 4 5% 4 7% 5% 42%	2730
	11th to 20th	Night	18% 7% 2 5% 4 3 5% 3 5% 5% 42%	2658
	21st to end	Night	14% 8% 3 5% 4 4 4 3 5% 7% 43%	2039
March	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 5% 47%	5028
	11th to 20th	Night	9% 5% 2 4 4 4 4 3 6% 7% 52%	5090
	21st to end	Night	10% 7% 2 6% 4 4 4 4 6% 7% 46%	5588
April	1st to 10th	Night	13% 6% 3 6% 4 4 4 4 4 6% 43%	2111
	11th to 20th	Night	11% 5% 3 5% 4 3 4 4 5% 8% 48%	2075
	21st to end	Night	15% 4 2 5% 3 5% 4 4 5% 7% 45%	2095
May	1st to 10th	Night	17% 7% 3 7% 5% 4 5% 4 6% 8% 34%	5237
	11th to 20th	Night	20% 6% 2 5% 4 5% 5% 4 6% 8% 37%	5141
	21st to end	Night	21% 6% 2 6% 5% 4 4 4 5% 7% 35%	5108
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 7% 8% 45%	1680
	11th to 20th	Night	13% 5% 2 5% 4 5% 5% 4 7% 7% 43%	1680
	21st to end	Night	9% 3 2 6% 5% 5% 5% 5% 7% 9% 44%	1680
July	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 4 7% 10% 40%	4678
	11th to 20th	Night	9% 7% 3 9% 6% 6% 7% 5% 8% 9% 31%	5275
	21st to end	Night	8% 6% 2 7% 5% 6% 5% 5% 7% 9% 41%	5736
August	1st to 10th	Night	14% 6% 3 7% 5% 6% 6% 5% 9% 10% 29%	4135
	11th to 20th	Night	10% 6% 3 8% 6% 7% 5% 4 7% 10% 35%	4184
	21st to end	Night	12% 5% 2 5% 4 4 4 4 6% 8% 46%	5045
September	1st to 10th	Night	14% 4 2 5% 3 3 4 4 5% 8% 47%	2100
	11th to 20th	Night	14% 6% 2 6% 3 3 4 3 5% 7% 49%	2139
	21st to end	Night	24% 6% 2 5% 4 3 4 3 4 5% 40%	2404
October	1st to 10th	Night	17% 6% 2 6% 3 3 4 3 4 5% 48%	2520
	11th to 20th	Night	15% 4 2 5% 4 3 3 3 4 6% 52%	2520
	21st to end	Night	13% 4 2 4 3 4 3 3 4 6% 53%	2737
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 54%	2479
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 4 6% 50%	2809
	21st to end	Night	13% 5% 2 4 4 4 4 4 5% 7% 48%	2940
December	1st to 10th	Night	9% 5% 1 5% 4 4 3 3 5% 7% 54%	4720
	11th to 20th	Night	17% 7% 2 5% 4 4 4 3 5% 6% 43%	5345
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 6% 8% 41%	7150

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	36% 5% 10% 23% 19% 6%	5082
	11th to 20th	Day	36% 3 8% 18% 21% 13%	5280
	21st to end	Day	40% 3 10% 29% 15% 4	5780
February	1st to 10th	Day	41% 6% 11% 24% 14% 3	1890
	11th to 20th	Day	44% 5% 10% 22% 17% 3	1857
	21st to end	Day	44% 5% 13% 19% 15% 5%	1763
March	1st to 10th	Day	40% 4 11% 19% 17% 9%	5045
	11th to 20th	Day	32% 5% 9% 26% 21% 7%	5090
	21st to end	Day	41% 5% 12% 17% 17% 7%	5586
April	1st to 10th	Day	38% 6% 13% 18% 18% 7%	2509
	11th to 20th	Day	36% 5% 12% 21% 20% 6%	2494
	21st to end	Day	43% 6% 11% 22% 14% 4	2542
May	1st to 10th	Day	44% 8% 13% 20% 13% 2	7651
	11th to 20th	Day	42% 8% 13% 25% 12% 1	7707
	21st to end	Day	44% 7% 12% 16% 17% 3	9061
June	1st to 10th	Day	36% 8% 14% 22% 18% 2	2940
	11th to 20th	Day	39% 9% 15% 21% 14% 2	2940
	21st to end	Day	34% 9% 15% 24% 16% 2	2940
July	1st to 10th	Day	36% 10% 15% 22% 13% 3	8193
	11th to 20th	Day	40% 10% 14% 27% 8% 1	7611
