Cape Orion Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cape Orion Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	15% 5% 2 5% 4 4 4 4 6% 7% 43%	131302
	Night	16% 5% 2 5% 4 4 4 3 5% 7% 45%	130199
Transparency	Day	37% 8% 12% 19% 18% 7%	135039
	Night	37% 7% 11% 18% 20% 7%	134532
Seeing	Day	18% 11% 24% 24% 16% 8%	16759
	Night	17% 8% 22% 30% 19% 4	26889

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	19% 5% 2 4 2 2 2 2 3 5% 52%	5258
	11th to 20th	Day	11% 5% 2 4 3 2 2 2 2 3 65%	5280
	21st to end	Day	23% 6% 3 6% 3 3 4 2 4 4 42%	5780
February	1st to 10th	Day	16% 6% 2 6% 4 3 3 3 5% 6% 46%	2053
	11th to 20th	Day	22% 5% 1 4 4 3 3 3 5% 6% 43%	2048
	21st to end	Day	20% 6% 2 6% 3 4 3 3 4 5% 44%	1730
March	1st to 10th	Day	23% 5% 2 5% 3 4 3 3 5% 5% 41%	4862
	11th to 20th	Day	12% 4 2 4 3 3 3 3 4 6% 56%	5057
	21st to end	Day	20% 4 2 5% 4 4 3 3 4 5% 46%	5465
April	1st to 10th	Day	15% 3 2 4 3 5% 4 5% 6% 7% 46%	2200
	11th to 20th	Day	14% 4 2 4 5% 5% 4 4 5% 6% 48%	2384
	21st to end	Day	13% 4 2 5% 4 4 4 4 5% 7% 49%	2600
May	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 8% 36%	5862
	11th to 20th	Day	20% 7% 2 7% 5% 5% 5% 5% 7% 8% 29%	5861
	21st to end	Day	15% 5% 2 6% 4 4 4 4 6% 8% 41%	6433
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 6% 6% 9% 10% 34%	2600
	11th to 20th	Day	7% 5% 1 5% 5% 6% 7% 6% 9% 11% 38%	2600
	21st to end	Day	9% 3 1 5% 5% 6% 7% 5% 8% 11% 39%	2600
July	1st to 10th	Day	7% 4 2 7% 6% 5% 7% 5% 9% 12% 37%	5900
	11th to 20th	Day	7% 4 2 7% 6% 6% 8% 6% 9% 13% 32%	5900
	21st to end	Day	10% 6% 3 8% 7% 7% 8% 6% 8% 10% 26%	6424
August	1st to 10th	Day	13% 6% 2 6% 5% 5% 6% 5% 8% 10% 32%	4234
	11th to 20th	Day	10% 4 2 6% 5% 5% 6% 5% 7% 11% 38%	4174
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 6% 7% 50%	4225
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 4 8% 8% 39%	2200
	11th to 20th	Day	17% 5% 2 5% 5% 5% 6% 4 6% 8% 37%	2200
	21st to end	Day	17% 5% 2 5% 4 5% 4 3 5% 8% 43%	2200
October	1st to 10th	Day	16% 4 2 4 2 3 3 2 4 6% 54%	2200
	11th to 20th	Day	14% 4 1 4 3 2 2 2 4 5% 59%	2007
	21st to end	Day	16% 4 1 4 3 2 2 3 3 4 58%	2200
November	1st to 10th	Day	14% 4 1 4 3 2 3 2 4 4 59%	1937
	11th to 20th	Day	15% 5% 1 3 2 2 2 3 4 6% 56%	1890
	21st to end	Day	16% 4 2 4 3 3 3 3 5% 6% 51%	1890
December	1st to 10th	Day	20% 5% 2 4 3 3 3 2 5% 5% 50%	3020
	11th to 20th	Day	20% 4 2 5% 4 3 4 3 5% 5% 45%	3463
	21st to end	Day	25% 4 1 2 2 2 3 2 4 7% 48%	4565
January	1st to 10th	Night	18% 3 1 3 3 3 3 3 4 4 54%	7635
	11th to 20th	Night	12% 5% 2 5% 3 3 3 2 4 4 57%	7680
	21st to end	Night	21% 6% 2 4 3 3 3 2 4 5% 47%	8401
February	1st to 10th	Night	15% 6% 2 4 3 3 3 3 4 6% 49%	2562
	11th to 20th	Night	22% 6% 2 4 4 3 2 3 5% 4 44%	2458
	21st to end	Night	20% 5% 1 5% 4 4 4 3 4 7% 42%	2076
March	1st to 10th	Night	22% 5% 2 5% 3 4 4 3 5% 7% 41%	5197
