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 2024-11-30.

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	18% 15% 3 6% 4 3 3 3 4 5% 36%	105585
	Night	20% 18% 3 6% 4 3 3 3 4 5% 32%	102627
Transparency	Day	52% 5% 9% 17% 12% 5%	105351
	Night	55% 6% 9% 16% 10% 4	102507
Seeing	Day	29% 13% 26% 21% 10% 2	14594
	Night	30% 7% 18% 28% 15% 2	23579

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	26% 38% 5% 7% 3 2 2 2 3 4 9%	2539
	11th to 20th	Day	29% 21% 3 8% 3 3 3 2 3 5% 20%	2516
	21st to end	Day	39% 23% 3 6% 3 3 3 2 3 4 12%	2935
February	1st to 10th	Day	26% 25% 4 7% 4 4 3 3 4 4 16%	1710
	11th to 20th	Day	23% 30% 3 7% 4 3 2 2 3 3 19%	1665
	21st to end	Day	23% 32% 4 7% 4 3 2 2 3 4 16%	1516
March	1st to 10th	Day	17% 30% 6% 8% 4 3 3 2 4 4 19%	3200
	11th to 20th	Day	18% 22% 3 6% 4 3 2 2 2 2 37%	3490
	21st to end	Day	21% 30% 5% 10% 5% 4 3 2 2 4 13%	3968
April	1st to 10th	Day	25% 27% 4 8% 4 3 3 2 4 4 14%	2280
	11th to 20th	Day	22% 24% 4 8% 4 4 3 2 3 5% 22%	2252
	21st to end	Day	22% 22% 4 8% 5% 5% 4 3 4 5% 19%	2448
May	1st to 10th	Day	18% 18% 4 8% 4 4 3 2 3 4 32%	4420
	11th to 20th	Day	12% 10% 3 7% 5% 5% 4 4 6% 7% 37%	4377
	21st to end	Day	24% 20% 4 8% 5% 4 4 3 4 5% 20%	4813
June	1st to 10th	Day	27% 10% 3 7% 4 3 3 3 5% 5% 30%	2660
	11th to 20th	Day	22% 12% 4 8% 5% 4 4 4 5% 7% 24%	2660
	21st to end	Day	11% 7% 2 6% 4 3 4 3 5% 7% 47%	2660
July	1st to 10th	Day	4 3 1 3 2 2 3 2 4 4 71%	4420
	11th to 20th	Day	5% 3 1 2 2 2 2 2 3 4 74%	4420
	21st to end	Day	6% 4 1 4 2 2 2 2 3 4 69%	4790
August	1st to 10th	Day	4 4 2 3 2 3 3 3 4 5% 67%	4257
	11th to 20th	Day	5% 7% 2 7% 4 4 5% 3 5% 6% 52%	4026
	21st to end	Day	9% 7% 3 6% 4 4 3 3 4 6% 52%	4256
September	1st to 10th	Day	8% 6% 2 4 3 3 3 3 4 6% 59%	2280
	11th to 20th	Day	15% 7% 2 6% 4 3 4 4 4 6% 44%	2280
	21st to end	Day	19% 8% 2 5% 4 3 3 2 4 5% 45%	2018
October	1st to 10th	Day	17% 8% 1 4 3 3 3 2 5% 7% 47%	3180
	11th to 20th	Day	24% 10% 4 6% 4 3 4 3 5% 7% 30%	3161
	21st to end	Day	28% 17% 3 9% 4 3 3 2 3 4 23%	3273
November	1st to 10th	Day	33% 17% 4 8% 4 4 4 2 4 5% 15%	1681
	11th to 20th	Day	36% 20% 3 5% 3 2 2 3 4 5% 17%	1600
	21st to end	Day	26% 22% 3 7% 4 4 3 2 3 4 21%	1595
December	1st to 10th	Day	26% 21% 4 8% 4 3 3 2 3 3 24%	1520
	11th to 20th	Day	30% 22% 3 8% 4 3 3 3 4 7% 14%	1925
	21st to end	Day	26% 22% 3 9% 5% 4 4 3 5% 4 15%	2794
January	1st to 10th	Night	29% 32% 5% 7% 3 3 3 2 3 3 10%	4406
	11th to 20th	Night	29% 20% 5% 6% 3 3 3 2 4 6% 18%	4382
	21st to end	Night	37% 23% 3 6% 3 3 3 2 4 4 13%	4724
February	1st to 10th	Night	25% 25% 5% 7% 3 3 4 3 4 4 17%	2470
	11th to 20th	Night	23% 26% 4 6% 4 3 3 3 4 3 22%	2402
	21st to end	Night	20% 32% 5% 7% 3 2 4 3 3 3 18%	1937
March	1st to 10th	Night	17% 27% 7% 7% 4 4 4 3 5% 5% 19%	3658
