New Astronomy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the New Astronomy Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	22% 9% 3 7% 4 4 4 3 5% 6% 32%	131685
	Night	26% 10% 3 7% 4 4 4 3 4 5% 29%	127717
Transparency	Day	54% 6% 14% 18% 8% 1	131401
	Night	58% 5% 13% 16% 7% 1	127578
Seeing	Day	28% 9% 20% 26% 14% 3	16369
	Night	34% 6% 14% 26% 18% 2	24736

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	42% 14% 3 8% 5% 4 4 3 4 4 9%	3378
	11th to 20th	Day	22% 11% 3 8% 4 6% 4 4 5% 6% 27%	3616
	21st to end	Day	34% 9% 3 6% 3 3 3 3 3 4 29%	4073
February	1st to 10th	Day	32% 12% 4 8% 5% 4 4 3 5% 5% 17%	1800
	11th to 20th	Day	32% 15% 3 9% 5% 3 4 2 4 4 20%	1755
	21st to end	Day	31% 15% 3 7% 5% 4 3 3 5% 4 20%	1565
March	1st to 10th	Day	24% 14% 4 9% 6% 6% 5% 4 5% 6% 19%	4593
	11th to 20th	Day	23% 15% 4 8% 5% 4 5% 3 3 4 26%	5000
	21st to end	Day	29% 13% 3 9% 6% 6% 5% 4 5% 7% 12%	5699
April	1st to 10th	Day	28% 13% 4 9% 6% 5% 4 4 5% 6% 16%	2400
	11th to 20th	Day	24% 10% 3 9% 6% 4 5% 3 4 7% 24%	2382
	21st to end	Day	25% 11% 3 9% 6% 5% 5% 5% 6% 5% 21%	2665
May	1st to 10th	Day	17% 8% 3 7% 4 4 4 4 5% 6% 38%	6320
	11th to 20th	Day	25% 9% 3 7% 4 4 4 4 6% 8% 24%	6277
	21st to end	Day	25% 9% 3 7% 5% 3 4 3 6% 8% 28%	6903
June	1st to 10th	Day	25% 8% 3 7% 6% 4 5% 4 5% 6% 27%	2820
	11th to 20th	Day	27% 10% 3 8% 6% 5% 5% 5% 6% 6% 20%	3000
	21st to end	Day	15% 8% 2 8% 6% 4 4 5% 6% 7% 36%	3000
July	1st to 10th	Day	10% 4 2 4 2 3 3 3 4 6% 60%	6780
	11th to 20th	Day	5% 3 1 3 2 3 3 2 4 5% 68%	6417
	21st to end	Day	10% 4 1 4 3 3 3 3 5% 8% 54%	6874
August	1st to 10th	Day	7% 3 1 4 2 3 4 4 5% 8% 59%	4497
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 5% 7% 9% 38%	4156
	21st to end	Day	18% 8% 3 7% 3 3 3 2 5% 7% 41%	4450
September	1st to 10th	Day	11% 7% 3 7% 4 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	17% 7% 4 6% 5% 4 4 3 5% 7% 38%	2400
	21st to end	Day	20% 8% 2 6% 3 4 3 3 5% 5% 39%	2121
October	1st to 10th	Day	18% 7% 2 6% 3 4 5% 3 6% 10% 36%	3280
	11th to 20th	Day	34% 8% 3 7% 4 4 4 3 4 6% 25%	3261
	21st to end	Day	32% 11% 4 9% 5% 5% 4 3 5% 5% 18%	3330
November	1st to 10th	Day	40% 10% 2 6% 4 4 4 3 4 5% 17%	1706
	11th to 20th	Day	38% 11% 3 6% 4 4 3 3 4 4 20%	1680
	21st to end	Day	36% 12% 3 7% 4 4 3 3 4 4 21%	1680
December	1st to 10th	Day	26% 8% 2 6% 4 5% 5% 3 6% 7% 27%	2636
	11th to 20th	Day	35% 15% 4 8% 5% 4 4 3 4 4 13%	3025
	21st to end	Day	35% 14% 4 10% 5% 5% 4 3 4 4 12%	3746
January	1st to 10th	Night	39% 15% 4 8% 5% 5% 4 3 4 4 10%	6559
	11th to 20th	Night	22% 10% 3 6% 4 4 4 3 6% 8% 30%	6282
	21st to end	Night	31% 9% 3 6% 4 3 4 2 3 4 30%	6884
February	1st to 10th	Night	33% 14% 4 7% 5% 4 4 3 4 5% 18%	2600
	11th to 20th	Night	31% 14% 3 6% 4 4 4 2 4 5% 22%	2532
	21st to end	Night	33% 14% 3 7% 3 3 3 3 4 6% 20%	2065
