Mitchell School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mitchell School clear sky chart from 2005-11-06 to 2026-08-15.

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	6% 8% 3 6% 4 4 4 4 5% 6% 50%	152991
	Night	6% 10% 3 6% 4 4 4 4 5% 6% 47%	146216
Transparency	Day	33% 7% 10% 21% 20% 10%	147766
	Night	35% 8% 11% 21% 18% 7%	142441
Seeing	Day	13% 15% 28% 26% 15% 4	16792
	Night	13% 10% 19% 34% 20% 4	25805

By Month

Percentage Cloud Cover SW (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% 6% 10% 6% 6% 6% 4 6% 6% 21%	4722
	11th to 20th	Day	6% 10% 4 8% 4 4 4 3 4 5% 49%	4740
	21st to end	Day	6% 10% 3 7% 5% 5% 5% 5% 7% 8% 40%	5346
February	1st to 10th	Day	8% 15% 5% 9% 5% 4 5% 6% 7% 7% 28%	1890
	11th to 20th	Day	9% 16% 5% 9% 5% 5% 5% 4 5% 8% 30%	1904
	21st to end	Day	6% 17% 7% 11% 6% 5% 6% 5% 5% 6% 27%	1730
March	1st to 10th	Day	5% 11% 5% 10% 6% 5% 6% 5% 7% 9% 30%	4618
	11th to 20th	Day	4 13% 5% 9% 6% 6% 5% 4 5% 6% 37%	4992
	21st to end	Day	5% 15% 5% 11% 8% 7% 7% 6% 7% 7% 23%	5964
April	1st to 10th	Day	9% 16% 4 9% 6% 5% 6% 4 6% 8% 27%	2520
	11th to 20th	Day	9% 14% 5% 8% 6% 5% 4 4 5% 5% 35%	2489
	21st to end	Day	9% 13% 3 7% 4 4 4 4 5% 7% 40%	2593
May	1st to 10th	Day	6% 7% 2 5% 4 3 4 3 5% 6% 54%	8193
	11th to 20th	Day	7% 8% 2 5% 4 4 4 3 5% 7% 50%	8160
	21st to end	Day	8% 9% 2 5% 4 3 4 3 6% 8% 49%	8992
June	1st to 10th	Day	11% 9% 2 6% 5% 5% 4 4 5% 6% 41%	2940
	11th to 20th	Day	9% 7% 2 5% 4 3 4 3 6% 8% 49%	2940
	21st to end	Day	4 4 1 4 3 3 3 3 6% 6% 62%	2940
July	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 77%	8219
	11th to 20th	Day	2 2 1 3 2 2 2 3 3 4 77%	8220
	21st to end	Day	1 1 1 2 2 2 3 2 3 4 78%	8922
August	1st to 10th	Day	3 3 1 4 3 3 3 3 4 7% 65%	6272
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 5% 76%	5702
	21st to end	Day	4 4 2 4 3 4 4 3 5% 6% 62%	5984
September	1st to 10th	Day	3 2 1 3 2 3 3 3 4 6% 70%	2400
	11th to 20th	Day	5% 4 2 5% 4 4 3 3 5% 6% 58%	2390
	21st to end	Day	8% 6% 2 4 3 3 3 3 4 6% 58%	2178
October	1st to 10th	Day	7% 6% 1 4 3 2 3 3 4 5% 62%	3280
	11th to 20th	Day	5% 7% 2 7% 4 5% 5% 4 6% 8% 47%	3261
	21st to end	Day	9% 13% 3 8% 5% 6% 5% 4 6% 7% 35%	3297
November	1st to 10th	Day	11% 14% 4 8% 4 4 4 4 5% 7% 33%	1854
	11th to 20th	Day	10% 13% 4 9% 7% 5% 6% 5% 7% 8% 27%	1759
	21st to end	Day	7% 14% 4 8% 6% 5% 5% 5% 5% 8% 34%	1680
December	1st to 10th	Day	6% 11% 3 6% 3 3 4 3 5% 6% 50%	2700
	11th to 20th	Day	9% 16% 4 10% 6% 6% 6% 5% 7% 9% 24%	3108
	21st to end	Day	10% 16% 5% 10% 6% 6% 5% 4 7% 7% 24%	4092
January	1st to 10th	Night	9% 16% 6% 11% 6% 6% 6% 5% 7% 7% 21%	8172
	11th to 20th	Night	8% 11% 4 6% 4 3 3 3 5% 5% 49%	8220
	21st to end	Night	7% 13% 3 8% 5% 5% 5% 3 6% 6% 39%	8840
February	1st to 10th	Night	7% 15% 6% 9% 6% 6% 5% 4 5% 6% 31%	2730
	11th to 20th	Night	7% 14% 6% 10% 5% 5% 5% 4 6% 6% 32%	2601
