Kellogg Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Kellogg Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	16% 12% 4 9% 5% 5% 4 4 5% 7% 29%	131791
	Night	17% 13% 4 10% 6% 5% 4 3 5% 6% 27%	130350
Transparency	Day	54% 5% 12% 19% 9% 1	135384
	Night	58% 4 11% 17% 9% 1	134843
Seeing	Day	24% 17% 28% 19% 9% 3	16291
	Night	24% 15% 27% 24% 9% 1	25832

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	17% 21% 7% 15% 7% 6% 4 3 4 4 12%	4661
	11th to 20th	Day	11% 24% 8% 20% 11% 5% 5% 3 3 3 7%	5105
	21st to end	Day	15% 15% 7% 16% 11% 10% 6% 3 4 4 9%	5780
February	1st to 10th	Day	16% 19% 5% 14% 8% 6% 6% 3 4 5% 13%	1890
	11th to 20th	Day	14% 19% 7% 15% 8% 6% 5% 3 4 5% 16%	1843
	21st to end	Day	20% 15% 5% 12% 8% 4 5% 4 4 5% 17%	1636
March	1st to 10th	Day	17% 14% 6% 11% 6% 4 4 3 4 5% 26%	5042
	11th to 20th	Day	12% 17% 5% 8% 5% 5% 4 4 5% 7% 30%	5090
	21st to end	Day	21% 14% 4 9% 5% 4 4 3 5% 5% 26%	5586
April	1st to 10th	Day	19% 15% 4 7% 5% 4 4 3 4 5% 29%	2503
	11th to 20th	Day	19% 12% 3 7% 4 4 4 4 5% 5% 34%	2459
	21st to end	Day	21% 10% 3 7% 4 4 4 4 4 5% 34%	2600
May	1st to 10th	Day	23% 10% 3 8% 5% 4 4 3 5% 6% 28%	5888
	11th to 20th	Day	17% 10% 3 8% 5% 5% 4 4 5% 7% 30%	6290
	21st to end	Day	21% 8% 2 6% 3 4 4 4 6% 6% 36%	6949
June	1st to 10th	Day	16% 7% 2 7% 4 4 5% 4 6% 7% 38%	2800
	11th to 20th	Day	13% 5% 3 7% 5% 6% 5% 5% 7% 9% 38%	2800
	21st to end	Day	15% 6% 2 6% 4 4 4 4 7% 8% 42%	2800
July	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 6% 9% 44%	6360
	11th to 20th	Day	12% 6% 2 7% 4 5% 4 4 7% 9% 40%	6360
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 45%	6476
August	1st to 10th	Day	17% 6% 2 5% 4 4 5% 4 5% 9% 38%	4234
	11th to 20th	Day	13% 6% 3 7% 5% 5% 6% 4 8% 10% 33%	4174
	21st to end	Day	12% 6% 2 6% 4 4 4 4 6% 9% 44%	4420
September	1st to 10th	Day	14% 7% 2 6% 4 5% 5% 3 6% 7% 42%	2400
	11th to 20th	Day	11% 5% 2 6% 3 5% 4 4 6% 8% 46%	2321
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	2134
October	1st to 10th	Day	20% 7% 3 7% 4 4 5% 3 5% 7% 36%	2000
	11th to 20th	Day	20% 10% 3 8% 5% 5% 5% 5% 5% 6% 28%	2000
	21st to end	Day	26% 13% 4 9% 5% 5% 4 4 4 5% 22%	2200
November	1st to 10th	Day	16% 11% 4 8% 6% 6% 4 3 5% 6% 31%	1836
	11th to 20th	Day	15% 14% 5% 9% 5% 6% 4 4 5% 6% 27%	1680
	21st to end	Day	21% 17% 4 9% 7% 5% 5% 3 5% 6% 18%	1680
December	1st to 10th	Day	20% 19% 5% 12% 6% 6% 5% 5% 5% 6% 12%	2680
	11th to 20th	Day	22% 20% 5% 10% 5% 4 3 3 4 5% 17%	3066
	21st to end	Day	23% 22% 6% 12% 6% 5% 4 3 3 4 12%	4048
January	1st to 10th	Night	17% 20% 7% 14% 7% 5% 3 3 4 5% 16%	8234
	11th to 20th	Night	12% 20% 8% 21% 10% 7% 4 3 3 3 9%	7777
	21st to end	Night	16% 18% 7% 16% 9% 8% 6% 3 4 4 9%	8401
February	1st to 10th	Night	18% 19% 6% 13% 7% 5% 4 3 4 4 17%	2730
	11th to 20th	Night	16% 16% 6% 12% 7% 5% 5% 4 4 5% 19%	2663
