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 2025-01-22.

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 9% 5% 4 4 4 5% 7% 31%	105176
	Night	17% 13% 4 9% 5% 4 4 3 5% 6% 30%	104086
Transparency	Day	52% 5% 11% 20% 11% 1	108853
	Night	54% 5% 11% 19% 10% 1	108549
Seeing	Day	21% 14% 28% 22% 12% 4	15099
	Night	22% 11% 27% 26% 12% 1	24768

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	18% 23% 8% 15% 4 4 3 2 3 4 14%	2926
	11th to 20th	Day	16% 24% 8% 13% 7% 6% 5% 3 3 4 11%	2930
	21st to end	Day	27% 21% 11% 13% 8% 4 2 2 3 2 7%	3133
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	13% 6% 3 7% 5% 5% 5% 4 7% 9% 37%	4104
	11th to 20th	Day	12% 7% 2 7% 4 5% 5% 5% 7% 9% 36%	4044
	21st to end	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	4479
September	1st to 10th	Day	14% 6% 2 6% 4 4 5% 5% 6% 7% 42%	2157
	11th to 20th	Day	12% 7% 3 6% 5% 3 4 4 6% 8% 44%	2090
	21st to end	Day	20% 8% 3 6% 4 4 3 3 5% 7% 37%	2090
October	1st to 10th	Day	15% 7% 3 7% 4 4 4 4 6% 7% 39%	1976
	11th to 20th	Day	20% 9% 3 8% 4 4 3 3 5% 6% 34%	1900
	21st to end	Day	24% 11% 3 7% 5% 4 5% 3 5% 6% 28%	2062
November	1st to 10th	Day	15% 11% 3 7% 5% 4 4 3 5% 6% 36%	1762
	11th to 20th	Day	19% 18% 2 9% 4 4 4 3 5% 5% 27%	1800
	21st to end	Day	20% 19% 7% 10% 5% 5% 4 3 5% 6% 18%	1600
December	1st to 10th	Day	17% 18% 7% 13% 7% 5% 4 3 4 5% 16%	2600
	11th to 20th	Day	22% 21% 8% 12% 5% 3 3 3 4 5% 15%	2990
	21st to end	Day	23% 23% 7% 14% 5% 4 4 3 4 4 9%	4234
January	1st to 10th	Night	16% 25% 10% 14% 5% 3 3 2 4 4 14%	4239
	11th to 20th	Night	14% 21% 10% 18% 6% 4 6% 4 4 3 10%	4250
	21st to end	Night	27% 21% 7% 14% 5% 4 2 2 3 3 13%	4546
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	11% 7% 2 8% 4 5% 5% 4 6% 9% 40%	2825
	11th to 20th	Night	14% 8% 3 7% 4 4 3 3 5% 7% 42%	2785
	21st to end	Night	8% 6% 3 7% 5% 5% 6% 5% 6% 8% 43%	3143
September	1st to 10th	Night	12% 6% 2 5% 5% 4 4 4 6% 8% 44%	2019
	11th to 20th	Night	11% 4 2 5% 3 4 4 3 5% 7% 51%	2090
	21st to end	Night	17% 7% 2 7% 5% 5% 5% 3 4 6% 39%	2090
October	1st to 10th	Night	14% 6% 2 6% 4 4 5% 3 6% 7% 43%	2204
	11th to 20th	Night	21% 9% 2 7% 4 4 4 3 5% 6% 34%	2280
	21st to end	Night	26% 10% 3 7% 5% 4 3 3 5% 6% 28%	2532
November	1st to 10th	Night	16% 11% 3 7% 5% 4 3 3 5% 6% 37%	2536
	11th to 20th	Night	19% 18% 3 9% 5% 4 4 3 5% 5% 27%	2600
	21st to end	Night	23% 18% 4 8% 5% 4 4 3 5% 5% 22%	2796
December	1st to 10th	Night	16% 18% 6% 12% 6% 5% 4 3 5% 6% 18%	4580
	11th to 20th	Night	20% 22% 7% 10% 4 4 4 3 4 4 18%	5205
	21st to end	Night	22% 21% 6% 13% 5% 5% 4 3 4 5% 12%	6703