March	1st to 10th	Night	21% 15% 4 9% 5% 5% 5% 4 5% 7% 19%	5298
	11th to 20th	Night	27% 14% 3 7% 4 3 3 2 4 5% 28%	4999
	21st to end	Night	36% 14% 4 8% 5% 4 5% 3 5% 4 14%	5275
April	1st to 10th	Night	28% 11% 3 9% 5% 5% 5% 5% 6% 5% 17%	2000
	11th to 20th	Night	24% 10% 3 7% 5% 4 5% 4 5% 5% 28%	1968
	21st to end	Night	28% 10% 3 7% 6% 5% 4 3 5% 7% 22%	1735
May	1st to 10th	Night	18% 8% 3 8% 4 4 3 2 4 4 41%	3640
	11th to 20th	Night	27% 11% 4 7% 4 3 3 3 4 5% 28%	3616
	21st to end	Night	27% 9% 4 9% 5% 4 4 4 5% 6% 23%	3972
June	1st to 10th	Night	23% 8% 3 7% 6% 5% 5% 3 5% 6% 31%	1575
	11th to 20th	Night	28% 11% 3 8% 5% 4 5% 3 4 6% 25%	1400
	21st to end	Night	15% 7% 3 6% 5% 5% 4 4 5% 6% 41%	1400
July	1st to 10th	Night	14% 5% 2 4 2 2 3 2 3 4 58%	3180
	11th to 20th	Night	4 3 1 3 2 2 2 2 4 4 73%	3538
	21st to end	Night	10% 4 2 4 3 3 3 3 5% 6% 58%	3969
August	1st to 10th	Night	6% 3 1 3 2 2 3 3 5% 6% 67%	2639
	11th to 20th	Night	16% 7% 3 6% 3 3 3 2 4 6% 47%	2902
	21st to end	Night	21% 7% 2 5% 3 3 2 2 4 5% 47%	3466
September	1st to 10th	Night	12% 5% 2 6% 3 4 3 3 5% 6% 52%	2000
	11th to 20th	Night	18% 7% 4 7% 4 3 3 4 4 4 42%	2000
	21st to end	Night	20% 8% 3 7% 4 4 3 3 5% 5% 40%	2275
October	1st to 10th	Night	18% 7% 2 6% 4 4 5% 4 6% 9% 36%	3920
	11th to 20th	Night	31% 8% 3 5% 3 3 4 3 4 5% 29%	3897
	21st to end	Night	33% 14% 3 8% 4 3 3 3 4 5% 21%	4473
November	1st to 10th	Night	37% 11% 2 6% 4 4 3 3 5% 5% 19%	2810
	11th to 20th	Night	39% 11% 3 7% 4 4 3 2 3 4 21%	2940
	21st to end	Night	36% 10% 3 7% 5% 4 4 3 4 4 21%	2940
December	1st to 10th	Night	26% 10% 3 6% 4 4 3 3 5% 6% 30%	4588
	11th to 20th	Night	36% 15% 4 9% 4 4 4 3 4 4 13%	5180
	21st to end	Night	34% 17% 4 10% 6% 5% 4 3 4 4 11%	7200

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	78% 3 6% 9% 3	3378
	11th to 20th	Day	57% 5% 12% 18% 8% 1	3640
	21st to end	Day	61% 4 7% 16% 12%	4073
February	1st to 10th	Day	71% 7% 9% 8% 3 1	1800
	11th to 20th	Day	70% 5% 9% 8% 7% 1	1764
	21st to end	Day	70% 5% 10% 11% 4	1565
March	1st to 10th	Day	64% 6% 12% 12% 5%	4593
	11th to 20th	Day	64% 5% 11% 18% 1	5000
	21st to end	Day	72% 7% 12% 8% 1	5699
April	1st to 10th	Day	70% 7% 14% 8%	2400
	11th to 20th	Day	63% 6% 11% 13% 5% 1	2382
	21st to end	Day	66% 7% 12% 10% 4 1	2665
May	1st to 10th	Day	49% 5% 16% 23% 8%	6320
	11th to 20th	Day	59% 7% 19% 13% 2	6280
	21st to end	Day	56% 8% 21% 14% 1	6904
June	1st to 10th	Day	59% 7% 18% 12% 5%	2820
	11th to 20th	Day	65% 7% 15% 11% 3	3000
	21st to end	Day	47% 10% 18% 18% 7% 1	3000
July	1st to 10th	Day	27% 5% 17% 22% 19% 10%	6706
	11th to 20th	Day	20% 5% 15% 30% 29% 2	6339
	21st to end	Day	30% 5% 22% 31% 10% 1	6681
August	1st to 10th	Day	25% 5% 13% 39% 16% 1	4480
	11th to 20th	Day	44% 9% 18% 20% 7% 1	4195
	21st to end	Day	45% 4 12% 27% 11% 1	4449
