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 2025-04-11.

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% 9% 3 7% 4 4 4 4 5% 6% 47%	117408
	Night	7% 11% 3 7% 4 4 4 4 5% 6% 45%	119673
Transparency	Day	36% 7% 10% 19% 17% 10%	117151
	Night	37% 9% 12% 20% 16% 7%	119519
Seeing	Day	13% 15% 27% 25% 15% 4	15020
	Night	13% 10% 19% 34% 21% 4	24266

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	12% 19% 7% 10% 6% 5% 5% 4 6% 5% 21%	3639
	11th to 20th	Day	8% 13% 5% 10% 5% 4 4 3 4 5% 38%	3640
	21st to end	Day	7% 11% 3 7% 4 5% 4 4 6% 6% 42%	4121
February	1st to 10th	Day	9% 15% 6% 9% 6% 5% 5% 6% 7% 7% 27%	1800
	11th to 20th	Day	9% 16% 4 8% 5% 5% 5% 4 5% 7% 31%	1811
	21st to end	Day	6% 17% 7% 10% 6% 5% 6% 5% 5% 6% 27%	1650
March	1st to 10th	Day	5% 11% 5% 10% 6% 5% 6% 5% 7% 9% 30%	4518
	11th to 20th	Day	4 14% 5% 9% 5% 6% 5% 4 5% 6% 37%	4885
	21st to end	Day	5% 15% 5% 11% 8% 7% 7% 6% 7% 7% 23%	5835
April	1st to 10th	Day	9% 16% 5% 10% 6% 5% 6% 4 6% 8% 26%	2400
	11th to 20th	Day	9% 14% 5% 8% 6% 5% 5% 4 5% 5% 34%	2251
	21st to end	Day	10% 13% 3 7% 4 5% 4 4 5% 7% 37%	2346
May	1st to 10th	Day	8% 12% 3 8% 5% 4 5% 3 4 5% 42%	4418
	11th to 20th	Day	7% 5% 2 5% 4 3 3 3 4 5% 60%	4377
	21st to end	Day	9% 12% 3 6% 4 3 4 4 5% 8% 41%	4813
June	1st to 10th	Day	12% 10% 2 7% 5% 5% 5% 4 5% 7% 38%	2660
	11th to 20th	Day	10% 7% 2 5% 4 3 4 3 7% 7% 48%	2660
	21st to end	Day	5% 4 1 4 3 3 3 3 6% 6% 63%	2660
July	1st to 10th	Day	1 1 1 1 1 2 2 2 3 4 82%	4420
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 80%	4420
	21st to end	Day	1 1 1 2 2 2 2 2 4 4 78%	4790
August	1st to 10th	Day	3 3 1 4 3 3 3 3 4 5% 68%	4349
	11th to 20th	Day	2 2 1 3 2 2 3 3 4 6% 73%	3918
	21st to end	Day	3 2 1 3 3 3 3 2 4 5% 71%	4180
September	1st to 10th	Day	3 2 1 2 2 2 2 3 4 6% 73%	2280
	11th to 20th	Day	5% 4 2 5% 4 4 3 3 5% 6% 58%	2271
	21st to end	Day	8% 7% 2 5% 3 3 3 3 4 6% 57%	2069
October	1st to 10th	Day	7% 6% 1 4 3 2 3 3 4 5% 62%	3180
	11th to 20th	Day	5% 7% 2 6% 4 5% 5% 5% 6% 8% 47%	3161
	21st to end	Day	9% 13% 3 8% 6% 6% 5% 4 6% 6% 34%	3195
November	1st to 10th	Day	12% 14% 4 8% 4 4 4 4 5% 7% 33%	1764
	11th to 20th	Day	10% 13% 4 9% 7% 5% 6% 5% 7% 7% 27%	1675
	21st to end	Day	7% 13% 4 8% 6% 5% 6% 5% 6% 7% 34%	1600
December	1st to 10th	Day	5% 11% 3 6% 3 3 4 3 5% 6% 52%	2620
	11th to 20th	Day	9% 17% 4 10% 6% 6% 6% 5% 7% 8% 23%	3028
	21st to end	Day	10% 16% 5% 10% 6% 6% 5% 4 7% 7% 24%	4004
January	1st to 10th	Night	10% 18% 6% 12% 7% 6% 6% 4 6% 6% 20%	6306
	11th to 20th	Night	10% 14% 5% 8% 4 4 4 3 5% 5% 39%	6320
	21st to end	Night	7% 13% 3 8% 5% 5% 5% 3 5% 5% 41%	6837
February	1st to 10th	Night	8% 15% 6% 10% 6% 6% 5% 4 5% 6% 30%	2600
	11th to 20th	Night	7% 15% 6% 9% 5% 4 5% 4 6% 6% 32%	2474
	21st to end	Night	7% 16% 6% 9% 7% 6% 5% 4 6% 7% 26%	1980
March	1st to 10th	Night	4 11% 3 7% 4 5% 6% 5% 8% 10% 37%	5376