	21st to end	Night	18% 17% 5% 14% 7% 4 4 3 5% 5% 18%	2166
March	1st to 10th	Night	19% 12% 5% 9% 7% 4 4 3 4 4 28%	5018
	11th to 20th	Night	16% 14% 3 7% 4 4 4 3 5% 7% 33%	5089
	21st to end	Night	19% 12% 4 8% 4 4 4 3 5% 5% 33%	5588
April	1st to 10th	Night	22% 12% 4 8% 5% 4 4 3 5% 4 29%	2116
	11th to 20th	Night	19% 9% 4 7% 4 4 4 4 4 7% 34%	2036
	21st to end	Night	18% 8% 3 6% 4 4 5% 4 4 7% 35%	1805
May	1st to 10th	Night	27% 11% 4 7% 4 3 3 3 4 5% 29%	4053
	11th to 20th	Night	14% 7% 3 7% 6% 6% 5% 4 6% 8% 34%	3586
	21st to end	Night	19% 8% 3 7% 4 3 4 4 6% 7% 36%	3926
June	1st to 10th	Night	12% 7% 3 7% 6% 6% 6% 5% 6% 7% 36%	1600
	11th to 20th	Night	13% 6% 2 5% 5% 5% 6% 4 6% 8% 41%	1600
	21st to end	Night	15% 5% 2 6% 4 4 5% 3 6% 7% 41%	1600
July	1st to 10th	Night	11% 6% 2 7% 5% 4 4 4 6% 9% 42%	3600
	11th to 20th	Night	11% 6% 2 7% 5% 4 4 4 6% 8% 44%	3600
	21st to end	Night	10% 6% 2 5% 4 5% 4 4 6% 7% 49%	4372
August	1st to 10th	Night	11% 7% 2 7% 5% 4 4 3 6% 9% 41%	2915
	11th to 20th	Night	16% 7% 2 7% 4 5% 5% 4 6% 8% 38%	2875
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 4 7% 9% 45%	3444
September	1st to 10th	Night	14% 6% 2 6% 4 3 4 3 5% 8% 44%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 8% 52%	2075
	21st to end	Night	17% 8% 2 6% 5% 4 5% 4 6% 7% 36%	2265
October	1st to 10th	Night	17% 6% 3 6% 5% 4 4 3 4 7% 40%	2400
	11th to 20th	Night	22% 10% 3 7% 4 5% 5% 3 5% 6% 30%	2400
	21st to end	Night	26% 9% 3 6% 5% 5% 4 4 5% 6% 27%	2640
November	1st to 10th	Night	18% 9% 3 7% 4 4 4 3 5% 6% 37%	2678
	11th to 20th	Night	16% 13% 4 9% 6% 5% 5% 3 5% 6% 28%	2940
	21st to end	Night	19% 17% 4 9% 5% 5% 5% 5% 5% 5% 21%	2940
December	1st to 10th	Night	18% 21% 7% 13% 6% 5% 5% 3 5% 5% 12%	4720
	11th to 20th	Night	21% 22% 5% 10% 6% 5% 4 3 4 4 17%	5348
	21st to end	Night	25% 18% 5% 11% 6% 5% 5% 3 4 4 13%	7150

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	76% 5% 9% 7% 3	4662
	11th to 20th	Day	83% 4 6% 5% 1	5139
	21st to end	Day	80% 4 10% 5% 1	5780
February	1st to 10th	Day	75% 4 11% 9% 1	1890
	11th to 20th	Day	73% 4 9% 12% 3	1852
	21st to end	Day	69% 4 10% 13% 4	1636
March	1st to 10th	Day	61% 4 14% 16% 5%	5045
	11th to 20th	Day	53% 4 19% 16% 7% 1	5090
	21st to end	Day	61% 3 8% 20% 8% 1	5586
April	1st to 10th	Day	58% 3 10% 19% 9% 1	2503
	11th to 20th	Day	52% 4 10% 21% 12% 2	2459
	21st to end	Day	53% 3 10% 18% 13% 2	2600
May	1st to 10th	Day	56% 5% 10% 16% 9% 3	5897
	11th to 20th	Day	52% 6% 11% 19% 11% 1	6294
	21st to end	Day	49% 4 12% 22% 11% 2	6949
June	1st to 10th	Day	45% 6% 12% 21% 16% 1	2800
	11th to 20th	Day	42% 6% 15% 24% 12% 1	2800
	21st to end	Day	40% 6% 14% 25% 14% 1	2800
July	1st to 10th	Day	36% 5% 13% 28% 17% 1	6290
	11th to 20th	Day	40% 5% 14% 24% 16% 1	6285
	21st to end	Day	35% 6% 15% 27% 17% 1	6290
August	1st to 10th	Day	43% 6% 14% 23% 13% 1	4221
