Arch Cape Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arch Cape Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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	23% 12% 3 7% 4 4 4 3 5% 7% 27%	105813
	Night	27% 14% 4 8% 4 4 3 3 4 5% 24%	101797
Transparency	Day	55% 9% 11% 13% 9% 3	105573
	Night	59% 8% 10% 12% 8% 3	101684
Seeing	Day	28% 9% 21% 26% 14% 3	15319
	Night	33% 7% 14% 23% 19% 3	23654

By Month

Percentage Cloud Cover NW (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	38% 17% 6% 11% 5% 4 3 2 3 3 9%	2283
	11th to 20th	Day	41% 12% 4 6% 4 3 3 2 3 3 20%	2516
	21st to end	Day	43% 14% 4 8% 4 3 3 2 3 3 13%	3145
February	1st to 10th	Day	40% 11% 3 6% 3 3 4 3 5% 5% 17%	1710
	11th to 20th	Day	34% 13% 3 9% 5% 4 4 3 3 4 19%	1665
	21st to end	Day	31% 15% 5% 9% 4 4 4 3 3 5% 17%	1431
March	1st to 10th	Day	22% 19% 4 9% 5% 4 4 4 5% 6% 18%	3384
	11th to 20th	Day	26% 12% 4 7% 3 3 3 2 3 5% 31%	3490
	21st to end	Day	28% 18% 4 9% 5% 4 4 3 5% 5% 15%	3837
April	1st to 10th	Day	33% 13% 3 9% 5% 3 4 4 5% 7% 15%	2280
	11th to 20th	Day	29% 13% 3 8% 4 3 4 3 5% 5% 22%	2323
	21st to end	Day	26% 13% 4 8% 6% 5% 4 4 6% 6% 19%	2470
May	1st to 10th	Day	21% 11% 3 7% 4 4 4 3 4 5% 33%	4265
	11th to 20th	Day	14% 12% 4 8% 5% 5% 5% 5% 8% 8% 26%	4377
	21st to end	Day	27% 13% 4 7% 5% 4 4 4 5% 8% 19%	4813
June	1st to 10th	Day	27% 11% 3 7% 5% 4 5% 4 5% 7% 23%	2682
	11th to 20th	Day	22% 13% 4 10% 5% 5% 4 4 6% 7% 20%	2850
	21st to end	Day	16% 9% 3 8% 4 5% 4 4 6% 8% 32%	2850
July	1st to 10th	Day	8% 8% 2 5% 3 3 3 3 5% 7% 51%	4740
	11th to 20th	Day	8% 10% 3 7% 3 3 4 3 8% 10% 41%	4521
	21st to end	Day	13% 11% 3 7% 4 4 4 3 7% 10% 34%	4730
August	1st to 10th	Day	8% 11% 3 8% 5% 4 5% 5% 7% 9% 34%	4092
	11th to 20th	Day	14% 9% 3 7% 4 5% 5% 4 8% 9% 32%	4026
	21st to end	Day	13% 7% 3 7% 4 4 5% 3 7% 9% 37%	4398
September	1st to 10th	Day	12% 6% 2 6% 5% 4 4 3 6% 8% 44%	2280
	11th to 20th	Day	16% 11% 3 6% 5% 5% 4 4 6% 8% 33%	2211
	21st to end	Day	20% 7% 2 6% 4 3 4 3 5% 7% 39%	2043
October	1st to 10th	Day	20% 6% 2 4 3 2 3 3 5% 7% 44%	3180
	11th to 20th	Day	32% 10% 4 8% 4 4 3 3 4 6% 23%	3161
	21st to end	Day	28% 10% 3 7% 4 4 5% 3 6% 7% 24%	3448
November	1st to 10th	Day	35% 11% 3 9% 6% 5% 5% 4 3 4 15%	1529
	11th to 20th	Day	39% 10% 3 7% 4 3 3 3 3 6% 19%	1520
	21st to end	Day	34% 11% 3 9% 4 3 3 3 3 4 23%	1520
December	1st to 10th	Day	32% 11% 3 6% 4 4 4 3 4 6% 24%	1520
	11th to 20th	Day	37% 11% 3 8% 4 5% 4 3 4 4 18%	1925
	21st to end	Day	36% 19% 6% 8% 4 3 2 2 3 4 12%	2598
January	1st to 10th	Night	38% 18% 5% 10% 4 3 3 2 3 3 10%	4655
	11th to 20th	Night	36% 15% 4 8% 3 3 3 2 3 4 18%	4382
	21st to end	Night	47% 14% 3 6% 3 3 3 2 3 3 14%	4521
February	1st to 10th	Night	33% 16% 4 8% 5% 4 4 3 3 5% 16%	2470
	11th to 20th	Night	30% 16% 4 7% 4 4 3 2 4 4 21%	2402
	21st to end	Night	33% 15% 4 8% 5% 3 4 3 3 4 17%	2022
