McClellan Overlook
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McClellan Overlook clear sky chart from 2005-11-06 to 2026-02-16.

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	17% 15% 3 7% 4 3 3 3 4 5% 37%	135058
	Night	18% 18% 4 6% 3 3 3 3 4 5% 33%	134529
Transparency	Day	51% 5% 9% 18% 13% 5%	134783
	Night	55% 5% 8% 16% 12% 4	134382
Seeing	Day	29% 12% 26% 21% 9% 2	16030
	Night	30% 7% 18% 28% 14% 2	25126

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	25% 35% 6% 9% 4 3 3 2 3 4 9%	4723
	11th to 20th	Day	16% 15% 3 5% 3 3 3 2 4 6% 42%	4716
	21st to end	Day	27% 16% 2 6% 3 3 3 3 4 4 30%	5465
February	1st to 10th	Day	25% 24% 5% 8% 4 4 3 3 4 5% 15%	1890
	11th to 20th	Day	22% 29% 4 8% 5% 3 2 2 3 3 19%	1801
	21st to end	Day	23% 32% 4 7% 4 4 2 2 2 4 17%	1594
March	1st to 10th	Day	15% 24% 5% 8% 5% 4 5% 4 5% 5% 21%	4606
	11th to 20th	Day	17% 27% 5% 10% 5% 3 2 2 2 2 26%	5000
	21st to end	Day	23% 27% 5% 10% 5% 5% 4 3 3 4 11%	5694
April	1st to 10th	Day	25% 26% 4 8% 4 3 4 3 4 4 14%	2400
	11th to 20th	Day	21% 23% 4 8% 4 4 3 2 3 4 25%	2372
	21st to end	Day	21% 21% 4 8% 5% 5% 4 3 4 5% 21%	2577
May	1st to 10th	Day	14% 14% 3 7% 4 3 3 3 4 5% 41%	6320
	11th to 20th	Day	15% 16% 5% 10% 5% 5% 4 3 4 5% 26%	6277
	21st to end	Day	19% 16% 3 7% 4 4 4 3 5% 6% 29%	6903
June	1st to 10th	Day	25% 10% 3 6% 4 3 3 3 5% 5% 32%	2800
	11th to 20th	Day	21% 11% 4 8% 5% 5% 5% 4 5% 7% 25%	2800
	21st to end	Day	11% 7% 3 6% 4 3 4 3 5% 7% 47%	2800
July	1st to 10th	Day	4 3 1 3 2 2 3 2 4 4 71%	6320
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 3 78%	6320
	21st to end	Day	5% 3 1 4 2 2 2 2 3 5% 70%	6874
August	1st to 10th	Day	4 4 2 4 2 3 3 3 4 5% 66%	4392
	11th to 20th	Day	5% 6% 2 7% 4 4 5% 3 6% 6% 52%	4156
	21st to end	Day	9% 7% 3 6% 4 4 3 3 4 6% 51%	4391
September	1st to 10th	Day	8% 6% 2 4 3 3 3 3 4 6% 58%	2400
	11th to 20th	Day	14% 7% 2 6% 4 3 4 4 4 6% 46%	2400
	21st to end	Day	19% 8% 2 5% 4 3 3 2 4 5% 45%	2124
October	1st to 10th	Day	17% 8% 2 4 3 3 3 2 5% 6% 47%	3280
	11th to 20th	Day	24% 11% 4 6% 4 3 4 3 5% 7% 30%	3261
	21st to end	Day	28% 17% 4 9% 4 3 4 2 3 4 23%	3378
November	1st to 10th	Day	33% 17% 4 8% 4 4 4 2 4 5% 15%	1767
	11th to 20th	Day	36% 20% 3 5% 3 3 2 3 4 5% 17%	1680
	21st to end	Day	26% 21% 3 7% 4 4 3 2 4 5% 21%	1680
December	1st to 10th	Day	21% 16% 3 6% 3 3 3 3 4 5% 35%	2700
	11th to 20th	Day	28% 24% 5% 10% 4 4 3 3 3 5% 10%	3105
	21st to end	Day	28% 26% 4 9% 4 4 4 2 4 3 11%	4092
January	1st to 10th	Night	25% 31% 5% 9% 4 4 3 2 3 3 11%	8172
	11th to 20th	Night	16% 15% 5% 5% 3 2 3 2 4 6% 39%	8182
	21st to end	Night	24% 16% 3 6% 3 3 3 2 4 5% 31%	8728
February	1st to 10th	Night	24% 24% 5% 8% 4 4 4 3 4 4 17%	2730
	11th to 20th	Night	22% 27% 4 6% 4 3 3 3 3 3 22%	2597
	21st to end	Night	20% 31% 5% 7% 3 2 4 3 3 4 19%	2035