	11th to 20th	Day	44% 7% 17% 24% 8% 1	4213
	21st to end	Day	37% 4 13% 34% 11% 1	4420
September	1st to 10th	Day	41% 6% 13% 27% 11% 1	2400
	11th to 20th	Day	36% 5% 14% 31% 13% 1	2321
	21st to end	Day	50% 5% 15% 21% 9% 1	2134
October	1st to 10th	Day	41% 5% 15% 25% 13% 1	3260
	11th to 20th	Day	46% 5% 16% 20% 12% 1	3240
	21st to end	Day	52% 4 14% 21% 8% 1	3538
November	1st to 10th	Day	54% 6% 10% 20% 8% 1	1836
	11th to 20th	Day	57% 5% 15% 16% 5% 2	1680
	21st to end	Day	68% 4 13% 12% 3	1680
December	1st to 10th	Day	73% 6% 13% 7% 1	2680
	11th to 20th	Day	71% 4 10% 11% 4	3066
	21st to end	Day	78% 4 9% 7% 2	4048
January	1st to 10th	Night	73% 5% 8% 11% 3	8234
	11th to 20th	Night	82% 4 7% 5% 2	7818
	21st to end	Night	79% 4 9% 6% 2	8401
February	1st to 10th	Night	73% 4 10% 11% 3	2730
	11th to 20th	Night	69% 3 11% 13% 3	2665
	21st to end	Night	69% 4 9% 12% 5%	2166
March	1st to 10th	Night	60% 4 11% 17% 7% 1	5028
	11th to 20th	Night	52% 3 16% 19% 8% 2	5090
	21st to end	Night	54% 4 9% 19% 12% 1	5588
April	1st to 10th	Night	59% 4 9% 18% 10% 1	2116
	11th to 20th	Night	51% 4 11% 19% 13% 2	2036
	21st to end	Night	50% 5% 10% 20% 14% 2	1805
May	1st to 10th	Night	59% 4 8% 14% 10% 5%	4060
	11th to 20th	Night	47% 6% 14% 21% 11% 1	3587
	21st to end	Night	48% 5% 10% 21% 14% 2	3927
June	1st to 10th	Night	45% 6% 11% 21% 15% 2	1600
	11th to 20th	Night	41% 5% 14% 21% 17% 2	1600
	21st to end	Night	41% 6% 14% 22% 15% 2	1600
July	1st to 10th	Night	38% 4 13% 25% 17% 2	3560
	11th to 20th	Night	38% 6% 14% 26% 16% 1	3552
	21st to end	Night	34% 5% 14% 24% 21% 2	4243
August	1st to 10th	Night	40% 5% 16% 26% 12% 1	2904
	11th to 20th	Night	45% 5% 16% 25% 9%	2902
	21st to end	Night	35% 5% 14% 32% 13% 1	3444
September	1st to 10th	Night	38% 5% 13% 28% 14% 1	2000
	11th to 20th	Night	33% 4 13% 32% 17% 1	2075
	21st to end	Night	46% 6% 13% 22% 12% 1	2265
October	1st to 10th	Night	43% 4 10% 25% 17% 1	3920
	11th to 20th	Night	49% 4 13% 19% 13%	3899
	21st to end	Night	46% 4 16% 24% 8% 1	4252
November	1st to 10th	Night	50% 5% 11% 23% 10% 2	2678
	11th to 20th	Night	59% 4 13% 16% 7% 1	2940
	21st to end	Night	65% 5% 15% 13% 3	2940
December	1st to 10th	Night	75% 5% 12% 7% 1	4720
	11th to 20th	Night	73% 4 9% 10% 3	5348
	21st to end	Night	75% 4 9% 9% 2	7150

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	27% 30% 27% 10% 4 1	220
	11th to 20th	Day	30% 29% 27% 10% 4 1	305
	21st to end	Day	28% 26% 26% 16% 3	363
February	1st to 10th	Day	32% 25% 28% 12% 2	330
	11th to 20th	Day	32% 31% 28% 8% 1	330
	21st to end	Day	39% 21% 29% 10% 1	336
March	1st to 10th	Day	29% 21% 30% 15% 4 1	540
	11th to 20th	Day	34% 16% 31% 14% 4 1	540
	21st to end	Day	32% 20% 25% 16% 5% 1	594
April	1st to 10th	Day	28% 22% 28% 15% 5% 1	540
	11th to 20th	Day	33% 21% 23% 15% 6% 2	516
	21st to end	Day	29% 18% 29% 15% 7% 1	500
