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 2024-01-12.

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	15% 8% 3 6% 6% 3 5% 5% 6% 7% 35%	74895
	Night	15% 8% 3 6% 5% 3 4 5% 5% 7% 37%	74826
Transparency	Day	30% 39% 7% 12% 9% 2	74618
	Night	30% 39% 7% 12% 9% 2	74622
Seeing	Day	22% 12% 21% 22% 16% 7%	13921
	Night	19% 9% 20% 31% 18% 2	24056

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% 11% 5% 10% 7% 2 4 3 4 5% 30%	1706
	11th to 20th	Day	20% 13% 5% 9% 5% 3 4 4 4 5% 27%	1629
	21st to end	Day	17% 12% 5% 6% 6% 3 5% 3 5% 5% 33%	1782
February	1st to 10th	Day	20% 12% 4 8% 5% 3 4 5% 4 6% 30%	1642
	11th to 20th	Day	23% 11% 5% 9% 5% 3 3 3 4 6% 28%	1748
	21st to end	Day	24% 11% 4 8% 3 3 3 3 4 6% 32%	1480
March	1st to 10th	Day	15% 10% 3 5% 5% 3 3 4 5% 7% 40%	1944
	11th to 20th	Day	19% 9% 4 7% 5% 3 4 4 6% 6% 31%	1980
	21st to end	Day	19% 11% 3 6% 5% 3 5% 4 6% 6% 33%	2178
April	1st to 10th	Day	14% 7% 3 5% 6% 3 4 4 7% 7% 39%	1980
	11th to 20th	Day	16% 6% 3 5% 4 3 4 5% 5% 7% 42%	2176
	21st to end	Day	16% 8% 3 5% 6% 3 6% 5% 5% 6% 37%	2340
May	1st to 10th	Day	16% 6% 3 6% 7% 4 6% 5% 6% 8% 33%	2340
	11th to 20th	Day	19% 8% 3 6% 6% 4 6% 5% 6% 7% 31%	2301
	21st to end	Day	12% 6% 3 6% 7% 4 5% 5% 7% 9% 35%	2531
June	1st to 10th	Day	11% 7% 3 6% 6% 4 6% 6% 7% 9% 36%	2340
	11th to 20th	Day	8% 6% 2 8% 8% 5% 6% 6% 8% 9% 33%	2340
	21st to end	Day	10% 6% 3 7% 7% 4 6% 7% 8% 9% 35%	2340
July	1st to 10th	Day	10% 7% 3 7% 7% 4 7% 7% 8% 9% 32%	2340
	11th to 20th	Day	13% 6% 3 6% 6% 4 6% 7% 8% 10% 31%	2340
	21st to end	Day	13% 7% 3 8% 7% 4 6% 7% 6% 10% 28%	2547
August	1st to 10th	Day	11% 6% 3 6% 8% 4 6% 7% 8% 10% 30%	2327
	11th to 20th	Day	13% 8% 2 8% 7% 4 6% 6% 7% 10% 29%	2303
	21st to end	Day	10% 5% 2 6% 5% 4 7% 6% 6% 9% 39%	2447
September	1st to 10th	Day	11% 5% 2 6% 5% 4 6% 6% 6% 9% 40%	1980
	11th to 20th	Day	13% 6% 3 5% 6% 3 5% 5% 6% 9% 40%	1980
	21st to end	Day	14% 6% 2 6% 6% 3 5% 4 6% 7% 40%	1980
October	1st to 10th	Day	13% 5% 3 5% 4 3 3 4 5% 7% 48%	1921
	11th to 20th	Day	15% 6% 2 5% 4 3 4 4 5% 5% 48%	1800
	21st to end	Day	18% 7% 2 5% 4 2 3 4 4 6% 44%	1980
November	1st to 10th	Day	12% 6% 2 4 5% 2 3 3 5% 6% 52%	1766
	11th to 20th	Day	12% 8% 3 5% 4 3 4 4 5% 7% 43%	1710
	21st to end	Day	15% 8% 3 6% 4 3 4 4 5% 6% 42%	1710
December	1st to 10th	Day	21% 12% 5% 7% 5% 3 4 4 4 5% 30%	1710
	11th to 20th	Day	17% 14% 4 6% 4 2 4 3 6% 8% 33%	2153
	21st to end	Day	17% 15% 6% 6% 5% 4 6% 5% 5% 6% 24%	3124
January	1st to 10th	Night	18% 12% 5% 8% 6% 3 4 4 5% 5% 30%	2459
	11th to 20th	Night	21% 13% 5% 9% 5% 3 4 4 4 5% 27%	2354
	21st to end	Night	17% 11% 5% 8% 5% 3 5% 4 3 5% 34%	2574
February	1st to 10th	Night	18% 10% 3 7% 5% 3 4 4 5% 7% 33%	2315
	11th to 20th	Night	22% 10% 5% 7% 6% 3 4 4 4 5% 30%	2098
	21st to end	Night	24% 11% 4 8% 5% 3 3 4 4 6% 29%	1776