	11th to 20th	Night	4 9% 4 9% 5% 5% 5% 5% 7% 7% 40%	5100
	21st to end	Night	6% 15% 5% 10% 6% 6% 6% 5% 7% 8% 27%	5138
April	1st to 10th	Night	9% 14% 4 8% 5% 5% 4 3 6% 6% 36%	2000
	11th to 20th	Night	8% 13% 4 9% 6% 5% 4 3 5% 6% 38%	1883
	21st to end	Night	8% 13% 3 6% 5% 3 4 5% 7% 8% 38%	1833
May	1st to 10th	Night	8% 15% 5% 8% 4 3 4 3 5% 5% 40%	2540
	11th to 20th	Night	7% 7% 2 4 3 2 2 3 4 5% 61%	2516
	21st to end	Night	11% 14% 2 4 2 3 3 3 4 5% 48%	2762
June	1st to 10th	Night	11% 7% 2 4 4 3 3 4 6% 8% 46%	1520
	11th to 20th	Night	10% 8% 2 5% 3 2 2 3 4 6% 55%	1520
	21st to end	Night	5% 3 1 4 3 3 3 4 4 5% 66%	1520
July	1st to 10th	Night	1 1 1 2 1 2 2 3 3 84%	2540
	11th to 20th	Night	1 1 2 3 2 3 2 3 4 79%	2540
	21st to end	Night	2 1 1 2 2 2 2 2 3 4 80%	2761
August	1st to 10th	Night	2 2 1 4 4 3 4 3 4 6% 67%	2580
	11th to 20th	Night	2 2 1 2 2 2 2 3 4 5% 75%	2922
	21st to end	Night	2 3 1 3 2 2 2 2 3 5% 75%	3498
September	1st to 10th	Night	2 2 1 3 2 2 2 2 4 4 76%	1900
	11th to 20th	Night	6% 4 1 3 3 3 4 4 5% 6% 62%	1909
	21st to end	Night	7% 5% 2 5% 3 3 4 3 4 6% 58%	2109
October	1st to 10th	Night	5% 6% 2 4 3 2 2 2 3 5% 66%	3800
	11th to 20th	Night	7% 8% 2 6% 4 3 4 3 6% 7% 51%	3777
	21st to end	Night	7% 12% 3 7% 5% 6% 4 4 6% 7% 40%	4366
November	1st to 10th	Night	11% 14% 3 6% 5% 5% 5% 4 6% 6% 34%	2535
	11th to 20th	Night	11% 12% 3 8% 5% 5% 5% 5% 7% 7% 32%	2723
	21st to end	Night	7% 14% 3 7% 5% 4 5% 4 7% 8% 35%	2800
December	1st to 10th	Night	5% 9% 3 5% 4 4 4 3 4 6% 53%	4560
	11th to 20th	Night	10% 15% 5% 10% 6% 7% 6% 5% 7% 7% 21%	5176
	21st to end	Night	9% 15% 5% 10% 5% 5% 5% 4 6% 6% 29%	6952

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	60% 12% 10% 11% 6%	3639
	11th to 20th	Day	47% 8% 11% 17% 15% 2	3640
	21st to end	Day	40% 10% 9% 14% 18% 8%	4121
February	1st to 10th	Day	52% 11% 14% 15% 7% 1	1800
	11th to 20th	Day	51% 10% 12% 16% 9% 2	1820
	21st to end	Day	56% 10% 13% 12% 7% 2	1650
March	1st to 10th	Day	46% 11% 12% 19% 9% 4	4518
	11th to 20th	Day	43% 11% 9% 26% 11%	4885
	21st to end	Day	50% 14% 14% 16% 5%	5835
April	1st to 10th	Day	55% 8% 13% 15% 8% 1	2400
	11th to 20th	Day	49% 8% 12% 17% 11% 2	2254
	21st to end	Day	45% 8% 12% 22% 11% 1	2346
May	1st to 10th	Day	40% 9% 11% 26% 12% 1	4418
	11th to 20th	Day	27% 4 9% 29% 27% 4	4380
	21st to end	Day	37% 11% 20% 23% 8% 1	4814
June	1st to 10th	Day	44% 7% 11% 27% 11% 1	2660
	11th to 20th	Day	34% 7% 12% 27% 17% 3	2660
	21st to end	Day	21% 5% 12% 27% 26% 9%	2660
July	1st to 10th	Day	9% 2 6% 12% 21% 50%	4351
	11th to 20th	Day	10% 2 6% 7% 23% 52%	4345
	21st to end	Day	11% 2 4 16% 39% 27%	4639
August	1st to 10th	Day	19% 3 6% 13% 33% 26%	4333
	11th to 20th	Day	13% 4 4 27% 37% 15%	3957
	21st to end	Day	16% 4 7% 28% 25% 20%	4179
September	1st to 10th	Day	14% 4 7% 25% 32% 19%	2280