March	1st to 10th	Night	26% 18% 5% 8% 4 4 5% 4 6% 7% 15%	3477
	11th to 20th	Night	23% 14% 4 7% 3 2 3 2 3 5% 33%	3489
	21st to end	Night	32% 21% 5% 9% 4 3 4 2 3 4 13%	3815
April	1st to 10th	Night	33% 16% 5% 8% 4 4 3 2 4 5% 17%	1900
	11th to 20th	Night	26% 14% 4 9% 4 4 5% 3 5% 6% 21%	1804
	21st to end	Night	30% 14% 3 9% 4 4 4 3 6% 5% 17%	1710
May	1st to 10th	Night	28% 11% 3 7% 3 3 3 3 3 6% 31%	2700
	11th to 20th	Night	17% 25% 5% 8% 4 4 3 3 5% 5% 22%	2516
	21st to end	Night	23% 17% 5% 8% 6% 5% 5% 4 5% 4 17%	2762
June	1st to 10th	Night	25% 11% 5% 8% 6% 5% 5% 3 4 6% 22%	1494
	11th to 20th	Night	25% 16% 5% 10% 4 5% 4 4 5% 5% 18%	1330
	21st to end	Night	17% 15% 4 8% 4 3 4 3 5% 5% 33%	1330
July	1st to 10th	Night	9% 16% 3 5% 3 2 2 2 3 6% 48%	2220
	11th to 20th	Night	9% 19% 3 8% 4 4 3 3 5% 8% 33%	2435
	21st to end	Night	14% 19% 4 7% 4 3 4 3 4 5% 34%	2820
August	1st to 10th	Night	8% 23% 4 7% 4 4 5% 4 6% 7% 28%	2848
	11th to 20th	Night	18% 13% 3 7% 4 3 3 3 4 7% 34%	2812
	21st to end	Night	17% 9% 2 7% 4 3 3 3 4 6% 41%	3272
September	1st to 10th	Night	13% 8% 2 7% 3 3 3 3 5% 5% 47%	1900
	11th to 20th	Night	18% 10% 4 9% 4 4 3 2 4 5% 37%	1966
	21st to end	Night	19% 8% 2 6% 5% 3 3 3 5% 6% 39%	2137
October	1st to 10th	Night	17% 7% 3 5% 4 3 3 3 5% 7% 43%	3800
	11th to 20th	Night	28% 13% 3 8% 4 4 3 3 4 6% 24%	3777
	21st to end	Night	27% 10% 4 8% 5% 4 4 3 4 5% 25%	4121
November	1st to 10th	Night	36% 10% 3 8% 5% 4 4 4 5% 5% 17%	2615
	11th to 20th	Night	36% 11% 3 8% 5% 4 3 3 4 4 20%	2660
	21st to end	Night	29% 12% 4 8% 4 4 4 2 4 4 24%	2660
December	1st to 10th	Night	30% 11% 3 7% 4 4 4 4 4 6% 23%	2660
	11th to 20th	Night	35% 11% 4 7% 5% 4 4 3 4 5% 18%	3280
	21st to end	Night	36% 17% 5% 10% 5% 4 3 2 3 3 13%	5035

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	81% 7% 4 4 3 1	2283
	11th to 20th	Day	70% 5% 7% 10% 4 4	2540
	21st to end	Day	77% 6% 4 6% 6% 1	3145
February	1st to 10th	Day	72% 6% 10% 7% 5%	1710
	11th to 20th	Day	74% 5% 6% 9% 4 2	1674
	21st to end	Day	74% 6% 7% 7% 6% 1	1431
March	1st to 10th	Day	65% 12% 13% 8% 3	3384
	11th to 20th	Day	54% 10% 8% 18% 9% 1	3490
	21st to end	Day	72% 11% 10% 6% 2	3837
April	1st to 10th	Day	71% 9% 11% 8% 1	2280
	11th to 20th	Day	69% 7% 7% 9% 7% 1	2323
	21st to end	Day	67% 9% 9% 9% 5%	2470
May	1st to 10th	Day	52% 10% 17% 10% 11%	4265
	11th to 20th	Day	44% 18% 21% 12% 6% 1	4380
	21st to end	Day	63% 15% 15% 7%	4814
June	1st to 10th	Day	62% 9% 13% 10% 6%	2682
	11th to 20th	Day	64% 11% 12% 10% 3	2850
	21st to end	Day	47% 10% 16% 17% 7% 2	2850
July	1st to 10th	Day	29% 7% 9% 14% 16% 25%	4666
	11th to 20th	Day	28% 6% 10% 18% 31% 7%	4442
	21st to end	Day	37% 9% 13% 25% 15% 2	4581
August	1st to 10th	Day	30% 8% 14% 21% 22% 5%	4076
	11th to 20th	Day	44% 12% 16% 18% 8% 2	4065
	21st to end	Day	41% 8% 15% 19% 14% 4	4397
