Lookum Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lookum 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	14% 12% 5% 10% 5% 4 4 3 5% 6% 32%	126722
	Night	14% 13% 6% 13% 6% 4 4 3 5% 6% 28%	124210
Transparency	Day	52% 6% 12% 20% 9% 1	130176
	Night	57% 6% 12% 17% 7% 1	128843
Seeing	Day	21% 17% 32% 21% 8% 2	17266
	Night	21% 14% 32% 25% 9% 1	26997

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	8% 16% 11% 28% 9% 4 3 2 4 4 10%	3617
	11th to 20th	Day	9% 15% 8% 22% 12% 10% 4 3 4 4 9%	3589
	21st to end	Day	8% 16% 10% 17% 9% 5% 5% 3 5% 4 17%	3982
February	1st to 10th	Day	10% 14% 5% 16% 12% 8% 5% 4 6% 4 17%	1620
	11th to 20th	Day	10% 16% 7% 17% 9% 8% 5% 3 4 4 17%	1939
	21st to end	Day	11% 17% 7% 16% 8% 6% 5% 3 4 6% 18%	1650
March	1st to 10th	Day	12% 11% 5% 11% 8% 4 4 3 5% 7% 29%	4478
	11th to 20th	Day	11% 15% 5% 18% 7% 4 4 3 5% 6% 21%	4721
	21st to end	Day	19% 17% 9% 17% 6% 4 4 2 3 3 16%	5867
April	1st to 10th	Day	16% 17% 4 9% 5% 4 4 3 5% 5% 27%	2400
	11th to 20th	Day	19% 14% 6% 8% 3 3 3 3 5% 5% 31%	2447
	21st to end	Day	15% 13% 4 8% 4 4 4 4 5% 7% 33%	2600
May	1st to 10th	Day	13% 9% 3 6% 4 3 3 3 4 6% 47%	5998
	11th to 20th	Day	14% 9% 3 6% 4 4 4 3 5% 7% 43%	6319
	21st to end	Day	11% 9% 3 8% 4 4 4 3 6% 7% 40%	6949
June	1st to 10th	Day	14% 6% 3 6% 4 4 5% 5% 6% 8% 39%	2910
	11th to 20th	Day	14% 8% 3 7% 4 4 5% 4 6% 8% 38%	3000
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 7% 7% 38%	2910
July	1st to 10th	Day	13% 7% 2 5% 4 4 3 3 5% 7% 46%	6359
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 5% 8% 50%	6360
	21st to end	Day	12% 5% 2 4 3 3 4 4 6% 8% 50%	6926
August	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 4 6% 7% 43%	4563
	11th to 20th	Day	17% 6% 2 5% 3 3 3 3 5% 6% 48%	4186
	21st to end	Day	18% 6% 2 5% 3 3 3 3 5% 7% 45%	4415
September	1st to 10th	Day	19% 6% 1 6% 4 4 4 3 6% 7% 39%	2400
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 4 6% 9% 39%	2235
	21st to end	Day	19% 8% 3 6% 5% 4 4 3 6% 7% 34%	2200
October	1st to 10th	Day	19% 9% 3 8% 4 5% 3 4 5% 6% 32%	2180
	11th to 20th	Day	24% 16% 3 8% 5% 4 3 5% 5% 6% 20%	1850
	21st to end	Day	22% 24% 3 8% 5% 4 5% 3 5% 4 17%	1980
November	1st to 10th	Day	18% 19% 5% 7% 4 3 4 4 5% 6% 25%	1852
	11th to 20th	Day	16% 22% 9% 10% 5% 4 5% 3 5% 5% 16%	1890
	21st to end	Day	16% 24% 10% 13% 4 4 3 3 4 5% 14%	1594
December	1st to 10th	Day	12% 26% 10% 17% 7% 5% 4 3 3 4 10%	2294
	11th to 20th	Day	10% 24% 10% 18% 6% 5% 4 3 3 4 12%	2621
	21st to end	Day	19% 25% 12% 16% 6% 3 3 2 3 3 8%	3821
January	1st to 10th	Night	10% 16% 9% 31% 7% 4 2 2 2 3 13%	6348
	11th to 20th	Night	8% 17% 7% 23% 12% 7% 5% 3 4 4 11%	6316
	21st to end	Night	9% 14% 9% 19% 8% 6% 5% 4 6% 6% 15%	6996
February	1st to 10th	Night	8% 14% 6% 15% 11% 8% 5% 4 5% 5% 19%	2777
	11th to 20th	Night	10% 14% 5% 16% 9% 6% 4 4 5% 6% 21%	2346
