KepCo Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the KepCo Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	9% 10% 3 7% 5% 4 5% 4 6% 7% 41%	135356
	Night	10% 12% 4 7% 4 4 4 3 5% 6% 39%	135811
Transparency	Day	25% 39% 8% 16% 10% 3	135083
	Night	28% 37% 8% 16% 9% 2	135662
Seeing	Day	17% 14% 28% 25% 13% 3	16679
	Night	18% 8% 21% 33% 18% 3	25679

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	13% 23% 6% 12% 5% 5% 5% 4 5% 7% 16%	4723
	11th to 20th	Day	9% 9% 3 5% 4 4 3 3 5% 8% 47%	4716
	21st to end	Day	15% 19% 4 6% 3 3 4 3 4 6% 31%	5186
February	1st to 10th	Day	13% 16% 5% 9% 5% 4 4 4 6% 7% 28%	1727
	11th to 20th	Day	9% 16% 4 10% 5% 5% 5% 4 6% 5% 30%	1860
	21st to end	Day	9% 15% 4 11% 7% 6% 5% 5% 6% 6% 26%	1730
March	1st to 10th	Day	7% 13% 4 8% 5% 6% 6% 5% 6% 8% 33%	4618
	11th to 20th	Day	7% 14% 4 8% 5% 4 4 3 5% 6% 40%	4828
	21st to end	Day	13% 15% 4 9% 6% 6% 6% 5% 7% 6% 23%	6087
April	1st to 10th	Day	10% 12% 5% 11% 6% 5% 6% 5% 5% 7% 27%	2520
	11th to 20th	Day	9% 11% 4 9% 6% 5% 6% 4 6% 8% 31%	2515
	21st to end	Day	8% 12% 4 9% 6% 4 5% 5% 6% 8% 33%	2824
May	1st to 10th	Day	7% 7% 4 8% 5% 4 5% 3 5% 7% 44%	6335
	11th to 20th	Day	8% 11% 4 9% 5% 6% 6% 4 7% 9% 30%	6277
	21st to end	Day	10% 8% 3 8% 5% 5% 5% 5% 7% 9% 35%	6903
June	1st to 10th	Day	14% 9% 3 7% 6% 5% 5% 5% 6% 7% 33%	2964
	11th to 20th	Day	11% 9% 3 8% 6% 6% 6% 6% 7% 7% 30%	3000
	21st to end	Day	7% 6% 3 7% 5% 4 5% 4 8% 7% 45%	3170
July	1st to 10th	Day	2 2 1 3 2 2 3 3 4 7% 70%	6325
	11th to 20th	Day	2 3 1 3 2 2 3 2 3 5% 73%	6320
	21st to end	Day	3 3 1 4 3 3 3 3 5% 8% 63%	6874
August	1st to 10th	Day	2 3 1 3 3 3 4 4 6% 8% 63%	4541
	11th to 20th	Day	3 3 2 4 4 4 5% 4 6% 8% 59%	4432
	21st to end	Day	5% 5% 2 6% 5% 4 5% 3 5% 7% 54%	4312
September	1st to 10th	Day	5% 4 2 5% 4 3 3 2 4 7% 60%	2400
	11th to 20th	Day	9% 6% 3 5% 4 5% 4 3 5% 7% 49%	2360
	21st to end	Day	9% 6% 3 7% 3 4 4 3 5% 7% 50%	2200
October	1st to 10th	Day	6% 6% 2 6% 4 4 4 3 5% 8% 50%	3290
	11th to 20th	Day	10% 10% 4 8% 5% 6% 5% 4 5% 6% 39%	3145
	21st to end	Day	13% 11% 7% 9% 4 4 3 3 5% 6% 35%	3200
November	1st to 10th	Day	19% 13% 3 7% 5% 4 5% 3 6% 7% 27%	1807
	11th to 20th	Day	17% 18% 4 8% 5% 4 4 3 4 5% 28%	1680
	21st to end	Day	17% 17% 6% 10% 6% 3 4 4 5% 5% 24%	1587
December	1st to 10th	Day	10% 11% 3 8% 5% 4 3 5% 7% 10% 34%	2360
	11th to 20th	Day	14% 23% 8% 11% 5% 4 5% 4 5% 4 17%	2712
	21st to end	Day	14% 22% 7% 14% 6% 5% 5% 4 5% 5% 13%	3828
January	1st to 10th	Night	15% 25% 7% 12% 5% 5% 4 3 3 4 17%	8172
	11th to 20th	Night	9% 10% 3 5% 4 3 4 3 5% 8% 46%	8182
	21st to end	Night	15% 17% 6% 8% 3 3 4 3 3 5% 32%	8994
February	1st to 10th	Night	13% 15% 5% 10% 5% 5% 5% 4 5% 6% 27%	2890
	11th to 20th	Night	10% 14% 5% 9% 5% 4 4 4 5% 6% 35%	2646
	21st to end	Night	9% 16% 6% 9% 5% 6% 5% 4 6% 8% 27%	2076