	11th to 20th	Night	17% 22% 3 5% 3 2 2 2 3 3 38%	3489
	21st to end	Night	23% 34% 5% 7% 4 3 3 2 3 4 14%	3684
April	1st to 10th	Night	21% 26% 5% 8% 5% 4 3 2 3 4 18%	1900
	11th to 20th	Night	18% 22% 4 7% 4 4 4 2 4 5% 27%	1879
	21st to end	Night	20% 21% 3 7% 5% 3 3 3 5% 6% 23%	1727
May	1st to 10th	Night	19% 21% 4 7% 4 3 2 3 3 4 32%	2540
	11th to 20th	Night	11% 13% 3 6% 4 4 3 3 5% 5% 43%	2516
	21st to end	Night	15% 21% 3 7% 4 4 4 4 5% 7% 26%	2762
June	1st to 10th	Night	22% 11% 4 6% 4 3 5% 4 4 5% 32%	1520
	11th to 20th	Night	18% 13% 4 7% 4 3 3 3 5% 6% 34%	1520
	21st to end	Night	9% 8% 3 6% 3 3 3 3 4 5% 54%	1520
July	1st to 10th	Night	3 3 1 3 2 2 2 2 2 3 78%	2540
	11th to 20th	Night	4 2 1 3 2 2 2 2 2 4 75%	2540
	21st to end	Night	5% 2 1 3 1 2 2 2 2 5% 76%	2762
August	1st to 10th	Night	2 4 1 4 3 2 2 2 3 5% 72%	2674
	11th to 20th	Night	5% 7% 2 5% 3 3 4 4 4 6% 56%	2812
	21st to end	Night	12% 7% 2 4 3 2 2 2 3 5% 57%	3420
September	1st to 10th	Night	8% 5% 1 4 2 2 3 3 4 5% 62%	1900
	11th to 20th	Night	14% 8% 2 6% 4 4 4 3 4 5% 46%	1900
	21st to end	Night	16% 8% 2 5% 4 2 3 3 4 5% 47%	2160
October	1st to 10th	Night	15% 7% 2 4 4 3 3 2 4 7% 49%	3800
	11th to 20th	Night	21% 12% 3 7% 4 3 4 4 5% 6% 32%	3777
	21st to end	Night	26% 18% 3 7% 4 3 3 3 4 5% 24%	4289
November	1st to 10th	Night	34% 16% 3 7% 4 3 3 3 3 4 19%	2615
	11th to 20th	Night	35% 20% 3 6% 4 3 3 2 3 4 19%	2800
	21st to end	Night	25% 20% 3 6% 4 3 3 3 4 4 23%	2800
December	1st to 10th	Night	26% 19% 4 6% 4 4 4 3 4 4 23%	2660
	11th to 20th	Night	27% 22% 4 7% 4 4 4 4 5% 4 14%	3280
	21st to end	Night	26% 26% 5% 10% 5% 4 4 2 3 3 11%	4862

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% 7% 4 5% 1	2539
	11th to 20th	Day	70% 7% 7% 11% 3 1	2540
	21st to end	Day	80% 5% 5% 7% 3	2935
February	1st to 10th	Day	72% 5% 10% 10% 3	1710
	11th to 20th	Day	72% 6% 7% 11% 4	1674
	21st to end	Day	74% 5% 7% 7% 7% 1	1516
March	1st to 10th	Day	69% 4 8% 13% 5%	3200
	11th to 20th	Day	56% 4 4 31% 5%	3490
	21st to end	Day	75% 6% 8% 8% 3	3968
April	1st to 10th	Day	74% 5% 9% 9% 2	2280
	11th to 20th	Day	67% 4 9% 10% 8% 1	2252
	21st to end	Day	66% 6% 10% 11% 6% 1	2448
May	1st to 10th	Day	55% 5% 11% 18% 11%	4420
	11th to 20th	Day	41% 7% 20% 22% 10%	4380
	21st to end	Day	65% 8% 13% 12% 2	4814
June	1st to 10th	Day	56% 5% 14% 17% 8% 1	2660
	11th to 20th	Day	58% 5% 16% 13% 7% 1	2660
	21st to end	Day	35% 6% 15% 28% 13% 3	2660
July	1st to 10th	Day	17% 3 8% 17% 24% 31%	4351
	11th to 20th	Day	16% 3 6% 14% 26% 35%	4345
	21st to end	Day	19% 3 8% 24% 35% 11%	4639
August	1st to 10th	Day	20% 4 7% 21% 29% 20%	4240
	11th to 20th	Day	30% 5% 12% 34% 15% 4	4065
	21st to end	Day	34% 5% 8% 23% 25% 6%	4255
September	1st to 10th	Day	26% 3 11% 25% 23% 11%	2280