	11th to 20th	Night	11% 3 1 4 3 3 3 3 5% 7% 57%	5058
	21st to end	Night	19% 4 2 5% 4 3 3 3 4 6% 46%	5467
April	1st to 10th	Night	15% 4 2 4 3 3 4 3 5% 5% 51%	2200
	11th to 20th	Night	14% 4 2 5% 3 3 3 3 5% 6% 52%	1980
	21st to end	Night	13% 5% 2 5% 3 4 3 3 5% 6% 51%	1800
May	1st to 10th	Night	15% 6% 2 8% 4 4 5% 4 7% 8% 37%	4033
	11th to 20th	Night	25% 5% 2 6% 4 4 5% 4 5% 7% 32%	4032
	21st to end	Night	16% 4 2 5% 4 5% 4 4 6% 9% 42%	4421
June	1st to 10th	Night	11% 6% 2 7% 5% 6% 6% 5% 8% 8% 37%	1800
	11th to 20th	Night	9% 6% 2 5% 6% 5% 5% 5% 6% 11% 42%	1800
	21st to end	Night	8% 4 2 5% 5% 4 6% 5% 8% 11% 41%	1800
July	1st to 10th	Night	11% 7% 3 8% 6% 6% 6% 5% 7% 10% 30%	4058
	11th to 20th	Night	10% 6% 2 7% 6% 6% 7% 5% 9% 12% 29%	4057
	21st to end	Night	14% 8% 4 8% 6% 6% 5% 5% 7% 9% 28%	4427
August	1st to 10th	Night	15% 6% 3 6% 5% 5% 5% 4 8% 10% 33%	2915
	11th to 20th	Night	17% 8% 4 8% 4 5% 5% 5% 6% 8% 30%	2875
	21st to end	Night	10% 5% 3 6% 5% 5% 4 4 6% 9% 43%	3635
September	1st to 10th	Night	12% 6% 3 6% 5% 5% 6% 4 7% 9% 37%	2200
	11th to 20th	Night	18% 6% 2 7% 4 5% 5% 4 5% 6% 38%	2200
	21st to end	Night	17% 5% 2 6% 5% 4 4 4 4 6% 44%	2200
October	1st to 10th	Night	15% 4 1 4 3 3 3 3 4 6% 52%	2200
	11th to 20th	Night	14% 4 2 5% 3 3 3 3 4 4 55%	2391
	21st to end	Night	16% 3 1 3 3 2 3 2 4 4 58%	2640
November	1st to 10th	Night	15% 4 2 4 3 2 3 2 3 4 56%	2576
	11th to 20th	Night	16% 6% 2 4 3 2 3 3 4 5% 54%	2730
	21st to end	Night	17% 4 1 4 3 3 4 4 5% 6% 49%	2730
December	1st to 10th	Night	18% 4 2 4 3 3 3 3 4 5% 51%	4380
	11th to 20th	Night	21% 5% 2 5% 3 3 3 3 4 4 46%	4952
	21st to end	Night	24% 5% 1 4 3 2 3 3 4 5% 45%	6633

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% 4 7% 22% 25% 7%	5259
	11th to 20th	Day	27% 3 4 12% 36% 19%	5280
	21st to end	Day	44% 4 5% 15% 19% 13%	5780
February	1st to 10th	Day	40% 5% 8% 22% 20% 7%	2053
	11th to 20th	Day	42% 5% 8% 18% 21% 7%	2058
	21st to end	Day	43% 4 8% 14% 20% 10%	1730
March	1st to 10th	Day	43% 5% 7% 15% 16% 13%	4862
	11th to 20th	Day	30% 5% 6% 12% 26% 21%	5057
	21st to end	Day	40% 5% 6% 23% 19% 6%	5465
April	1st to 10th	Day	34% 7% 11% 21% 21% 6%	2200
	11th to 20th	Day	36% 6% 8% 19% 22% 9%	2384
	21st to end	Day	33% 7% 12% 26% 19% 4	2600
May	1st to 10th	Day	42% 10% 14% 19% 13% 2	5897
	11th to 20th	Day	48% 11% 14% 16% 10% 1	5865
	21st to end	Day	38% 9% 14% 18% 19% 2	6447
June	1st to 10th	Day	39% 12% 17% 22% 7% 2	2600
	11th to 20th	Day	33% 12% 21% 21% 11% 2	2600
	21st to end	Day	34% 13% 17% 20% 13% 3	2600
July	1st to 10th	Day	34% 15% 20% 24% 7% 1	5835
	11th to 20th	Day	35% 17% 29% 15% 3 1	5831
	21st to end	Day	46% 15% 17% 17% 5%	6240
August	1st to 10th	Day	41% 11% 16% 26% 5%	4221
	11th to 20th	Day	34% 12% 24% 17% 12% 1	4213
	21st to end	Day	29% 10% 15% 24% 20% 1	4225
September	1st to 10th	Day	37% 11% 15% 17% 17% 3	2200