	21st to end	Night	12% 16% 7% 14% 7% 6% 4 4 4 6% 19%	1980
March	1st to 10th	Night	13% 10% 5% 13% 8% 4 4 3 5% 7% 29%	5362
	11th to 20th	Night	11% 14% 6% 11% 6% 4 5% 4 6% 8% 26%	5237
	21st to end	Night	20% 12% 6% 14% 5% 4 5% 3 4 5% 21%	5108
April	1st to 10th	Night	17% 15% 4 9% 5% 5% 5% 4 5% 5% 26%	2000
	11th to 20th	Night	22% 10% 4 6% 3 4 4 3 4 5% 34%	1902
	21st to end	Night	15% 13% 5% 7% 6% 4 4 3 5% 7% 34%	1800
May	1st to 10th	Night	16% 8% 3 5% 2 3 4 2 4 6% 46%	3933
	11th to 20th	Night	14% 8% 2 6% 4 4 5% 4 5% 7% 41%	3596
	21st to end	Night	15% 9% 3 6% 5% 5% 4 4 5% 7% 37%	3949
June	1st to 10th	Night	13% 7% 2 7% 5% 4 5% 4 6% 6% 41%	1485
	11th to 20th	Night	15% 6% 2 5% 5% 5% 5% 5% 6% 8% 37%	1400
	21st to end	Night	17% 7% 2 5% 5% 3 4 3 6% 8% 41%	1482
July	1st to 10th	Night	13% 6% 3 7% 4 3 4 3 5% 8% 44%	3620
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 4 7% 50%	3620
	21st to end	Night	8% 6% 2 6% 3 4 3 3 6% 8% 51%	3948
August	1st to 10th	Night	11% 7% 2 6% 5% 4 4 3 6% 8% 44%	2597
	11th to 20th	Night	17% 5% 2 5% 3 3 4 2 5% 7% 48%	2862
	21st to end	Night	13% 6% 2 6% 3 4 3 3 5% 6% 49%	3480
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 44%	2000
	11th to 20th	Night	16% 7% 2 7% 4 4 4 3 5% 7% 42%	2145
	21st to end	Night	17% 8% 2 7% 4 4 4 4 6% 7% 37%	2200
October	1st to 10th	Night	19% 9% 2 6% 4 4 5% 3 6% 8% 34%	2218
	11th to 20th	Night	20% 14% 4 7% 4 5% 4 3 4 6% 29%	2550
	21st to end	Night	23% 18% 4 9% 5% 4 4 3 4 5% 22%	2860
November	1st to 10th	Night	16% 21% 4 7% 4 4 4 3 5% 6% 26%	2666
	11th to 20th	Night	16% 22% 6% 11% 6% 5% 3 2 4 4 20%	2730
	21st to end	Night	16% 20% 11% 12% 5% 4 4 3 3 5% 16%	3018
December	1st to 10th	Night	13% 22% 11% 17% 5% 4 4 3 4 4 11%	4941
	11th to 20th	Night	11% 16% 8% 23% 6% 5% 4 3 4 5% 15%	5586
	21st to end	Night	20% 21% 12% 17% 6% 3 2 2 2 3 11%	7152

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	77% 7% 9% 6% 1	3618
	11th to 20th	Day	78% 6% 9% 7%	3620
	21st to end	Day	70% 5% 10% 8% 6%	3982
February	1st to 10th	Day	65% 9% 12% 10% 4	1620
	11th to 20th	Day	69% 7% 11% 8% 4	1949
	21st to end	Day	66% 8% 9% 12% 6% 1	1650
March	1st to 10th	Day	53% 6% 14% 14% 11% 1	4480
	11th to 20th	Day	63% 5% 14% 13% 5% 1	4721
	21st to end	Day	71% 6% 8% 11% 4	5867
April	1st to 10th	Day	58% 7% 10% 18% 7%	2400
	11th to 20th	Day	55% 5% 9% 20% 10% 1	2447
	21st to end	Day	50% 8% 10% 18% 12% 2	2600
May	1st to 10th	Day	39% 5% 10% 27% 18% 2	6007
	11th to 20th	Day	40% 6% 11% 26% 14% 3	6323
	21st to end	Day	41% 7% 11% 22% 16% 2	6949
June	1st to 10th	Day	40% 9% 15% 22% 13% 1	2910
	11th to 20th	Day	43% 8% 13% 22% 13% 1	3000
	21st to end	Day	42% 8% 17% 21% 10% 1	2910
July	1st to 10th	Day	38% 4 15% 31% 11%	6291
	11th to 20th	Day	34% 5% 17% 31% 11% 1	6285
	21st to end	Day	33% 4 14% 33% 15%	6728
August	1st to 10th	Day	40% 5% 17% 28% 10% 1	4548