March	1st to 10th	Night	5% 11% 4 9% 6% 5% 5% 4 6% 9% 36%	5496
	11th to 20th	Night	9% 14% 4 6% 4 4 4 3 5% 6% 42%	5390
	21st to end	Night	11% 18% 5% 11% 7% 5% 6% 4 5% 5% 23%	5129
April	1st to 10th	Night	10% 10% 3 7% 5% 4 6% 5% 6% 8% 35%	2100
	11th to 20th	Night	7% 9% 3 8% 6% 5% 6% 4 6% 7% 38%	2055
	21st to end	Night	6% 10% 4 7% 6% 5% 5% 4 7% 8% 40%	1792
May	1st to 10th	Night	5% 8% 4 7% 5% 4 4 3 5% 7% 49%	3648
	11th to 20th	Night	7% 8% 3 8% 5% 5% 5% 4 6% 7% 42%	3616
	21st to end	Night	8% 5% 2 7% 5% 4 5% 4 7% 9% 44%	3972
June	1st to 10th	Night	12% 7% 2 5% 5% 5% 5% 5% 7% 8% 39%	1434
	11th to 20th	Night	9% 7% 3 8% 5% 6% 6% 5% 6% 8% 38%	1400
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 6% 9% 49%	1211
July	1st to 10th	Night	2 3 1 2 2 2 2 2 5% 6% 73%	3608
	11th to 20th	Night	2 2 1 3 2 2 3 2 4 5% 75%	3640
	21st to end	Night	2 2 1 4 2 3 3 3 5% 7% 68%	3969
August	1st to 10th	Night	2 3 2 5% 3 3 4 3 5% 6% 64%	2609
	11th to 20th	Night	2 2 1 3 4 3 3 3 7% 9% 61%	2612
	21st to end	Night	6% 5% 2 5% 3 3 4 3 6% 7% 56%	3608
September	1st to 10th	Night	3 4 2 4 3 3 3 4 6% 7% 62%	2000
	11th to 20th	Night	8% 5% 1 5% 3 3 4 3 5% 8% 54%	2040
	21st to end	Night	8% 6% 3 6% 4 4 4 3 5% 8% 51%	2200
October	1st to 10th	Night	7% 5% 2 5% 4 5% 5% 4 5% 6% 51%	3907
	11th to 20th	Night	12% 11% 3 8% 4 4 4 3 5% 6% 41%	4009
	21st to end	Night	16% 9% 3 7% 4 4 4 3 5% 5% 40%	4600
November	1st to 10th	Night	19% 15% 3 7% 4 4 4 3 6% 6% 27%	2707
	11th to 20th	Night	20% 15% 3 8% 4 4 4 4 5% 6% 27%	2940
	21st to end	Night	18% 18% 4 11% 5% 4 4 3 4 5% 24%	3008
December	1st to 10th	Night	9% 10% 4 9% 5% 5% 4 3 5% 8% 37%	5040
	11th to 20th	Night	17% 23% 5% 11% 6% 5% 4 3 4 5% 16%	5700
	21st to end	Night	16% 24% 6% 10% 5% 4 5% 4 5% 4 17%	7411

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	47% 25% 10% 12% 6%	4723
	11th to 20th	Day	24% 23% 14% 28% 9% 3	4740
	21st to end	Day	41% 26% 9% 14% 9%	5187
February	1st to 10th	Day	24% 61% 3 5% 6% 1	1727
	11th to 20th	Day	24% 60% 4 8% 4	1869
	21st to end	Day	19% 61% 6% 8% 5%	1730
March	1st to 10th	Day	31% 33% 8% 13% 15% 1	4618
	11th to 20th	Day	30% 30% 10% 23% 7%	4828
	21st to end	Day	40% 32% 9% 11% 8%	6087
April	1st to 10th	Day	19% 61% 8% 9% 3	2520
	11th to 20th	Day	17% 61% 6% 12% 4 1	2515
	21st to end	Day	18% 61% 4 9% 7% 1	2824
May	1st to 10th	Day	24% 31% 5% 21% 18% 1	6335
	11th to 20th	Day	35% 32% 8% 18% 7%	6280
	21st to end	Day	29% 31% 14% 20% 6%	6904
June	1st to 10th	Day	16% 63% 4 9% 6% 1	2964
	11th to 20th	Day	17% 64% 4 9% 6% 1	3000
	21st to end	Day	15% 62% 5% 11% 7% 1	3170
July	1st to 10th	Day	9% 27% 8% 16% 23% 16%	6256
	11th to 20th	Day	9% 26% 6% 22% 20% 18%	6245
	21st to end	Day	15% 28% 6% 25% 18% 7%	6681
August	1st to 10th	Day	10% 37% 3 18% 25% 7%	4524
	11th to 20th	Day	15% 40% 10% 28% 7% 1	4474
	21st to end	Day	19% 38% 9% 18% 13% 3	4311