September	1st to 10th	Day	38% 6% 16% 24% 12% 4	2400
	11th to 20th	Day	48% 5% 12% 18% 14% 2	2400
	21st to end	Day	48% 6% 17% 18% 8% 2	2121
October	1st to 10th	Day	44% 6% 15% 29% 6%	3280
	11th to 20th	Day	62% 5% 9% 15% 8%	3261
	21st to end	Day	68% 5% 11% 9% 7% 1	3330
November	1st to 10th	Day	70% 5% 12% 9% 3	1706
	11th to 20th	Day	70% 4 9% 8% 6% 2	1680
	21st to end	Day	69% 5% 6% 11% 7% 2	1680
December	1st to 10th	Day	56% 5% 8% 18% 14% 1	2636
	11th to 20th	Day	77% 4 8% 9% 1 1	3028
	21st to end	Day	76% 6% 9% 7% 2	3746
January	1st to 10th	Night	77% 4 6% 9% 3 1	6559
	11th to 20th	Night	51% 5% 14% 21% 7% 1	6320
	21st to end	Night	59% 4 10% 17% 10% 1	6884
February	1st to 10th	Night	70% 6% 10% 9% 5% 1	2600
	11th to 20th	Night	66% 7% 10% 9% 7% 2	2534
	21st to end	Night	69% 5% 10% 11% 4	2065
March	1st to 10th	Night	62% 7% 14% 12% 6%	5298
	11th to 20th	Night	62% 4 15% 16% 2	5000
	21st to end	Night	73% 7% 12% 7%	5275
April	1st to 10th	Night	68% 9% 12% 10% 1	2000
	11th to 20th	Night	59% 6% 15% 11% 8% 1	1968
	21st to end	Night	65% 7% 11% 12% 5% 1	1735
May	1st to 10th	Night	49% 4 15% 24% 8%	3637
	11th to 20th	Night	60% 6% 16% 15% 2	3616
	21st to end	Night	63% 8% 18% 10%	3972
June	1st to 10th	Night	56% 6% 17% 15% 5%	1575
	11th to 20th	Night	63% 7% 15% 13% 2	1400
	21st to end	Night	45% 8% 20% 19% 6% 2	1400
July	1st to 10th	Night	32% 4 15% 19% 16% 14%	3145
	11th to 20th	Night	17% 3 17% 29% 33% 1	3493
	21st to end	Night	27% 6% 22% 31% 13% 2	3857
August	1st to 10th	Night	20% 4 14% 40% 20% 2	2631
	11th to 20th	Night	41% 7% 20% 22% 9% 2	2929
	21st to end	Night	43% 4 13% 30% 9% 1	3466
September	1st to 10th	Night	34% 6% 16% 26% 13% 4	2000
	11th to 20th	Night	47% 6% 11% 18% 14% 3	2000
	21st to end	Night	48% 7% 15% 18% 10% 2	2275
October	1st to 10th	Night	45% 6% 14% 29% 5%	3920
	11th to 20th	Night	58% 5% 10% 17% 9%	3897
	21st to end	Night	68% 4 9% 11% 7%	4473
November	1st to 10th	Night	68% 6% 13% 9% 3 1	2810
	11th to 20th	Night	70% 4 9% 10% 4 2	2940
	21st to end	Night	69% 5% 7% 10% 8% 1	2940
December	1st to 10th	Night	56% 5% 8% 18% 12% 1	4588
	11th to 20th	Night	76% 4 7% 10% 2	5176
	21st to end	Night	78% 5% 9% 6% 2	7200

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	44% 9% 15% 16% 11% 6%	200
	11th to 20th	Day	37% 13% 13% 20% 17% 2	200
	21st to end	Day	46% 8% 13% 16% 12% 4	250
February	1st to 10th	Day	42% 11% 20% 18% 8% 2	400
	11th to 20th	Day	44% 12% 21% 16% 7%	395
	21st to end	Day	40% 13% 18% 22% 6% 1	371
March	1st to 10th	Day	38% 13% 20% 19% 9% 1	500
	11th to 20th	Day	33% 13% 18% 23% 10% 3	500
	21st to end	Day	40% 12% 25% 15% 7% 1	550
April	1st to 10th	Day	36% 11% 24% 17% 9% 2	500
	11th to 20th	Day	29% 15% 27% 17% 9% 2	492
	21st to end	Day	31% 12% 24% 21% 10% 2	500
May	1st to 10th	Day	22% 11% 28% 26% 11% 2	503
	11th to 20th	Day	33% 9% 20% 21% 14% 4	528
