Kresge Environmental Education Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Kresge Environmental Education Center clear sky chart from 2005-11-06 to 2025-03-12.

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% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	108312
	Night	14% 11% 4 9% 5% 5% 4 4 5% 6% 34%	110773
Transparency	Day	49% 6% 12% 20% 12% 1	111911
	Night	49% 6% 12% 21% 11% 1	115314
Seeing	Day	18% 11% 29% 23% 13% 6%	15766
	Night	19% 10% 30% 29% 11% 1	25562

By Month

Percentage Cloud Cover SE (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	11% 17% 7% 15% 6% 4 4 3 4 4 24%	3617
	11th to 20th	Day	13% 17% 6% 17% 10% 8% 6% 3 4 4 11%	3620
	21st to end	Day	20% 17% 6% 13% 6% 4 5% 3 4 5% 17%	4349
February	1st to 10th	Day	13% 15% 5% 15% 8% 5% 4 3 4 5% 22%	1800
	11th to 20th	Day	13% 15% 5% 12% 8% 6% 5% 3 3 6% 23%	1867
	21st to end	Day	14% 16% 5% 12% 7% 5% 4 3 5% 5% 24%	1650
March	1st to 10th	Day	17% 14% 5% 9% 5% 4 4 3 4 5% 29%	3307
	11th to 20th	Day	11% 15% 6% 10% 6% 5% 5% 4 5% 6% 29%	3491
	21st to end	Day	18% 13% 4 9% 5% 4 5% 3 4 6% 29%	3815
April	1st to 10th	Day	16% 13% 4 9% 5% 4 4 3 5% 6% 30%	2207
	11th to 20th	Day	17% 10% 3 6% 5% 4 4 4 6% 7% 35%	2423
	21st to end	Day	16% 9% 4 8% 6% 5% 4 3 5% 6% 34%	2470
May	1st to 10th	Day	17% 9% 3 7% 4 4 5% 4 5% 7% 35%	4082
	11th to 20th	Day	17% 7% 2 7% 5% 5% 5% 4 6% 9% 34%	4081
	21st to end	Day	10% 6% 2 7% 4 4 4 3 5% 8% 46%	4508
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	2845
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 4 7% 7% 43%	2850
	21st to end	Day	10% 7% 2 6% 5% 5% 5% 5% 8% 9% 37%	2818
July	1st to 10th	Day	10% 5% 2 8% 5% 5% 6% 5% 7% 8% 37%	4440
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 7% 10% 46%	4440
	21st to end	Day	8% 5% 2 6% 6% 6% 6% 5% 7% 9% 41%	4573
August	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	4104
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 9% 40%	4044
	21st to end	Day	9% 5% 3 7% 5% 5% 6% 5% 6% 9% 40%	4518
September	1st to 10th	Day	9% 7% 2 8% 5% 6% 6% 5% 6% 9% 38%	2147
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 6% 8% 47%	2090
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 5% 5% 8% 34%	2090
October	1st to 10th	Day	15% 7% 3 7% 5% 4 5% 3 6% 7% 40%	2025
	11th to 20th	Day	18% 10% 3 7% 5% 5% 5% 4 5% 6% 32%	1900
	21st to end	Day	20% 12% 3 7% 4 4 5% 3 5% 7% 29%	1932
November	1st to 10th	Day	12% 11% 4 8% 6% 5% 4 4 5% 7% 35%	1762
	11th to 20th	Day	14% 12% 5% 10% 6% 5% 4 4 6% 7% 27%	1800
	21st to end	Day	17% 16% 5% 9% 6% 5% 4 4 4 6% 25%	1620
December	1st to 10th	Day	14% 16% 6% 11% 6% 5% 6% 5% 6% 7% 19%	2600
	11th to 20th	Day	17% 20% 6% 9% 4 4 5% 3 5% 5% 22%	2699
	21st to end	Day	25% 20% 6% 12% 6% 4 4 3 4 5% 12%	3728
January	1st to 10th	Night	13% 12% 5% 11% 5% 5% 5% 4 5% 6% 28%	6348
	11th to 20th	Night	11% 15% 6% 14% 9% 9% 6% 4 4 6% 16%	6360
	21st to end	Night	15% 15% 8% 10% 6% 5% 5% 3 5% 6% 22%	6610
