Millard Oakley STEM Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Millard Oakley STEM Center clear sky chart from 2005-11-06 to 2025-09-19.

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% 8% 3 7% 5% 4 5% 4 6% 7% 37%	125940
	Night	14% 8% 3 7% 4 4 4 3 5% 7% 40%	123106
Transparency	Day	44% 7% 11% 21% 15% 3	129652
	Night	42% 6% 11% 21% 16% 4	127464
Seeing	Day	19% 12% 24% 22% 16% 7%	15782
	Night	18% 10% 24% 27% 17% 3	25757

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	16% 14% 8% 12% 4 4 3 3 4 4 28%	4056
	11th to 20th	Day	18% 11% 6% 12% 3 2 3 2 3 4 36%	4060
	21st to end	Day	21% 10% 5% 6% 3 3 3 3 5% 6% 36%	4673
February	1st to 10th	Day	20% 12% 5% 8% 4 4 4 3 4 5% 33%	2000
	11th to 20th	Day	22% 11% 5% 7% 6% 3 4 3 4 5% 30%	1948
	21st to end	Day	21% 10% 3 7% 4 4 3 3 5% 6% 35%	1650
March	1st to 10th	Day	19% 11% 4 7% 5% 4 4 3 4 6% 34%	4480
	11th to 20th	Day	14% 8% 3 7% 4 4 4 3 4 5% 45%	4925
	21st to end	Day	14% 8% 2 7% 4 4 4 3 5% 6% 42%	5503
April	1st to 10th	Day	14% 7% 2 6% 4 4 4 3 6% 7% 42%	2400
	11th to 20th	Day	14% 8% 2 5% 4 4 5% 4 5% 6% 42%	2374
	21st to end	Day	15% 8% 3 6% 5% 5% 5% 4 5% 7% 38%	2527
May	1st to 10th	Day	21% 9% 3 7% 5% 4 5% 4 7% 8% 28%	5862
	11th to 20th	Day	16% 10% 3 9% 6% 5% 5% 4 6% 7% 29%	5861
	21st to end	Day	18% 7% 3 7% 5% 5% 4 4 5% 6% 36%	6433
June	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	2600
	11th to 20th	Day	7% 5% 2 5% 5% 7% 5% 5% 7% 10% 43%	2600
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 6% 8% 11% 39%	2600
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 39%	5900
	11th to 20th	Day	9% 6% 2 8% 5% 6% 7% 5% 9% 11% 32%	5900
	21st to end	Day	14% 8% 3 8% 6% 6% 6% 6% 7% 11% 24%	6424
August	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 5% 7% 11% 44%	4234
	11th to 20th	Day	7% 5% 2 6% 6% 6% 5% 6% 9% 10% 38%	3856
	21st to end	Day	7% 4 1 5% 4 5% 5% 5% 7% 8% 49%	4312
September	1st to 10th	Day	14% 5% 2 6% 6% 5% 5% 4 6% 6% 39%	2302
	11th to 20th	Day	13% 7% 3 7% 4 4 3 3 5% 8% 43%	2178
	21st to end	Day	14% 5% 2 5% 3 3 3 4 5% 7% 48%	2090
October	1st to 10th	Day	10% 5% 1 5% 3 4 3 3 4 6% 55%	2090
	11th to 20th	Day	14% 5% 2 5% 3 3 4 2 4 7% 52%	2028
	21st to end	Day	16% 6% 2 5% 3 3 3 3 6% 6% 46%	1943
November	1st to 10th	Day	12% 5% 2 5% 3 3 3 4 4 6% 53%	1762
	11th to 20th	Day	15% 8% 3 4 3 3 4 3 4 5% 47%	1800
	21st to end	Day	16% 9% 3 6% 4 3 3 3 5% 5% 43%	1800
December	1st to 10th	Day	16% 10% 3 6% 4 4 4 3 5% 5% 40%	2930
	11th to 20th	Day	18% 15% 5% 10% 5% 4 4 4 4 4 27%	3373
	21st to end	Day	19% 13% 5% 9% 4 4 5% 4 5% 7% 26%	4466
January	1st to 10th	Night	17% 13% 5% 9% 4 3 3 2 4 5% 35%	5889
	11th to 20th	Night	19% 12% 3 6% 5% 5% 3 2 3 3 38%	5900
	21st to end	Night	19% 11% 4 7% 4 3 3 2 4 5% 39%	6279
February	1st to 10th	Night	22% 9% 4 8% 5% 4 4 4 5% 5% 31%	2400
	11th to 20th	Night	24% 9% 5% 8% 4 4 3 2 3 5% 32%	2338