	11th to 20th	Day	27% 5% 10% 22% 24% 12%	2271
	21st to end	Day	29% 6% 11% 23% 22% 10%	2069
October	1st to 10th	Day	27% 4 9% 18% 30% 12%	3180
	11th to 20th	Day	34% 6% 15% 25% 18% 2	3161
	21st to end	Day	47% 7% 9% 20% 17% 1	3195
November	1st to 10th	Day	50% 7% 13% 19% 10% 1	1764
	11th to 20th	Day	53% 11% 13% 16% 8%	1675
	21st to end	Day	47% 10% 12% 14% 15% 1	1600
December	1st to 10th	Day	33% 9% 10% 19% 24% 5%	2620
	11th to 20th	Day	56% 10% 15% 14% 5% 1	3028
	21st to end	Day	56% 11% 13% 15% 5%	4004
January	1st to 10th	Night	57% 13% 12% 12% 6%	6306
	11th to 20th	Night	44% 9% 10% 19% 17% 2	6320
	21st to end	Night	42% 10% 10% 11% 17% 10%	6837
February	1st to 10th	Night	52% 12% 14% 13% 9% 1	2600
	11th to 20th	Night	49% 10% 13% 17% 9% 3	2476
	21st to end	Night	54% 11% 10% 15% 9% 2	1980
March	1st to 10th	Night	37% 10% 12% 24% 12% 5%	5376
	11th to 20th	Night	36% 9% 15% 28% 11% 1	5100
	21st to end	Night	46% 17% 13% 19% 5%	5138
April	1st to 10th	Night	47% 8% 18% 20% 7% 1	2000
	11th to 20th	Night	46% 8% 14% 21% 9% 1	1883
	21st to end	Night	41% 10% 16% 21% 11% 2	1833
May	1st to 10th	Night	37% 11% 14% 26% 11%	2540
	11th to 20th	Night	21% 6% 13% 39% 19% 1	2516
	21st to end	Night	32% 11% 26% 25% 6%	2762
June	1st to 10th	Night	36% 8% 14% 28% 13% 1	1520
	11th to 20th	Night	31% 5% 15% 31% 15% 4	1520
	21st to end	Night	20% 5% 13% 29% 26% 8%	1520
July	1st to 10th	Night	7% 3 7% 14% 22% 47%	2500
	11th to 20th	Night	11% 2 6% 13% 22% 47%	2492
	21st to end	Night	10% 2 4 18% 37% 29%	2674
August	1st to 10th	Night	19% 3 7% 15% 35% 21%	2572
	11th to 20th	Night	12% 4 6% 34% 33% 10%	2949
	21st to end	Night	13% 4 10% 28% 26% 19%	3498
September	1st to 10th	Night	14% 3 7% 27% 33% 18%	1900
	11th to 20th	Night	22% 6% 10% 24% 25% 13%	1909
	21st to end	Night	27% 5% 9% 25% 22% 12%	2109
October	1st to 10th	Night	23% 3 7% 22% 34% 11%	3800
	11th to 20th	Night	32% 6% 16% 22% 20% 3	3777
	21st to end	Night	41% 8% 9% 23% 18% 2	4366
November	1st to 10th	Night	48% 10% 12% 15% 13% 2	2535
	11th to 20th	Night	48% 12% 12% 18% 10% 1	2723
	21st to end	Night	44% 9% 13% 15% 17% 2	2800
December	1st to 10th	Night	32% 7% 11% 18% 28% 5%	4560
	11th to 20th	Night	57% 11% 11% 14% 6%	5176
	21st to end	Night	50% 12% 17% 14% 7% 1	6952

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% 20% 25% 18% 13% 2	200
	11th to 20th	Day	19% 20% 23% 22% 15% 3	200
	21st to end	Day	13% 22% 26% 23% 14% 2	272
February	1st to 10th	Day	23% 22% 27% 21% 7% 1	400
	11th to 20th	Day	23% 20% 29% 18% 8% 1	423
	21st to end	Day	21% 19% 27% 26% 5% 1	414
March	1st to 10th	Day	17% 22% 33% 21% 7%	500
	11th to 20th	Day	18% 20% 29% 21% 11% 2	500
	21st to end	Day	19% 21% 29% 23% 7%	550
April	1st to 10th	Day	19% 20% 26% 20% 11% 3	500
	11th to 20th	Day	17% 21% 29% 19% 11% 3	453
	21st to end	Day	15% 20% 30% 18% 13% 3	460
