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 2024-08-15.

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% 12% 4 8% 5% 4 4 4 5% 7% 31%	95386
	Night	17% 13% 4 9% 5% 4 4 3 5% 6% 31%	93239
Transparency	Day	53% 5% 11% 19% 11% 1	95158
	Night	54% 5% 11% 18% 10% 1	93072
Seeing	Day	21% 13% 28% 22% 12% 4	14579
	Night	22% 11% 27% 26% 12% 1	24146

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	19% 23% 8% 14% 4 4 3 2 3 4 14%	2836
	11th to 20th	Day	17% 24% 8% 13% 7% 6% 5% 3 3 4 11%	2840
	21st to end	Day	27% 22% 11% 13% 8% 4 2 2 3 2 7%	3124
February	1st to 10th	Day	17% 20% 5% 12% 7% 3 4 3 4 4 21%	1710
	11th to 20th	Day	14% 16% 6% 15% 8% 5% 4 3 4 5% 19%	1732
	21st to end	Day	19% 15% 5% 12% 5% 4 5% 3 5% 6% 22%	1570
March	1st to 10th	Day	22% 14% 6% 8% 4 3 3 3 4 6% 26%	3334
	11th to 20th	Day	13% 16% 5% 10% 5% 4 4 3 4 5% 32%	3480
	21st to end	Day	21% 13% 6% 8% 5% 4 4 3 4 5% 28%	3815
April	1st to 10th	Day	16% 13% 4 7% 5% 4 5% 4 5% 7% 30%	2114
	11th to 20th	Day	19% 11% 3 6% 4 4 4 3 5% 6% 34%	2434
	21st to end	Day	22% 10% 3 7% 4 4 4 3 4 7% 33%	2470
May	1st to 10th	Day	19% 10% 3 8% 5% 5% 5% 4 5% 6% 31%	4082
	11th to 20th	Day	19% 9% 2 6% 4 3 4 4 6% 7% 35%	4081
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	4476
June	1st to 10th	Day	15% 7% 2 6% 4 5% 5% 4 7% 8% 37%	2652
	11th to 20th	Day	13% 6% 3 7% 5% 4 5% 4 6% 8% 40%	2673
	21st to end	Day	12% 6% 3 6% 4 5% 5% 5% 7% 9% 39%	2800
July	1st to 10th	Day	10% 6% 2 8% 5% 4 5% 4 7% 11% 39%	4290
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 10% 45%	4120
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 8% 40%	4466
August	1st to 10th	Day	10% 5% 2 7% 5% 5% 5% 4 7% 9% 39%	2457
	11th to 20th	Day	10% 6% 2 7% 5% 6% 5% 5% 7% 9% 37%	2357
	21st to end	Day	12% 5% 2 7% 4 5% 5% 5% 7% 10% 39%	2549
September	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 5% 6% 7% 40%	2044
	11th to 20th	Day	12% 7% 3 6% 5% 3 4 4 6% 8% 42%	1980
	21st to end	Day	18% 7% 3 6% 4 4 3 4 5% 8% 38%	1980
October	1st to 10th	Day	15% 7% 3 7% 4 4 4 4 6% 7% 38%	1872
	11th to 20th	Day	21% 8% 3 8% 4 4 4 4 6% 6% 33%	1800
	21st to end	Day	25% 12% 3 8% 5% 4 4 3 5% 5% 26%	1954
November	1st to 10th	Day	15% 12% 4 7% 4 4 4 3 5% 6% 36%	1672
	11th to 20th	Day	19% 18% 2 9% 4 4 4 3 5% 5% 28%	1710
	21st to end	Day	19% 18% 7% 10% 5% 5% 4 3 5% 6% 18%	1520
December	1st to 10th	Day	17% 24% 7% 11% 6% 4 4 2 4 4 16%	1520
	11th to 20th	Day	18% 18% 7% 15% 5% 3 4 3 5% 6% 15%	1910
	21st to end	Day	23% 25% 8% 12% 5% 4 4 3 4 4 9%	2962
January	1st to 10th	Night	17% 25% 10% 13% 5% 4 3 3 4 4 14%	4109
	11th to 20th	Night	15% 21% 10% 18% 6% 4 6% 4 3 3 10%	4120
	21st to end	Night	27% 21% 7% 14% 5% 4 2 2 3 3 13%	4532
February	1st to 10th	Night	20% 16% 5% 12% 7% 3 3 3 4 4 21%	2470
	11th to 20th	Night	17% 17% 6% 11% 5% 5% 5% 3 4 4 22%	2332