March	1st to 10th	Night	17% 11% 4 5% 4 3 4 4 4 5% 39%	2015
	11th to 20th	Night	19% 9% 3 8% 6% 2 4 4 6% 7% 32%	1980
	21st to end	Night	15% 9% 4 6% 5% 4 5% 5% 5% 6% 36%	2178
April	1st to 10th	Night	14% 8% 3 6% 5% 2 4 5% 5% 7% 41%	1980
	11th to 20th	Night	14% 6% 2 6% 6% 3 4 4 5% 7% 42%	1746
	21st to end	Night	14% 6% 3 6% 5% 3 5% 6% 5% 6% 41%	1620
May	1st to 10th	Night	17% 7% 3 5% 6% 3 5% 5% 5% 8% 36%	1620
	11th to 20th	Night	15% 7% 4 6% 6% 4 5% 5% 7% 10% 32%	1592
	21st to end	Night	8% 7% 4 7% 6% 3 5% 6% 7% 10% 37%	1744
June	1st to 10th	Night	11% 5% 2 6% 4 3 6% 5% 7% 11% 39%	1620
	11th to 20th	Night	6% 5% 3 5% 6% 4 6% 7% 7% 9% 42%	1620
	21st to end	Night	11% 6% 3 6% 6% 4 5% 5% 6% 12% 37%	1620
July	1st to 10th	Night	11% 6% 3 7% 6% 3 4 5% 9% 9% 36%	1620
	11th to 20th	Night	9% 6% 2 5% 5% 3 6% 6% 7% 12% 37%	1620
	21st to end	Night	10% 5% 2 5% 7% 4 6% 7% 8% 11% 34%	1771
August	1st to 10th	Night	10% 7% 2 5% 7% 3 5% 6% 8% 12% 35%	1611
	11th to 20th	Night	12% 7% 3 7% 7% 4 6% 6% 7% 8% 34%	1598
	21st to end	Night	10% 5% 2 5% 5% 3 5% 6% 8% 9% 42%	1888
September	1st to 10th	Night	11% 6% 3 6% 6% 3 5% 4 5% 7% 43%	1980
	11th to 20th	Night	13% 6% 3 5% 4 3 4 4 6% 8% 44%	1980
	21st to end	Night	13% 6% 2 5% 5% 3 3 5% 5% 6% 47%	1980
October	1st to 10th	Night	14% 5% 2 5% 4 2 4 4 5% 6% 49%	2038
	11th to 20th	Night	16% 6% 2 4 4 3 3 4 4 6% 48%	2160
	21st to end	Night	16% 7% 3 4 3 3 4 3 4 7% 45%	2376
November	1st to 10th	Night	13% 7% 3 4 3 2 4 3 4 5% 52%	2309
	11th to 20th	Night	15% 8% 3 5% 3 2 4 3 4 7% 45%	2470
	21st to end	Night	14% 9% 3 5% 4 2 4 4 4 5% 45%	2470
December	1st to 10th	Night	21% 12% 4 7% 6% 3 4 3 4 7% 29%	2470
	11th to 20th	Night	18% 11% 5% 8% 5% 3 3 4 4 5% 34%	3042
	21st to end	Night	17% 15% 5% 7% 6% 4 5% 5% 5% 7% 23%	4532

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	38% 41% 5% 11% 5% 1	1707
	11th to 20th	Day	38% 42% 5% 9% 6% 1	1629
	21st to end	Day	36% 37% 5% 13% 8% 1	1782
February	1st to 10th	Day	36% 38% 5% 11% 9%	1642
	11th to 20th	Day	37% 37% 5% 11% 7% 1	1758
	21st to end	Day	37% 36% 6% 10% 8% 3	1480
March	1st to 10th	Day	27% 37% 5% 17% 13% 1	1944
	11th to 20th	Day	37% 38% 7% 10% 6% 2	1980
	21st to end	Day	36% 38% 7% 13% 5%	2178
April	1st to 10th	Day	25% 38% 8% 16% 10% 3	1980
	11th to 20th	Day	27% 37% 6% 13% 15% 3	2176
	21st to end	Day	30% 40% 7% 14% 7% 2	2340
May	1st to 10th	Day	33% 40% 7% 12% 8% 1	2340
	11th to 20th	Day	36% 41% 8% 10% 4 2	2305
	21st to end	Day	26% 42% 11% 15% 6% 1	2531
June	1st to 10th	Day	27% 42% 10% 13% 8%	2340
	11th to 20th	Day	29% 41% 8% 11% 9% 1	2340
	21st to end	Day	27% 41% 8% 12% 11% 1	2340
July	1st to 10th	Day	28% 44% 10% 10% 7% 1	2275
	11th to 20th	Day	30% 43% 9% 8% 8% 2	2271
	21st to end	Day	31% 43% 10% 10% 5%	2405
August	1st to 10th	Day	28% 42% 11% 13% 5%	2314
	11th to 20th	Day	30% 41% 10% 11% 7% 1	2303