	11th to 20th	Day	41% 6% 11% 17% 17% 7%	2280
	21st to end	Day	43% 4 12% 21% 15% 4	2018
October	1st to 10th	Day	38% 4 6% 26% 24% 3	3180
	11th to 20th	Day	55% 5% 11% 17% 12% 1	3161
	21st to end	Day	65% 4 5% 15% 9% 1	3273
November	1st to 10th	Day	72% 6% 7% 12% 4	1681
	11th to 20th	Day	72% 5% 8% 9% 4 1	1600
	21st to end	Day	69% 4 6% 12% 8% 1	1595
December	1st to 10th	Day	68% 6% 7% 12% 7%	1520
	11th to 20th	Day	72% 10% 8% 9% 1	1928
	21st to end	Day	73% 9% 8% 8% 2	2794
January	1st to 10th	Night	82% 7% 5% 4 1	4406
	11th to 20th	Night	70% 7% 9% 10% 3 1	4420
	21st to end	Night	77% 7% 6% 7% 3 1	4724
February	1st to 10th	Night	70% 6% 9% 11% 3	2470
	11th to 20th	Night	68% 6% 8% 12% 6%	2404
	21st to end	Night	71% 4 9% 10% 6%	1937
March	1st to 10th	Night	67% 5% 11% 13% 4	3658
	11th to 20th	Night	53% 4 10% 27% 6% 1	3490
	21st to end	Night	77% 5% 7% 10% 2	3684
April	1st to 10th	Night	70% 6% 10% 10% 4	1900
	11th to 20th	Night	59% 7% 11% 13% 7% 3	1879
	21st to end	Night	60% 7% 13% 14% 6% 1	1727
May	1st to 10th	Night	54% 5% 11% 18% 12% 1	2540
	11th to 20th	Night	38% 6% 17% 27% 10% 1	2516
	21st to end	Night	52% 13% 20% 12% 4	2762
June	1st to 10th	Night	52% 6% 13% 22% 7% 1	1520
	11th to 20th	Night	49% 5% 17% 20% 9%	1520
	21st to end	Night	33% 4 15% 31% 14% 3	1520
July	1st to 10th	Night	14% 3 7% 19% 26% 30%	2500
	11th to 20th	Night	16% 2 5% 16% 25% 35%	2492
	21st to end	Night	14% 3 10% 20% 38% 15%	2674
August	1st to 10th	Night	17% 2 8% 24% 30% 19%	2666
	11th to 20th	Night	28% 5% 14% 32% 16% 5%	2839
	21st to end	Night	31% 4 8% 26% 24% 7%	3420
September	1st to 10th	Night	23% 4 10% 27% 26% 11%	1900
	11th to 20th	Night	39% 6% 10% 20% 15% 10%	1900
	21st to end	Night	39% 4 14% 21% 17% 6%	2160
October	1st to 10th	Night	36% 4 6% 28% 23% 2	3800
	11th to 20th	Night	52% 5% 11% 19% 12% 1	3777
	21st to end	Night	63% 5% 5% 16% 10% 1	4289
November	1st to 10th	Night	69% 5% 8% 13% 6%	2615
	11th to 20th	Night	72% 5% 8% 9% 5% 1	2800
	21st to end	Night	65% 5% 7% 13% 9% 1	2800
December	1st to 10th	Night	67% 7% 9% 11% 6%	2660
	11th to 20th	Night	73% 10% 5% 8% 3	3276
	21st to end	Night	78% 9% 6% 5% 2	4862

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% 10% 22% 13% 6% 2	180
	11th to 20th	Day	49% 11% 23% 10% 8%	180
	21st to end	Day	46% 8% 19% 18% 8% 1	287
February	1st to 10th	Day	43% 15% 21% 16% 5% 1	370
	11th to 20th	Day	43% 18% 23% 14% 2	365
	21st to end	Day	43% 15% 21% 17% 4	357
March	1st to 10th	Day	40% 14% 28% 13% 3	460
	11th to 20th	Day	37% 14% 25% 15% 7% 2	460
	21st to end	Day	41% 15% 25% 15% 3 1	506
April	1st to 10th	Day	43% 15% 23% 12% 6% 2	460
	11th to 20th	Day	40% 20% 21% 11% 5% 2	452
	21st to end	Day	35% 16% 28% 15% 5% 1	460
May	1st to 10th	Day	28% 16% 30% 17% 8% 1	463