March	1st to 10th	Night	14% 26% 6% 8% 4 4 4 3 5% 5% 21%	5272
	11th to 20th	Night	17% 28% 5% 7% 3 3 2 2 3 3 27%	4999
	21st to end	Night	26% 31% 4 7% 3 3 3 2 4 5% 12%	5280
April	1st to 10th	Night	21% 26% 5% 8% 5% 4 4 2 3 4 18%	2000
	11th to 20th	Night	18% 21% 4 7% 4 4 4 2 4 5% 29%	1979
	21st to end	Night	19% 20% 3 7% 5% 3 4 3 5% 6% 24%	1818
May	1st to 10th	Night	15% 17% 4 7% 3 3 3 3 4 5% 38%	3640
	11th to 20th	Night	12% 24% 4 7% 4 4 3 2 4 5% 31%	3616
	21st to end	Night	13% 17% 3 7% 4 4 4 4 5% 8% 31%	3972
June	1st to 10th	Night	21% 11% 4 6% 4 3 5% 4 5% 5% 34%	1600
	11th to 20th	Night	17% 13% 4 7% 4 3 4 3 5% 6% 35%	1600
	21st to end	Night	9% 8% 3 6% 3 3 3 3 4 5% 54%	1600
July	1st to 10th	Night	2 3 1 3 2 2 2 3 4 4 75%	3640
	11th to 20th	Night	3 2 1 2 2 2 2 1 2 3 80%	3640
	21st to end	Night	4 2 1 2 1 2 2 2 3 6% 75%	3969
August	1st to 10th	Night	2 4 1 4 3 2 2 2 3 5% 72%	2759
	11th to 20th	Night	6% 7% 2 5% 3 3 4 4 4 6% 55%	2902
	21st to end	Night	12% 7% 2 5% 3 3 3 2 3 5% 57%	3527
September	1st to 10th	Night	7% 6% 1 4 3 3 3 3 4 6% 61%	2000
	11th to 20th	Night	14% 8% 2 6% 4 4 4 3 4 5% 47%	2000
	21st to end	Night	16% 8% 2 5% 3 3 3 3 4 5% 48%	2274
October	1st to 10th	Night	15% 7% 2 4 3 3 3 2 4 7% 49%	3920
	11th to 20th	Night	21% 12% 3 7% 4 3 4 4 5% 6% 32%	3897
	21st to end	Night	26% 19% 3 7% 4 3 3 3 4 5% 24%	4426
November	1st to 10th	Night	34% 16% 3 7% 4 3 3 3 3 4 19%	2749
	11th to 20th	Night	34% 19% 3 6% 4 3 3 2 3 4 19%	2940
	21st to end	Night	26% 20% 3 6% 5% 4 3 3 4 4 23%	2940
December	1st to 10th	Night	17% 18% 4 5% 3 3 3 3 4 5% 35%	4700
	11th to 20th	Night	27% 26% 6% 10% 4 4 4 3 4 3 10%	5320
	21st to end	Night	29% 28% 5% 10% 4 4 3 2 3 3 9%	7106

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	82% 5% 3 5% 4	4723
	11th to 20th	Day	47% 5% 7% 14% 19% 8%	4740
	21st to end	Day	60% 4 4 11% 15% 5%	5466
February	1st to 10th	Day	73% 5% 10% 10% 3	1890
	11th to 20th	Day	72% 6% 7% 11% 5% 1	1810
	21st to end	Day	74% 5% 6% 7% 7% 1	1594
March	1st to 10th	Day	64% 5% 9% 12% 8% 1	4606
	11th to 20th	Day	67% 3 4 22% 4	5000
	21st to end	Day	77% 6% 8% 6% 2	5694
April	1st to 10th	Day	74% 5% 9% 10% 2	2400
	11th to 20th	Day	64% 4 9% 13% 8% 1	2372
	21st to end	Day	64% 6% 10% 13% 6% 1	2577
May	1st to 10th	Day	46% 4 9% 25% 15% 1	6320
	11th to 20th	Day	56% 7% 15% 16% 7%	6280
	21st to end	Day	54% 7% 16% 18% 3 1	6904
June	1st to 10th	Day	53% 5% 14% 18% 9% 1	2800
	11th to 20th	Day	56% 5% 16% 13% 9% 1	2800
	21st to end	Day	36% 6% 15% 28% 12% 3	2800
July	1st to 10th	Day	17% 3 8% 26% 23% 23%	6251
	11th to 20th	Day	14% 3 6% 19% 32% 28%	6245
	21st to end	Day	17% 3 11% 34% 27% 7%	6681
August	1st to 10th	Day	21% 4 8% 21% 28% 19%	4375
	11th to 20th	Day	30% 5% 12% 34% 15% 5%	4195
	21st to end	Day	34% 5% 8% 23% 24% 6%	4390