	21st to end	Night	20% 15% 5% 13% 6% 4 4 2 4 4 23%	1884
March	1st to 10th	Night	22% 15% 4 6% 4 3 3 2 4 5% 33%	3519
	11th to 20th	Night	16% 11% 4 7% 4 5% 4 4 5% 6% 35%	3480
	21st to end	Night	18% 12% 5% 8% 5% 4 4 3 5% 6% 33%	3817
April	1st to 10th	Night	17% 14% 3 7% 6% 4 3 4 6% 6% 31%	2066
	11th to 20th	Night	16% 10% 3 6% 6% 4 4 4 6% 6% 33%	1711
	21st to end	Night	16% 9% 3 7% 4 4 4 3 7% 7% 36%	1710
May	1st to 10th	Night	21% 10% 4 8% 4 3 4 4 6% 7% 29%	2813
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 4 5% 8% 36%	2812
	21st to end	Night	11% 5% 2 4 4 4 5% 5% 6% 9% 46%	3079
June	1st to 10th	Night	11% 6% 3 7% 4 4 6% 4 5% 7% 43%	1520
	11th to 20th	Night	10% 5% 3 7% 5% 5% 5% 4 7% 9% 40%	1495
	21st to end	Night	10% 5% 2 5% 5% 5% 4 5% 7% 10% 43%	1374
July	1st to 10th	Night	10% 5% 2 6% 3 3 4 4 6% 9% 47%	2654
	11th to 20th	Night	9% 4 2 5% 4 4 5% 4 7% 8% 48%	2840
	21st to end	Night	7% 6% 2 6% 5% 5% 5% 4 7% 9% 44%	3087
August	1st to 10th	Night	8% 5% 2 8% 3 4 5% 4 6% 9% 47%	1701
	11th to 20th	Night	9% 6% 2 6% 5% 4 4 4 6% 8% 44%	1643
	21st to end	Night	10% 6% 3 7% 4 4 5% 4 5% 8% 45%	1787
September	1st to 10th	Night	12% 6% 2 6% 5% 4 4 4 6% 8% 43%	1912
	11th to 20th	Night	11% 4 2 5% 3 4 4 3 5% 8% 50%	1980
	21st to end	Night	16% 7% 2 7% 5% 4 5% 3 4 6% 40%	1980
October	1st to 10th	Night	15% 6% 2 6% 4 4 5% 3 6% 7% 42%	2088
	11th to 20th	Night	21% 9% 2 7% 5% 4 4 3 5% 6% 33%	2160
	21st to end	Night	28% 10% 3 8% 5% 4 3 3 4 6% 27%	2398
November	1st to 10th	Night	17% 11% 3 7% 5% 4 3 3 5% 6% 36%	2406
	11th to 20th	Night	18% 17% 3 9% 5% 4 4 3 5% 5% 27%	2470
	21st to end	Night	23% 17% 4 8% 5% 4 4 3 5% 5% 22%	2656
December	1st to 10th	Night	20% 21% 7% 10% 5% 4 4 3 4 5% 18%	2660
	11th to 20th	Night	18% 21% 6% 11% 4 4 3 3 4 5% 21%	3285
	21st to end	Night	24% 22% 7% 11% 5% 5% 4 3 4 5% 11%	4689

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	76% 3 10% 9% 2	2837
	11th to 20th	Day	79% 2 8% 9% 2	2840
	21st to end	Day	86% 2 5% 5% 2	3124
February	1st to 10th	Day	68% 3 10% 13% 5%	1710
	11th to 20th	Day	68% 5% 7% 14% 6% 1	1741
	21st to end	Day	64% 4 12% 13% 6% 1	1570
March	1st to 10th	Day	60% 4 9% 23% 5%	3334
	11th to 20th	Day	55% 4 12% 22% 6% 1	3480
	21st to end	Day	58% 5% 10% 16% 10% 1	3815
April	1st to 10th	Day	53% 5% 10% 18% 12% 1	2114
	11th to 20th	Day	51% 5% 11% 18% 13% 2	2434
	21st to end	Day	52% 4 11% 21% 10% 2	2470
May	1st to 10th	Day	54% 5% 8% 23% 10% 1	4120
	11th to 20th	Day	48% 5% 12% 23% 10% 1	4085
	21st to end	Day	40% 6% 13% 20% 16% 5%	4489
June	1st to 10th	Day	44% 6% 11% 25% 14% 1	2652
	11th to 20th	Day	41% 6% 14% 22% 14% 2	2673
	21st to end	Day	38% 7% 13% 24% 16% 2	2800
July	1st to 10th	Day	39% 6% 11% 24% 17% 2	4225
	11th to 20th	Day	34% 6% 12% 28% 19% 1	4051
	21st to end	Day	39% 8% 13% 19% 19% 2	4324
August	1st to 10th	Day	40% 7% 12% 27% 13%	2444
	11th to 20th	Day	41% 6% 14% 25% 12% 2	2357