	11th to 20th	Day	42% 9% 13% 18% 17% 2	2200
	21st to end	Day	38% 7% 13% 20% 17% 6%	2200
October	1st to 10th	Day	25% 5% 8% 18% 35% 9%	3590
	11th to 20th	Day	20% 3 6% 15% 29% 26%	3247
	21st to end	Day	29% 4 6% 21% 25% 15%	3538
November	1st to 10th	Day	31% 4 7% 19% 27% 12%	1937
	11th to 20th	Day	30% 4 7% 22% 28% 9%	1890
	21st to end	Day	33% 5% 8% 19% 27% 8%	1890
December	1st to 10th	Day	38% 3 5% 17% 25% 13%	3020
	11th to 20th	Day	40% 6% 8% 15% 20% 11%	3460
	21st to end	Day	37% 5% 9% 26% 20% 3	4565
January	1st to 10th	Night	32% 5% 10% 22% 23% 9%	7636
	11th to 20th	Night	31% 4 5% 14% 33% 14%	7680
	21st to end	Night	39% 5% 6% 15% 23% 12%	8401
February	1st to 10th	Night	36% 5% 9% 22% 21% 7%	2562
	11th to 20th	Night	43% 4 6% 18% 22% 7%	2459
	21st to end	Night	41% 5% 8% 16% 21% 9%	2076
March	1st to 10th	Night	40% 6% 8% 15% 23% 8%	5197
	11th to 20th	Night	27% 5% 7% 15% 27% 19%	5058
	21st to end	Night	39% 4 6% 22% 20% 9%	5467
April	1st to 10th	Night	34% 5% 10% 20% 25% 6%	2200
	11th to 20th	Night	33% 5% 8% 22% 23% 9%	1980
	21st to end	Night	34% 6% 12% 26% 20% 3	1800
May	1st to 10th	Night	42% 10% 14% 21% 12% 1	4060
	11th to 20th	Night	49% 8% 16% 17% 10%	4033
	21st to end	Night	35% 10% 14% 24% 14% 2	4429
June	1st to 10th	Night	40% 12% 16% 20% 10% 2	1800
	11th to 20th	Night	34% 10% 20% 22% 12% 1	1800
	21st to end	Night	32% 14% 19% 18% 14% 4	1800
July	1st to 10th	Night	44% 14% 17% 19% 5%	4015
	11th to 20th	Night	41% 17% 24% 15% 2 1	4006
	21st to end	Night	49% 13% 17% 16% 5% 1	4297
August	1st to 10th	Night	43% 11% 22% 20% 5%	2904
	11th to 20th	Night	47% 12% 16% 14% 10% 1	2902
	21st to end	Night	34% 11% 15% 24% 14% 2	3635
September	1st to 10th	Night	41% 10% 14% 19% 13% 3	2200
	11th to 20th	Night	44% 10% 13% 16% 15% 2	2200
	21st to end	Night	40% 6% 12% 19% 18% 6%	2200
October	1st to 10th	Night	26% 7% 14% 13% 32% 8%	3590
	11th to 20th	Night	20% 4 6% 17% 30% 23%	3890
	21st to end	Night	29% 4 6% 18% 27% 16%	4252
November	1st to 10th	Night	32% 5% 8% 18% 26% 11%	2576
	11th to 20th	Night	33% 4 6% 17% 29% 12%	2730
	21st to end	Night	35% 6% 6% 21% 25% 8%	2730
December	1st to 10th	Night	35% 4 6% 18% 27% 9%	4380
	11th to 20th	Night	42% 5% 9% 15% 21% 9%	4954
	21st to end	Night	40% 5% 8% 25% 19% 4	6633

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	25% 18% 39% 14% 2	330
	11th to 20th	Day	28% 24% 28% 16% 3	330
	21st to end	Day	25% 27% 34% 11% 3	363
February	1st to 10th	Day	21% 19% 41% 16% 3	330
	11th to 20th	Day	38% 18% 34% 10%	330
	21st to end	Day	32% 22% 35% 9% 2	273
March	1st to 10th	Day	24% 17% 32% 21% 7%	456
	11th to 20th	Day	23% 20% 33% 18% 5% 1	528
	21st to end	Day	24% 20% 29% 20% 5% 1	550
April	1st to 10th	Day	24% 14% 29% 23% 9% 1	500
	11th to 20th	Day	15% 16% 34% 22% 11% 1	492
	21st to end	Day	19% 10% 26% 26% 14% 4	500
May	1st to 10th	Day	12% 15% 31% 26% 10% 6%	503