	21st to end	Day	37% 8% 11% 24% 19% 1	8156
August	1st to 10th	Day	47% 12% 17% 19% 5%	6001
	11th to 20th	Day	39% 10% 17% 24% 10%	5855
	21st to end	Day	30% 8% 19% 27% 15% 1	6080
September	1st to 10th	Day	36% 7% 14% 18% 20% 4	2520
	11th to 20th	Day	35% 6% 10% 27% 19% 3	2477
	21st to end	Day	49% 6% 9% 19% 15% 3	2214
October	1st to 10th	Day	34% 4 8% 20% 28% 6%	3360
	11th to 20th	Day	24% 4 9% 27% 24% 12%	3340
	21st to end	Day	21% 3 10% 30% 25% 10%	3618
November	1st to 10th	Day	31% 4 11% 25% 24% 5%	2036
	11th to 20th	Day	34% 5% 11% 21% 22% 6%	1807
	21st to end	Day	35% 5% 11% 27% 19% 3	1680
December	1st to 10th	Day	32% 6% 11% 27% 19% 5%	2680
	11th to 20th	Day	40% 4 14% 26% 12% 4	3066
	21st to end	Day	42% 7% 16% 20% 14% 1	4048
January	1st to 10th	Night	34% 4 12% 24% 22% 4	7796
	11th to 20th	Night	34% 3 8% 21% 24% 10%	7680
	21st to end	Night	36% 3 9% 28% 21% 4	8401
February	1st to 10th	Night	41% 6% 13% 22% 16% 2	2730
	11th to 20th	Night	42% 4 11% 21% 16% 5%	2660
	21st to end	Night	41% 5% 13% 21% 15% 6%	2039
March	1st to 10th	Night	37% 5% 10% 22% 17% 9%	5028
	11th to 20th	Night	30% 5% 10% 24% 25% 6%	5090
	21st to end	Night	36% 6% 13% 21% 19% 6%	5588
April	1st to 10th	Night	40% 6% 11% 19% 20% 4	2111
	11th to 20th	Night	35% 5% 14% 22% 19% 5%	2075
	21st to end	Night	37% 6% 13% 26% 14% 4	2095
May	1st to 10th	Night	47% 7% 13% 19% 11% 4	5264
	11th to 20th	Night	42% 7% 14% 25% 11% 1	5142
	21st to end	Night	48% 8% 12% 18% 11% 3	5115
June	1st to 10th	Night	33% 9% 13% 26% 17% 2	1680
	11th to 20th	Night	38% 8% 16% 23% 13% 2	1680
	21st to end	Night	34% 9% 15% 22% 18% 2	1680
July	1st to 10th	Night	38% 11% 17% 18% 15% 2	4640
	11th to 20th	Night	45% 12% 15% 17% 10% 1	5225
	21st to end	Night	36% 8% 16% 21% 18%	5606
August	1st to 10th	Night	47% 10% 15% 21% 6%	4124
	11th to 20th	Night	42% 9% 16% 20% 13%	4211
	21st to end	Night	35% 7% 17% 26% 14% 1	5045
September	1st to 10th	Night	35% 7% 14% 22% 18% 5%	2100
	11th to 20th	Night	35% 6% 10% 27% 20% 2	2139
	21st to end	Night	47% 5% 10% 19% 16% 3	2404
October	1st to 10th	Night	39% 4 10% 20% 22% 5%	4040
	11th to 20th	Night	24% 4 10% 22% 30% 10%	4019
	21st to end	Night	22% 3 10% 31% 26% 8%	4349
November	1st to 10th	Night	32% 4 10% 24% 23% 6%	2479
	11th to 20th	Night	36% 4 7% 24% 21% 7%	2809
	21st to end	Night	35% 5% 12% 24% 20% 4	2940
December	1st to 10th	Night	30% 4 13% 29% 22% 3	4720
	11th to 20th	Night	42% 5% 13% 19% 17% 5%	5348
	21st to end	Night	39% 7% 16% 21% 16% 1	7150

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	297
	11th to 20th	Day	34% 25% 26% 13% 2	330
	21st to end	Day	25% 29% 35% 9% 2 1	363
February	1st to 10th	Day	25% 29% 32% 12% 2	330
	11th to 20th	Day	39% 24% 32% 4	335
	21st to end	Day	28% 19% 34% 17% 2	446
March	1st to 10th	Day	22% 22% 32% 19% 4	540
	11th to 20th	Day	25% 24% 29% 16% 5% 1	540
	21st to end	Day	25% 21% 33% 17% 4 1	594
April	1st to 10th	Day	24% 25% 26% 17% 7% 1	540