	21st to end	Day	40% 6% 13% 23% 15% 3	2549
September	1st to 10th	Day	40% 7% 14% 21% 15% 3	2044
	11th to 20th	Day	39% 6% 14% 24% 15% 2	1980
	21st to end	Day	45% 4 10% 23% 17% 1	1980
October	1st to 10th	Day	44% 6% 10% 23% 14% 1	1872
	11th to 20th	Day	51% 5% 12% 21% 10% 1	1800
	21st to end	Day	59% 5% 12% 13% 9% 1	1954
November	1st to 10th	Day	49% 5% 11% 21% 13% 1	1672
	11th to 20th	Day	59% 3 10% 17% 8% 2	1710
	21st to end	Day	68% 4 9% 13% 6%	1520
December	1st to 10th	Day	73% 3 11% 10% 3	1520
	11th to 20th	Day	72% 3 10% 12% 3	1906
	21st to end	Day	80% 3 9% 6% 1	2962
January	1st to 10th	Night	75% 3 8% 10% 3	4109
	11th to 20th	Night	82% 3 6% 7% 1	4120
	21st to end	Night	79% 2 8% 9% 2	4532
February	1st to 10th	Night	68% 4 10% 13% 5%	2470
	11th to 20th	Night	65% 5% 8% 16% 6%	2334
	21st to end	Night	65% 3 9% 14% 7% 1	1884
March	1st to 10th	Night	56% 2 11% 22% 8%	3519
	11th to 20th	Night	50% 4 15% 20% 9% 1	3480
	21st to end	Night	53% 5% 9% 24% 9% 1	3817
April	1st to 10th	Night	53% 5% 11% 17% 12% 2	2066
	11th to 20th	Night	50% 5% 11% 17% 14% 3	1711
	21st to end	Night	46% 5% 11% 21% 14% 3	1710
May	1st to 10th	Night	54% 5% 9% 20% 10% 2	2840
	11th to 20th	Night	46% 5% 12% 23% 13% 2	2813
	21st to end	Night	32% 6% 15% 26% 18% 4	3087
June	1st to 10th	Night	41% 6% 10% 26% 15% 3	1520
	11th to 20th	Night	38% 6% 15% 20% 18% 3	1495
	21st to end	Night	36% 7% 13% 26% 16% 2	1374
July	1st to 10th	Night	35% 6% 11% 25% 18% 6%	2610
	11th to 20th	Night	32% 7% 12% 28% 19% 3	2786
	21st to end	Night	35% 8% 13% 20% 23% 2	2988
August	1st to 10th	Night	33% 6% 15% 28% 18% 1	1690
	11th to 20th	Night	36% 6% 15% 28% 13% 2	1643
	21st to end	Night	36% 6% 12% 25% 18% 4	1787
September	1st to 10th	Night	38% 6% 12% 26% 14% 4	1912
	11th to 20th	Night	33% 5% 13% 27% 21% 2	1980
	21st to end	Night	44% 4 11% 22% 18% 1	1980
October	1st to 10th	Night	40% 5% 13% 22% 19% 1	2088
	11th to 20th	Night	51% 4 11% 21% 12% 1	2160
	21st to end	Night	59% 4 9% 18% 9% 1	2398
November	1st to 10th	Night	48% 5% 12% 20% 13% 2	2406
	11th to 20th	Night	59% 3 11% 17% 8% 2	2470
	21st to end	Night	64% 4 11% 15% 5%	2656
December	1st to 10th	Night	69% 4 11% 11% 4	2660
	11th to 20th	Night	67% 4 14% 14% 2	3288
	21st to end	Night	77% 4 11% 7% 1	4689

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	27% 22% 33% 11% 6% 1	270
	11th to 20th	Day	31% 24% 32% 10% 3 1	270
	21st to end	Day	29% 26% 30% 14% 2	297
February	1st to 10th	Day	31% 20% 35% 11% 2	270
	11th to 20th	Day	29% 26% 32% 12% 1	270
	21st to end	Day	31% 29% 30% 9%	225
March	1st to 10th	Day	27% 20% 30% 18% 5% 1	393
	11th to 20th	Day	27% 17% 34% 17% 5% 1	460
	21st to end	Day	33% 17% 27% 18% 6%	506
April	1st to 10th	Day	22% 20% 33% 18% 6% 1	460
	11th to 20th	Day	28% 18% 24% 19% 8% 3	452
	21st to end	Day	28% 15% 33% 16% 6% 2	460
