Burrell Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Burrell Memorial Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	16% 11% 4 10% 5% 4 4 4 5% 7% 30%	132021
	Night	16% 13% 5% 10% 5% 4 4 3 5% 6% 29%	130744
Transparency	Day	53% 5% 11% 20% 11% 1	135654
	Night	55% 5% 10% 19% 10% 1	135182
Seeing	Day	22% 14% 28% 21% 11% 4	16833
	Night	23% 12% 27% 26% 12% 1	26484

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	15% 21% 7% 23% 6% 4 3 2 3 4 11%	5258
	11th to 20th	Day	12% 21% 8% 18% 10% 5% 5% 2 3 4 11%	5280
	21st to end	Day	19% 18% 8% 14% 8% 7% 4 3 4 3 11%	5780
February	1st to 10th	Day	17% 19% 5% 13% 6% 4 4 3 4 4 22%	1890
	11th to 20th	Day	14% 16% 7% 15% 9% 5% 4 3 4 5% 19%	1920
	21st to end	Day	18% 14% 5% 13% 5% 4 5% 3 5% 6% 21%	1730
March	1st to 10th	Day	17% 13% 6% 13% 4 4 4 3 4 6% 27%	4898
	11th to 20th	Day	12% 14% 4 11% 4 4 4 3 4 5% 35%	5090
	21st to end	Day	20% 14% 6% 9% 5% 4 4 3 4 5% 27%	5586
April	1st to 10th	Day	17% 13% 4 8% 5% 4 5% 4 5% 7% 29%	2336
	11th to 20th	Day	18% 12% 3 6% 4 4 4 3 5% 6% 34%	2694
	21st to end	Day	21% 11% 3 7% 4 4 4 3 4 7% 33%	2730
May	1st to 10th	Day	21% 10% 3 8% 5% 5% 4 4 5% 5% 30%	5992
	11th to 20th	Day	17% 9% 3 7% 5% 4 5% 4 6% 7% 33%	5939
	21st to end	Day	17% 7% 3 7% 5% 4 4 4 6% 8% 35%	6433
June	1st to 10th	Day	15% 7% 3 6% 4 5% 4 4 7% 8% 37%	2792
	11th to 20th	Day	13% 6% 3 7% 5% 4 5% 4 6% 8% 39%	2814
	21st to end	Day	11% 5% 3 6% 4 5% 5% 5% 7% 10% 39%	2947
July	1st to 10th	Day	9% 5% 2 7% 5% 4 5% 4 7% 11% 42%	6140
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 10% 39%	5900
	21st to end	Day	12% 6% 2 7% 4 5% 5% 4 6% 8% 41%	6424
August	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 7% 9% 38%	4234
	11th to 20th	Day	11% 7% 2 7% 4 5% 5% 5% 7% 10% 36%	4174
	21st to end	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	4620
September	1st to 10th	Day	13% 6% 2 5% 4 4 5% 5% 5% 7% 43%	2270
	11th to 20th	Day	11% 6% 3 6% 5% 3 4 4 6% 8% 45%	2200
	21st to end	Day	19% 8% 3 6% 4 4 3 4 5% 7% 37%	2200
October	1st to 10th	Day	14% 7% 3 7% 4 4 4 4 6% 7% 41%	2080
	11th to 20th	Day	20% 8% 3 8% 4 4 3 3 5% 6% 36%	2000
	21st to end	Day	23% 11% 3 8% 4 4 4 3 5% 5% 28%	2170
November	1st to 10th	Day	16% 11% 4 7% 5% 4 4 3 5% 6% 35%	1852
	11th to 20th	Day	18% 18% 2 9% 4 4 4 3 5% 5% 27%	1890
	21st to end	Day	20% 18% 7% 10% 5% 5% 4 3 4 6% 17%	1680
December	1st to 10th	Day	18% 18% 7% 13% 7% 5% 4 3 5% 5% 16%	2680
	11th to 20th	Day	21% 21% 7% 12% 5% 3 3 3 4 5% 15%	3070
	21st to end	Day	23% 23% 7% 14% 5% 4 4 3 4 4 9%	4328
January	1st to 10th	Night	16% 21% 8% 23% 6% 4 3 2 3 3 11%	7635
	11th to 20th	Night	10% 20% 11% 19% 8% 5% 6% 4 4 3 11%	7680
	21st to end	Night	18% 18% 6% 13% 6% 5% 4 3 4 5% 19%	8401
February	1st to 10th	Night	19% 16% 6% 13% 7% 4 3 3 4 4 21%	2730
	11th to 20th	Night	17% 16% 6% 12% 5% 5% 5% 3 4 4 21%	2584
