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 2025-08-05.

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% 28%	130448
	Night	27% 14% 4 8% 4 3 3 3 4 5% 25%	126331
Transparency	Day	55% 8% 11% 14% 10% 2	130166
	Night	60% 8% 9% 12% 9% 3	126191
Seeing	Day	28% 9% 21% 25% 14% 3	16256
	Night	33% 7% 15% 23% 19% 3	24566

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% 16% 5% 10% 5% 4 3 2 3 3 10%	3285
	11th to 20th	Day	29% 11% 3 6% 3 4 3 3 4 5% 29%	3616
	21st to end	Day	35% 11% 3 6% 3 3 2 2 3 4 29%	4509
February	1st to 10th	Day	38% 12% 3 7% 4 3 4 3 5% 5% 17%	1800
	11th to 20th	Day	34% 14% 3 9% 5% 4 4 2 3 4 19%	1755
	21st to end	Day	32% 15% 5% 9% 4 4 4 3 3 5% 17%	1504
March	1st to 10th	Day	23% 18% 4 9% 5% 4 4 4 5% 6% 18%	4868
	11th to 20th	Day	30% 15% 4 7% 4 3 3 2 3 5% 23%	5000
	21st to end	Day	32% 16% 5% 10% 5% 5% 4 3 4 5% 11%	5498
April	1st to 10th	Day	33% 12% 3 9% 5% 3 4 4 5% 7% 15%	2400
	11th to 20th	Day	28% 12% 3 8% 4 3 4 3 5% 5% 24%	2447
	21st to end	Day	25% 12% 4 8% 6% 5% 4 4 6% 6% 20%	2600
May	1st to 10th	Day	20% 10% 3 6% 3 4 4 4 4 6% 36%	6100
	11th to 20th	Day	18% 13% 5% 10% 5% 5% 5% 5% 7% 7% 19%	6277
	21st to end	Day	22% 12% 3 7% 4 4 4 3 5% 9% 26%	6903
June	1st to 10th	Day	26% 11% 3 7% 5% 4 5% 3 5% 7% 25%	2823
	11th to 20th	Day	21% 13% 4 10% 5% 5% 4 4 6% 7% 20%	3000
	21st to end	Day	16% 10% 3 8% 4 5% 4 4 6% 8% 32%	3000
July	1st to 10th	Day	8% 9% 3 6% 3 3 4 3 6% 8% 47%	6780
	11th to 20th	Day	6% 11% 3 6% 3 3 3 3 9% 10% 42%	6463
	21st to end	Day	10% 11% 3 7% 4 4 4 4 7% 9% 36%	6788
August	1st to 10th	Day	8% 11% 3 8% 5% 4 5% 5% 7% 9% 34%	4145
	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	34% 11% 3 9% 6% 5% 5% 4 3 4 15%	1585
	11th to 20th	Day	39% 10% 3 8% 4 3 3 3 3 6% 19%	1600
	21st to end	Day	33% 11% 3 9% 5% 3 4 3 3 4 23%	1600
December	1st to 10th	Day	26% 8% 3 6% 4 4 4 3 4 6% 35%	2620
	11th to 20th	Day	40% 13% 3 9% 5% 5% 4 3 4 4 12%	3025
	21st to end	Day	39% 18% 6% 8% 4 3 3 2 3 4 10%	3710
January	1st to 10th	Night	40% 16% 5% 9% 4 3 3 2 3 4 11%	6653
	11th to 20th	Night	26% 13% 4 7% 3 3 4 2 4 5% 30%	6282
	21st to end	Night	36% 12% 3 5% 2 2 2 2 2 3 30%	6468
February	1st to 10th	Night	32% 17% 4 8% 5% 4 4 3 4 5% 16%	2600
	11th to 20th	Night	31% 16% 4 7% 4 4 3 2 3 4 21%	2532
	21st to end	Night	33% 15% 4 8% 5% 3 4 3 4 4 17%	2125
March	1st to 10th	Night	25% 18% 5% 10% 5% 4 4 4 5% 6% 14%	5013
	11th to 20th	Night	27% 16% 4 9% 4 3 4 3 3 4 24%	4999
	21st to end	Night	35% 19% 5% 9% 4 3 5% 3 4 4 10%	5476
April	1st to 10th	Night	32% 16% 5% 8% 4 4 3 2 4 5% 17%	2000
	11th to 20th	Night	26% 13% 4 8% 4 4 5% 3 5% 7% 22%	1900
	21st to end	Night	29% 15% 3 9% 4 4 4 3 6% 5% 18%	1800
May	1st to 10th	Night	21% 10% 3 7% 3 3 4 3 4 6% 35%	3870
	11th to 20th	Night	23% 24% 5% 9% 5% 4 4 2 4 4 16%	3616
	21st to end	Night	22% 16% 4 7% 5% 4 4 3 6% 6% 23%	3972
June	1st to 10th	Night	24% 11% 5% 8% 5% 5% 5% 3 4 6% 23%	1572
	11th to 20th	Night	24% 16% 5% 10% 4 4 4 4 5% 5% 19%	1400
	21st to end	Night	17% 15% 3 8% 4 3 4 3 5% 5% 33%	1400
July	1st to 10th	Night	9% 17% 4 5% 3 2 3 2 4 6% 43%	3180
	11th to 20th	Night	7% 19% 3 7% 4 3 3 3 6% 8% 37%	3493
	21st to end	Night	12% 22% 4 6% 4 4 4 3 4 6% 31%	4055
August	1st to 10th	Night	9% 24% 4 7% 4 4 5% 4 6% 7% 28%	2884
	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%	2707
	11th to 20th	Night	37% 11% 3 8% 5% 4 3 3 4 4 19%	2800
	21st to end	Night	29% 11% 4 8% 5% 4 4 2 4 4 24%	2800
December	1st to 10th	Night	24% 9% 3 6% 3 3 4 3 4 8% 33%	4560
	11th to 20th	Night	43% 13% 4 8% 4 3 3 2 3 4 12%	5180
	21st to end	Night	40% 16% 5% 9% 4 4 3 2 3 3 11%	7209