February	1st to 10th	Night	15% 15% 5% 11% 7% 5% 6% 4 5% 5% 22%	2600
	11th to 20th	Night	13% 14% 5% 10% 6% 5% 5% 4 5% 5% 27%	2415
	21st to end	Night	17% 11% 5% 9% 5% 5% 5% 4 5% 6% 28%	1980
March	1st to 10th	Night	14% 14% 4 8% 4 3 3 3 4 6% 37%	3765
	11th to 20th	Night	9% 9% 4 7% 5% 4 4 4 5% 8% 40%	3498
	21st to end	Night	15% 10% 3 9% 5% 5% 4 3 5% 5% 36%	3817
April	1st to 10th	Night	18% 11% 3 7% 4 4 3 3 5% 6% 35%	1990
	11th to 20th	Night	19% 8% 2 7% 4 5% 4 3 5% 7% 37%	1705
	21st to end	Night	17% 9% 2 6% 4 4 4 4 6% 7% 37%	1710
May	1st to 10th	Night	15% 8% 2 7% 4 5% 4 4 6% 8% 37%	2813
	11th to 20th	Night	16% 7% 3 6% 4 4 4 3 4 7% 42%	2812
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 7% 50%	3031
June	1st to 10th	Night	12% 5% 2 6% 3 5% 3 4 6% 8% 46%	1330
	11th to 20th	Night	10% 5% 2 6% 3 4 5% 4 6% 9% 46%	1330
	21st to end	Night	9% 6% 2 6% 4 4 4 4 6% 8% 49%	1356
July	1st to 10th	Night	7% 5% 2 5% 4 4 5% 3 8% 10% 46%	2530
	11th to 20th	Night	9% 3 2 4 3 4 4 4 6% 8% 53%	2530
	21st to end	Night	5% 4 2 6% 5% 5% 6% 5% 7% 8% 47%	2982
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 5% 8% 47%	2825
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 7% 8% 47%	2785
	21st to end	Night	7% 5% 3 7% 4 5% 5% 4 5% 7% 47%	3124
September	1st to 10th	Night	10% 5% 2 7% 5% 6% 4 4 5% 7% 47%	2012
	11th to 20th	Night	10% 5% 2 5% 4 3 4 3 5% 6% 54%	2090
	21st to end	Night	15% 7% 3 7% 4 4 5% 3 5% 6% 43%	2090
October	1st to 10th	Night	15% 6% 2 6% 4 3 3 4 5% 7% 45%	2153
	11th to 20th	Night	19% 10% 3 7% 4 4 5% 4 5% 6% 34%	2280
	21st to end	Night	21% 9% 3 9% 5% 5% 4 3 5% 6% 31%	2665
November	1st to 10th	Night	13% 9% 3 6% 4 4 4 3 5% 7% 42%	2536
	11th to 20th	Night	14% 12% 4 9% 5% 4 6% 5% 6% 7% 28%	2600
	21st to end	Night	19% 15% 3 8% 5% 4 5% 4 5% 6% 26%	2775
December	1st to 10th	Night	12% 16% 6% 12% 6% 5% 4 4 6% 7% 22%	4580
	11th to 20th	Night	14% 18% 6% 12% 6% 5% 4 3 5% 5% 22%	5501
	21st to end	Night	22% 21% 7% 11% 5% 4 3 3 3 4 16%	7245

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	64% 5% 10% 14% 6% 1	3618
	11th to 20th	Day	76% 5% 9% 6% 4	3620
	21st to end	Day	71% 4 11% 9% 5% 1	4349
February	1st to 10th	Day	64% 6% 9% 15% 7%	1800
	11th to 20th	Day	64% 4 9% 14% 9% 1	1876
	21st to end	Day	61% 6% 10% 15% 7% 2	1650
March	1st to 10th	Day	56% 7% 11% 19% 6% 1	3307
	11th to 20th	Day	54% 6% 11% 20% 8% 2	3491
	21st to end	Day	56% 5% 13% 14% 10% 2	3815
April	1st to 10th	Day	54% 6% 11% 19% 10% 1	2207
	11th to 20th	Day	48% 6% 12% 20% 13% 2	2423
	21st to end	Day	50% 6% 13% 18% 13% 1	2470
May	1st to 10th	Day	45% 8% 12% 21% 12% 3	4120
	11th to 20th	Day	43% 9% 14% 23% 10% 1	4085
	21st to end	Day	36% 6% 11% 19% 23% 5%	4521
June	1st to 10th	Day	38% 8% 12% 25% 15% 2	2845
	11th to 20th	Day	37% 8% 13% 24% 17% 2	2850
	21st to end	Day	39% 9% 12% 23% 15% 1	2818
July	1st to 10th	Day	41% 7% 14% 20% 16% 2	4371