	21st to end	Night	19% 16% 5% 13% 6% 4 4 3 4 4 23%	2076
March	1st to 10th	Night	21% 14% 5% 9% 4 3 3 2 4 5% 31%	5175
	11th to 20th	Night	13% 11% 3 7% 4 4 4 3 5% 7% 38%	5090
	21st to end	Night	17% 12% 4 9% 5% 4 4 3 4 5% 32%	5588
April	1st to 10th	Night	18% 14% 3 7% 6% 4 3 4 5% 6% 29%	2283
	11th to 20th	Night	16% 10% 4 7% 6% 5% 4 4 6% 6% 32%	1893
	21st to end	Night	16% 9% 3 6% 4 4 4 3 7% 6% 37%	1890
May	1st to 10th	Night	22% 11% 4 8% 4 4 5% 4 5% 6% 27%	4123
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 4 5% 8% 37%	4086
	21st to end	Night	17% 6% 2 5% 4 4 5% 4 6% 8% 37%	4421
June	1st to 10th	Night	11% 6% 3 7% 4 4 6% 4 5% 6% 43%	1600
	11th to 20th	Night	10% 5% 3 7% 5% 5% 5% 5% 7% 9% 39%	1573
	21st to end	Night	10% 5% 2 5% 5% 4 4 5% 7% 9% 43%	1447
July	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 7% 10% 44%	3794
	11th to 20th	Night	9% 5% 2 6% 5% 5% 5% 4 7% 8% 43%	4057
	21st to end	Night	8% 6% 2 6% 4 4 4 3 6% 8% 47%	4427
August	1st to 10th	Night	10% 7% 2 8% 4 5% 5% 3 6% 9% 41%	2915
	11th to 20th	Night	14% 7% 3 7% 4 4 3 3 5% 7% 43%	2875
	21st to end	Night	8% 6% 3 7% 5% 5% 6% 5% 6% 8% 43%	3243
September	1st to 10th	Night	11% 6% 2 5% 5% 4 4 4 6% 8% 45%	2125
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 5% 8% 52%	2200
	21st to end	Night	17% 8% 2 7% 5% 4 5% 4 4 5% 39%	2200
October	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	2320
	11th to 20th	Night	21% 9% 2 7% 4 4 4 3 5% 6% 35%	2400
	21st to end	Night	26% 10% 3 7% 5% 4 3 3 5% 6% 29%	2665
November	1st to 10th	Night	16% 11% 3 7% 5% 4 4 3 5% 6% 37%	2666
	11th to 20th	Night	18% 17% 3 9% 5% 4 4 3 5% 5% 27%	2730
	21st to end	Night	23% 18% 4 8% 5% 4 4 3 4 5% 22%	2936
December	1st to 10th	Night	16% 18% 7% 12% 6% 5% 4 3 5% 6% 18%	4720
	11th to 20th	Night	19% 22% 7% 10% 5% 4 4 3 4 4 18%	5345
	21st to end	Night	22% 21% 7% 13% 5% 5% 4 3 4 5% 12%	6851

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	79% 3 8% 7% 2	5259
	11th to 20th	Day	79% 3 7% 7% 4	5280
	21st to end	Day	79% 3 6% 9% 3	5780
February	1st to 10th	Day	67% 3 10% 14% 6%	1890
	11th to 20th	Day	68% 5% 8% 13% 5% 1	1929
	21st to end	Day	63% 4 12% 14% 6% 1	1730
March	1st to 10th	Day	59% 4 10% 21% 5%	4898
	11th to 20th	Day	52% 4 13% 24% 7% 1	5090
	21st to end	Day	59% 4 8% 17% 10% 1	5586
April	1st to 10th	Day	54% 5% 10% 18% 12% 1	2336
	11th to 20th	Day	51% 5% 11% 18% 13% 2	2694
	21st to end	Day	51% 5% 11% 21% 10% 2	2730
May	1st to 10th	Day	56% 5% 8% 18% 11% 2	6027
	11th to 20th	Day	49% 6% 11% 21% 11% 1	5943
	21st to end	Day	46% 6% 11% 21% 13% 4	6447
June	1st to 10th	Day	44% 5% 11% 25% 14% 1	2792
	11th to 20th	Day	43% 6% 14% 22% 14% 2	2814
	21st to end	Day	38% 7% 13% 24% 16% 2	2947
July	1st to 10th	Day	36% 6% 11% 29% 17% 1	6075
	11th to 20th	Day	38% 7% 13% 25% 16% 1	5831
	21st to end	Day	38% 7% 11% 22% 20% 1	6240
August	1st to 10th	Day	41% 7% 14% 26% 12%	4221