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	80% 6% 3 6% 4 1	3285
	11th to 20th	Day	58% 5% 6% 15% 13% 3	3640
	21st to end	Day	61% 5% 4 11% 16% 4	4509
February	1st to 10th	Day	72% 6% 10% 7% 5%	1800
	11th to 20th	Day	74% 5% 5% 9% 5% 2	1764
	21st to end	Day	73% 6% 7% 7% 6% 1	1504
March	1st to 10th	Day	67% 10% 12% 8% 3	4868
	11th to 20th	Day	65% 8% 6% 13% 7%	5000
	21st to end	Day	76% 10% 8% 5% 1	5498
April	1st to 10th	Day	71% 9% 10% 8% 1	2400
	11th to 20th	Day	66% 7% 8% 11% 7% 1	2447
	21st to end	Day	66% 9% 9% 11% 5%	2600
May	1st to 10th	Day	48% 8% 16% 18% 10%	6100
	11th to 20th	Day	56% 15% 16% 9% 4	6280
	21st to end	Day	55% 12% 19% 11% 1	6904
June	1st to 10th	Day	60% 9% 13% 12% 6%	2823
	11th to 20th	Day	64% 11% 12% 11% 3	3000
	21st to end	Day	48% 10% 16% 17% 7% 2	3000
July	1st to 10th	Day	31% 7% 11% 20% 14% 17%	6706
	11th to 20th	Day	27% 6% 11% 20% 32% 5%	6384
	21st to end	Day	37% 8% 16% 27% 11% 1	6597
August	1st to 10th	Day	30% 8% 14% 21% 22% 5%	4129
	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% 6% 10% 3	1585
	11th to 20th	Day	70% 7% 6% 8% 6% 3	1600
	21st to end	Day	67% 5% 6% 8% 11% 2	1600
December	1st to 10th	Day	54% 5% 6% 9% 22% 4	2620
	11th to 20th	Day	77% 6% 8% 6% 3	3028
	21st to end	Day	79% 6% 7% 5% 2	3710
January	1st to 10th	Night	79% 5% 5% 6% 4 2	6653
	11th to 20th	Night	58% 5% 6% 16% 13% 3	6320
	21st to end	Night	61% 4 4 10% 16% 6%	6468
February	1st to 10th	Night	72% 7% 6% 10% 4 1	2600
	11th to 20th	Night	69% 6% 7% 9% 7% 2	2534
	21st to end	Night	70% 6% 8% 9% 5% 1	2125
March	1st to 10th	Night	66% 10% 10% 10% 4	5013
	11th to 20th	Night	64% 8% 8% 12% 8% 1	5000
	21st to end	Night	77% 7% 8% 7% 1	5476
April	1st to 10th	Night	71% 7% 11% 9% 2	2000
	11th to 20th	Night	61% 8% 11% 12% 6% 2	1900
	21st to end	Night	65% 8% 10% 11% 5% 2	1800
May	1st to 10th	Night	47% 9% 15% 19% 10%	3867
	11th to 20th	Night	59% 19% 9% 6% 6% 1	3616
	21st to end	Night	59% 12% 16% 10% 2	3972
June	1st to 10th	Night	58% 8% 15% 15% 4	1572
	11th to 20th	Night	59% 13% 13% 13% 1	1400
	21st to end	Night	47% 10% 16% 19% 5% 3	1400
July	1st to 10th	Night	37% 7% 8% 17% 14% 17%	3145
	11th to 20th	Night	33% 8% 10% 22% 21% 6%	3449
	21st to end	Night	43% 8% 14% 22% 9% 3	3942
August	1st to 10th	Night	40% 9% 15% 18% 14% 4	2875
	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% 6% 1	2707
	11th to 20th	Night	71% 6% 7% 8% 5% 3	2800
	21st to end	Night	65% 5% 8% 8% 10% 5%	2800
December	1st to 10th	Night	51% 6% 7% 13% 20% 3	4560
	11th to 20th	Night	78% 6% 5% 6% 4	5176
	21st to end	Night	77% 8% 8% 5% 2 1	7209