	21st to end	Night	23% 12% 3 7% 3 4 3 3 5% 6% 31%	1980
March	1st to 10th	Night	19% 13% 3 6% 4 4 4 3 5% 6% 34%	5373
	11th to 20th	Night	14% 6% 2 4 3 4 3 3 5% 6% 49%	5034
	21st to end	Night	15% 8% 3 7% 5% 4 5% 3 5% 6% 39%	5467
April	1st to 10th	Night	14% 6% 2 6% 5% 5% 5% 3 5% 8% 42%	2000
	11th to 20th	Night	16% 7% 2 4 4 2 4 3 5% 6% 47%	1975
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 7% 42%	1870
May	1st to 10th	Night	17% 9% 3 8% 6% 5% 5% 4 6% 8% 29%	4033
	11th to 20th	Night	15% 9% 3 9% 6% 6% 6% 4 6% 7% 29%	4032
	21st to end	Night	14% 7% 2 7% 5% 5% 5% 4 7% 8% 37%	4421
June	1st to 10th	Night	7% 6% 2 7% 5% 5% 5% 4 6% 9% 44%	1800
	11th to 20th	Night	5% 4 1 5% 4 4 5% 4 8% 11% 50%	1800
	21st to end	Night	5% 5% 1 5% 5% 5% 5% 6% 8% 12% 42%	1750
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 5% 8% 10% 41%	4054
	11th to 20th	Night	9% 6% 2 6% 5% 5% 5% 5% 8% 12% 38%	4057
	21st to end	Night	9% 7% 4 10% 7% 6% 7% 5% 7% 11% 27%	4427
August	1st to 10th	Night	6% 4 2 6% 4 4 5% 3 6% 10% 52%	2915
	11th to 20th	Night	7% 4 2 6% 4 5% 6% 4 7% 10% 45%	3191
	21st to end	Night	5% 3 1 3 3 4 4 4 5% 9% 58%	3586
September	1st to 10th	Night	11% 6% 3 6% 4 3 5% 3 5% 7% 46%	2076
	11th to 20th	Night	11% 6% 2 7% 4 4 5% 3 5% 7% 46%	2179
	21st to end	Night	12% 5% 2 4 3 3 4 3 5% 7% 52%	2090
October	1st to 10th	Night	10% 5% 2 4 2 2 2 3 4 6% 60%	2090
	11th to 20th	Night	17% 5% 1 5% 3 3 3 3 4 5% 51%	2152
	21st to end	Night	16% 6% 2 5% 3 3 3 3 4 6% 49%	2650
November	1st to 10th	Night	12% 7% 3 4 3 3 4 3 4 6% 51%	2536
	11th to 20th	Night	14% 9% 2 4 3 3 3 3 4 7% 47%	2600
	21st to end	Night	17% 10% 3 5% 3 3 3 3 4 6% 43%	2600
December	1st to 10th	Night	20% 12% 3 8% 4 4 4 2 3 4 36%	4250
	11th to 20th	Night	22% 11% 4 8% 4 3 3 2 3 5% 34%	4822
	21st to end	Night	15% 13% 5% 9% 5% 6% 5% 4 5% 6% 27%	6490

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	59% 4 9% 15% 12% 1	4057
	11th to 20th	Day	54% 3 5% 16% 20% 2	4060
	21st to end	Day	50% 4 8% 22% 14% 3	4673
February	1st to 10th	Day	55% 5% 7% 16% 15% 2	2000
	11th to 20th	Day	58% 4 7% 16% 13% 3	1958
	21st to end	Day	51% 4 10% 14% 16% 5%	1650
March	1st to 10th	Day	52% 4 6% 13% 16% 8%	4480
	11th to 20th	Day	42% 4 8% 20% 19% 8%	4925
	21st to end	Day	41% 6% 10% 27% 15%	5505
April	1st to 10th	Day	41% 7% 11% 22% 16% 3	2400
	11th to 20th	Day	41% 6% 7% 21% 22% 3	2374
	21st to end	Day	44% 7% 10% 25% 13% 1	2527
May	1st to 10th	Day	51% 9% 12% 19% 8% 1	5897
	11th to 20th	Day	53% 7% 13% 18% 8%	5865
	21st to end	Day	47% 8% 10% 19% 14% 2	6447
June	1st to 10th	Day	40% 9% 14% 21% 14% 2	2600
	11th to 20th	Day	33% 10% 18% 21% 16% 2	2600
	21st to end	Day	35% 12% 17% 18% 15% 3	2600
July	1st to 10th	Day	36% 10% 18% 22% 13% 1	5835
	11th to 20th	Day	42% 11% 19% 22% 5% 1	5831
	21st to end	Day	50% 10% 17% 19% 3 1	6240
August	1st to 10th	Day	32% 9% 15% 29% 13% 1	4221