	11th to 20th	Day	31% 13% 25% 17% 11% 2	488
	21st to end	Day	31% 10% 26% 21% 10% 2	529
June	1st to 10th	Day	29% 12% 24% 21% 10% 3	490
	11th to 20th	Day	25% 21% 30% 16% 7% 1	490
	21st to end	Day	13% 11% 32% 26% 14% 3	490
July	1st to 10th	Day	11% 10% 34% 31% 11% 2	486
	11th to 20th	Day	6% 12% 33% 32% 14% 3	484
	21st to end	Day	7% 8% 31% 33% 17% 5%	531
August	1st to 10th	Day	9% 7% 28% 37% 14% 5%	487
	11th to 20th	Day	7% 8% 28% 35% 18% 4	477
	21st to end	Day	11% 13% 30% 29% 12% 4	539
September	1st to 10th	Day	12% 11% 32% 30% 11% 4	490
	11th to 20th	Day	18% 12% 27% 30% 12% 1	490
	21st to end	Day	24% 11% 25% 28% 10% 2	490
October	1st to 10th	Day	28% 10% 25% 20% 14% 2	490
	11th to 20th	Day	34% 11% 20% 19% 13% 3	490
	21st to end	Day	41% 11% 18% 17% 10% 2	439
November	1st to 10th	Day	45% 9% 19% 19% 7%	257
	11th to 20th	Day	49% 10% 20% 13% 8% 1	200
	21st to end	Day	40% 10% 19% 22% 10%	199
December	1st to 10th	Day	46% 12% 13% 16% 11% 2	180
	11th to 20th	Day	46% 11% 19% 18% 6%	180
	21st to end	Day	39% 13% 23% 18% 5% 1	198
January	1st to 10th	Night	48% 11% 14% 18% 8% 1	760
	11th to 20th	Night	41% 8% 20% 21% 9% 1	760
	21st to end	Night	38% 7% 19% 25% 10% 1	836
February	1st to 10th	Night	38% 8% 21% 24% 8% 1	760
	11th to 20th	Night	36% 9% 19% 26% 9%	744
	21st to end	Night	39% 10% 19% 26% 6%	628
March	1st to 10th	Night	34% 12% 16% 25% 12% 1	760
	11th to 20th	Night	35% 9% 21% 25% 10%	760
	21st to end	Night	40% 11% 19% 20% 9%	836
April	1st to 10th	Night	35% 11% 19% 23% 11% 1	760
	11th to 20th	Night	28% 12% 23% 21% 12% 3	751
	21st to end	Night	30% 9% 17% 30% 13% 1	587
May	1st to 10th	Night	23% 10% 21% 30% 15% 2	380
	11th to 20th	Night	25% 4 15% 33% 21% 2	374
	21st to end	Night	21% 8% 19% 28% 22% 2	411
June	1st to 10th	Night	27% 6% 18% 29% 17% 2	380
	11th to 20th	Night	22% 8% 22% 33% 14%	380
	21st to end	Night	12% 6% 15% 43% 22% 1	380
July	1st to 10th	Night	8% 4 17% 41% 30% 1	370
	11th to 20th	Night	7% 2 21% 45% 24% 2	368
	21st to end	Night	2 2 19% 45% 28% 4	416
August	1st to 10th	Night	8% 1 14% 36% 38% 3	458
	11th to 20th	Night	7% 2 10% 38% 37% 6%	564
	21st to end	Night	9% 6% 18% 41% 23% 4	784
September	1st to 10th	Night	9% 4 19% 41% 24% 3	760
	11th to 20th	Night	18% 3 13% 32% 30% 4	760
	21st to end	Night	21% 4 17% 34% 21% 3	760
October	1st to 10th	Night	26% 4 16% 33% 18% 3	760
	11th to 20th	Night	31% 4 13% 31% 17% 3	760
	21st to end	Night	36% 7% 17% 27% 11% 2	836
November	1st to 10th	Night	44% 6% 16% 28% 6% 1	780
	11th to 20th	Night	47% 7% 16% 22% 7% 1	800
	21st to end	Night	37% 9% 18% 26% 10% 1	800
December	1st to 10th	Night	36% 7% 22% 25% 8% 1	760
	11th to 20th	Night	44% 7% 19% 24% 7%	760
	21st to end	Night	49% 11% 19% 16% 5%	836

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