September	1st to 10th	Day	27% 3 12% 25% 23% 10%	2400
	11th to 20th	Day	39% 6% 11% 18% 18% 7%	2400
	21st to end	Day	43% 4 11% 21% 16% 4	2124
October	1st to 10th	Day	38% 4 6% 26% 24% 3	3280
	11th to 20th	Day	54% 5% 11% 17% 12% 1	3261
	21st to end	Day	66% 4 5% 15% 9% 1	3378
November	1st to 10th	Day	71% 5% 7% 12% 4	1767
	11th to 20th	Day	73% 5% 8% 10% 4 1	1680
	21st to end	Day	68% 4 7% 13% 7% 1	1680
December	1st to 10th	Day	55% 5% 6% 13% 20% 1	2700
	11th to 20th	Day	79% 8% 6% 7% 1	3108
	21st to end	Day	78% 7% 6% 7% 1	4092
January	1st to 10th	Night	81% 5% 4 6% 3	8172
	11th to 20th	Night	48% 5% 8% 13% 18% 8%	8220
	21st to end	Night	59% 4 4 10% 17% 6%	8728
February	1st to 10th	Night	70% 6% 9% 11% 3 1	2730
	11th to 20th	Night	68% 5% 8% 12% 6% 1	2599
	21st to end	Night	70% 4 9% 10% 7%	2035
March	1st to 10th	Night	65% 5% 9% 13% 6% 3	5272
	11th to 20th	Night	65% 3 8% 19% 4 1	5000
	21st to end	Night	77% 5% 8% 10% 1	5280
April	1st to 10th	Night	70% 6% 10% 11% 4	2000
	11th to 20th	Night	58% 7% 11% 15% 8% 2	1979
	21st to end	Night	59% 7% 13% 15% 6% 1	1818
May	1st to 10th	Night	46% 4 9% 21% 19%	3637
	11th to 20th	Night	52% 6% 14% 20% 7% 1	3616
	21st to end	Night	47% 11% 18% 18% 5% 1	3972
June	1st to 10th	Night	50% 6% 13% 23% 7% 1	1600
	11th to 20th	Night	48% 5% 16% 21% 9%	1600
	21st to end	Night	33% 4 16% 30% 14% 3	1600
July	1st to 10th	Night	14% 4 8% 26% 26% 23%	3600
	11th to 20th	Night	13% 2 6% 18% 35% 27%	3592
	21st to end	Night	13% 3 12% 32% 30% 10%	3857
August	1st to 10th	Night	18% 2 8% 24% 29% 19%	2751
	11th to 20th	Night	28% 5% 14% 32% 16% 5%	2929
	21st to end	Night	31% 4 8% 26% 24% 7%	3527
September	1st to 10th	Night	24% 4 11% 27% 24% 10%	2000
	11th to 20th	Night	38% 6% 10% 21% 16% 9%	2000
	21st to end	Night	38% 4 13% 21% 17% 5%	2274
October	1st to 10th	Night	37% 4 7% 29% 23% 2	3920
	11th to 20th	Night	52% 5% 11% 19% 13% 1	3897
	21st to end	Night	63% 5% 5% 16% 10% 1	4426
November	1st to 10th	Night	69% 5% 8% 12% 6%	2749
	11th to 20th	Night	72% 5% 8% 9% 5% 1	2940
	21st to end	Night	65% 5% 7% 13% 9% 1	2940
December	1st to 10th	Night	53% 5% 8% 12% 20% 2	4700
	11th to 20th	Night	80% 8% 4 6% 2	5316
	21st to end	Night	82% 7% 4 5% 1	7106

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	50% 9% 22% 13% 5% 1	220
	11th to 20th	Day	41% 12% 25% 14% 8%	220
	21st to end	Day	43% 9% 21% 19% 8% 1	341
February	1st to 10th	Day	41% 16% 22% 15% 4 1	430
	11th to 20th	Day	43% 17% 25% 13% 2	411
	21st to end	Day	44% 14% 21% 17% 4	387
March	1st to 10th	Day	40% 14% 29% 14% 3	500
	11th to 20th	Day	39% 14% 25% 14% 7% 2	500
	21st to end	Day	42% 15% 25% 14% 3 1	550
April	1st to 10th	Day	42% 14% 24% 12% 6% 2	500
	11th to 20th	Day	38% 20% 24% 12% 5% 2	492
	21st to end	Day	33% 15% 29% 16% 7% 1	500
May	1st to 10th	Day	27% 16% 29% 18% 8% 1	503