	11th to 20th	Day	36% 8% 18% 24% 14% 1	3892
	21st to end	Day	30% 8% 12% 34% 15% 1	4311
September	1st to 10th	Day	42% 9% 11% 17% 16% 4	2302
	11th to 20th	Day	40% 7% 13% 21% 17% 2	2178
	21st to end	Day	36% 7% 11% 23% 19% 4	2090
October	1st to 10th	Day	24% 4 7% 18% 36% 10%	3480
	11th to 20th	Day	24% 4 9% 22% 31% 11%	3346
	21st to end	Day	30% 4 9% 25% 23% 9%	3180
November	1st to 10th	Day	33% 4 9% 23% 27% 4	1762
	11th to 20th	Day	39% 4 7% 22% 23% 5%	1800
	21st to end	Day	43% 5% 7% 21% 20% 4	1800
December	1st to 10th	Day	47% 4 5% 21% 21% 1	2930
	11th to 20th	Day	60% 4 7% 17% 10% 2	3370
	21st to end	Day	55% 5% 11% 19% 8% 2	4466
January	1st to 10th	Night	52% 4 10% 18% 15% 1	5889
	11th to 20th	Night	53% 4 5% 15% 19% 5%	5900
	21st to end	Night	50% 3 6% 18% 18% 5%	6279
February	1st to 10th	Night	54% 4 9% 17% 14% 1	2400
	11th to 20th	Night	57% 4 6% 17% 12% 4	2339
	21st to end	Night	54% 5% 6% 15% 16% 4	1980
March	1st to 10th	Night	52% 5% 8% 14% 17% 5%	5373
	11th to 20th	Night	35% 5% 8% 21% 20% 11%	5034
	21st to end	Night	45% 6% 9% 26% 13% 2	5467
April	1st to 10th	Night	39% 6% 10% 22% 19% 3	2000
	11th to 20th	Night	37% 6% 9% 21% 22% 5%	1975
	21st to end	Night	40% 6% 11% 28% 14% 1	1870
May	1st to 10th	Night	51% 8% 14% 19% 8% 1	4060
	11th to 20th	Night	53% 8% 14% 15% 9%	4033
	21st to end	Night	42% 10% 15% 20% 12% 1	4429
June	1st to 10th	Night	35% 9% 13% 28% 13% 2	1800
	11th to 20th	Night	26% 10% 20% 26% 17% 2	1800
	21st to end	Night	30% 12% 17% 24% 13% 3	1750
July	1st to 10th	Night	35% 10% 19% 22% 13% 1	4011
	11th to 20th	Night	36% 10% 21% 25% 6% 2	4006
	21st to end	Night	48% 11% 18% 17% 5% 1	4297
August	1st to 10th	Night	28% 6% 15% 35% 16% 1	2904
	11th to 20th	Night	32% 8% 20% 27% 12% 1	3221
	21st to end	Night	22% 7% 13% 38% 19% 2	3586
September	1st to 10th	Night	36% 8% 13% 20% 18% 5%	2076
	11th to 20th	Night	37% 7% 11% 22% 22% 1	2179
	21st to end	Night	32% 7% 11% 23% 23% 5%	2090
October	1st to 10th	Night	23% 3 7% 22% 34% 11%	3480
	11th to 20th	Night	23% 3 6% 24% 28% 15%	3573
	21st to end	Night	25% 3 8% 24% 29% 12%	4363
November	1st to 10th	Night	35% 4 7% 23% 24% 7%	2536
	11th to 20th	Night	37% 4 8% 20% 24% 7%	2600
	21st to end	Night	44% 4 7% 23% 19% 4	2600
December	1st to 10th	Night	53% 3 5% 15% 22% 1	4250
	11th to 20th	Night	53% 4 6% 19% 15% 2	4824
	21st to end	Night	55% 6% 11% 18% 9% 1	6490

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	25% 27% 35% 9% 3 1	300
	11th to 20th	Day	37% 26% 25% 9% 3	300
	21st to end	Day	26% 25% 39% 7% 2	330
February	1st to 10th	Day	31% 27% 26% 14% 2	300
	11th to 20th	Day	38% 26% 30% 4 2	300
	21st to end	Day	36% 26% 30% 6% 1	249
March	1st to 10th	Day	30% 23% 30% 15% 2	300
	11th to 20th	Day	27% 22% 32% 15% 4 1	471
	21st to end	Day	29% 19% 33% 15% 5%	550
April	1st to 10th	Day	25% 21% 27% 18% 8% 1	500
	11th to 20th	Day	21% 17% 33% 20% 8% 1	492