May	1st to 10th	Day	24% 16% 27% 23% 6% 3	463
	11th to 20th	Day	21% 12% 30% 23% 11% 4	487
	21st to end	Day	14% 9% 26% 29% 17% 5%	529
June	1st to 10th	Day	16% 7% 30% 31% 12% 3	490
	11th to 20th	Day	17% 8% 24% 33% 15% 4	503
	21st to end	Day	15% 8% 33% 29% 13% 3	680
July	1st to 10th	Day	13% 5% 24% 28% 21% 9%	590
	11th to 20th	Day	15% 6% 22% 30% 20% 7%	484
	21st to end	Day	11% 5% 26% 31% 20% 7%	532
August	1st to 10th	Day	13% 7% 20% 29% 23% 9%	486
	11th to 20th	Day	14% 5% 23% 29% 21% 9%	456
	21st to end	Day	12% 6% 22% 28% 23% 9%	495
September	1st to 10th	Day	16% 5% 23% 26% 20% 10%	450
	11th to 20th	Day	14% 4 19% 26% 25% 12%	450
	21st to end	Day	20% 12% 25% 21% 13% 8%	450
October	1st to 10th	Day	14% 9% 26% 25% 19% 6%	450
	11th to 20th	Day	22% 16% 33% 20% 8% 1	450
	21st to end	Day	32% 18% 28% 17% 5%	469
November	1st to 10th	Day	20% 23% 30% 16% 10% 1	270
	11th to 20th	Day	31% 21% 31% 11% 5% 1	270
	21st to end	Day	29% 21% 34% 16% 1	180
December	1st to 10th	Day	23% 27% 30% 13% 5% 2	180
	11th to 20th	Day	23% 29% 34% 12% 2	180
	21st to end	Day	29% 23% 33% 14% 1	252
January	1st to 10th	Night	30% 21% 31% 14% 4	760
	11th to 20th	Night	32% 18% 30% 15% 4	760
	21st to end	Night	29% 16% 31% 21% 3	836
February	1st to 10th	Night	30% 16% 30% 22% 3	760
	11th to 20th	Night	26% 17% 34% 20% 3	743
	21st to end	Night	28% 17% 34% 19% 2	628
March	1st to 10th	Night	24% 15% 33% 23% 5%	760
	11th to 20th	Night	23% 15% 31% 23% 7% 1	760
	21st to end	Night	26% 12% 27% 26% 9% 1	836
April	1st to 10th	Night	22% 16% 30% 27% 5% 1	760
	11th to 20th	Night	21% 12% 33% 23% 10% 1	587
	21st to end	Night	19% 10% 34% 28% 8% 1	570
May	1st to 10th	Night	22% 8% 26% 33% 9% 1	570
	11th to 20th	Night	20% 6% 24% 35% 12% 2	561
	21st to end	Night	12% 3 22% 37% 22% 3	617
June	1st to 10th	Night	11% 5% 27% 40% 17% 1	380
	11th to 20th	Night	11% 5% 25% 41% 17% 1	380
	21st to end	Night	10% 4 27% 38% 21%	424
July	1st to 10th	Night	12% 1 15% 39% 30% 3	555
	11th to 20th	Night	11% 3 21% 36% 26% 4	552
	21st to end	Night	10% 4 16% 40% 29% 2	624
August	1st to 10th	Night	11% 3 14% 42% 26% 4	567
	11th to 20th	Night	12% 4 13% 39% 29% 4	549
	21st to end	Night	12% 3 18% 32% 30% 5%	599
September	1st to 10th	Night	15% 6% 15% 34% 25% 5%	720
	11th to 20th	Night	13% 4 17% 31% 29% 5%	720
	21st to end	Night	18% 7% 24% 28% 18% 5%	720
October	1st to 10th	Night	18% 8% 22% 29% 20% 3	720
	11th to 20th	Night	25% 14% 28% 23% 8% 1	720
	21st to end	Night	34% 13% 27% 22% 5%	792
November	1st to 10th	Night	23% 14% 29% 24% 10% 1	740
	11th to 20th	Night	30% 16% 30% 17% 6% 1	760
	21st to end	Night	33% 16% 29% 17% 5%	760
December	1st to 10th	Night	32% 16% 32% 15% 5%	760
	11th to 20th	Night	29% 19% 31% 18% 3	760
	21st to end	Night	30% 16% 33% 18% 2	836

Page generated at 2024-08-12 09:52:54. Version ac07

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