Burke Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Burke Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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% 9% 3 7% 5% 5% 5% 4 6% 8% 35%	126403
	Night	15% 9% 3 8% 4 4 4 4 5% 7% 37%	122957
Transparency	Day	29% 40% 7% 13% 9% 2	130088
	Night	29% 40% 7% 14% 9% 2	127341
Seeing	Day	21% 12% 22% 22% 16% 7%	16277
	Night	19% 9% 21% 31% 18% 2	26310

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% 16% 5% 11% 5% 3 3 2 4 5% 29%	4056
	11th to 20th	Day	14% 18% 9% 9% 6% 3 3 2 3 4 28%	4060
	21st to end	Day	15% 14% 3 9% 4 4 4 3 4 5% 35%	4466
February	1st to 10th	Day	19% 12% 4 8% 4 4 5% 4 4 6% 31%	1825
	11th to 20th	Day	23% 12% 4 10% 4 3 3 2 4 6% 28%	1948
	21st to end	Day	22% 11% 3 7% 3 3 3 3 4 6% 34%	1650
March	1st to 10th	Day	17% 15% 3 6% 4 4 4 3 4 6% 34%	4847
	11th to 20th	Day	12% 8% 3 6% 4 4 4 3 5% 6% 45%	4980
	21st to end	Day	16% 8% 3 7% 5% 4 4 3 5% 6% 39%	5465
April	1st to 10th	Day	14% 8% 3 7% 5% 4 4 3 7% 8% 38%	2200
	11th to 20th	Day	16% 7% 2 5% 4 4 4 4 5% 6% 42%	2423
	21st to end	Day	16% 9% 3 6% 5% 4 5% 4 5% 6% 37%	2600
May	1st to 10th	Day	16% 9% 3 9% 6% 6% 6% 4 6% 9% 25%	5862
	11th to 20th	Day	17% 9% 3 9% 6% 6% 6% 5% 7% 7% 26%	5861
	21st to end	Day	15% 7% 2 8% 5% 5% 5% 5% 7% 10% 30%	6433
June	1st to 10th	Day	11% 7% 2 6% 5% 5% 5% 5% 7% 9% 36%	2600
	11th to 20th	Day	8% 6% 2 8% 7% 6% 6% 5% 9% 9% 33%	2600
	21st to end	Day	9% 6% 2 7% 6% 5% 5% 5% 7% 9% 37%	2600
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 6% 9% 11% 39%	5900
	11th to 20th	Day	12% 6% 3 8% 5% 5% 6% 5% 8% 11% 32%	5900
	21st to end	Day	16% 8% 3 9% 6% 6% 6% 5% 6% 9% 25%	6424
August	1st to 10th	Day	12% 7% 3 8% 6% 6% 6% 5% 8% 11% 28%	4234
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 5% 8% 11% 35%	4174
	21st to end	Day	6% 3 1 4 3 4 4 4 6% 9% 55%	4415
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 5% 6% 9% 42%	2200
	11th to 20th	Day	13% 6% 3 7% 5% 4 5% 3 6% 9% 40%	2200
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 7% 7% 38%	2200
October	1st to 10th	Day	13% 5% 2 5% 3 4 3 3 5% 6% 49%	2060
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 5% 50%	1900
	21st to end	Day	18% 8% 2 6% 3 3 3 3 4 6% 46%	2090
November	1st to 10th	Day	12% 7% 2 5% 4 3 3 3 5% 6% 50%	1861
	11th to 20th	Day	12% 10% 2 6% 4 4 4 3 6% 7% 42%	1800
	21st to end	Day	15% 9% 3 7% 4 4 3 4 5% 6% 41%	1800
December	1st to 10th	Day	15% 12% 4 11% 5% 3 4 3 4 4 35%	2930
	11th to 20th	Day	19% 14% 4 9% 4 4 4 3 6% 7% 26%	3373
	21st to end	Day	14% 13% 6% 7% 5% 6% 6% 5% 7% 7% 26%	4466
January	1st to 10th	Night	16% 15% 6% 11% 4 4 3 3 4 5% 27%	5889
	11th to 20th	Night	17% 14% 5% 11% 4 3 3 2 3 4 34%	5900
	21st to end	Night	18% 10% 4 7% 4 4 4 3 4 6% 37%	6490
February	1st to 10th	Night	18% 10% 3 8% 4 4 3 4 5% 6% 34%	2572
	11th to 20th	Night	22% 11% 5% 8% 5% 4 4 3 4 5% 30%	2338