	11th to 20th	Day	21% 8% 20% 25% 19% 7%	527
	21st to end	Day	17% 8% 20% 26% 18% 11%	573
June	1st to 10th	Day	15% 5% 19% 30% 20% 12%	530
	11th to 20th	Day	7% 6% 17% 30% 23% 17%	530
	21st to end	Day	10% 3 18% 32% 22% 15%	530
July	1st to 10th	Day	8% 4 11% 26% 31% 20%	525
	11th to 20th	Day	7% 3 11% 25% 31% 23%	524
	21st to end	Day	6% 3 10% 30% 30% 20%	576
August	1st to 10th	Day	19% 4 14% 23% 23% 17%	526
	11th to 20th	Day	10% 4 12% 28% 29% 16%	519
	21st to end	Day	15% 3 11% 25% 31% 15%	583
September	1st to 10th	Day	9% 3 17% 30% 30% 11%	530
	11th to 20th	Day	17% 4 14% 22% 32% 10%	530
	21st to end	Day	18% 7% 16% 29% 23% 6%	530
October	1st to 10th	Day	18% 5% 18% 33% 18% 8%	530
	11th to 20th	Day	14% 7% 24% 34% 18% 3	530
	21st to end	Day	19% 9% 26% 28% 15% 3	583
November	1st to 10th	Day	21% 8% 30% 25% 13% 3	415
	11th to 20th	Day	17% 18% 34% 25% 6%	330
	21st to end	Day	15% 15% 36% 24% 9% 1	330
December	1st to 10th	Day	27% 20% 33% 17% 3	330
	11th to 20th	Day	24% 14% 36% 21% 5% 1	330
	21st to end	Day	31% 15% 31% 16% 5% 2	363
January	1st to 10th	Night	21% 14% 34% 25% 6%	840
	11th to 20th	Night	20% 13% 34% 27% 5%	840
	21st to end	Night	20% 14% 38% 24% 3	924
February	1st to 10th	Night	16% 13% 32% 34% 4	840
	11th to 20th	Night	24% 16% 34% 24% 2	823
	21st to end	Night	21% 15% 36% 25% 3	692
March	1st to 10th	Night	18% 13% 32% 34% 4	840
	11th to 20th	Night	17% 12% 34% 28% 8% 1	829
	21st to end	Night	20% 12% 30% 29% 9%	880
April	1st to 10th	Night	16% 13% 28% 32% 9%	800
	11th to 20th	Night	15% 11% 24% 34% 14% 1	651
	21st to end	Night	13% 7% 21% 37% 20% 2	600
May	1st to 10th	Night	13% 7% 24% 33% 21% 1	600
	11th to 20th	Night	18% 6% 19% 31% 22% 4	591
	21st to end	Night	16% 2 13% 30% 31% 8%	650
June	1st to 10th	Night	12% 3 13% 34% 32% 7%	600
	11th to 20th	Night	9% 2 14% 29% 32% 14%	600
	21st to end	Night	8% 1 10% 32% 41% 8%	600
July	1st to 10th	Night	12% 3 7% 25% 41% 12%	585
	11th to 20th	Night	11% 2 7% 23% 41% 15%	582
	21st to end	Night	8% 2 8% 27% 44% 12%	657
August	1st to 10th	Night	18% 2 7% 24% 37% 11%	597
	11th to 20th	Night	15% 4 9% 27% 35% 10%	594
	21st to end	Night	13% 3 8% 26% 38% 11%	690
September	1st to 10th	Night	13% 3 11% 30% 35% 9%	800
	11th to 20th	Night	19% 3 13% 27% 32% 6%	800
	21st to end	Night	18% 3 15% 35% 27% 3	800
October	1st to 10th	Night	16% 5% 17% 31% 26% 6%	800
	11th to 20th	Night	14% 5% 23% 35% 20% 3	800
	21st to end	Night	16% 6% 21% 37% 17% 3	880
November	1st to 10th	Night	16% 7% 21% 35% 18% 3	820
	11th to 20th	Night	18% 9% 26% 34% 12% 1	840
	21st to end	Night	18% 9% 28% 33% 13% 1	840
December	1st to 10th	Night	24% 10% 27% 31% 7%	840
	11th to 20th	Night	20% 8% 30% 32% 9% 1	840
	21st to end	Night	27% 10% 26% 32% 5%	924

Page generated at 2026-03-15 02:23:51. Version ac07

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