	21st to end	Night	7% 16% 6% 9% 7% 6% 5% 4 6% 7% 26%	2076
March	1st to 10th	Night	4 11% 4 7% 4 5% 6% 5% 8% 10% 37%	5496
	11th to 20th	Night	4 9% 4 9% 5% 5% 5% 4 7% 7% 40%	5213
	21st to end	Night	6% 15% 5% 10% 6% 6% 6% 5% 7% 8% 27%	5251
April	1st to 10th	Night	9% 14% 4 8% 5% 5% 4 3 6% 6% 36%	2100
	11th to 20th	Night	8% 12% 4 8% 6% 5% 5% 3 5% 6% 38%	2079
	21st to end	Night	8% 12% 3 6% 4 3 4 5% 6% 8% 40%	2026
May	1st to 10th	Night	5% 9% 3 5% 3 3 4 3 5% 7% 54%	4723
	11th to 20th	Night	8% 12% 2 4 3 3 3 3 4 5% 54%	4700
	21st to end	Night	10% 8% 1 3 2 2 3 3 5% 6% 56%	5181
June	1st to 10th	Night	10% 7% 2 4 4 3 3 4 6% 7% 48%	1680
	11th to 20th	Night	10% 8% 2 5% 3 3 3 3 4 6% 55%	1680
	21st to end	Night	5% 4 1 4 3 3 3 3 4 5% 65%	1680
July	1st to 10th	Night	1 1 1 2 2 2 2 2 4 4 79%	4740
	11th to 20th	Night	2 2 1 3 3 3 3 2 3 4 75%	4740
	21st to end	Night	3 2 1 2 2 2 2 2 4 5% 76%	5153
August	1st to 10th	Night	2 2 1 4 4 3 4 3 4 7% 64%	3741
	11th to 20th	Night	1 1 1 2 2 2 3 2 5% 5% 76%	4240
	21st to end	Night	3 3 1 4 2 3 3 3 4 5% 68%	5015
September	1st to 10th	Night	2 2 1 4 2 2 3 3 4 4 73%	2000
	11th to 20th	Night	6% 4 1 3 3 3 4 4 5% 6% 61%	2010
	21st to end	Night	7% 5% 2 5% 3 3 4 3 4 6% 58%	2220
October	1st to 10th	Night	5% 6% 2 4 3 2 2 2 3 5% 66%	3920
	11th to 20th	Night	7% 8% 2 6% 4 3 4 3 6% 7% 51%	3897
	21st to end	Night	7% 11% 3 7% 5% 6% 5% 4 6% 7% 40%	4506
November	1st to 10th	Night	11% 14% 3 6% 5% 5% 5% 4 6% 6% 33%	2665
	11th to 20th	Night	11% 12% 3 8% 5% 5% 5% 5% 7% 7% 32%	2859
	21st to end	Night	8% 14% 3 7% 5% 4 5% 4 7% 8% 35%	2940
December	1st to 10th	Night	5% 9% 3 5% 4 4 4 3 5% 6% 53%	4700
	11th to 20th	Night	10% 15% 5% 10% 6% 7% 6% 5% 7% 8% 22%	5316
	21st to end	Night	9% 15% 5% 10% 5% 5% 5% 4 6% 6% 29%	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	58% 11% 12% 13% 6%	4723
	11th to 20th	Day	37% 6% 9% 16% 22% 10%	4740
	21st to end	Day	39% 10% 9% 12% 22% 8%	5346
February	1st to 10th	Day	51% 11% 14% 16% 8% 1	1890
	11th to 20th	Day	50% 10% 12% 17% 9% 2	1913
	21st to end	Day	56% 10% 12% 12% 8% 3	1730
March	1st to 10th	Day	46% 11% 12% 19% 8% 4	4618
	11th to 20th	Day	44% 10% 9% 26% 11%	4992
	21st to end	Day	50% 14% 14% 16% 5%	5964
April	1st to 10th	Day	54% 8% 12% 17% 8% 1	2520
	11th to 20th	Day	48% 8% 12% 19% 11% 2	2492
	21st to end	Day	43% 7% 12% 24% 12% 1	2593
May	1st to 10th	Day	29% 7% 9% 31% 23% 2	8193
	11th to 20th	Day	33% 6% 11% 27% 21% 2	8163
	21st to end	Day	31% 8% 15% 31% 13% 2	8994
June	1st to 10th	Day	42% 6% 10% 28% 13% 1	2940
	11th to 20th	Day	32% 6% 11% 26% 20% 4	2940
	21st to end	Day	22% 5% 12% 27% 26% 9%	2940
July	1st to 10th	Day	12% 3 5% 21% 31% 29%	8151
	11th to 20th	Day	14% 2 5% 8% 29% 43%	8145
	21st to end	Day	12% 3 4 26% 34% 21%	8723
August	1st to 10th	Day	18% 3 6% 14% 33% 26%	4609
	11th to 20th	Day	13% 4 4 27% 38% 15%	4137
	21st to end	Day	17% 4 7% 28% 24% 20%	4311