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	2927
	11th to 20th	Day	78% 3 8% 9% 2	2930
	21st to end	Day	85% 2 5% 5% 2	3133
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	42% 7% 13% 25% 12%	4091
	11th to 20th	Day	41% 6% 11% 30% 10% 1	4083
	21st to end	Day	38% 6% 14% 29% 11% 2	4479
September	1st to 10th	Day	39% 7% 14% 22% 16% 3	2157
	11th to 20th	Day	38% 6% 13% 24% 17% 2	2090
	21st to end	Day	47% 4 10% 22% 17% 1	2090
October	1st to 10th	Day	35% 6% 13% 28% 19% 1	3288
	11th to 20th	Day	47% 4 10% 20% 17% 2	3140
	21st to end	Day	44% 4 13% 23% 15% 1	3376
November	1st to 10th	Day	49% 4 11% 21% 13% 1	1762
	11th to 20th	Day	60% 4 10% 16% 9% 2	1800
	21st to end	Day	69% 4 9% 12% 6%	1600
December	1st to 10th	Day	69% 6% 13% 10% 2	2600
	11th to 20th	Day	73% 3 9% 12% 3	2986
	21st to end	Day	80% 4 7% 7% 1	4234
January	1st to 10th	Night	76% 3 8% 10% 3	4239
	11th to 20th	Night	82% 4 6% 7% 1	4250
	21st to end	Night	79% 2 8% 9% 2	4546
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	40% 7% 12% 26% 14% 1	2814
	11th to 20th	Night	41% 6% 10% 29% 13% 1	2812
	21st to end	Night	37% 7% 12% 28% 14% 2	3143
September	1st to 10th	Night	37% 5% 12% 27% 15% 4	2019
	11th to 20th	Night	31% 5% 13% 27% 21% 2	2090
	21st to end	Night	46% 4 11% 21% 17% 1	2090
October	1st to 10th	Night	31% 5% 9% 31% 23% 1	3672
	11th to 20th	Night	48% 4 9% 19% 18% 1	3779
	21st to end	Night	43% 4 10% 25% 16% 1	4168
November	1st to 10th	Night	47% 5% 12% 21% 13% 2	2536
	11th to 20th	Night	60% 4 10% 16% 8% 2	2600
	21st to end	Night	65% 4 11% 15% 4	2796
December	1st to 10th	Night	65% 6% 13% 13% 3	4580
	11th to 20th	Night	70% 4 10% 13% 2	5208
	21st to end	Night	75% 5% 10% 9% 1	6703

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% 34% 11% 6% 1	300
	11th to 20th	Day	30% 26% 32% 9% 2 1	300
	21st to end	Day	30% 25% 29% 14% 2	300
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% 6% 23% 29% 20% 8%	479
	21st to end	Day	12% 6% 22% 28% 23% 8%	539
September	1st to 10th	Day	15% 6% 23% 26% 20% 9%	490
	11th to 20th	Day	13% 4 18% 28% 25% 12%	490
	21st to end	Day	22% 13% 25% 20% 12% 8%	490
October	1st to 10th	Day	13% 10% 26% 25% 19% 6%	490
	11th to 20th	Day	23% 16% 32% 20% 8% 2	490
	21st to end	Day	30% 18% 29% 18% 5%	511
November	1st to 10th	Day	20% 24% 31% 15% 9% 1	300
	11th to 20th	Day	31% 21% 32% 10% 5% 1	300
	21st to end	Day	31% 23% 32% 14% 1	200
December	1st to 10th	Day	24% 26% 32% 12% 5% 2	200
	11th to 20th	Day	26% 27% 33% 13% 2	200
	21st to end	Day	30% 24% 32% 13% 1	280
January	1st to 10th	Night	30% 20% 32% 14% 4	800
	11th to 20th	Night	32% 19% 30% 15% 4	800
	21st to end	Night	29% 17% 31% 21% 3	841
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	13% 4 13% 38% 28% 4	564
	21st to end	Night	12% 3 18% 33% 30% 4	633
September	1st to 10th	Night	15% 6% 16% 34% 25% 5%	760
	11th to 20th	Night	12% 4 18% 32% 29% 5%	760
	21st to end	Night	19% 7% 24% 28% 18% 4	760
October	1st to 10th	Night	18% 8% 22% 29% 19% 3	760
	11th to 20th	Night	26% 13% 28% 23% 9% 1	760
	21st to end	Night	32% 13% 27% 23% 5%	836
November	1st to 10th	Night	23% 14% 30% 23% 10% 1	780
	11th to 20th	Night	31% 16% 30% 17% 6% 1	800
	21st to end	Night	34% 16% 29% 16% 5%	800
December	1st to 10th	Night	32% 16% 32% 15% 5%	800
	11th to 20th	Night	30% 20% 30% 18% 3	800
	21st to end	Night	30% 16% 34% 18% 2	880

Page generated at 2025-01-19 03:06:46. Version ac07

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