	21st to end	Day	24% 12% 32% 21% 10% 1	500
May	1st to 10th	Day	23% 14% 31% 20% 8% 4	503
	11th to 20th	Day	16% 11% 30% 24% 12% 7%	527
	21st to end	Day	16% 9% 24% 23% 17% 10%	573
June	1st to 10th	Day	13% 6% 23% 27% 19% 11%	530
	11th to 20th	Day	7% 5% 18% 32% 26% 12%	530
	21st to end	Day	10% 4 20% 28% 22% 15%	530
July	1st to 10th	Day	13% 3 13% 29% 27% 16%	525
	11th to 20th	Day	13% 3 13% 25% 30% 16%	524
	21st to end	Day	14% 3 11% 27% 30% 16%	576
August	1st to 10th	Day	12% 2 17% 26% 31% 12%	526
	11th to 20th	Day	11% 3 17% 33% 28% 9%	519
	21st to end	Day	12% 2 12% 27% 30% 16%	583
September	1st to 10th	Day	14% 3 15% 26% 31% 12%	530
	11th to 20th	Day	14% 4 13% 27% 29% 13%	522
	21st to end	Day	16% 5% 19% 29% 24% 7%	490
October	1st to 10th	Day	12% 5% 22% 32% 21% 8%	490
	11th to 20th	Day	15% 12% 29% 24% 17% 3	490
	21st to end	Day	22% 11% 29% 26% 10% 2	392
November	1st to 10th	Day	20% 10% 35% 25% 8% 2	300
	11th to 20th	Day	19% 18% 34% 24% 4	300
	21st to end	Day	22% 19% 34% 15% 9% 1	300
December	1st to 10th	Day	31% 21% 31% 13% 4	300
	11th to 20th	Day	31% 21% 27% 15% 6%	300
	21st to end	Day	30% 21% 31% 14% 4	330
January	1st to 10th	Night	26% 19% 32% 21% 4	800
	11th to 20th	Night	29% 21% 29% 17% 4	800
	21st to end	Night	23% 18% 38% 20% 2	880
February	1st to 10th	Night	26% 17% 33% 21% 3	800
	11th to 20th	Night	31% 19% 34% 15% 1	783
	21st to end	Night	30% 21% 31% 16% 1	660
March	1st to 10th	Night	24% 17% 33% 23% 3	800
	11th to 20th	Night	26% 17% 31% 20% 6% 1	800
	21st to end	Night	23% 16% 31% 23% 7%	880
April	1st to 10th	Night	18% 14% 32% 28% 8%	600
	11th to 20th	Night	20% 11% 34% 25% 10%	593
	21st to end	Night	17% 10% 32% 31% 10% 1	600
May	1st to 10th	Night	16% 9% 31% 28% 14% 2	600
	11th to 20th	Night	14% 7% 23% 34% 19% 3	591
	21st to end	Night	9% 3 22% 35% 26% 5%	650
June	1st to 10th	Night	9% 5% 16% 38% 29% 4	600
	11th to 20th	Night	7% 1 15% 38% 34% 7%	600
	21st to end	Night	6% 1 14% 39% 34% 6%	600
July	1st to 10th	Night	10% 2 10% 29% 39% 10%	585
	11th to 20th	Night	10% 3 9% 27% 43% 8%	582
	21st to end	Night	7% 2 9% 33% 40% 8%	657
August	1st to 10th	Night	10% 2 7% 31% 44% 7%	597
	11th to 20th	Night	8% 1 10% 39% 37% 5%	594
	21st to end	Night	8% 2 7% 33% 40% 8%	660
September	1st to 10th	Night	10% 3 11% 35% 37% 5%	676
	11th to 20th	Night	12% 3 13% 32% 34% 6%	793
	21st to end	Night	14% 2 19% 32% 29% 5%	760
October	1st to 10th	Night	10% 6% 21% 30% 28% 6%	760
	11th to 20th	Night	19% 8% 22% 33% 16% 2	760
	21st to end	Night	19% 9% 27% 28% 15% 2	836
November	1st to 10th	Night	15% 10% 24% 32% 16% 2	780
	11th to 20th	Night	18% 14% 32% 25% 10% 1	800
	21st to end	Night	23% 13% 28% 27% 9% 1	800
December	1st to 10th	Night	29% 16% 27% 23% 5%	800
	11th to 20th	Night	25% 14% 29% 25% 6% 1	800
	21st to end	Night	26% 19% 28% 23% 4	880

Page generated at 2025-09-16 00:49:11. Version ac07

Copyright © 2025 Attilla Danko.