	11th to 20th	Day	41% 6% 11% 31% 10% 1	4213
	21st to end	Day	38% 6% 14% 29% 11% 2	4620
September	1st to 10th	Day	38% 7% 14% 23% 16% 3	2270
	11th to 20th	Day	36% 6% 14% 26% 16% 2	2200
	21st to end	Day	47% 4 10% 21% 17% 1	2200
October	1st to 10th	Day	34% 6% 12% 28% 19% 1	3392
	11th to 20th	Day	47% 4 10% 20% 17% 2	3240
	21st to end	Day	44% 4 13% 23% 16% 1	3484
November	1st to 10th	Day	50% 4 11% 21% 13% 1	1852
	11th to 20th	Day	60% 4 10% 16% 8% 2	1890
	21st to end	Day	69% 4 9% 13% 5%	1680
December	1st to 10th	Day	69% 5% 13% 10% 2	2680
	11th to 20th	Day	73% 3 9% 12% 3	3066
	21st to end	Day	80% 4 7% 7% 1	4328
January	1st to 10th	Night	80% 3 6% 9% 2	7636
	11th to 20th	Night	79% 4 6% 8% 3	7680
	21st to end	Night	69% 4 9% 13% 5%	8401
February	1st to 10th	Night	67% 4 9% 14% 5%	2730
	11th to 20th	Night	66% 4 9% 15% 6%	2586
	21st to end	Night	65% 3 9% 14% 7% 1	2076
March	1st to 10th	Night	58% 2 12% 20% 8%	5175
	11th to 20th	Night	45% 4 14% 26% 10% 1	5090
	21st to end	Night	53% 5% 8% 21% 12% 1	5588
April	1st to 10th	Night	55% 5% 10% 16% 12% 2	2283
	11th to 20th	Night	51% 5% 11% 18% 13% 2	1893
	21st to end	Night	46% 5% 10% 21% 15% 3	1890
May	1st to 10th	Night	57% 5% 9% 15% 10% 5%	4150
	11th to 20th	Night	45% 6% 10% 24% 14% 1	4087
	21st to end	Night	42% 6% 13% 23% 14% 3	4429
June	1st to 10th	Night	41% 6% 10% 26% 15% 3	1600
	11th to 20th	Night	39% 6% 14% 20% 18% 2	1573
	21st to end	Night	36% 7% 13% 26% 17% 2	1447
July	1st to 10th	Night	34% 7% 13% 25% 17% 4	3752
	11th to 20th	Night	35% 8% 13% 27% 15% 3	4006
	21st to end	Night	33% 7% 11% 21% 26% 2	4297
August	1st to 10th	Night	39% 7% 12% 26% 14% 1	2904
	11th to 20th	Night	40% 6% 10% 30% 13% 1	2902
	21st to end	Night	37% 8% 12% 28% 14% 2	3243
September	1st to 10th	Night	36% 5% 12% 28% 15% 4	2125
	11th to 20th	Night	30% 5% 13% 28% 21% 2	2200
	21st to end	Night	46% 4 11% 20% 18% 1	2200
October	1st to 10th	Night	31% 5% 9% 30% 24% 1	3788
	11th to 20th	Night	47% 4 8% 19% 19% 1	3899
	21st to end	Night	43% 4 10% 25% 16% 1	4301
November	1st to 10th	Night	47% 5% 12% 21% 13% 2	2666
	11th to 20th	Night	59% 4 11% 17% 8% 2	2730
	21st to end	Night	65% 4 11% 15% 4	2936
December	1st to 10th	Night	65% 5% 13% 13% 3	4720
	11th to 20th	Night	70% 4 10% 13% 2	5348
	21st to end	Night	75% 5% 10% 9% 1	6851

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	26% 23% 35% 10% 5% 1	330
	11th to 20th	Day	34% 26% 29% 8% 2 1	330
	21st to end	Day	30% 26% 30% 13% 1	363
February	1st to 10th	Day	30% 23% 33% 12% 2	330
	11th to 20th	Day	31% 25% 29% 13% 2	330
	21st to end	Day	29% 30% 33% 8%	273
March	1st to 10th	Day	27% 21% 30% 17% 4	466
	11th to 20th	Day	29% 18% 33% 15% 4 1	540
	21st to end	Day	33% 19% 28% 16% 5%	594
April	1st to 10th	Day	25% 21% 30% 17% 5% 1	540
	11th to 20th	Day	28% 20% 23% 18% 7% 3	532
	21st to end	Day	28% 15% 33% 16% 6% 2	540