September	1st to 10th	Day	13% 57% 6% 14% 9% 2	2400
	11th to 20th	Day	16% 59% 5% 12% 7% 1	2360
	21st to end	Day	16% 58% 8% 12% 5% 1	2200
October	1st to 10th	Day	18% 38% 14% 24% 6%	3290
	11th to 20th	Day	29% 40% 5% 18% 8%	3145
	21st to end	Day	31% 39% 7% 16% 7%	3200
November	1st to 10th	Day	25% 56% 8% 9% 2	1807
	11th to 20th	Day	28% 55% 6% 6% 3 1	1680
	21st to end	Day	27% 55% 6% 6% 5% 1	1587
December	1st to 10th	Day	28% 38% 19% 13% 2	2360
	11th to 20th	Day	49% 33% 7% 6% 4	2714
	21st to end	Day	57% 28% 7% 6% 2	3828
January	1st to 10th	Night	54% 23% 8% 11% 3	8172
	11th to 20th	Night	24% 24% 10% 27% 12% 3	8220
	21st to end	Night	41% 25% 9% 12% 12%	8994
February	1st to 10th	Night	25% 60% 3 5% 5% 1	2890
	11th to 20th	Night	23% 59% 5% 8% 4 2	2648
	21st to end	Night	19% 60% 5% 9% 6%	2076
March	1st to 10th	Night	28% 31% 7% 17% 15% 1	5496
	11th to 20th	Night	29% 29% 9% 22% 11%	5391
	21st to end	Night	46% 31% 7% 10% 5%	5129
April	1st to 10th	Night	17% 61% 8% 12% 3 1	2100
	11th to 20th	Night	17% 61% 5% 11% 6%	2055
	21st to end	Night	15% 61% 4 12% 8% 1	1792
May	1st to 10th	Night	22% 31% 4 22% 20% 1	3645
	11th to 20th	Night	28% 30% 9% 26% 5% 2	3616
	21st to end	Night	21% 30% 17% 26% 5% 1	3972
June	1st to 10th	Night	12% 63% 6% 12% 7% 1	1434
	11th to 20th	Night	15% 63% 4 9% 8% 1	1400
	21st to end	Night	13% 61% 5% 15% 6% 1	1211
July	1st to 10th	Night	8% 25% 9% 16% 28% 13%	3568
	11th to 20th	Night	10% 26% 9% 19% 19% 18%	3592
	21st to end	Night	13% 26% 5% 28% 20% 8%	3857
August	1st to 10th	Night	13% 36% 3 16% 27% 5%	2601
	11th to 20th	Night	12% 39% 11% 27% 10% 1	2636
	21st to end	Night	18% 37% 9% 22% 12% 2	3608
September	1st to 10th	Night	10% 58% 7% 13% 11% 1	2000
	11th to 20th	Night	14% 58% 5% 14% 8%	2040
	21st to end	Night	16% 58% 7% 13% 5%	2200
October	1st to 10th	Night	16% 37% 15% 24% 8%	3907
	11th to 20th	Night	28% 37% 7% 18% 11%	4009
	21st to end	Night	26% 38% 10% 21% 5%	4600
November	1st to 10th	Night	26% 56% 8% 8% 3	2707
	11th to 20th	Night	26% 56% 6% 6% 4 2	2940
	21st to end	Night	27% 56% 6% 6% 4 1	3008
December	1st to 10th	Night	25% 37% 15% 18% 5%	5040
	11th to 20th	Night	48% 34% 8% 6% 4	5697
	21st to end	Night	49% 28% 8% 10% 5%	7411

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	26% 14% 23% 28% 7% 3	220
	11th to 20th	Day	27% 6% 27% 25% 13% 2	220
	21st to end	Day	25% 10% 28% 24% 10% 2	242
February	1st to 10th	Day	17% 16% 33% 27% 6% 1	267
	11th to 20th	Day	18% 17% 32% 24% 9% 1	433
	21st to end	Day	15% 15% 36% 25% 9% 1	446
March	1st to 10th	Day	23% 20% 29% 17% 9% 1	540
	11th to 20th	Day	25% 17% 25% 19% 13% 1	540
	21st to end	Day	24% 20% 26% 18% 10% 2	594
April	1st to 10th	Day	25% 16% 29% 19% 9% 2	540
	11th to 20th	Day	22% 20% 29% 17% 8% 4	532
	21st to end	Day	18% 19% 27% 21% 10% 5%	540
May	1st to 10th	Day	17% 17% 31% 23% 9% 3	508
	11th to 20th	Day	22% 11% 27% 27% 10% 4	528