	21st to end	Night	22% 11% 3 8% 4 4 3 3 4 6% 31%	1980
March	1st to 10th	Night	20% 11% 3 6% 4 4 3 3 3 5% 38%	4993
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 51%	4980
	21st to end	Night	12% 7% 2 6% 4 5% 5% 5% 6% 7% 41%	5467
April	1st to 10th	Night	14% 9% 3 7% 4 4 4 4 5% 7% 39%	2200
	11th to 20th	Night	14% 7% 2 7% 5% 4 5% 4 5% 7% 42%	1941
	21st to end	Night	13% 7% 2 7% 5% 5% 5% 5% 5% 7% 40%	1800
May	1st to 10th	Night	18% 10% 4 8% 5% 5% 5% 4 6% 9% 27%	4033
	11th to 20th	Night	16% 9% 4 10% 6% 5% 4 4 6% 8% 28%	4032
	21st to end	Night	17% 8% 3 8% 4 5% 5% 4 6% 8% 31%	4421
June	1st to 10th	Night	11% 6% 1 8% 4 4 6% 5% 7% 10% 38%	1800
	11th to 20th	Night	6% 5% 2 6% 6% 6% 6% 5% 8% 10% 40%	1800
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 7% 12% 38%	1800
July	1st to 10th	Night	11% 6% 2 7% 5% 5% 5% 5% 9% 11% 35%	4058
	11th to 20th	Night	12% 7% 2 8% 5% 4 6% 4 7% 11% 32%	4057
	21st to end	Night	15% 10% 4 11% 7% 6% 5% 4 6% 8% 24%	4427
August	1st to 10th	Night	12% 7% 3 7% 6% 5% 5% 5% 7% 11% 31%	2915
	11th to 20th	Night	13% 7% 2 7% 5% 5% 5% 4 7% 9% 36%	2875
	21st to end	Night	6% 4 1 4 3 4 4 5% 7% 9% 53%	3419
September	1st to 10th	Night	11% 6% 3 7% 5% 5% 4 4 5% 8% 44%	2200
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 6% 8% 44%	2200
	21st to end	Night	14% 7% 2 6% 4 4 4 4 6% 6% 44%	2200
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 6% 49%	2186
	11th to 20th	Night	15% 7% 2 5% 3 3 3 3 4 6% 49%	2280
	21st to end	Night	15% 8% 2 5% 3 4 3 2 5% 7% 46%	2508
November	1st to 10th	Night	13% 7% 3 5% 3 3 4 3 4 5% 50%	2434
	11th to 20th	Night	15% 9% 3 6% 3 3 4 3 4 7% 44%	2600
	21st to end	Night	14% 10% 2 6% 3 3 4 4 4 5% 45%	2600
December	1st to 10th	Night	17% 12% 4 8% 5% 5% 4 2 3 5% 35%	4250
	11th to 20th	Night	20% 12% 6% 11% 5% 4 4 3 3 5% 28%	4822
	21st to end	Night	15% 14% 4 8% 5% 5% 4 4 5% 8% 29%	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	44% 28% 3 14% 10% 2	4057
	11th to 20th	Day	45% 28% 3 10% 14% 1	4060
	21st to end	Day	38% 29% 6% 17% 10%	4466
February	1st to 10th	Day	21% 62% 3 8% 6% 1	1825
	11th to 20th	Day	26% 60% 2 6% 6% 1	1958
	21st to end	Day	19% 62% 3 6% 9% 2	1650
March	1st to 10th	Day	40% 31% 4 8% 12% 6%	4847
	11th to 20th	Day	24% 30% 4 15% 22% 5%	4980
	21st to end	Day	30% 30% 5% 22% 12% 2	5465
April	1st to 10th	Day	21% 62% 4 7% 5%	2200
	11th to 20th	Day	17% 62% 4 9% 5% 1	2423
	21st to end	Day	19% 62% 4 9% 5% 1	2600
May	1st to 10th	Day	40% 33% 8% 13% 5%	5897
	11th to 20th	Day	39% 33% 10% 14% 3	5865
	21st to end	Day	34% 34% 9% 13% 8% 2	6447
June	1st to 10th	Day	16% 64% 6% 9% 5%	2600
	11th to 20th	Day	17% 64% 7% 7% 5%	2600
	21st to end	Day	16% 62% 8% 9% 5%	2600
July	1st to 10th	Day	24% 33% 16% 16% 10%	5835
	11th to 20th	Day	33% 31% 17% 17% 2	5831
	21st to end	Day	42% 30% 8% 12% 8%	6240
August	1st to 10th	Day	31% 41% 10% 13% 4	4221
