Michael L. Britton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Michael L. Britton Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	127021
	Night	14% 7% 2 7% 5% 4 5% 4 6% 7% 39%	123340
Transparency	Day	43% 5% 12% 23% 15% 2	130615
	Night	43% 4 12% 24% 16% 2	127815
Seeing	Day	22% 15% 27% 20% 12% 4	16204
	Night	20% 14% 28% 27% 11% 1	25612

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	12% 7% 3 10% 7% 5% 4 4 4 5% 38%	3597
	11th to 20th	Day	11% 10% 4 11% 6% 6% 6% 4 6% 6% 32%	4014
	21st to end	Day	14% 9% 3 9% 5% 5% 5% 4 5% 6% 35%	4466
February	1st to 10th	Day	15% 11% 3 7% 5% 6% 5% 4 5% 6% 35%	1800
	11th to 20th	Day	15% 8% 2 7% 6% 5% 4 4 5% 7% 36%	1783
	21st to end	Day	17% 10% 3 7% 5% 4 4 4 4 7% 35%	1650
March	1st to 10th	Day	18% 6% 2 6% 4 5% 4 4 5% 6% 40%	4815
	11th to 20th	Day	9% 7% 3 7% 5% 4 5% 4 6% 8% 42%	4980
	21st to end	Day	14% 8% 3 6% 4 5% 5% 4 5% 6% 40%	5574
April	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 4 5% 7% 36%	2400
	11th to 20th	Day	14% 8% 2 7% 4 5% 5% 4 6% 8% 40%	2374
	21st to end	Day	18% 6% 2 6% 4 4 4 4 5% 7% 39%	2401
May	1st to 10th	Day	24% 6% 2 6% 5% 4 5% 4 6% 8% 30%	5862
	11th to 20th	Day	24% 6% 2 7% 5% 4 5% 4 6% 7% 30%	6290
	21st to end	Day	17% 5% 2 7% 4 4 5% 5% 7% 9% 35%	6934
June	1st to 10th	Day	13% 6% 3 6% 4 4 5% 4 7% 9% 40%	2800
	11th to 20th	Day	14% 5% 2 6% 6% 5% 4 4 7% 9% 38%	2800
	21st to end	Day	10% 4 2 5% 4 5% 6% 4 7% 11% 43%	2800
July	1st to 10th	Day	5% 5% 2 7% 5% 6% 7% 5% 7% 10% 41%	6360
	11th to 20th	Day	6% 6% 3 8% 6% 6% 7% 6% 9% 12% 32%	6360
	21st to end	Day	9% 6% 3 7% 4 5% 5% 4 6% 9% 41%	6425
August	1st to 10th	Day	16% 5% 2 6% 5% 5% 6% 5% 6% 9% 34%	4234
	11th to 20th	Day	10% 6% 3 7% 4 5% 5% 5% 7% 8% 40%	4047
	21st to end	Day	11% 5% 2 6% 3 4 4 3 6% 8% 48%	4312
September	1st to 10th	Day	15% 5% 2 5% 3 4 4 3 6% 7% 46%	2400
	11th to 20th	Day	13% 5% 2 4 3 3 4 4 5% 8% 49%	2400
	21st to end	Day	21% 8% 2 6% 4 4 3 3 5% 6% 38%	2101
October	1st to 10th	Day	16% 5% 1 5% 4 4 4 3 5% 6% 46%	2000
	11th to 20th	Day	16% 5% 2 6% 4 4 4 3 5% 7% 44%	2000
	21st to end	Day	14% 6% 3 6% 5% 4 4 4 5% 7% 43%	2200
November	1st to 10th	Day	11% 6% 2 4 3 3 3 3 6% 8% 50%	1905
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 3 5% 7% 42%	1787
	21st to end	Day	16% 6% 3 5% 5% 4 4 5% 6% 7% 40%	1600
December	1st to 10th	Day	14% 6% 2 8% 6% 5% 6% 4 7% 7% 36%	2600
	11th to 20th	Day	19% 11% 3 5% 3 5% 4 4 6% 6% 34%	2990
	21st to end	Day	15% 9% 4 10% 6% 5% 6% 5% 6% 7% 27%	3960
January	1st to 10th	Night	11% 9% 3 10% 6% 5% 5% 4 5% 7% 36%	6349
	11th to 20th	Night	12% 8% 2 7% 5% 5% 5% 5% 7% 7% 37%	5930
	21st to end	Night	16% 8% 2 7% 4 4 4 4 6% 7% 38%	6490
February	1st to 10th	Night	16% 8% 2 6% 5% 4 5% 4 5% 7% 36%	2600
	11th to 20th	Night	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	2501
	21st to end	Night	15% 9% 3 8% 4 4 5% 4 5% 6% 37%	1980