	21st to end	Day	27% 41% 9% 13% 8% 2	2447
September	1st to 10th	Day	27% 40% 8% 13% 9% 3	1980
	11th to 20th	Day	27% 39% 6% 15% 11% 1	1980
	21st to end	Day	27% 39% 7% 11% 13% 3	1980
October	1st to 10th	Day	22% 38% 6% 15% 16% 3	1921
	11th to 20th	Day	26% 38% 6% 16% 12% 2	1800
	21st to end	Day	26% 39% 4 15% 11% 4	1980
November	1st to 10th	Day	17% 41% 5% 13% 20% 4	1766
	11th to 20th	Day	25% 40% 6% 13% 14% 3	1710
	21st to end	Day	26% 40% 5% 13% 13% 3	1710
December	1st to 10th	Day	35% 40% 6% 12% 7%	1710
	11th to 20th	Day	35% 33% 8% 10% 13% 1	2150
	21st to end	Day	44% 29% 7% 15% 5%	3124
January	1st to 10th	Night	36% 41% 7% 9% 7% 1	2459
	11th to 20th	Night	37% 42% 5% 7% 7% 1	2354
	21st to end	Night	36% 39% 5% 12% 7% 1	2574
February	1st to 10th	Night	34% 36% 7% 13% 9% 1	2315
	11th to 20th	Night	36% 38% 4 12% 7% 3	2099
	21st to end	Night	38% 37% 6% 11% 6% 2	1776
March	1st to 10th	Night	30% 37% 5% 12% 14% 2	2015
	11th to 20th	Night	34% 39% 8% 13% 6% 1	1980
	21st to end	Night	32% 39% 8% 13% 7% 1	2178
April	1st to 10th	Night	27% 38% 8% 14% 11% 3	1980
	11th to 20th	Night	25% 38% 6% 14% 13% 4	1746
	21st to end	Night	29% 38% 8% 16% 8% 1	1620
May	1st to 10th	Night	32% 40% 9% 12% 7% 1	1620
	11th to 20th	Night	32% 41% 9% 12% 5% 1	1593
	21st to end	Night	23% 42% 13% 14% 6% 1	1745
June	1st to 10th	Night	25% 41% 11% 12% 10%	1620
	11th to 20th	Night	24% 42% 11% 13% 9% 1	1620
	21st to end	Night	25% 41% 9% 13% 9% 3	1620
July	1st to 10th	Night	25% 42% 11% 11% 9% 2	1575
	11th to 20th	Night	26% 43% 11% 9% 6% 4	1566
	21st to end	Night	25% 44% 13% 13% 5%	1672
August	1st to 10th	Night	26% 42% 13% 13% 7% 1	1600
	11th to 20th	Night	29% 41% 10% 11% 8% 1	1598
	21st to end	Night	25% 40% 11% 16% 7% 1	1888
September	1st to 10th	Night	29% 39% 7% 12% 10% 3	1980
	11th to 20th	Night	26% 40% 8% 14% 12% 1	1980
	21st to end	Night	25% 39% 8% 13% 13% 2	1980
October	1st to 10th	Night	24% 37% 6% 14% 16% 3	2038
	11th to 20th	Night	26% 38% 7% 16% 11% 2	2160
	21st to end	Night	26% 38% 5% 14% 13% 4	2376
November	1st to 10th	Night	18% 41% 5% 12% 19% 4	2309
	11th to 20th	Night	25% 40% 6% 12% 13% 3	2470
	21st to end	Night	26% 40% 4 15% 13% 3	2470
December	1st to 10th	Night	34% 40% 6% 11% 8% 1	2470
	11th to 20th	Night	38% 34% 4 10% 12% 2	3044
	21st to end	Night	43% 30% 8% 12% 8%	4532

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	30% 29% 27% 11% 3 1	270
	11th to 20th	Day	37% 23% 26% 12% 1	243
	21st to end	Day	38% 23% 25% 11% 2	264
February	1st to 10th	Day	34% 21% 33% 10% 3	240
	11th to 20th	Day	47% 23% 23% 7% 1	240
	21st to end	Day	43% 19% 30% 7% 1	198
March	1st to 10th	Day	25% 21% 27% 22% 5%	384
	11th to 20th	Day	30% 19% 28% 17% 4 1	420
	21st to end	Day	32% 16% 24% 19% 8%	462
April	1st to 10th	Day	26% 17% 23% 22% 10% 2	420
	11th to 20th	Day	22% 17% 26% 19% 13% 2	412
	21st to end	Day	25% 15% 25% 20% 10% 4	420