September	1st to 10th	Day	15% 4 7% 24% 31% 18%	2400
	11th to 20th	Day	27% 5% 10% 22% 25% 12%	2390
	21st to end	Day	28% 5% 11% 23% 23% 10%	2178
October	1st to 10th	Day	27% 4 9% 18% 30% 11%	3280
	11th to 20th	Day	34% 6% 15% 25% 18% 2	3261
	21st to end	Day	46% 7% 9% 21% 16% 1	3297
November	1st to 10th	Day	50% 7% 13% 19% 9% 1	1854
	11th to 20th	Day	52% 11% 13% 16% 8%	1759
	21st to end	Day	47% 10% 12% 15% 14% 1	1680
December	1st to 10th	Day	35% 8% 10% 19% 23% 5%	2700
	11th to 20th	Day	55% 10% 15% 14% 5% 1	3108
	21st to end	Day	55% 11% 13% 15% 6%	4092
January	1st to 10th	Night	53% 12% 14% 15% 5%	8172
	11th to 20th	Night	35% 7% 8% 18% 21% 11%	8220
	21st to end	Night	43% 9% 9% 11% 18% 11%	8840
February	1st to 10th	Night	51% 11% 13% 14% 9% 1	2730
	11th to 20th	Night	49% 10% 13% 18% 9% 3	2603
	21st to end	Night	53% 10% 10% 15% 10% 2	2076
March	1st to 10th	Night	37% 10% 12% 24% 12% 5%	5496
	11th to 20th	Night	36% 9% 15% 28% 11% 1	5213
	21st to end	Night	46% 16% 13% 19% 5%	5251
April	1st to 10th	Night	46% 8% 18% 21% 7% 1	2100
	11th to 20th	Night	45% 8% 14% 22% 10% 1	2079
	21st to end	Night	39% 9% 15% 22% 13% 1	2026
May	1st to 10th	Night	26% 8% 11% 29% 26% 1	4721
	11th to 20th	Night	30% 6% 14% 35% 15% 1	4700
	21st to end	Night	26% 8% 20% 32% 14% 1	5182
June	1st to 10th	Night	34% 8% 14% 28% 15% 1	1680
	11th to 20th	Night	30% 5% 14% 29% 18% 4	1680
	21st to end	Night	21% 5% 13% 28% 26% 8%	1680
July	1st to 10th	Night	10% 3 5% 26% 29% 27%	4700
	11th to 20th	Night	15% 3 5% 11% 26% 39%	4692
	21st to end	Night	13% 3 3 27% 36% 18%	5040
August	1st to 10th	Night	18% 3 7% 16% 34% 21%	2735
	11th to 20th	Night	12% 4 6% 34% 34% 10%	3078
	21st to end	Night	14% 4 9% 28% 26% 19%	3608
September	1st to 10th	Night	15% 3 7% 26% 32% 17%	2000
	11th to 20th	Night	22% 6% 10% 24% 25% 13%	2010
	21st to end	Night	26% 5% 9% 25% 23% 12%	2220
October	1st to 10th	Night	23% 3 7% 22% 33% 11%	3920
	11th to 20th	Night	32% 6% 16% 22% 20% 3	3897
	21st to end	Night	40% 8% 9% 24% 18% 2	4506
November	1st to 10th	Night	47% 10% 12% 16% 12% 2	2665
	11th to 20th	Night	48% 11% 12% 18% 10% 1	2859
	21st to end	Night	44% 9% 13% 16% 17% 2	2940
December	1st to 10th	Night	32% 8% 11% 18% 27% 5%	4700
	11th to 20th	Night	57% 11% 11% 14% 6%	5316
	21st to end	Night	50% 12% 17% 15% 7% 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	24% 21% 24% 18% 11% 2	220
	11th to 20th	Day	17% 18% 22% 23% 16% 3	220
	21st to end	Day	12% 21% 27% 24% 13% 2	297
February	1st to 10th	Day	22% 21% 27% 21% 8% 1	430
	11th to 20th	Day	22% 20% 30% 18% 8% 1	456
	21st to end	Day	22% 20% 27% 25% 5% 1	446
March	1st to 10th	Day	17% 22% 32% 21% 7%	540
	11th to 20th	Day	18% 20% 31% 19% 10% 2	540
	21st to end	Day	19% 21% 29% 23% 7%	594
April	1st to 10th	Day	19% 19% 26% 21% 13% 3	540
	11th to 20th	Day	16% 21% 28% 21% 11% 3	532
	21st to end	Day	14% 18% 31% 21% 12% 3	540