March	1st to 10th	Night	16% 8% 2 7% 5% 4 4 3 5% 6% 42%	5035
	11th to 20th	Night	9% 6% 3 6% 5% 4 4 3 5% 7% 49%	4980
	21st to end	Night	12% 8% 2 7% 4 5% 5% 4 5% 6% 42%	5358
April	1st to 10th	Night	17% 9% 3 7% 6% 4 5% 3 6% 6% 34%	2000
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 5% 7% 42%	1975
	21st to end	Night	17% 6% 2 6% 3 3 4 4 6% 7% 43%	2000
May	1st to 10th	Night	25% 8% 3 8% 4 5% 4 4 6% 7% 28%	4061
	11th to 20th	Night	24% 8% 2 5% 4 4 4 4 5% 8% 32%	3587
	21st to end	Night	18% 7% 3 6% 4 4 4 3 6% 8% 36%	3920
June	1st to 10th	Night	14% 6% 2 6% 3 4 5% 4 6% 8% 45%	1600
	11th to 20th	Night	13% 4 2 6% 6% 4 5% 4 7% 9% 39%	1600
	21st to end	Night	10% 5% 2 6% 4 5% 4 4 7% 10% 44%	1600
July	1st to 10th	Night	7% 5% 3 6% 6% 6% 5% 4 6% 10% 41%	3598
	11th to 20th	Night	7% 7% 3 9% 6% 7% 6% 5% 9% 10% 31%	3597
	21st to end	Night	9% 6% 2 7% 4 5% 5% 4 6% 8% 44%	4421
August	1st to 10th	Night	17% 5% 2 7% 4 5% 5% 4 6% 8% 37%	2915
	11th to 20th	Night	10% 7% 3 7% 5% 5% 4 3 6% 9% 41%	2969
	21st to end	Night	12% 6% 2 5% 4 3 5% 4 5% 7% 46%	3586
September	1st to 10th	Night	14% 5% 2 5% 4 4 4 3 5% 7% 48%	2000
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 5% 8% 51%	2000
	21st to end	Night	19% 7% 2 6% 4 4 4 4 5% 7% 38%	2292
October	1st to 10th	Night	18% 6% 2 5% 3 3 4 3 5% 5% 47%	2400
	11th to 20th	Night	15% 5% 2 5% 4 3 4 3 5% 7% 48%	2400
	21st to end	Night	16% 6% 2 5% 3 4 4 3 5% 7% 45%	2640
November	1st to 10th	Night	10% 6% 2 5% 3 4 4 3 5% 6% 52%	2612
	11th to 20th	Night	14% 8% 2 5% 4 4 4 4 5% 7% 43%	2763
	21st to end	Night	14% 6% 2 6% 5% 5% 5% 3 6% 8% 41%	2800
December	1st to 10th	Night	11% 7% 3 8% 5% 5% 6% 4 6% 7% 40%	4580
	11th to 20th	Night	19% 8% 2 6% 4 4 4 5% 6% 7% 36%	5205
	21st to end	Night	16% 10% 4 9% 6% 5% 6% 4 6% 6% 29%	6996

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	49% 3 10% 23% 14% 2	3598
	11th to 20th	Day	51% 3 10% 17% 16% 4	4014
	21st to end	Day	49% 4 11% 22% 13% 1	4466
February	1st to 10th	Day	51% 3 11% 23% 10% 2	1800
	11th to 20th	Day	49% 4 10% 23% 12% 1	1792
	21st to end	Day	50% 3 10% 22% 11% 3	1650
March	1st to 10th	Day	45% 3 9% 23% 14% 6%	4815
	11th to 20th	Day	39% 4 11% 30% 13% 5%	4980
	21st to end	Day	45% 3 10% 19% 18% 4	5574
April	1st to 10th	Day	48% 3 11% 19% 15% 4	2400
	11th to 20th	Day	43% 4 11% 21% 17% 4	2374
	21st to end	Day	44% 4 8% 25% 16% 3	2401
May	1st to 10th	Day	51% 5% 13% 19% 11% 1	5897
	11th to 20th	Day	53% 4 10% 22% 10% 1	6294
	21st to end	Day	43% 5% 13% 22% 15% 1	6949
June	1st to 10th	Day	39% 6% 13% 24% 16% 1	2800
	11th to 20th	Day	41% 5% 15% 24% 13% 2	2800
	21st to end	Day	35% 6% 13% 29% 16% 1	2800
July	1st to 10th	Day	36% 9% 12% 24% 16% 2	6290
	11th to 20th	Day	40% 9% 17% 22% 11% 1	6285
	21st to end	Day	38% 6% 11% 22% 22% 1	6240
August	1st to 10th	Day	45% 7% 13% 25% 9%	4221