September	1st to 10th	Day	36% 7% 14% 21% 14% 8%	2280
	11th to 20th	Day	47% 10% 13% 14% 12% 4	2211
	21st to end	Day	45% 9% 14% 16% 13% 4	2043
October	1st to 10th	Day	40% 5% 9% 26% 19% 1	3180
	11th to 20th	Day	61% 6% 11% 13% 7% 1	3161
	21st to end	Day	60% 6% 10% 13% 9% 1	3448
November	1st to 10th	Day	73% 7% 7% 10% 4	1529
	11th to 20th	Day	69% 7% 7% 8% 6% 3	1520
	21st to end	Day	68% 5% 5% 8% 12% 3	1520
December	1st to 10th	Day	64% 6% 8% 9% 12% 2	1520
	11th to 20th	Day	70% 7% 10% 8% 5%	1928
	21st to end	Day	75% 7% 8% 6% 3 1	2598
January	1st to 10th	Night	79% 6% 5% 4 3 2	4655
	11th to 20th	Night	71% 5% 7% 10% 4 3	4420
	21st to end	Night	78% 5% 4 5% 6% 2	4521
February	1st to 10th	Night	73% 6% 6% 9% 4 1	2470
	11th to 20th	Night	69% 6% 7% 9% 7% 2	2404
	21st to end	Night	71% 6% 8% 9% 5% 1	2022
March	1st to 10th	Night	63% 11% 12% 11% 3	3477
	11th to 20th	Night	52% 9% 11% 16% 11% 1	3490
	21st to end	Night	75% 8% 8% 7% 1	3815
April	1st to 10th	Night	70% 7% 11% 9% 2	1900
	11th to 20th	Night	63% 9% 11% 10% 6% 2	1804
	21st to end	Night	66% 8% 10% 10% 5% 2	1710
May	1st to 10th	Night	52% 10% 15% 14% 8% 1	2700
	11th to 20th	Night	45% 25% 12% 9% 9% 1	2516
	21st to end	Night	65% 14% 14% 5% 1	2762
June	1st to 10th	Night	59% 8% 14% 14% 4	1494
	11th to 20th	Night	59% 14% 14% 11% 1	1330
	21st to end	Night	47% 10% 17% 18% 5% 3	1330
July	1st to 10th	Night	34% 8% 8% 11% 15% 24%	2185
	11th to 20th	Night	35% 10% 12% 17% 18% 9%	2391
	21st to end	Night	41% 8% 14% 20% 12% 5%	2731
August	1st to 10th	Night	40% 9% 15% 18% 14% 4	2839
	11th to 20th	Night	45% 8% 17% 20% 7% 4	2839
	21st to end	Night	42% 7% 14% 16% 17% 4	3272
September	1st to 10th	Night	33% 8% 13% 22% 16% 8%	1900
	11th to 20th	Night	46% 8% 13% 14% 13% 5%	1966
	21st to end	Night	43% 9% 12% 15% 15% 6%	2137
October	1st to 10th	Night	39% 7% 10% 24% 18% 1	3800
	11th to 20th	Night	58% 7% 11% 13% 9% 2	3777
	21st to end	Night	61% 6% 9% 14% 8% 2	4121
November	1st to 10th	Night	68% 7% 8% 10% 5% 1	2615
	11th to 20th	Night	70% 6% 7% 8% 6% 3	2660
	21st to end	Night	65% 5% 7% 7% 11% 5%	2660
December	1st to 10th	Night	64% 7% 7% 9% 10% 2	2660
	11th to 20th	Night	69% 8% 8% 8% 7% 1	3276
	21st to end	Night	73% 10% 8% 7% 2 1	5035

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	47% 12% 18% 14% 6% 2	180
	11th to 20th	Day	45% 8% 17% 14% 12% 3	180
	21st to end	Day	43% 9% 15% 20% 10% 3	406
February	1st to 10th	Day	47% 9% 16% 16% 11% 1	370
	11th to 20th	Day	48% 10% 18% 15% 8% 1	365
	21st to end	Day	41% 12% 23% 15% 8% 1	316
March	1st to 10th	Day	39% 13% 18% 22% 7%	460
	11th to 20th	Day	36% 11% 19% 21% 10% 3	460
	21st to end	Day	41% 14% 17% 19% 8% 1	506
April	1st to 10th	Day	37% 14% 19% 19% 10% 2	460
	11th to 20th	Day	35% 15% 23% 16% 8% 2	452
	21st to end	Day	30% 11% 28% 21% 9% 1	460
May	1st to 10th	Day	24% 14% 23% 24% 13% 2	463