May	1st to 10th	Day	12% 16% 31% 23% 14% 3	543
	11th to 20th	Day	21% 16% 26% 21% 12% 4	568
	21st to end	Day	15% 12% 24% 28% 13% 8%	617
June	1st to 10th	Day	14% 13% 25% 26% 16% 7%	570
	11th to 20th	Day	12% 15% 33% 25% 11% 4	570
	21st to end	Day	4 11% 28% 32% 18% 6%	570
July	1st to 10th	Day	3 8% 33% 35% 15% 6%	566
	11th to 20th	Day	2 11% 36% 28% 15% 8%	564
	21st to end	Day	2 11% 31% 31% 17% 8%	619
August	1st to 10th	Day	4 7% 29% 32% 18% 10%	567
	11th to 20th	Day	3 6% 25% 31% 23% 12%	534
	21st to end	Day	5% 9% 25% 31% 23% 7%	583
September	1st to 10th	Day	4 6% 28% 30% 27% 6%	530
	11th to 20th	Day	8% 8% 24% 32% 22% 6%	530
	21st to end	Day	10% 10% 24% 31% 20% 4	530
October	1st to 10th	Day	15% 12% 22% 28% 19% 4	530
	11th to 20th	Day	14% 13% 25% 25% 19% 4	530
	21st to end	Day	15% 16% 27% 21% 17% 4	423
November	1st to 10th	Day	17% 18% 27% 25% 13%	330
	11th to 20th	Day	21% 16% 25% 21% 16% 2	261
	21st to end	Day	16% 17% 23% 26% 14% 4	220
December	1st to 10th	Day	17% 21% 20% 21% 15% 6%	220
	11th to 20th	Day	25% 17% 34% 18% 5%	220
	21st to end	Day	21% 24% 22% 19% 11% 2	242
January	1st to 10th	Night	19% 17% 25% 22% 14% 3	840
	11th to 20th	Night	19% 14% 21% 25% 17% 4	840
	21st to end	Night	18% 10% 21% 31% 17% 3	924
February	1st to 10th	Night	17% 15% 22% 30% 15% 2	840
	11th to 20th	Night	15% 13% 25% 29% 17% 2	824
	21st to end	Night	16% 17% 27% 29% 9% 1	692
March	1st to 10th	Night	15% 15% 25% 31% 11% 2	840
	11th to 20th	Night	13% 14% 24% 32% 15% 2	840
	21st to end	Night	14% 14% 27% 32% 12% 2	924
April	1st to 10th	Night	16% 12% 22% 31% 17% 3	840
	11th to 20th	Night	14% 9% 24% 32% 17% 4	831
	21st to end	Night	14% 10% 20% 35% 18% 2	766
May	1st to 10th	Night	10% 9% 21% 39% 19% 3	420
	11th to 20th	Night	15% 6% 14% 38% 24% 4	414
	21st to end	Night	15% 7% 14% 37% 23% 4	455
June	1st to 10th	Night	12% 4 17% 40% 25% 2	420
	11th to 20th	Night	14% 7% 16% 44% 19% 1	420
	21st to end	Night	7% 2 13% 48% 26% 3	420
July	1st to 10th	Night	2 1 9% 55% 29% 3	410
	11th to 20th	Night	2 1 13% 56% 26% 2	408
	21st to end	Night	2 1 10% 51% 32% 4	460
August	1st to 10th	Night	2 8% 47% 39% 3	446
	11th to 20th	Night	2 1 8% 39% 40% 10%	667
	21st to end	Night	3 4 12% 45% 32% 5%	880
September	1st to 10th	Night	4 2 10% 41% 38% 5%	800
	11th to 20th	Night	7% 4 12% 37% 31% 11%	800
	21st to end	Night	10% 4 13% 35% 32% 7%	800
October	1st to 10th	Night	14% 4 16% 37% 26% 5%	800
	11th to 20th	Night	11% 9% 18% 35% 21% 6%	800
	21st to end	Night	14% 10% 21% 32% 17% 6%	880
November	1st to 10th	Night	18% 11% 19% 35% 16% 1	820
	11th to 20th	Night	19% 16% 21% 28% 14% 2	840
	21st to end	Night	17% 13% 20% 29% 16% 5%	840
December	1st to 10th	Night	17% 9% 23% 30% 18% 3	840
	11th to 20th	Night	19% 13% 25% 27% 14% 2	840
	21st to end	Night	22% 19% 28% 21% 10% 1	924

Page generated at 2026-08-12 00:37:19. Version ac07

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