	11th to 20th	Day	34% 13% 24% 16% 11% 2	528
	21st to end	Day	30% 10% 26% 22% 10% 2	573
June	1st to 10th	Day	28% 12% 25% 21% 11% 3	530
	11th to 20th	Day	25% 21% 31% 16% 6% 1	530
	21st to end	Day	14% 11% 32% 25% 15% 3	530
July	1st to 10th	Day	11% 10% 34% 32% 10% 3	526
	11th to 20th	Day	6% 12% 33% 32% 14% 3	524
	21st to end	Day	6% 7% 30% 34% 17% 5%	575
August	1st to 10th	Day	9% 7% 27% 37% 14% 5%	527
	11th to 20th	Day	8% 7% 29% 33% 18% 4	517
	21st to end	Day	11% 12% 30% 30% 13% 5%	583
September	1st to 10th	Day	11% 11% 32% 31% 11% 5%	530
	11th to 20th	Day	17% 12% 28% 30% 12% 1	530
	21st to end	Day	24% 11% 26% 28% 10% 2	530
October	1st to 10th	Day	27% 10% 26% 21% 13% 2	530
	11th to 20th	Day	34% 11% 20% 20% 12% 3	530
	21st to end	Day	41% 11% 19% 18% 9% 2	478
November	1st to 10th	Day	47% 10% 20% 17% 6%	283
	11th to 20th	Day	47% 9% 21% 14% 9% 1	220
	21st to end	Day	40% 10% 21% 20% 9%	220
December	1st to 10th	Day	45% 10% 15% 18% 10% 2	220
	11th to 20th	Day	49% 10% 20% 16% 5%	220
	21st to end	Day	42% 13% 24% 17% 4 1	242
January	1st to 10th	Night	48% 10% 15% 18% 8% 1	840
	11th to 20th	Night	38% 8% 24% 22% 8% 1	840
	21st to end	Night	37% 7% 21% 24% 9% 1	924
February	1st to 10th	Night	37% 8% 23% 24% 7% 1	840
	11th to 20th	Night	37% 9% 20% 26% 8%	804
	21st to end	Night	39% 10% 20% 25% 6%	660
March	1st to 10th	Night	35% 11% 17% 25% 11% 1	800
	11th to 20th	Night	36% 9% 21% 25% 9%	800
	21st to end	Night	41% 11% 19% 20% 9%	880
April	1st to 10th	Night	35% 11% 20% 23% 10% 1	800
	11th to 20th	Night	27% 12% 24% 22% 12% 3	791
	21st to end	Night	29% 9% 17% 31% 13% 1	618
May	1st to 10th	Night	22% 10% 21% 31% 15% 2	400
	11th to 20th	Night	26% 4 15% 33% 20% 2	394
	21st to end	Night	21% 8% 20% 28% 21% 2	433
June	1st to 10th	Night	26% 6% 19% 30% 18% 2	400
	11th to 20th	Night	22% 8% 22% 34% 14%	400
	21st to end	Night	12% 6% 15% 44% 23% 1	400
July	1st to 10th	Night	7% 3 17% 42% 29% 1	390
	11th to 20th	Night	6% 2 21% 45% 24% 2	388
	21st to end	Night	2 2 18% 46% 29% 3	438
August	1st to 10th	Night	8% 1 14% 36% 37% 3	483
	11th to 20th	Night	8% 2 10% 39% 36% 6%	594
	21st to end	Night	9% 6% 17% 42% 23% 3	825
September	1st to 10th	Night	9% 4 18% 41% 25% 3	800
	11th to 20th	Night	17% 3 14% 33% 29% 4	800
	21st to end	Night	21% 4 17% 35% 21% 3	800
October	1st to 10th	Night	26% 4 17% 33% 18% 2	800
	11th to 20th	Night	31% 4 14% 31% 16% 3	800
	21st to end	Night	36% 6% 19% 26% 11% 2	880
November	1st to 10th	Night	45% 6% 16% 27% 5%	820
	11th to 20th	Night	47% 7% 17% 23% 7% 1	840
	21st to end	Night	37% 8% 18% 26% 10% 1	840
December	1st to 10th	Night	37% 7% 21% 25% 9% 1	840
	11th to 20th	Night	45% 7% 19% 23% 6%	840
	21st to end	Night	49% 10% 20% 15% 5%	924

Page generated at 2026-02-12 22:42:12. Version ac07

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