May	1st to 10th	Day	26% 17% 28% 21% 5% 3	543
	11th to 20th	Day	21% 14% 30% 22% 10% 3	551
	21st to end	Day	17% 9% 26% 27% 15% 5%	573
June	1st to 10th	Day	16% 8% 31% 30% 12% 3	530
	11th to 20th	Day	17% 9% 24% 32% 14% 3	544
	21st to end	Day	14% 7% 32% 30% 14% 3	730
July	1st to 10th	Day	13% 5% 24% 30% 20% 9%	635
	11th to 20th	Day	16% 6% 22% 31% 19% 7%	524
	21st to end	Day	11% 5% 25% 31% 20% 7%	576
August	1st to 10th	Day	12% 6% 20% 30% 24% 8%	526
	11th to 20th	Day	14% 5% 23% 29% 20% 8%	519
	21st to end	Day	11% 7% 25% 27% 22% 8%	583
September	1st to 10th	Day	15% 7% 23% 25% 20% 10%	530
	11th to 20th	Day	12% 4 18% 28% 26% 13%	530
	21st to end	Day	21% 13% 25% 21% 13% 8%	530
October	1st to 10th	Day	13% 10% 26% 26% 19% 7%	530
	11th to 20th	Day	22% 16% 33% 19% 8% 2	530
	21st to end	Day	30% 18% 29% 18% 4	553
November	1st to 10th	Day	22% 23% 31% 15% 8% 1	330
	11th to 20th	Day	32% 22% 31% 9% 5% 1	330
	21st to end	Day	32% 22% 32% 13%	220
December	1st to 10th	Day	25% 26% 31% 13% 5% 1	220
	11th to 20th	Day	28% 27% 31% 12% 2	220
	21st to end	Day	33% 23% 31% 12% 1	308
January	1st to 10th	Night	30% 20% 32% 15% 4	840
	11th to 20th	Night	32% 20% 29% 14% 4	840
	21st to end	Night	29% 18% 30% 20% 3	924
February	1st to 10th	Night	30% 15% 30% 22% 3	840
	11th to 20th	Night	26% 17% 33% 20% 3	823
	21st to end	Night	28% 17% 35% 18% 2	692
March	1st to 10th	Night	25% 15% 33% 22% 5%	840
	11th to 20th	Night	25% 16% 30% 22% 7% 1	840
	21st to end	Night	26% 12% 29% 25% 8% 1	924
April	1st to 10th	Night	25% 16% 29% 25% 5%	840
	11th to 20th	Night	22% 13% 33% 23% 9% 1	649
	21st to end	Night	20% 11% 34% 27% 8% 1	630
May	1st to 10th	Night	22% 9% 27% 32% 8% 1	630
	11th to 20th	Night	20% 7% 25% 34% 12% 2	609
	21st to end	Night	13% 4 22% 37% 21% 3	650
June	1st to 10th	Night	11% 6% 28% 39% 16% 1	400
	11th to 20th	Night	11% 5% 26% 40% 17% 1	400
	21st to end	Night	9% 4 26% 38% 22%	447
July	1st to 10th	Night	11% 1 16% 40% 29% 3	585
	11th to 20th	Night	11% 3 21% 36% 26% 3	582
	21st to end	Night	11% 4 16% 40% 28% 2	657
August	1st to 10th	Night	10% 3 14% 44% 25% 4	597
	11th to 20th	Night	13% 4 13% 38% 28% 4	594
	21st to end	Night	11% 3 18% 34% 30% 4	667
September	1st to 10th	Night	14% 6% 17% 34% 25% 5%	800
	11th to 20th	Night	12% 3 17% 32% 31% 5%	800
	21st to end	Night	19% 7% 23% 28% 18% 4	800
October	1st to 10th	Night	18% 8% 22% 30% 20% 3	800
	11th to 20th	Night	25% 13% 28% 23% 9% 1	800
	21st to end	Night	32% 13% 27% 23% 5%	880
November	1st to 10th	Night	22% 15% 30% 23% 9%	820
	11th to 20th	Night	30% 16% 29% 17% 6% 1	840
	21st to end	Night	34% 16% 29% 15% 5%	840
December	1st to 10th	Night	31% 16% 32% 15% 5%	840
	11th to 20th	Night	30% 20% 30% 17% 3	840
	21st to end	Night	31% 16% 34% 17% 2	924

Page generated at 2026-05-14 01:59:00. Version ac07

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