May	1st to 10th	Day	13% 17% 30% 22% 14% 3	463
	11th to 20th	Day	20% 15% 25% 22% 13% 5%	488
	21st to end	Day	16% 11% 23% 28% 15% 7%	529
June	1st to 10th	Day	14% 12% 25% 26% 17% 6%	490
	11th to 20th	Day	14% 15% 32% 23% 12% 3	490
	21st to end	Day	4 10% 28% 33% 18% 6%	490
July	1st to 10th	Day	3 9% 32% 35% 15% 6%	486
	11th to 20th	Day	1 10% 37% 29% 15% 8%	484
	21st to end	Day	2 10% 32% 32% 18% 7%	531
August	1st to 10th	Day	4 7% 28% 33% 18% 11%	487
	11th to 20th	Day	3 6% 24% 32% 23% 12%	477
	21st to end	Day	5% 9% 25% 31% 23% 7%	539
September	1st to 10th	Day	4 7% 28% 29% 27% 6%	490
	11th to 20th	Day	8% 8% 25% 32% 22% 6%	490
	21st to end	Day	11% 10% 25% 31% 20% 3	490
October	1st to 10th	Day	14% 12% 21% 29% 20% 4	490
	11th to 20th	Day	14% 13% 23% 25% 21% 4	490
	21st to end	Day	15% 16% 27% 20% 17% 4	387
November	1st to 10th	Day	17% 18% 25% 25% 14%	300
	11th to 20th	Day	22% 15% 24% 22% 16% 2	237
	21st to end	Day	16% 17% 22% 28% 14% 4	200
December	1st to 10th	Day	15% 21% 20% 22% 16% 7%	200
	11th to 20th	Day	26% 17% 34% 17% 6% 1	200
	21st to end	Day	20% 25% 23% 19% 11% 2	220
January	1st to 10th	Night	19% 17% 24% 22% 14% 3	800
	11th to 20th	Night	20% 15% 20% 25% 17% 4	800
	21st to end	Night	19% 10% 20% 31% 17% 3	880
February	1st to 10th	Night	17% 15% 22% 30% 15% 2	800
	11th to 20th	Night	16% 13% 23% 29% 17% 2	784
	21st to end	Night	16% 17% 26% 31% 9% 1	660
March	1st to 10th	Night	15% 15% 25% 32% 12% 2	800
	11th to 20th	Night	13% 13% 23% 33% 16% 2	800
	21st to end	Night	14% 14% 26% 31% 13% 2	880
April	1st to 10th	Night	17% 12% 22% 30% 17% 3	800
	11th to 20th	Night	15% 10% 23% 31% 18% 4	753
	21st to end	Night	14% 11% 20% 34% 18% 3	693
May	1st to 10th	Night	11% 9% 21% 36% 20% 3	380
	11th to 20th	Night	15% 5% 11% 38% 27% 4	374
	21st to end	Night	17% 7% 14% 35% 24% 4	411
June	1st to 10th	Night	12% 4 16% 40% 25% 3	380
	11th to 20th	Night	15% 7% 16% 41% 20% 1	380
	21st to end	Night	6% 3 13% 48% 27% 3	380
July	1st to 10th	Night	2 2 9% 55% 29% 3	370
	11th to 20th	Night	1 1 12% 56% 27% 3	368
	21st to end	Night	2 1 9% 50% 32% 5%	416
August	1st to 10th	Night	2 8% 46% 40% 3	403
	11th to 20th	Night	3 1 8% 36% 42% 10%	622
	21st to end	Night	2 4 12% 44% 33% 5%	836
September	1st to 10th	Night	4 2 10% 40% 38% 5%	760
	11th to 20th	Night	7% 4 11% 36% 31% 11%	760
	21st to end	Night	10% 4 13% 35% 31% 7%	760
October	1st to 10th	Night	14% 3 15% 37% 26% 5%	760
	11th to 20th	Night	11% 9% 18% 35% 21% 7%	760
	21st to end	Night	15% 10% 20% 32% 18% 6%	836
November	1st to 10th	Night	18% 11% 18% 35% 16% 1	780
	11th to 20th	Night	20% 16% 20% 28% 15% 2	800
	21st to end	Night	16% 14% 20% 28% 17% 6%	800
December	1st to 10th	Night	16% 9% 22% 31% 19% 3	800
	11th to 20th	Night	19% 13% 24% 28% 14% 2	800
	21st to end	Night	22% 18% 27% 22% 10% 1	880

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