	21st to end	Day	19% 10% 27% 25% 15% 4	573
June	1st to 10th	Day	18% 12% 30% 27% 11% 3	694
	11th to 20th	Day	17% 13% 28% 26% 13% 3	730
	21st to end	Day	11% 12% 26% 29% 19% 3	900
July	1st to 10th	Day	7% 13% 31% 34% 12% 3	531
	11th to 20th	Day	6% 16% 29% 31% 14% 4	524
	21st to end	Day	6% 8% 30% 32% 19% 5%	575
August	1st to 10th	Day	6% 8% 27% 35% 18% 6%	527
	11th to 20th	Day	6% 11% 28% 31% 17% 8%	517
	21st to end	Day	11% 13% 25% 29% 16% 6%	583
September	1st to 10th	Day	10% 11% 27% 29% 17% 6%	530
	11th to 20th	Day	17% 10% 26% 27% 17% 4	530
	21st to end	Day	18% 15% 30% 22% 12% 3	530
October	1st to 10th	Day	14% 16% 30% 25% 12% 2	530
	11th to 20th	Day	18% 16% 28% 23% 13% 2	465
	21st to end	Day	25% 9% 33% 22% 9% 1	363
November	1st to 10th	Day	25% 14% 29% 24% 9% 1	305
	11th to 20th	Day	28% 17% 24% 19% 12%	220
	21st to end	Day	20% 12% 33% 18% 13% 4	220
December	1st to 10th	Day	22% 5% 26% 21% 19% 6%	220
	11th to 20th	Day	23% 13% 30% 24% 8% 2	220
	21st to end	Day	24% 12% 23% 29% 11% 2	242
January	1st to 10th	Night	29% 10% 22% 27% 10% 1	840
	11th to 20th	Night	25% 8% 23% 31% 12% 2	840
	21st to end	Night	22% 5% 21% 38% 13% 1	924
February	1st to 10th	Night	20% 9% 25% 32% 11% 2	840
	11th to 20th	Night	16% 7% 24% 37% 14% 2	824
	21st to end	Night	18% 9% 23% 31% 17% 3	692
March	1st to 10th	Night	18% 10% 22% 32% 16% 3	840
	11th to 20th	Night	19% 10% 23% 32% 15% 2	840
	21st to end	Night	20% 12% 23% 28% 14% 3	924
April	1st to 10th	Night	17% 8% 20% 34% 19% 3	840
	11th to 20th	Night	14% 11% 25% 32% 16% 3	807
	21st to end	Night	12% 10% 26% 33% 17% 3	532
May	1st to 10th	Night	10% 11% 22% 35% 19% 2	402
	11th to 20th	Night	13% 7% 18% 34% 25% 4	394
	21st to end	Night	13% 8% 19% 31% 24% 5%	433
June	1st to 10th	Night	16% 7% 21% 36% 18% 2	400
	11th to 20th	Night	14% 7% 26% 33% 19% 2	400
	21st to end	Night	9% 7% 22% 33% 27% 3	400
July	1st to 10th	Night	5% 7% 22% 33% 30% 4	390
	11th to 20th	Night	4 9% 20% 41% 23% 3	388
	21st to end	Night	4 6% 14% 39% 32% 5%	438
August	1st to 10th	Night	2 6% 17% 34% 34% 8%	398
	11th to 20th	Night	3 4 19% 33% 35% 6%	405
	21st to end	Night	7% 6% 16% 38% 28% 4	880
September	1st to 10th	Night	5% 6% 17% 38% 28% 7%	800
	11th to 20th	Night	12% 5% 13% 32% 30% 8%	800
	21st to end	Night	12% 6% 20% 36% 22% 4	800
October	1st to 10th	Night	17% 7% 18% 32% 24% 2	800
	11th to 20th	Night	20% 9% 20% 31% 18% 3	800
	21st to end	Night	24% 8% 22% 31% 13% 3	880
November	1st to 10th	Night	28% 8% 25% 31% 7% 1	820
	11th to 20th	Night	30% 10% 20% 24% 15% 1	840
	21st to end	Night	27% 8% 20% 31% 12% 3	908
December	1st to 10th	Night	21% 8% 21% 35% 13% 1	1050
	11th to 20th	Night	28% 9% 22% 30% 10%	1050
	21st to end	Night	28% 8% 26% 29% 8% 1	1060

Page generated at 2026-04-28 23:21:46. Version ac07

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