May	1st to 10th	Day	28% 15% 30% 16% 8% 3	503
	11th to 20th	Day	21% 14% 31% 20% 10% 3	527
	21st to end	Day	20% 13% 30% 22% 10% 5%	573
June	1st to 10th	Day	23% 15% 32% 22% 7% 1	530
	11th to 20th	Day	15% 16% 32% 24% 10% 3	530
	21st to end	Day	20% 15% 31% 25% 8% 2	530
July	1st to 10th	Day	11% 10% 27% 31% 17% 4	525
	11th to 20th	Day	16% 11% 30% 26% 13% 5%	524
	21st to end	Day	11% 9% 35% 28% 12% 5%	576
August	1st to 10th	Day	15% 8% 23% 30% 16% 8%	526
	11th to 20th	Day	13% 8% 28% 26% 16% 8%	519
	21st to end	Day	15% 8% 32% 23% 15% 7%	583
September	1st to 10th	Day	16% 13% 23% 23% 17% 8%	530
	11th to 20th	Day	11% 7% 21% 25% 24% 13%	530
	21st to end	Day	18% 15% 28% 22% 12% 6%	530
October	1st to 10th	Day	24% 11% 26% 20% 14% 5%	530
	11th to 20th	Day	26% 20% 29% 18% 5% 2	530
	21st to end	Day	35% 19% 24% 15% 7%	583
November	1st to 10th	Day	27% 21% 29% 16% 6% 1	376
	11th to 20th	Day	34% 24% 27% 10% 3 2	220
	21st to end	Day	35% 28% 25% 10% 2	220
December	1st to 10th	Day	33% 24% 29% 10% 4	220
	11th to 20th	Day	35% 26% 25% 13% 2	220
	21st to end	Day	36% 29% 21% 10% 2 1	242
January	1st to 10th	Night	31% 25% 28% 13% 4	840
	11th to 20th	Night	33% 24% 25% 15% 3	840
	21st to end	Night	26% 23% 31% 16% 5%	924
February	1st to 10th	Night	27% 25% 28% 16% 2	840
	11th to 20th	Night	27% 23% 30% 17% 3	823
	21st to end	Night	29% 22% 29% 17% 2	692
March	1st to 10th	Night	25% 19% 31% 21% 5%	840
	11th to 20th	Night	29% 17% 30% 19% 5%	840
	21st to end	Night	26% 14% 31% 21% 7% 1	924
April	1st to 10th	Night	27% 16% 32% 21% 3	840
	11th to 20th	Night	25% 16% 28% 23% 7% 1	816
	21st to end	Night	23% 9% 31% 28% 8%	605
May	1st to 10th	Night	23% 11% 23% 31% 11% 2	600
	11th to 20th	Night	16% 8% 32% 36% 8%	405
	21st to end	Night	15% 10% 25% 34% 15% 1	433
June	1st to 10th	Night	14% 8% 32% 34% 12% 1	400
	11th to 20th	Night	15% 10% 29% 35% 12%	400
	21st to end	Night	17% 7% 33% 32% 11%	400
July	1st to 10th	Night	10% 5% 26% 42% 16% 1	390
	11th to 20th	Night	12% 7% 28% 33% 19% 1	388
	21st to end	Night	13% 4 25% 39% 18% 2	627
August	1st to 10th	Night	12% 4 20% 40% 23% 2	597
	11th to 20th	Night	11% 6% 20% 34% 26% 3	594
	21st to end	Night	14% 5% 24% 32% 23% 2	790
September	1st to 10th	Night	17% 8% 22% 33% 18% 3	800
	11th to 20th	Night	14% 5% 21% 30% 27% 5%	800
	21st to end	Night	20% 10% 24% 28% 16% 3	800
October	1st to 10th	Night	19% 10% 27% 26% 16% 2	800
	11th to 20th	Night	28% 15% 25% 23% 8% 1	800
	21st to end	Night	32% 14% 27% 23% 4	880
November	1st to 10th	Night	25% 16% 27% 23% 9% 1	820
	11th to 20th	Night	25% 21% 28% 17% 8% 1	840
	21st to end	Night	28% 22% 28% 16% 5%	840
December	1st to 10th	Night	34% 23% 23% 15% 4	840
	11th to 20th	Night	30% 27% 24% 15% 3	840
	21st to end	Night	35% 20% 27% 16% 2	924

Page generated at 2026-04-14 02:44:25. Version ac07

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