Courthouse Hill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Courthouse Hill clear sky chart from 2005-11-06 to 2026-03-03.

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	20% 13% 4 9% 5% 4 4 3 5% 6% 27%	132032
	Night	20% 13% 4 10% 6% 4 4 3 4 5% 26%	129286
Transparency	Day	58% 5% 11% 18% 8% 1	135599
	Night	59% 5% 10% 17% 8% 1	133807
Seeing	Day	30% 18% 30% 16% 5% 1	19145
	Night	28% 16% 30% 21% 5%	26642

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	14% 20% 8% 16% 9% 6% 4 2 3 3 17%	4690
	11th to 20th	Day	18% 20% 10% 19% 6% 4 3 2 3 3 11%	4774
	21st to end	Day	14% 12% 6% 13% 11% 8% 6% 3 4 5% 19%	5781
February	1st to 10th	Day	18% 16% 7% 12% 9% 6% 4 2 4 4 18%	1890
	11th to 20th	Day	16% 16% 5% 14% 7% 5% 3 3 4 5% 21%	1843
	21st to end	Day	12% 16% 6% 10% 6% 4 3 3 4 6% 29%	1594
March	1st to 10th	Day	23% 16% 4 13% 5% 5% 3 2 3 4 23%	4897
	11th to 20th	Day	13% 20% 7% 10% 5% 3 3 3 4 6% 26%	4980
	21st to end	Day	25% 19% 5% 9% 4 4 4 3 3 4 20%	5483
April	1st to 10th	Day	25% 18% 4 8% 4 4 4 2 3 4 24%	2400
	11th to 20th	Day	21% 14% 3 6% 5% 4 4 3 5% 6% 30%	2434
	21st to end	Day	24% 15% 3 7% 4 4 3 4 5% 5% 27%	2600
May	1st to 10th	Day	31% 16% 3 7% 4 4 4 3 5% 5% 19%	6088
	11th to 20th	Day	23% 12% 3 8% 5% 4 4 3 4 6% 28%	6318
	21st to end	Day	20% 12% 3 7% 5% 5% 4 3 5% 7% 29%	6948
June	1st to 10th	Day	27% 11% 3 7% 5% 4 5% 3 4 6% 25%	2800
	11th to 20th	Day	23% 8% 3 6% 5% 4 4 3 4 7% 32%	2894
	21st to end	Day	30% 9% 2 6% 4 5% 3 3 5% 7% 26%	3000
July	1st to 10th	Day	21% 9% 3 7% 5% 5% 5% 4 6% 8% 27%	6618
	11th to 20th	Day	16% 9% 3 8% 4 4 5% 3 6% 8% 34%	6360
	21st to end	Day	13% 8% 3 6% 4 4 5% 5% 7% 9% 36%	6798
August	1st to 10th	Day	23% 10% 3 8% 5% 5% 4 3 5% 6% 27%	4235
	11th to 20th	Day	15% 8% 3 8% 5% 5% 4 4 7% 8% 32%	4175
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 9% 44%	4479
September	1st to 10th	Day	19% 6% 2 6% 4 3 5% 3 6% 7% 40%	2400
	11th to 20th	Day	20% 7% 2 6% 4 3 3 3 5% 6% 41%	2344
	21st to end	Day	20% 7% 3 5% 4 4 3 3 5% 7% 41%	2123
October	1st to 10th	Day	19% 7% 3 7% 4 3 5% 4 5% 7% 37%	2000
	11th to 20th	Day	20% 11% 3 7% 4 4 5% 4 5% 7% 31%	2000
	21st to end	Day	21% 12% 4 9% 4 3 3 3 5% 5% 29%	2195
November	1st to 10th	Day	23% 14% 4 7% 4 5% 4 4 4 4 27%	1695
	11th to 20th	Day	21% 22% 5% 8% 3 3 4 3 5% 5% 23%	1680
	21st to end	Day	19% 18% 5% 10% 7% 5% 4 3 4 4 21%	1680
December	1st to 10th	Day	18% 16% 8% 15% 5% 4 3 3 4 5% 18%	2690
	11th to 20th	Day	27% 15% 6% 12% 6% 4 4 2 3 3 19%	3076
	21st to end	Day	19% 15% 10% 17% 4 3 3 3 4 4 20%	4070
January	1st to 10th	Night	14% 19% 8% 16% 9% 5% 4 3 3 4 16%	8234
	11th to 20th	Night	20% 17% 7% 19% 5% 5% 3 3 3 3 14%	8132
	21st to end	Night	11% 12% 5% 16% 14% 8% 6% 3 4 5% 16%	8400
February	1st to 10th	Night	19% 15% 5% 14% 10% 6% 5% 3 4 3 17%	2730
	11th to 20th	Night	18% 14% 5% 11% 8% 5% 5% 4 5% 5% 21%	2663