	11th to 20th	Day	29% 10% 23% 23% 11% 4	488
	21st to end	Day	27% 9% 23% 26% 12% 3	529
June	1st to 10th	Day	28% 8% 22% 26% 15% 1	512
	11th to 20th	Day	27% 9% 29% 26% 8% 1	680
	21st to end	Day	15% 7% 24% 37% 15% 2	680
July	1st to 10th	Day	15% 4 24% 38% 17% 1	671
	11th to 20th	Day	12% 5% 25% 39% 17% 2	542
	21st to end	Day	8% 4 19% 41% 24% 4	531
August	1st to 10th	Day	11% 4 19% 38% 22% 6%	487
	11th to 20th	Day	8% 3 16% 37% 30% 6%	477
	21st to end	Day	12% 6% 24% 36% 19% 4	539
September	1st to 10th	Day	14% 5% 22% 34% 22% 3	490
	11th to 20th	Day	22% 7% 19% 30% 19% 4	490
	21st to end	Day	21% 8% 24% 23% 21% 2	490
October	1st to 10th	Day	30% 7% 19% 20% 22% 2	490
	11th to 20th	Day	31% 9% 18% 20% 18% 5%	490
	21st to end	Day	33% 11% 22% 19% 13% 3	539
November	1st to 10th	Day	41% 8% 26% 13% 11% 2	198
	11th to 20th	Day	44% 8% 13% 20% 13% 3	180
	21st to end	Day	36% 9% 18% 20% 14% 2	180
December	1st to 10th	Day	39% 12% 17% 16% 13% 4	180
	11th to 20th	Day	42% 14% 18% 15% 9% 2	180
	21st to end	Day	42% 12% 17% 20% 8% 2	198
January	1st to 10th	Night	50% 10% 13% 14% 11% 1	828
	11th to 20th	Night	43% 9% 12% 18% 15% 3	760
	21st to end	Night	42% 6% 13% 23% 11% 3	836
February	1st to 10th	Night	41% 8% 16% 19% 13% 2	760
	11th to 20th	Night	38% 10% 17% 22% 12% 1	744
	21st to end	Night	40% 11% 15% 23% 11%	628
March	1st to 10th	Night	41% 8% 16% 20% 13% 2	760
	11th to 20th	Night	39% 11% 14% 22% 12% 2	760
	21st to end	Night	44% 11% 13% 19% 12% 2	836
April	1st to 10th	Night	41% 8% 15% 20% 14% 2	760
	11th to 20th	Night	33% 10% 16% 25% 13% 2	676
	21st to end	Night	36% 8% 15% 25% 14% 2	570
May	1st to 10th	Night	32% 7% 17% 25% 17% 3	475
	11th to 20th	Night	30% 6% 12% 22% 27% 3	374
	21st to end	Night	30% 9% 16% 19% 22% 3	411
June	1st to 10th	Night	27% 5% 14% 26% 27% 2	380
	11th to 20th	Night	27% 8% 15% 30% 19% 1	380
	21st to end	Night	19% 4 16% 27% 30% 3	380
July	1st to 10th	Night	12% 6% 13% 27% 37% 4	370
	11th to 20th	Night	13% 5% 13% 32% 33% 4	368
	21st to end	Night	10% 4 12% 30% 38% 5%	450
August	1st to 10th	Night	15% 5% 12% 23% 36% 9%	567
	11th to 20th	Night	15% 4 9% 21% 41% 10%	564
	21st to end	Night	18% 4 14% 27% 33% 4	701
September	1st to 10th	Night	14% 4 10% 31% 36% 5%	760
	11th to 20th	Night	20% 4 12% 24% 29% 12%	760
	21st to end	Night	21% 5% 14% 31% 26% 3	760
October	1st to 10th	Night	24% 5% 14% 27% 26% 5%	760
	11th to 20th	Night	30% 5% 13% 26% 20% 7%	760
	21st to end	Night	32% 9% 14% 23% 17% 4	836
November	1st to 10th	Night	41% 9% 15% 22% 12% 1	754
	11th to 20th	Night	42% 9% 15% 21% 12% 1	760
	21st to end	Night	35% 8% 18% 25% 12% 3	760
December	1st to 10th	Night	36% 8% 17% 24% 13% 2	760
	11th to 20th	Night	46% 7% 15% 21% 10% 1	760
	21st to end	Night	51% 11% 16% 14% 7% 1	886

Page generated at 2024-11-01 19:47:45. Version ac07

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