	11th to 20th	Day	24% 40% 11% 18% 6% 2	4213
	21st to end	Day	12% 37% 14% 27% 9%	4415
September	1st to 10th	Day	15% 59% 6% 10% 6% 3	2200
	11th to 20th	Day	18% 59% 5% 11% 7% 1	2200
	21st to end	Day	19% 59% 8% 9% 4	2200
October	1st to 10th	Day	17% 37% 4 13% 23% 6%	3398
	11th to 20th	Day	14% 37% 5% 16% 11% 16%	3140
	21st to end	Day	15% 36% 3 23% 10% 12%	3428
November	1st to 10th	Day	20% 56% 3 10% 10% 1	1861
	11th to 20th	Day	19% 57% 2 8% 12% 3	1800
	21st to end	Day	21% 57% 3 10% 8% 1	1800
December	1st to 10th	Day	34% 36% 3 11% 14% 2	2930
	11th to 20th	Day	42% 32% 5% 11% 9% 1	3370
	21st to end	Day	41% 31% 10% 12% 6% 1	4466
January	1st to 10th	Night	44% 30% 3 12% 10% 1	5889
	11th to 20th	Night	38% 29% 3 12% 15% 3	5900
	21st to end	Night	33% 30% 5% 18% 12% 2	6490
February	1st to 10th	Night	20% 62% 4 7% 6% 1	2572
	11th to 20th	Night	25% 61% 2 6% 5% 1	2339
	21st to end	Night	19% 62% 3 7% 6% 2	1980
March	1st to 10th	Night	37% 29% 4 16% 9% 4	4993
	11th to 20th	Night	20% 30% 5% 18% 22% 6%	4980
	21st to end	Night	28% 30% 8% 21% 12% 1	5467
April	1st to 10th	Night	18% 63% 5% 8% 5% 1	2200
	11th to 20th	Night	18% 63% 4 8% 6% 1	1941
	21st to end	Night	17% 62% 6% 10% 5%	1800
May	1st to 10th	Night	40% 34% 9% 12% 5%	4060
	11th to 20th	Night	39% 33% 10% 15% 3	4033
	21st to end	Night	37% 34% 10% 14% 5% 1	4429
June	1st to 10th	Night	15% 64% 7% 7% 5% 1	1800
	11th to 20th	Night	14% 65% 8% 9% 4 1	1800
	21st to end	Night	16% 63% 7% 10% 4	1800
July	1st to 10th	Night	29% 31% 14% 19% 6%	4015
	11th to 20th	Night	34% 31% 17% 15% 2	4006
	21st to end	Night	47% 30% 8% 10% 6%	4297
August	1st to 10th	Night	31% 41% 12% 14% 2	2904
	11th to 20th	Night	26% 40% 11% 16% 6%	2902
	21st to end	Night	13% 38% 12% 28% 9%	3419
September	1st to 10th	Night	16% 58% 8% 9% 8% 1	2200
	11th to 20th	Night	15% 58% 7% 11% 8% 1	2200
	21st to end	Night	19% 58% 7% 12% 5%	2200
October	1st to 10th	Night	23% 38% 4 12% 19% 4	3628
	11th to 20th	Night	15% 38% 4 14% 18% 11%	3779
	21st to end	Night	13% 36% 3 22% 16% 9%	4120
November	1st to 10th	Night	20% 56% 4 10% 8% 1	2434
	11th to 20th	Night	18% 57% 2 7% 12% 3	2600
	21st to end	Night	19% 57% 4 11% 8% 1	2600
December	1st to 10th	Night	34% 37% 4 14% 11% 1	4250
	11th to 20th	Night	43% 33% 5% 11% 7% 1	4824
	21st to end	Night	38% 31% 10% 13% 8%	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	31% 28% 27% 10% 3 1	300
	11th to 20th	Day	39% 25% 23% 11% 2	300
	21st to end	Day	35% 23% 27% 13% 3	330
February	1st to 10th	Day	30% 20% 35% 11% 4	300
	11th to 20th	Day	46% 24% 21% 7% 1	300
	21st to end	Day	39% 22% 30% 8% 2	249
March	1st to 10th	Day	27% 20% 25% 21% 5% 1	460
	11th to 20th	Day	29% 20% 29% 17% 3 1	500
	21st to end	Day	31% 17% 26% 19% 7%	550
April	1st to 10th	Day	28% 18% 23% 20% 9% 2	500
	11th to 20th	Day	22% 16% 29% 19% 11% 2	492