	21st to end	Night	12% 15% 6% 11% 8% 6% 4 4 4 4 27%	2211
March	1st to 10th	Night	24% 16% 5% 12% 8% 4 3 2 3 3 20%	4983
	11th to 20th	Night	18% 17% 6% 8% 4 4 4 3 4 5% 26%	4979
	21st to end	Night	28% 16% 5% 10% 4 3 3 2 3 3 23%	5449
April	1st to 10th	Night	24% 15% 4 7% 4 4 4 2 5% 5% 27%	2000
	11th to 20th	Night	23% 12% 3 7% 4 4 4 3 4 6% 31%	1913
	21st to end	Night	23% 14% 3 6% 4 4 4 3 4 6% 29%	1800
May	1st to 10th	Night	30% 18% 3 6% 4 3 4 3 4 5% 21%	3847
	11th to 20th	Night	22% 13% 3 6% 4 3 4 4 6% 6% 29%	3596
	21st to end	Night	18% 14% 3 8% 4 4 5% 3 5% 6% 30%	3948
June	1st to 10th	Night	27% 9% 3 6% 4 4 4 4 5% 6% 28%	1600
	11th to 20th	Night	23% 9% 2 5% 4 3 4 3 4 8% 35%	1517
	21st to end	Night	25% 10% 3 8% 5% 4 4 4 6% 7% 25%	1400
July	1st to 10th	Night	18% 10% 3 9% 6% 6% 6% 5% 6% 7% 23%	3298
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 6% 8% 36%	3620
	21st to end	Night	16% 7% 3 6% 4 4 4 3 5% 8% 39%	4067
August	1st to 10th	Night	22% 8% 3 6% 4 4 5% 3 5% 7% 33%	2915
	11th to 20th	Night	17% 8% 3 7% 4 4 4 4 6% 7% 35%	2872
	21st to end	Night	13% 5% 2 6% 3 3 4 3 6% 6% 48%	3418
September	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 4 7% 45%	2000
	11th to 20th	Night	18% 8% 2 5% 4 3 2 3 4 6% 44%	2053
	21st to end	Night	20% 8% 3 6% 4 3 3 3 5% 7% 40%	2276
October	1st to 10th	Night	18% 7% 2 6% 4 5% 5% 3 5% 6% 40%	2400
	11th to 20th	Night	19% 10% 3 6% 5% 4 4 3 4 7% 36%	2400
	21st to end	Night	28% 10% 3 6% 4 4 3 3 4 6% 30%	2643
November	1st to 10th	Night	21% 14% 3 7% 4 4 4 2 5% 5% 30%	2819
	11th to 20th	Night	20% 18% 4 8% 5% 4 4 3 5% 5% 23%	2940
	21st to end	Night	22% 16% 5% 8% 5% 4 4 3 4 5% 23%	2940
December	1st to 10th	Night	16% 14% 6% 17% 6% 5% 4 3 4 6% 19%	4720
	11th to 20th	Night	24% 16% 4 11% 5% 4 3 2 3 3 25%	5353
	21st to end	Night	22% 16% 6% 11% 5% 3 3 2 3 4 24%	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	75% 3 10% 8% 3	4691
	11th to 20th	Day	77% 7% 6% 9% 2	4803
	21st to end	Day	66% 6% 8% 9% 10% 2	5781
February	1st to 10th	Day	71% 4 8% 10% 7% 1	1890
	11th to 20th	Day	67% 5% 8% 14% 6% 1	1851
	21st to end	Day	57% 4 10% 17% 10% 2	1594
March	1st to 10th	Day	68% 3 6% 12% 7% 3	4901
	11th to 20th	Day	59% 6% 12% 17% 6% 1	4980
	21st to end	Day	70% 3 5% 14% 7% 1	5483
April	1st to 10th	Day	65% 3 6% 13% 10% 2	2400
	11th to 20th	Day	55% 5% 8% 20% 11% 1	2434
	21st to end	Day	58% 4 8% 17% 10% 2	2600
May	1st to 10th	Day	67% 4 8% 14% 6%	6099
	11th to 20th	Day	58% 5% 11% 21% 5%	6322
	21st to end	Day	53% 6% 13% 18% 9%	6948
June	1st to 10th	Day	60% 5% 10% 19% 6% 1	2800
	11th to 20th	Day	52% 5% 12% 20% 10% 1	2894
	21st to end	Day	59% 5% 11% 16% 9% 1	3000
July	1st to 10th	Day	53% 6% 13% 18% 9% 1	6547
	11th to 20th	Day	48% 6% 14% 23% 8%	6284
	21st to end	Day	42% 7% 17% 25% 8%	6603