	11th to 20th	Day	39% 7% 12% 27% 15% 1	4086
	21st to end	Day	34% 5% 11% 30% 18% 2	4312
September	1st to 10th	Day	38% 4 12% 21% 21% 3	2400
	11th to 20th	Day	33% 4 9% 28% 21% 3	2400
	21st to end	Day	48% 3 9% 18% 19% 3	2101
October	1st to 10th	Day	36% 4 8% 25% 22% 5%	3260
	11th to 20th	Day	30% 4 13% 27% 22% 4	3240
	21st to end	Day	27% 3 12% 31% 25% 2	3538
November	1st to 10th	Day	33% 4 14% 25% 21% 4	1905
	11th to 20th	Day	42% 3 11% 24% 18% 3	1787
	21st to end	Day	43% 4 13% 29% 10% 2	1600
December	1st to 10th	Day	45% 5% 16% 25% 7% 2	2600
	11th to 20th	Day	50% 3 14% 25% 7% 1	2986
	21st to end	Day	54% 4 15% 15% 9% 1	3960
January	1st to 10th	Night	48% 3 11% 23% 14% 2	6349
	11th to 20th	Night	43% 3 14% 22% 13% 5%	5930
	21st to end	Night	45% 3 11% 24% 14% 2	6490
February	1st to 10th	Night	47% 4 14% 24% 10% 1	2600
	11th to 20th	Night	47% 4 9% 21% 17% 2	2503
	21st to end	Night	48% 3 10% 22% 14% 3	1980
March	1st to 10th	Night	44% 2 10% 26% 12% 6%	5035
	11th to 20th	Night	36% 3 13% 31% 12% 5%	4980
	21st to end	Night	42% 3 10% 19% 23% 3	5358
April	1st to 10th	Night	51% 3 8% 19% 15% 3	2000
	11th to 20th	Night	41% 5% 11% 22% 19% 3	1975
	21st to end	Night	40% 5% 9% 24% 17% 4	2000
May	1st to 10th	Night	56% 4 9% 16% 11% 4	4088
	11th to 20th	Night	50% 5% 12% 22% 11% 1	3588
	21st to end	Night	47% 4 11% 23% 13% 2	3927
June	1st to 10th	Night	36% 6% 12% 25% 18% 3	1600
	11th to 20th	Night	40% 6% 13% 22% 17% 2	1600
	21st to end	Night	35% 6% 14% 26% 18% 1	1600
July	1st to 10th	Night	38% 7% 15% 20% 16% 4	3560
	11th to 20th	Night	44% 9% 15% 19% 12% 1	3552
	21st to end	Night	36% 6% 13% 22% 22% 1	4292
August	1st to 10th	Night	45% 6% 15% 23% 10% 1	2904
	11th to 20th	Night	39% 5% 11% 26% 17% 1	2996
	21st to end	Night	35% 5% 12% 30% 16% 2	3586
September	1st to 10th	Night	37% 4 12% 23% 21% 4	2000
	11th to 20th	Night	32% 4 9% 30% 23% 3	2000
	21st to end	Night	45% 6% 8% 20% 18% 2	2292
October	1st to 10th	Night	40% 4 10% 23% 22% 2	3920
	11th to 20th	Night	29% 4 10% 28% 25% 4	3899
	21st to end	Night	28% 3 11% 32% 24% 2	4252
November	1st to 10th	Night	34% 4 11% 26% 22% 4	2612
	11th to 20th	Night	40% 3 10% 25% 17% 4	2763
	21st to end	Night	42% 3 12% 27% 14% 2	2800
December	1st to 10th	Night	43% 5% 14% 29% 9% 1	4580
	11th to 20th	Night	47% 4 15% 22% 12% 2	5208
	21st to end	Night	55% 4 11% 19% 11%	6996

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	36% 24% 27% 10% 4	200
	11th to 20th	Day	37% 22% 30% 9% 2	290
	21st to end	Day	30% 27% 31% 9% 3 1	330
February	1st to 10th	Day	31% 28% 29% 11% 2	300
	11th to 20th	Day	40% 24% 31% 4 1	330
	21st to end	Day	33% 22% 30% 13% 2	414
March	1st to 10th	Day	31% 21% 27% 19% 3	500
	11th to 20th	Day	31% 23% 27% 14% 5% 1	500
	21st to end	Day	28% 24% 31% 13% 4 1	550
April	1st to 10th	Day	30% 22% 30% 12% 5% 1	500
	11th to 20th	Day	22% 22% 30% 16% 8% 2	492