	21st to end	Day	31% 9% 21% 24% 12% 3	573
June	1st to 10th	Day	27% 9% 21% 28% 13% 3	550
	11th to 20th	Day	28% 11% 24% 25% 10% 2	730
	21st to end	Day	17% 6% 21% 36% 17% 3	730
July	1st to 10th	Day	14% 5% 22% 38% 19% 2	721
	11th to 20th	Day	11% 6% 20% 38% 21% 4	566
	21st to end	Day	11% 4 17% 39% 23% 6%	575
August	1st to 10th	Day	14% 5% 16% 36% 21% 8%	527
	11th to 20th	Day	11% 4 18% 36% 25% 7%	517
	21st to end	Day	17% 6% 21% 34% 15% 7%	583
September	1st to 10th	Day	16% 9% 21% 33% 17% 5%	530
	11th to 20th	Day	20% 9% 18% 33% 18% 3	530
	21st to end	Day	23% 7% 21% 28% 17% 3	530
October	1st to 10th	Day	25% 7% 23% 23% 18% 4	530
	11th to 20th	Day	36% 7% 18% 21% 15% 4	530
	21st to end	Day	40% 11% 17% 19% 9% 4	443
November	1st to 10th	Day	42% 12% 14% 19% 11% 1	251
	11th to 20th	Day	50% 10% 11% 11% 14% 4	220
	21st to end	Day	41% 8% 16% 16% 10% 8%	220
December	1st to 10th	Day	44% 7% 12% 14% 13% 10%	204
	11th to 20th	Day	46% 12% 17% 17% 9%	200
	21st to end	Day	46% 6% 17% 20% 10% 1	220
January	1st to 10th	Night	50% 8% 14% 16% 11% 1	854
	11th to 20th	Night	40% 9% 13% 24% 12% 2	800
	21st to end	Night	41% 7% 13% 27% 13%	880
February	1st to 10th	Night	42% 7% 17% 25% 8% 1	800
	11th to 20th	Night	40% 8% 13% 29% 10% 1	784
	21st to end	Night	42% 8% 17% 24% 8% 1	660
March	1st to 10th	Night	38% 7% 17% 24% 13% 1	800
	11th to 20th	Night	37% 8% 18% 24% 11% 1	800
	21st to end	Night	41% 9% 18% 23% 8% 1	880
April	1st to 10th	Night	37% 9% 17% 25% 12% 1	800
	11th to 20th	Night	32% 7% 18% 29% 13% 1	780
	21st to end	Night	34% 8% 17% 27% 14% 1	535
May	1st to 10th	Night	24% 10% 12% 36% 18% 1	400
	11th to 20th	Night	27% 7% 12% 28% 23% 3	394
	21st to end	Night	27% 5% 16% 25% 23% 3	433
June	1st to 10th	Night	27% 5% 15% 31% 21% 2	400
	11th to 20th	Night	34% 6% 14% 27% 19% 1	400
	21st to end	Night	19% 3 14% 33% 28% 3	400
July	1st to 10th	Night	18% 3 11% 29% 35% 4	390
	11th to 20th	Night	12% 3 9% 36% 37% 3	388
	21st to end	Night	12% 2 7% 29% 45% 6%	438
August	1st to 10th	Night	11% 2 7% 24% 47% 7%	415
	11th to 20th	Night	14% 2 6% 26% 42% 10%	594
	21st to end	Night	18% 5% 7% 29% 36% 4	790
September	1st to 10th	Night	15% 3 9% 35% 32% 7%	800
	11th to 20th	Night	22% 5% 8% 27% 32% 6%	800
	21st to end	Night	25% 5% 12% 28% 26% 3	800
October	1st to 10th	Night	26% 5% 12% 29% 25% 3	800
	11th to 20th	Night	34% 6% 13% 26% 18% 3	800
	21st to end	Night	40% 6% 16% 24% 11% 2	880
November	1st to 10th	Night	44% 7% 19% 25% 6%	820
	11th to 20th	Night	47% 7% 17% 19% 9% 1	840
	21st to end	Night	41% 7% 17% 22% 11% 1	840
December	1st to 10th	Night	38% 6% 15% 25% 13% 2	808
	11th to 20th	Night	47% 9% 16% 22% 5% 1	800
	21st to end	Night	52% 8% 17% 19% 5%	933

Page generated at 2025-11-29 22:00:52. Version ac07

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