May	1st to 10th	Day	27% 16% 24% 18% 9% 5%	423
	11th to 20th	Day	21% 7% 24% 25% 15% 7%	447
	21st to end	Day	15% 7% 20% 22% 22% 14%	485
June	1st to 10th	Day	15% 5% 20% 29% 23% 8%	450
	11th to 20th	Day	9% 8% 19% 25% 27% 12%	450
	21st to end	Day	14% 8% 23% 24% 21% 9%	450
July	1st to 10th	Day	16% 3 14% 23% 27% 17%	445
	11th to 20th	Day	14% 3 12% 30% 26% 14%	444
	21st to end	Day	16% 4 14% 29% 26% 11%	488
August	1st to 10th	Day	14% 6% 17% 24% 24% 15%	446
	11th to 20th	Day	16% 3 13% 28% 25% 15%	439
	21st to end	Day	12% 3 12% 29% 26% 19%	495
September	1st to 10th	Day	12% 5% 12% 26% 30% 15%	450
	11th to 20th	Day	14% 6% 11% 23% 30% 15%	450
	21st to end	Day	18% 8% 17% 28% 22% 7%	450
October	1st to 10th	Day	20% 4 18% 30% 22% 7%	450
	11th to 20th	Day	18% 14% 21% 26% 18% 3	450
	21st to end	Day	23% 11% 23% 26% 13% 4	495
November	1st to 10th	Day	17% 13% 24% 28% 14% 3	364
	11th to 20th	Day	21% 20% 28% 21% 9% 1	270
	21st to end	Day	19% 19% 34% 19% 7% 1	270
December	1st to 10th	Day	35% 15% 30% 16% 4	270
	11th to 20th	Day	29% 18% 27% 18% 8%	270
	21st to end	Day	31% 18% 30% 14% 6% 1	297
January	1st to 10th	Night	24% 18% 30% 24% 4	760
	11th to 20th	Night	30% 15% 27% 23% 5%	725
	21st to end	Night	22% 18% 33% 24% 2	792
February	1st to 10th	Night	24% 16% 29% 28% 3	720
	11th to 20th	Night	31% 19% 30% 19% 1	703
	21st to end	Night	32% 17% 28% 21% 2	592
March	1st to 10th	Night	23% 15% 30% 28% 5%	720
	11th to 20th	Night	25% 16% 27% 25% 7%	720
	21st to end	Night	20% 12% 31% 29% 8%	792
April	1st to 10th	Night	20% 15% 25% 30% 10%	720
	11th to 20th	Night	19% 12% 25% 31% 12% 1	632
	21st to end	Night	18% 7% 25% 39% 11% 1	540
May	1st to 10th	Night	17% 11% 22% 34% 15% 2	540
	11th to 20th	Night	16% 6% 18% 36% 22% 2	531
	21st to end	Night	11% 3 12% 34% 34% 7%	584
June	1st to 10th	Night	12% 2 12% 38% 31% 5%	540
	11th to 20th	Night	8% 1 12% 36% 36% 7%	540
	21st to end	Night	11% 2 11% 36% 35% 4	540
July	1st to 10th	Night	16% 3 8% 29% 40% 4	525
	11th to 20th	Night	12% 2 6% 31% 42% 7%	522
	21st to end	Night	11% 2 8% 34% 36% 9%	591
August	1st to 10th	Night	12% 1 7% 31% 44% 4	537
	11th to 20th	Night	14% 2 5% 35% 37% 7%	534
	21st to end	Night	13% 2 6% 30% 41% 8%	648
September	1st to 10th	Night	14% 4 10% 31% 36% 6%	720
	11th to 20th	Night	16% 3 10% 29% 36% 5%	720
	21st to end	Night	15% 4 16% 33% 29% 3	720
October	1st to 10th	Night	16% 5% 17% 29% 28% 5%	720
	11th to 20th	Night	19% 8% 21% 34% 17% 1	720
	21st to end	Night	20% 10% 20% 34% 14% 2	792
November	1st to 10th	Night	19% 8% 17% 39% 16% 2	740
	11th to 20th	Night	20% 11% 23% 36% 10% 1	760
	21st to end	Night	19% 11% 24% 34% 11% 1	760
December	1st to 10th	Night	28% 13% 26% 25% 7%	760
	11th to 20th	Night	25% 13% 26% 31% 5% 1	760
	21st to end	Night	28% 11% 25% 30% 5%	836

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