	21st to end	Day	25% 13% 26% 20% 11% 4	500
May	1st to 10th	Day	26% 17% 24% 19% 9% 5%	503
	11th to 20th	Day	21% 10% 24% 24% 15% 6%	527
	21st to end	Day	17% 10% 21% 21% 20% 12%	573
June	1st to 10th	Day	14% 6% 22% 29% 22% 8%	530
	11th to 20th	Day	10% 8% 18% 27% 26% 11%	530
	21st to end	Day	13% 7% 21% 24% 25% 10%	530
July	1st to 10th	Day	14% 3 14% 26% 27% 16%	525
	11th to 20th	Day	14% 3 11% 32% 26% 14%	524
	21st to end	Day	17% 4 14% 27% 27% 11%	576
August	1st to 10th	Day	15% 6% 17% 24% 23% 14%	526
	11th to 20th	Day	15% 3 14% 29% 25% 14%	519
	21st to end	Day	11% 3 12% 28% 27% 20%	583
September	1st to 10th	Day	11% 5% 14% 28% 29% 13%	530
	11th to 20th	Day	13% 6% 12% 24% 30% 15%	530
	21st to end	Day	19% 9% 19% 27% 19% 6%	530
October	1st to 10th	Day	18% 4 18% 32% 23% 6%	502
	11th to 20th	Day	17% 15% 20% 26% 18% 4	490
	21st to end	Day	22% 10% 22% 28% 14% 4	539
November	1st to 10th	Day	16% 14% 25% 29% 13% 3	399
	11th to 20th	Day	20% 20% 29% 20% 9% 1	300
	21st to end	Day	21% 20% 33% 18% 6% 1	300
December	1st to 10th	Day	34% 16% 31% 15% 3	300
	11th to 20th	Day	29% 18% 27% 18% 8%	300
	21st to end	Day	30% 18% 30% 15% 6% 1	330
January	1st to 10th	Night	24% 18% 30% 24% 4	800
	11th to 20th	Night	29% 17% 27% 23% 4	800
	21st to end	Night	22% 18% 34% 25% 2	880
February	1st to 10th	Night	24% 15% 30% 29% 2	800
	11th to 20th	Night	31% 18% 31% 18% 1	783
	21st to end	Night	30% 17% 30% 22% 2	660
March	1st to 10th	Night	24% 15% 29% 28% 5%	800
	11th to 20th	Night	24% 16% 27% 26% 7%	800
	21st to end	Night	19% 12% 33% 28% 8%	880
April	1st to 10th	Night	20% 15% 27% 29% 9%	800
	11th to 20th	Night	19% 13% 25% 32% 11% 1	702
	21st to end	Night	17% 7% 25% 40% 11% 1	600
May	1st to 10th	Night	17% 10% 22% 35% 15% 2	600
	11th to 20th	Night	17% 6% 18% 37% 20% 2	591
	21st to end	Night	13% 4 14% 33% 30% 6%	650
June	1st to 10th	Night	13% 3 12% 39% 29% 4	600
	11th to 20th	Night	9% 2 12% 37% 35% 6%	600
	21st to end	Night	11% 3 10% 35% 37% 4	600
July	1st to 10th	Night	15% 3 8% 29% 41% 4	585
	11th to 20th	Night	12% 3 5% 30% 43% 7%	582
	21st to end	Night	13% 2 9% 32% 35% 9%	657
August	1st to 10th	Night	14% 2 7% 32% 42% 4	597
	11th to 20th	Night	14% 3 5% 35% 37% 7%	594
	21st to end	Night	12% 2 7% 32% 40% 7%	720
September	1st to 10th	Night	13% 4 9% 35% 34% 5%	800
	11th to 20th	Night	15% 3 10% 30% 37% 5%	800
	21st to end	Night	17% 5% 16% 33% 28% 2	800
October	1st to 10th	Night	16% 5% 16% 31% 28% 5%	773
	11th to 20th	Night	19% 7% 21% 35% 17% 1	760
	21st to end	Night	20% 9% 19% 36% 14% 1	836
November	1st to 10th	Night	18% 8% 17% 41% 15% 2	780
	11th to 20th	Night	20% 11% 24% 36% 10% 1	800
	21st to end	Night	20% 12% 25% 33% 10% 1	800
December	1st to 10th	Night	28% 13% 27% 26% 6%	800
	11th to 20th	Night	26% 13% 26% 30% 5% 1	800
	21st to end	Night	28% 11% 25% 31% 5%	880

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