August	1st to 10th	Day	57% 5% 11% 19% 8%	4222
	11th to 20th	Day	46% 7% 14% 24% 8%	4214
	21st to end	Day	35% 6% 14% 28% 16%	4479
September	1st to 10th	Day	43% 5% 12% 25% 13% 1	2400
	11th to 20th	Day	44% 4 13% 24% 14% 1	2344
	21st to end	Day	43% 4 11% 26% 16%	2123
October	1st to 10th	Day	46% 8% 11% 24% 11%	3260
	11th to 20th	Day	47% 5% 10% 26% 12%	3240
	21st to end	Day	49% 5% 14% 22% 11%	3522
November	1st to 10th	Day	60% 5% 8% 19% 7%	1695
	11th to 20th	Day	62% 6% 11% 14% 6% 1	1680
	21st to end	Day	66% 6% 11% 12% 4	1680
December	1st to 10th	Day	68% 6% 8% 16% 2	2690
	11th to 20th	Day	72% 4 5% 13% 5%	3075
	21st to end	Day	70% 3 8% 13% 6%	4070
January	1st to 10th	Night	72% 5% 9% 12% 2	8234
	11th to 20th	Night	75% 5% 7% 9% 3	8178
	21st to end	Night	69% 6% 8% 11% 6% 1	8400
February	1st to 10th	Night	71% 5% 7% 10% 6% 1	2730
	11th to 20th	Night	65% 5% 10% 14% 6% 1	2666
	21st to end	Night	61% 4 8% 18% 8% 1	2211
March	1st to 10th	Night	70% 4 5% 10% 10% 1	4992
	11th to 20th	Night	60% 5% 11% 16% 8% 1	4980
	21st to end	Night	68% 3 5% 16% 8% 1	5449
April	1st to 10th	Night	60% 5% 8% 14% 10% 3	2000
	11th to 20th	Night	55% 4 7% 20% 13% 1	1913
	21st to end	Night	57% 4 10% 17% 11% 2	1800
May	1st to 10th	Night	67% 3 9% 13% 7%	3853
	11th to 20th	Night	55% 5% 11% 22% 6% 1	3597
	21st to end	Night	54% 6% 14% 19% 6%	3949
June	1st to 10th	Night	56% 6% 11% 20% 7% 1	1600
	11th to 20th	Night	48% 5% 13% 23% 11% 1	1517
	21st to end	Night	58% 7% 12% 16% 7% 1	1400
July	1st to 10th	Night	57% 7% 13% 15% 7% 1	3260
	11th to 20th	Night	45% 6% 15% 23% 10% 1	3572
	21st to end	Night	43% 6% 16% 23% 11%	3948
August	1st to 10th	Night	51% 6% 14% 23% 6%	2903
	11th to 20th	Night	46% 8% 15% 24% 7%	2899
	21st to end	Night	34% 6% 17% 28% 14% 1	3417
September	1st to 10th	Night	40% 6% 11% 28% 14% 1	2000
	11th to 20th	Night	41% 5% 11% 29% 13% 1	2053
	21st to end	Night	44% 5% 10% 27% 12% 1	2276
October	1st to 10th	Night	48% 5% 12% 23% 11%	3920
	11th to 20th	Night	43% 5% 12% 27% 14%	3899
	21st to end	Night	47% 4 14% 22% 13%	4267
November	1st to 10th	Night	57% 5% 11% 18% 9% 1	2819
	11th to 20th	Night	62% 6% 10% 15% 7%	2940
	21st to end	Night	64% 6% 13% 13% 4	2940
December	1st to 10th	Night	64% 6% 11% 16% 2	4720
	11th to 20th	Night	65% 5% 7% 17% 6%	5355
	21st to end	Night	64% 5% 9% 15% 7%	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	36% 23% 29% 12% 1	430
	11th to 20th	Day	36% 23% 26% 10% 4	430
	21st to end	Day	28% 24% 33% 13% 2	473
February	1st to 10th	Day	32% 23% 30% 12% 3	430
	11th to 20th	Day	37% 26% 25% 10% 3	426
	21st to end	Day	29% 21% 35% 11% 3 1	355
March	1st to 10th	Day	33% 25% 33% 7% 2 1	493
	11th to 20th	Day	30% 29% 27% 9% 5% 1	500
	21st to end	Day	35% 28% 24% 10% 3 1	550
April	1st to 10th	Day	47% 23% 22% 7% 2	500
	11th to 20th	Day	37% 25% 24% 11% 2	492