	11th to 20th	Day	38% 5% 14% 32% 11%	4225
	21st to end	Day	41% 5% 15% 28% 10% 1	4414
September	1st to 10th	Day	43% 7% 14% 25% 10%	2400
	11th to 20th	Day	40% 8% 20% 26% 6%	2235
	21st to end	Day	47% 7% 17% 21% 8%	2200
October	1st to 10th	Day	42% 5% 13% 28% 11%	3557
	11th to 20th	Day	58% 5% 12% 16% 10%	2988
	21st to end	Day	66% 5% 14% 13% 2	3180
November	1st to 10th	Day	60% 6% 12% 16% 5%	1852
	11th to 20th	Day	69% 8% 12% 8% 3	1890
	21st to end	Day	73% 8% 12% 6% 2	1594
December	1st to 10th	Day	80% 5% 9% 5% 1	2294
	11th to 20th	Day	76% 5% 7% 9% 2	2621
	21st to end	Day	84% 4 4 5% 3	3821
January	1st to 10th	Night	78% 4 7% 10% 2	6348
	11th to 20th	Night	76% 7% 11% 5% 1	6360
	21st to end	Night	67% 7% 13% 10% 2	6996
February	1st to 10th	Night	65% 6% 11% 13% 5%	2777
	11th to 20th	Night	61% 6% 13% 15% 4	2347
	21st to end	Night	64% 6% 11% 14% 5%	1980
March	1st to 10th	Night	54% 5% 18% 17% 6% 1	5373
	11th to 20th	Night	55% 5% 16% 16% 6%	5238
	21st to end	Night	63% 7% 9% 15% 6%	5108
April	1st to 10th	Night	58% 6% 11% 15% 9% 1	2000
	11th to 20th	Night	52% 5% 9% 21% 11% 2	1902
	21st to end	Night	50% 6% 12% 18% 12% 3	1800
May	1st to 10th	Night	39% 5% 11% 27% 16% 3	3940
	11th to 20th	Night	41% 10% 10% 25% 13% 2	3597
	21st to end	Night	45% 5% 10% 22% 12% 5%	3950
June	1st to 10th	Night	40% 7% 12% 26% 13% 2	1485
	11th to 20th	Night	41% 10% 12% 21% 14% 1	1400
	21st to end	Night	42% 6% 14% 22% 14% 2	1482
July	1st to 10th	Night	40% 4 15% 26% 14% 1	3580
	11th to 20th	Night	35% 4 15% 34% 10% 1	3572
	21st to end	Night	32% 4 13% 29% 20% 2	3832
August	1st to 10th	Night	38% 6% 14% 28% 13% 1	2588
	11th to 20th	Night	37% 5% 15% 33% 10% 1	2889
	21st to end	Night	37% 5% 13% 29% 15% 2	3480
September	1st to 10th	Night	40% 6% 14% 27% 12% 1	2000
	11th to 20th	Night	42% 7% 18% 25% 9%	2145
	21st to end	Night	45% 8% 16% 22% 8% 1	2200
October	1st to 10th	Night	42% 5% 13% 30% 9% 1	3621
	11th to 20th	Night	50% 5% 13% 19% 13%	4151
	21st to end	Night	60% 5% 13% 19% 3	4610
November	1st to 10th	Night	58% 8% 12% 15% 6%	2666
	11th to 20th	Night	67% 6% 11% 10% 6%	2730
	21st to end	Night	71% 7% 12% 8% 1	3018
December	1st to 10th	Night	76% 7% 10% 7% 1	4941
	11th to 20th	Night	73% 5% 10% 8% 5%	5585
	21st to end	Night	80% 4 6% 6% 3	7152

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	18% 21% 43% 17% 1	300
	11th to 20th	Day	23% 18% 45% 12% 2	300
	21st to end	Day	19% 20% 43% 15% 2	330
February	1st to 10th	Day	24% 25% 34% 12% 4 1	300
	11th to 20th	Day	23% 29% 34% 11% 3	300
	21st to end	Day	26% 29% 37% 7% 1	249
March	1st to 10th	Day	23% 26% 37% 12% 3	300
	11th to 20th	Day	25% 22% 34% 16% 3 1	384
	21st to end	Day	29% 23% 25% 18% 5% 1	550
April	1st to 10th	Day	27% 19% 31% 19% 4 1	500
	11th to 20th	Day	27% 21% 27% 17% 7% 1	492
	21st to end	Day	23% 22% 29% 19% 5% 2	500