	21st to end	Day	26% 19% 31% 15% 7% 2	500
May	1st to 10th	Day	32% 15% 28% 15% 8% 2	503
	11th to 20th	Day	26% 16% 28% 17% 10% 2	527
	21st to end	Day	19% 10% 27% 24% 15% 5%	573
June	1st to 10th	Day	17% 15% 29% 19% 15% 5%	530
	11th to 20th	Day	19% 11% 29% 23% 14% 4	530
	21st to end	Day	11% 10% 31% 27% 16% 4	530
July	1st to 10th	Day	10% 5% 24% 31% 21% 8%	525
	11th to 20th	Day	7% 7% 25% 29% 24% 8%	524
	21st to end	Day	11% 6% 25% 32% 20% 6%	576
August	1st to 10th	Day	11% 5% 23% 30% 25% 7%	526
	11th to 20th	Day	14% 6% 22% 31% 20% 8%	519
	21st to end	Day	12% 5% 23% 32% 19% 9%	583
September	1st to 10th	Day	17% 7% 23% 25% 19% 9%	530
	11th to 20th	Day	15% 5% 20% 27% 24% 9%	530
	21st to end	Day	29% 10% 23% 21% 13% 4	530
October	1st to 10th	Day	21% 8% 23% 23% 17% 7%	530
	11th to 20th	Day	20% 17% 29% 21% 11% 2	530
	21st to end	Day	18% 16% 37% 18% 10% 1	583
November	1st to 10th	Day	20% 15% 29% 22% 12% 2	454
	11th to 20th	Day	25% 20% 28% 22% 6%	345
	21st to end	Day	27% 22% 32% 15% 5%	200
December	1st to 10th	Day	29% 24% 29% 14% 4 1	200
	11th to 20th	Day	32% 22% 31% 13% 3	200
	21st to end	Day	28% 19% 30% 16% 5% 1	220
January	1st to 10th	Night	22% 25% 34% 18% 2	800
	11th to 20th	Night	24% 18% 33% 22% 3	800
	21st to end	Night	20% 24% 34% 21% 1	880
February	1st to 10th	Night	23% 20% 30% 25% 2	800
	11th to 20th	Night	27% 25% 32% 14% 1	783
	21st to end	Night	27% 23% 32% 16% 2	660
March	1st to 10th	Night	23% 19% 36% 19% 3	800
	11th to 20th	Night	26% 20% 30% 18% 5%	800
	21st to end	Night	22% 18% 37% 18% 5%	880
April	1st to 10th	Night	26% 21% 32% 17% 4	800
	11th to 20th	Night	20% 17% 32% 23% 7% 1	791
	21st to end	Night	22% 16% 29% 26% 7%	800
May	1st to 10th	Night	23% 12% 28% 28% 9%	615
	11th to 20th	Night	20% 9% 33% 30% 9%	406
	21st to end	Night	16% 8% 24% 36% 15% 1	433
June	1st to 10th	Night	14% 4 29% 35% 18%	400
	11th to 20th	Night	17% 7% 26% 34% 17%	400
	21st to end	Night	11% 4 27% 44% 15%	400
July	1st to 10th	Night	12% 4 20% 37% 24% 2	390
	11th to 20th	Night	9% 3 19% 45% 23% 2	388
	21st to end	Night	10% 3 20% 39% 25% 1	651
August	1st to 10th	Night	12% 2 18% 40% 26% 3	597
	11th to 20th	Night	12% 5% 16% 39% 25% 3	649
	21st to end	Night	14% 4 18% 34% 27% 3	880
September	1st to 10th	Night	16% 3 21% 34% 22% 3	800
	11th to 20th	Night	13% 6% 18% 34% 27% 3	800
	21st to end	Night	23% 9% 20% 29% 18% 2	800
October	1st to 10th	Night	21% 8% 21% 27% 19% 3	800
	11th to 20th	Night	18% 14% 29% 30% 10% 1	800
	21st to end	Night	20% 15% 28% 27% 10%	880
November	1st to 10th	Night	15% 14% 31% 29% 10% 1	820
	11th to 20th	Night	22% 20% 29% 22% 6% 1	829
	21st to end	Night	20% 19% 30% 24% 6%	800
December	1st to 10th	Night	22% 20% 33% 20% 5% 1	800
	11th to 20th	Night	24% 17% 34% 22% 4	800
	21st to end	Night	20% 20% 33% 24% 3	880

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