	11th to 20th	Day	31% 7% 12% 34% 15% 1	4365
	21st to end	Day	37% 7% 15% 17% 22% 2	4428
August	1st to 10th	Day	40% 7% 13% 21% 17% 1	4091
	11th to 20th	Day	39% 6% 13% 28% 13% 1	4083
	21st to end	Day	39% 7% 13% 28% 12% 1	4517
September	1st to 10th	Day	41% 8% 14% 24% 12% 2	2147
	11th to 20th	Day	33% 6% 13% 31% 15% 1	2090
	21st to end	Day	46% 7% 13% 22% 11% 1	2090
October	1st to 10th	Day	33% 4 10% 33% 17% 2	3363
	11th to 20th	Day	42% 4 12% 25% 16%	3140
	21st to end	Day	48% 5% 13% 23% 11% 1	3156
November	1st to 10th	Day	48% 7% 11% 24% 10% 1	1762
	11th to 20th	Day	56% 5% 13% 17% 9%	1800
	21st to end	Day	61% 4 13% 15% 6% 1	1620
December	1st to 10th	Day	64% 6% 17% 11% 3	2600
	11th to 20th	Day	65% 5% 10% 13% 6%	2695
	21st to end	Day	76% 4 8% 8% 5%	3728
January	1st to 10th	Night	56% 6% 12% 18% 6% 2	6348
	11th to 20th	Night	68% 6% 10% 10% 5%	6360
	21st to end	Night	64% 4 10% 15% 6%	6610
February	1st to 10th	Night	63% 7% 9% 14% 7% 1	2600
	11th to 20th	Night	58% 6% 10% 18% 9% 1	2417
	21st to end	Night	56% 5% 12% 18% 7% 1	1980
March	1st to 10th	Night	49% 5% 14% 25% 7% 1	3765
	11th to 20th	Night	40% 7% 16% 22% 15% 1	3498
	21st to end	Night	48% 5% 12% 22% 11% 1	3817
April	1st to 10th	Night	50% 5% 10% 21% 12% 2	1990
	11th to 20th	Night	46% 5% 12% 18% 17% 2	1705
	21st to end	Night	44% 6% 12% 19% 15% 3	1710
May	1st to 10th	Night	43% 8% 11% 22% 13% 4	2840
	11th to 20th	Night	41% 6% 10% 28% 12% 3	2813
	21st to end	Night	33% 7% 11% 22% 20% 6%	3039
June	1st to 10th	Night	35% 6% 11% 29% 18% 2	1330
	11th to 20th	Night	34% 6% 15% 23% 19% 2	1330
	21st to end	Night	33% 6% 13% 26% 19% 1	1356
July	1st to 10th	Night	31% 6% 14% 26% 16% 6%	2490
	11th to 20th	Night	28% 6% 13% 33% 18% 1	2482
	21st to end	Night	31% 9% 18% 20% 21% 2	2887
August	1st to 10th	Night	34% 6% 14% 29% 16% 1	2814
	11th to 20th	Night	32% 7% 15% 31% 12% 2	2812
	21st to end	Night	34% 7% 13% 31% 14% 1	3124
September	1st to 10th	Night	35% 8% 12% 27% 16% 2	2012
	11th to 20th	Night	30% 7% 12% 31% 19% 2	2090
	21st to end	Night	41% 6% 11% 29% 13% 1	2090
October	1st to 10th	Night	27% 4 12% 32% 25% 1	3595
	11th to 20th	Night	44% 5% 11% 24% 16%	3779
	21st to end	Night	44% 4 12% 28% 11% 1	4391
November	1st to 10th	Night	42% 5% 12% 26% 14% 1	2536
	11th to 20th	Night	53% 6% 14% 18% 9% 1	2600
	21st to end	Night	58% 6% 15% 17% 5%	2775
December	1st to 10th	Night	61% 6% 17% 12% 3	4580
	11th to 20th	Night	64% 5% 10% 13% 7% 1	5504
	21st to end	Night	72% 5% 9% 8% 6%	7245

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	23% 16% 38% 15% 6% 1	300
	11th to 20th	Day	19% 15% 47% 15% 4 1	300
	21st to end	Day	28% 17% 34% 16% 4 1	330
February	1st to 10th	Day	24% 21% 37% 14% 3 1	300
	11th to 20th	Day	24% 21% 35% 19% 1	300
	21st to end	Day	27% 28% 31% 12% 2	249
March	1st to 10th	Day	28% 16% 38% 14% 3 1	348
	11th to 20th	Day	20% 15% 34% 23% 8%	464
	21st to end	Day	29% 11% 31% 22% 7% 1	506
April	1st to 10th	Day	21% 16% 33% 20% 9% 1	460