	21st to end	Day	33% 24% 26% 13% 4	500
May	1st to 10th	Day	40% 19% 26% 10% 3 1	588
	11th to 20th	Day	30% 17% 28% 19% 5% 1	724
	21st to end	Day	30% 18% 32% 16% 3 1	790
June	1st to 10th	Day	35% 16% 28% 16% 4	730
	11th to 20th	Day	28% 16% 32% 18% 5% 1	730
	21st to end	Day	35% 12% 29% 16% 6% 1	730
July	1st to 10th	Day	24% 11% 35% 22% 7% 1	720
	11th to 20th	Day	22% 9% 36% 24% 8% 2	718
	21st to end	Day	23% 9% 35% 25% 7% 1	741
August	1st to 10th	Day	18% 9% 38% 25% 9% 1	526
	11th to 20th	Day	20% 10% 33% 23% 11% 3	519
	21st to end	Day	17% 16% 34% 21% 9% 3	583
September	1st to 10th	Day	19% 14% 33% 25% 8% 1	530
	11th to 20th	Day	26% 14% 29% 20% 8% 3	530
	21st to end	Day	22% 18% 29% 20% 9% 2	487
October	1st to 10th	Day	24% 14% 31% 19% 10% 3	420
	11th to 20th	Day	26% 20% 28% 16% 7% 1	420
	21st to end	Day	30% 23% 27% 14% 5% 1	462
November	1st to 10th	Day	35% 18% 27% 14% 5% 1	425
	11th to 20th	Day	41% 20% 23% 11% 5%	430
	21st to end	Day	37% 22% 27% 10% 3 1	430
December	1st to 10th	Day	38% 19% 28% 12% 2	430
	11th to 20th	Day	43% 22% 24% 9% 1	430
	21st to end	Day	35% 23% 28% 11% 3	473
January	1st to 10th	Night	31% 23% 30% 14% 2	950
	11th to 20th	Night	34% 21% 27% 16% 3	931
	21st to end	Night	25% 24% 31% 17% 2	924
February	1st to 10th	Night	29% 21% 30% 17% 2	840
	11th to 20th	Night	34% 25% 26% 14% 2	823
	21st to end	Night	22% 23% 32% 20% 3	692
March	1st to 10th	Night	29% 24% 30% 16% 2	809
	11th to 20th	Night	26% 19% 31% 19% 5%	800
	21st to end	Night	30% 19% 30% 17% 4	875
April	1st to 10th	Night	36% 19% 28% 16% 2	600
	11th to 20th	Night	31% 18% 32% 15% 3 1	594
	21st to end	Night	32% 17% 28% 19% 4 1	600
May	1st to 10th	Night	35% 14% 30% 18% 4	600
	11th to 20th	Night	30% 12% 31% 22% 5%	592
	21st to end	Night	22% 15% 42% 18% 3	649
June	1st to 10th	Night	32% 10% 33% 22% 3	600
	11th to 20th	Night	26% 10% 34% 26% 4	517
	21st to end	Night	28% 6% 29% 29% 9%	400
July	1st to 10th	Night	20% 7% 33% 33% 7%	447
	11th to 20th	Night	17% 7% 36% 31% 8%	582
	21st to end	Night	20% 6% 31% 36% 8%	656
August	1st to 10th	Night	17% 6% 30% 38% 9% 1	598
	11th to 20th	Night	19% 7% 26% 37% 11% 1	592
	21st to end	Night	16% 8% 30% 33% 13%	660
September	1st to 10th	Night	17% 9% 32% 31% 11%	600
	11th to 20th	Night	21% 11% 24% 33% 11%	653
	21st to end	Night	25% 12% 29% 25% 9% 1	800
October	1st to 10th	Night	22% 11% 29% 27% 9% 1	800
	11th to 20th	Night	25% 14% 35% 17% 8% 1	800
	21st to end	Night	35% 17% 27% 17% 4	883
November	1st to 10th	Night	33% 16% 28% 18% 5% 1	930
	11th to 20th	Night	31% 21% 26% 17% 5%	950
	21st to end	Night	34% 22% 26% 16% 3	950
December	1st to 10th	Night	34% 19% 28% 16% 2	950
	11th to 20th	Night	36% 24% 27% 12% 2	950
	21st to end	Night	32% 21% 27% 17% 2	1045

Page generated at 2026-02-28 02:40:25. Version ac07

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