	11th to 20th	Day	20% 18% 30% 19% 11% 3	532
	21st to end	Day	25% 14% 33% 18% 9% 1	540
May	1st to 10th	Day	24% 17% 29% 19% 9% 3	543
	11th to 20th	Day	19% 13% 32% 22% 10% 4	567
	21st to end	Day	22% 8% 27% 24% 15% 5%	617
June	1st to 10th	Day	12% 10% 27% 29% 18% 5%	570
	11th to 20th	Day	17% 9% 26% 28% 13% 6%	570
	21st to end	Day	11% 7% 27% 32% 18% 5%	570
July	1st to 10th	Day	10% 4 19% 32% 26% 9%	565
	11th to 20th	Day	7% 2 21% 33% 28% 9%	564
	21st to end	Day	10% 4 23% 33% 23% 7%	620
August	1st to 10th	Day	10% 6% 22% 30% 21% 11%	566
	11th to 20th	Day	12% 5% 20% 33% 19% 11%	559
	21st to end	Day	8% 5% 22% 32% 21% 12%	627
September	1st to 10th	Day	16% 7% 18% 31% 17% 12%	570
	11th to 20th	Day	16% 4 21% 27% 23% 9%	570
	21st to end	Day	29% 10% 22% 23% 13% 4	570
October	1st to 10th	Day	18% 6% 24% 30% 16% 6%	570
	11th to 20th	Day	19% 14% 27% 25% 14% 2	570
	21st to end	Day	17% 15% 33% 23% 10% 2	615
November	1st to 10th	Day	15% 14% 33% 27% 9% 2	524
	11th to 20th	Day	23% 13% 32% 21% 9% 1	347
	21st to end	Day	21% 22% 31% 21% 4	220
December	1st to 10th	Day	21% 22% 37% 14% 5%	220
	11th to 20th	Day	22% 16% 38% 17% 7%	220
	21st to end	Day	28% 18% 32% 17% 5%	242
January	1st to 10th	Night	18% 18% 39% 22% 3	840
	11th to 20th	Night	20% 15% 35% 27% 3	840
	21st to end	Night	17% 14% 44% 23% 2	924
February	1st to 10th	Night	17% 14% 40% 26% 3	840
	11th to 20th	Night	25% 20% 36% 18% 2	823
	21st to end	Night	21% 14% 41% 20% 3	692
March	1st to 10th	Night	18% 17% 37% 25% 3	840
	11th to 20th	Night	21% 16% 35% 23% 5% 1	840
	21st to end	Night	18% 16% 40% 21% 5%	924
April	1st to 10th	Night	17% 19% 35% 24% 5%	840
	11th to 20th	Night	15% 13% 37% 27% 8% 1	831
	21st to end	Night	18% 11% 31% 32% 8%	835
May	1st to 10th	Night	20% 9% 27% 28% 15% 1	630
	11th to 20th	Night	20% 5% 29% 32% 13% 1	593
	21st to end	Night	18% 3 20% 38% 20% 1	455
June	1st to 10th	Night	10% 3 23% 43% 20% 1	420
	11th to 20th	Night	14% 5% 17% 42% 21% 1	420
	21st to end	Night	8% 2 18% 48% 23% 1	420
July	1st to 10th	Night	12% 3 15% 36% 30% 4	410
	11th to 20th	Night	7% 1 11% 37% 40% 4	611
	21st to end	Night	9% 3 13% 42% 29% 4	690
August	1st to 10th	Night	12% 3 10% 43% 30% 3	627
	11th to 20th	Night	12% 3 13% 39% 30% 3	661
	21st to end	Night	13% 2 16% 38% 27% 4	924
September	1st to 10th	Night	15% 3 14% 37% 27% 4	840
	11th to 20th	Night	15% 4 14% 37% 26% 4	840
	21st to end	Night	27% 4 19% 29% 20% 1	840
October	1st to 10th	Night	19% 6% 20% 32% 20% 3	840
	11th to 20th	Night	17% 9% 26% 33% 14% 1	840
	21st to end	Night	16% 9% 29% 33% 12% 1	913
November	1st to 10th	Night	15% 8% 31% 33% 11% 1	820
	11th to 20th	Night	17% 10% 35% 29% 8% 1	840
	21st to end	Night	15% 11% 38% 28% 9%	840
December	1st to 10th	Night	18% 13% 36% 25% 8%	840
	11th to 20th	Night	18% 10% 37% 31% 4	840
	21st to end	Night	18% 12% 33% 34% 3	924

Page generated at 2026-10-26 04:37:52. Version ac07

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