	11th to 20th	Day	27% 13% 31% 17% 8% 4	452
	21st to end	Day	21% 12% 30% 24% 10% 3	460
May	1st to 10th	Day	22% 12% 29% 23% 11% 3	463
	11th to 20th	Day	20% 9% 28% 23% 13% 7%	487
	21st to end	Day	15% 7% 25% 28% 17% 9%	548
June	1st to 10th	Day	13% 9% 30% 31% 13% 5%	680
	11th to 20th	Day	13% 8% 30% 29% 16% 5%	680
	21st to end	Day	13% 8% 29% 29% 16% 5%	680
July	1st to 10th	Day	9% 4 23% 30% 22% 11%	671
	11th to 20th	Day	10% 6% 27% 29% 19% 9%	668
	21st to end	Day	9% 4 24% 30% 23% 11%	600
August	1st to 10th	Day	10% 5% 20% 27% 23% 15%	486
	11th to 20th	Day	12% 5% 18% 29% 24% 13%	479
	21st to end	Day	11% 8% 19% 27% 22% 12%	539
September	1st to 10th	Day	11% 7% 24% 26% 17% 14%	490
	11th to 20th	Day	10% 4 18% 24% 28% 16%	490
	21st to end	Day	18% 12% 20% 24% 19% 8%	490
October	1st to 10th	Day	17% 9% 28% 23% 15% 8%	490
	11th to 20th	Day	21% 16% 32% 22% 8% 1	490
	21st to end	Day	28% 15% 33% 17% 6% 1	381
November	1st to 10th	Day	16% 20% 35% 21% 8% 1	300
	11th to 20th	Day	22% 20% 36% 15% 6% 2	300
	21st to end	Day	26% 23% 34% 14% 3	210
December	1st to 10th	Day	24% 18% 36% 16% 7% 1	200
	11th to 20th	Day	26% 24% 35% 12% 4 1	200
	21st to end	Day	29% 21% 36% 12% 3	275
January	1st to 10th	Night	21% 16% 37% 20% 5% 1	1000
	11th to 20th	Night	24% 15% 34% 22% 4 1	1000
	21st to end	Night	21% 12% 35% 27% 5%	935
February	1st to 10th	Night	23% 14% 39% 21% 3 1	800
	11th to 20th	Night	23% 17% 33% 23% 5%	783
	21st to end	Night	24% 18% 35% 22% 2	660
March	1st to 10th	Night	21% 14% 35% 24% 6%	800
	11th to 20th	Night	17% 15% 37% 26% 5% 1	767
	21st to end	Night	22% 11% 31% 28% 7% 1	836
April	1st to 10th	Night	24% 12% 35% 24% 5%	660
	11th to 20th	Night	23% 10% 35% 24% 7% 2	563
	21st to end	Night	20% 9% 33% 31% 7%	570
May	1st to 10th	Night	18% 7% 31% 33% 11% 1	570
	11th to 20th	Night	22% 4 26% 34% 12% 1	561
	21st to end	Night	11% 4 25% 38% 20% 2	617
June	1st to 10th	Night	11% 4 29% 44% 12%	380
	11th to 20th	Night	12% 6% 29% 39% 13%	380
	21st to end	Night	10% 5% 25% 45% 14%	406
July	1st to 10th	Night	8% 3 18% 43% 25% 3	555
	11th to 20th	Night	11% 3 18% 43% 21% 4	552
	21st to end	Night	7% 3 19% 45% 24% 2	624
August	1st to 10th	Night	9% 3 14% 43% 29% 2	567
	11th to 20th	Night	9% 3 17% 39% 28% 4	564
	21st to end	Night	10% 4 19% 37% 28% 3	627
September	1st to 10th	Night	12% 5% 20% 35% 22% 5%	684
	11th to 20th	Night	11% 5% 17% 35% 28% 4	760
	21st to end	Night	18% 6% 22% 32% 18% 4	760
October	1st to 10th	Night	18% 7% 26% 30% 18% 1	760
	11th to 20th	Night	23% 12% 30% 25% 10%	760
	21st to end	Night	25% 12% 33% 26% 5%	836
November	1st to 10th	Night	18% 12% 32% 26% 11% 1	780
	11th to 20th	Night	20% 16% 34% 23% 6% 1	800
	21st to end	Night	25% 15% 35% 20% 5%	800
December	1st to 10th	Night	23% 15% 35% 21% 5%	800
	11th to 20th	Night	22% 15% 39% 21% 3	945
	21st to end	Night	30% 12% 35% 20% 3	1100

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