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	49% 12% 19% 14% 6% 2	200
	11th to 20th	Day	42% 10% 18% 15% 13% 3	200
	21st to end	Day	41% 9% 16% 20% 11% 3	439
February	1st to 10th	Day	47% 10% 17% 15% 10% 2	400
	11th to 20th	Day	49% 10% 18% 14% 7% 1	395
	21st to end	Day	42% 12% 23% 15% 7% 1	341
March	1st to 10th	Day	39% 14% 19% 21% 7%	500
	11th to 20th	Day	38% 11% 19% 20% 9% 3	500
	21st to end	Day	42% 14% 17% 17% 7% 1	550
April	1st to 10th	Day	37% 14% 19% 19% 9% 2	500
	11th to 20th	Day	33% 15% 25% 17% 9% 2	492
	21st to end	Day	28% 11% 28% 22% 9% 1	500
May	1st to 10th	Day	23% 15% 23% 24% 13% 2	503
	11th to 20th	Day	31% 11% 22% 21% 11% 4	528
	21st to end	Day	27% 9% 24% 26% 12% 3	573
June	1st to 10th	Day	27% 9% 23% 26% 14% 1	553
	11th to 20th	Day	26% 10% 30% 25% 8% 1	730
	21st to end	Day	16% 7% 24% 37% 14% 2	730
July	1st to 10th	Day	15% 5% 24% 38% 17% 1	721
	11th to 20th	Day	12% 5% 26% 38% 17% 2	585
	21st to end	Day	8% 4 19% 41% 24% 4	575
August	1st to 10th	Day	11% 4 19% 38% 21% 6%	504
	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	40% 10% 24% 13% 12% 1	212
	11th to 20th	Day	47% 9% 12% 19% 12% 3	200
	21st to end	Day	35% 10% 20% 20% 15% 2	200
December	1st to 10th	Day	37% 11% 17% 17% 14% 5%	200
	11th to 20th	Day	44% 16% 18% 14% 8% 2	200
	21st to end	Day	45% 12% 16% 18% 7% 1	220
January	1st to 10th	Night	51% 10% 14% 14% 10% 1	875
	11th to 20th	Night	41% 10% 13% 19% 15% 3	800
	21st to end	Night	41% 6% 14% 23% 12% 3	880
February	1st to 10th	Night	40% 9% 17% 19% 12% 2	800
	11th to 20th	Night	39% 11% 17% 21% 11% 1	784
	21st to end	Night	40% 11% 16% 22% 10%	660
March	1st to 10th	Night	41% 8% 16% 21% 13% 2	800
	11th to 20th	Night	40% 10% 15% 22% 12% 2	800
	21st to end	Night	45% 10% 13% 18% 12% 2	880
April	1st to 10th	Night	40% 8% 15% 21% 14% 2	800
	11th to 20th	Night	32% 10% 17% 26% 13% 2	712
	21st to end	Night	37% 7% 15% 25% 15% 2	600
May	1st to 10th	Night	31% 7% 16% 27% 16% 2	500
	11th to 20th	Night	31% 6% 12% 21% 26% 3	394
	21st to end	Night	30% 8% 15% 21% 23% 3	433
June	1st to 10th	Night	27% 5% 14% 26% 27% 2	400
	11th to 20th	Night	26% 8% 15% 31% 19% 1	400
	21st to end	Night	19% 5% 17% 27% 30% 3	400
July	1st to 10th	Night	12% 6% 14% 27% 36% 4	390
	11th to 20th	Night	13% 4 14% 33% 32% 4	388
	21st to end	Night	9% 4 12% 30% 39% 5%	474
August	1st to 10th	Night	16% 5% 12% 23% 36% 9%	579
	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% 8% 15% 23% 12% 1	780
	11th to 20th	Night	44% 9% 15% 20% 11% 1	800
	21st to end	Night	35% 8% 18% 25% 12% 3	800
December	1st to 10th	Night	35% 8% 17% 24% 14% 2	800
	11th to 20th	Night	47% 7% 16% 20% 9% 1	800
	21st to end	Night	52% 11% 16% 13% 7% 1	936

Page generated at 2025-08-01 22:32:24. Version ac07

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