May	1st to 10th	Day	22% 21% 30% 17% 8% 2	548
	11th to 20th	Day	19% 18% 36% 18% 7% 2	725
	21st to end	Day	18% 14% 33% 24% 10% 2	789
June	1st to 10th	Day	16% 16% 35% 24% 8% 1	730
	11th to 20th	Day	19% 15% 35% 23% 7% 2	730
	21st to end	Day	19% 11% 34% 25% 9% 2	730
July	1st to 10th	Day	10% 8% 36% 30% 13% 3	721
	11th to 20th	Day	12% 11% 34% 29% 11% 3	718
	21st to end	Day	11% 7% 37% 28% 13% 4	794
August	1st to 10th	Day	11% 9% 31% 33% 13% 4	726
	11th to 20th	Day	17% 7% 23% 27% 19% 7%	531
	21st to end	Day	21% 12% 26% 25% 13% 4	583
September	1st to 10th	Day	22% 13% 29% 20% 12% 5%	530
	11th to 20th	Day	14% 9% 33% 28% 12% 4	530
	21st to end	Day	22% 12% 24% 30% 9% 2	530
October	1st to 10th	Day	25% 15% 29% 21% 8% 2	510
	11th to 20th	Day	35% 20% 31% 13% 1	330
	21st to end	Day	36% 22% 30% 9% 3	363
November	1st to 10th	Day	27% 22% 33% 15% 3	330
	11th to 20th	Day	26% 29% 32% 10% 2	330
	21st to end	Day	32% 25% 30% 11% 2	284
December	1st to 10th	Day	24% 24% 31% 14% 6%	204
	11th to 20th	Day	28% 24% 34% 10% 5% 1	200
	21st to end	Day	26% 25% 35% 10% 3	295
January	1st to 10th	Night	19% 21% 39% 19% 3	1000
	11th to 20th	Night	21% 20% 36% 18% 5% 1	1000
	21st to end	Night	19% 15% 39% 21% 5% 1	1100
February	1st to 10th	Night	16% 20% 35% 22% 6% 1	1000
	11th to 20th	Night	18% 19% 35% 23% 5%	791
	21st to end	Night	20% 17% 39% 20% 4	660
March	1st to 10th	Night	15% 16% 39% 23% 7%	800
	11th to 20th	Night	19% 19% 37% 19% 6% 1	800
	21st to end	Night	20% 14% 31% 26% 7% 1	728
April	1st to 10th	Night	24% 15% 30% 25% 6% 1	600
	11th to 20th	Night	27% 13% 25% 26% 8% 1	593
	21st to end	Night	22% 13% 30% 26% 8% 1	600
May	1st to 10th	Night	20% 12% 27% 31% 9% 1	600
	11th to 20th	Night	22% 11% 29% 28% 8% 2	591
	21st to end	Night	16% 8% 30% 32% 15%	650
June	1st to 10th	Night	14% 8% 29% 37% 11% 1	485
	11th to 20th	Night	17% 9% 28% 35% 12%	400
	21st to end	Night	18% 6% 28% 34% 12% 2	482
July	1st to 10th	Night	12% 3 26% 41% 17% 2	585
	11th to 20th	Night	13% 5% 27% 37% 16% 2	582
	21st to end	Night	11% 5% 24% 40% 18% 2	657
August	1st to 10th	Night	11% 3 26% 38% 21% 1	597
	11th to 20th	Night	12% 4 20% 37% 24% 3	594
	21st to end	Night	15% 5% 25% 36% 17% 2	660
September	1st to 10th	Night	18% 5% 26% 31% 18% 3	600
	11th to 20th	Night	18% 8% 28% 32% 13% 1	745
	21st to end	Night	20% 9% 27% 30% 13% 1	800
October	1st to 10th	Night	24% 12% 29% 24% 10% 1	800
	11th to 20th	Night	26% 15% 32% 22% 5%	800
	21st to end	Night	35% 13% 27% 19% 5%	880
November	1st to 10th	Night	31% 13% 34% 17% 5%	820
	11th to 20th	Night	28% 19% 31% 18% 4	840
	21st to end	Night	28% 20% 33% 16% 3	1046
December	1st to 10th	Night	22% 20% 38% 15% 4 1	1011
	11th to 20th	Night	21% 20% 36% 18% 5% 1	1000
	21st to end	Night	28% 19% 33% 16% 2	1100

Page generated at 2025-11-30 01:00:52. Version ac07

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