Behlen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Behlen 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	11% 7% 3 6% 5% 3 4 4 5% 7% 45%	74610
	Night	10% 7% 3 5% 4 2 4 4 5% 7% 49%	75128
Transparency	Day	39% 3 14% 27% 15% 2	74333
	Night	36% 3 13% 28% 17% 3	74925
Seeing	Day	15% 6% 28% 31% 16% 3	14627
	Night	14% 7% 31% 36% 11% 1	24638

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	10% 10% 4 6% 8% 4 5% 3 5% 7% 39%	1707
	11th to 20th	Day	10% 8% 4 9% 6% 3 4 5% 4 6% 40%	1634
	21st to end	Day	10% 10% 4 11% 7% 3 5% 4 5% 7% 34%	1782
February	1st to 10th	Day	10% 9% 5% 10% 6% 4 5% 4 5% 6% 34%	1620
	11th to 20th	Day	10% 10% 3 7% 6% 3 5% 5% 5% 7% 39%	1621
	21st to end	Day	11% 8% 4 6% 5% 3 5% 5% 5% 8% 41%	1480
March	1st to 10th	Day	10% 9% 5% 7% 4 3 4 4 4 7% 44%	1935
	11th to 20th	Day	15% 9% 3 7% 5% 2 4 4 4 6% 41%	1980
	21st to end	Day	19% 11% 4 6% 6% 3 3 5% 5% 7% 31%	2178
April	1st to 10th	Day	15% 7% 3 4 5% 3 3 4 5% 6% 44%	2020
	11th to 20th	Day	16% 9% 3 5% 5% 3 4 4 6% 5% 40%	2257
	21st to end	Day	20% 7% 3 5% 5% 3 4 4 4 7% 39%	2340
May	1st to 10th	Day	15% 8% 3 6% 6% 4 6% 5% 5% 6% 37%	2340
	11th to 20th	Day	14% 6% 3 6% 6% 3 4 4 6% 8% 40%	2302
	21st to end	Day	14% 7% 3 7% 5% 3 6% 5% 7% 8% 35%	2562
June	1st to 10th	Day	7% 4 2 4 5% 2 5% 5% 6% 9% 50%	2520
	11th to 20th	Day	9% 4 3 4 4 3 5% 5% 6% 9% 48%	2520
	21st to end	Day	7% 4 2 5% 5% 3 5% 5% 6% 8% 51%	2510
July	1st to 10th	Day	9% 4 2 3 4 3 4 5% 6% 10% 51%	2340
	11th to 20th	Day	5% 4 2 5% 5% 3 3 4 5% 9% 56%	2340
	21st to end	Day	5% 4 2 4 4 3 5% 6% 7% 10% 50%	2546
August	1st to 10th	Day	6% 5% 2 6% 6% 3 6% 6% 7% 10% 43%	2328
	11th to 20th	Day	7% 4 2 4 4 3 5% 5% 6% 10% 49%	2301
	21st to end	Day	8% 4 2 4 4 3 4 4 5% 8% 53%	2547
September	1st to 10th	Day	13% 4 2 4 5% 2 4 4 4 5% 53%	2070
	11th to 20th	Day	11% 5% 1 4 5% 2 4 4 5% 7% 53%	1980
	21st to end	Day	10% 3 2 4 5% 2 3 4 5% 6% 57%	1980
October	1st to 10th	Day	14% 6% 2 4 4 3 4 4 4 6% 49%	1854
	11th to 20th	Day	12% 5% 2 4 5% 2 3 4 4 4 55%	1800
	21st to end	Day	13% 7% 3 5% 4 3 4 4 4 6% 47%	1980
November	1st to 10th	Day	9% 5% 3 4 3 2 3 3 5% 6% 56%	1678
	11th to 20th	Day	10% 6% 3 6% 7% 3 4 4 4 6% 48%	1680
	21st to end	Day	14% 8% 3 4 5% 2 3 4 4 8% 45%	1520
December	1st to 10th	Day	9% 8% 3 6% 4 3 4 5% 5% 8% 47%	1520
	11th to 20th	Day	8% 9% 4 8% 6% 3 5% 5% 5% 7% 39%	1918
	21st to end	Day	15% 10% 5% 13% 7% 4 4 5% 4 6% 27%	2920
January	1st to 10th	Night	7% 8% 3 8% 7% 4 4 4 5% 7% 43%	2458
	11th to 20th	Night	9% 11% 4 8% 6% 3 4 4 5% 6% 41%	2366
	21st to end	Night	11% 9% 4 8% 5% 3 4 4 4 6% 42%	2574
February	1st to 10th	Night	12% 10% 4 8% 6% 3 4 4 5% 6% 38%	2340
	11th to 20th	Night	9% 7% 3 6% 5% 3 6% 4 6% 8% 42%	2241
	21st to end	Night	10% 7% 3 5% 4 3 4 5% 5% 6% 49%	1776
March	1st to 10th	Night	10% 7% 3 6% 5% 2 3 4 4 6% 49%	2023
	11th to 20th	Night	15% 9% 3 5% 3 2 3 4 3 7% 46%	1980
	21st to end	Night	17% 10% 4 6% 6% 3 4 4 6% 7% 33%	2178
April	1st to 10th	Night	12% 7% 3 5% 4 2 4 5% 4 7% 47%	1940
	11th to 20th	Night	12% 7% 3 4 5% 3 4 4 4 7% 47%	1651
	21st to end	Night	15% 7% 3 5% 4 3 4 4 4 7% 45%	1620
May	1st to 10th	Night	15% 6% 2 6% 5% 3 4 5% 6% 8% 39%	1620
	11th to 20th	Night	13% 6% 2 4 5% 3 4 4 5% 7% 46%	1593
	21st to end	Night	13% 7% 3 5% 5% 3 4 5% 5% 8% 42%	1709
June	1st to 10th	Night	4 4 2 4 5% 3 5% 5% 5% 9% 54%	1440
	11th to 20th	Night	7% 4 1 4 5% 3 5% 5% 6% 10% 51%	1440
	21st to end	Night	4 3 1 3 4 3 5% 4 7% 9% 58%	1444
July	1st to 10th	Night	6% 4 2 4 4 3 4 3 5% 9% 56%	1620
	11th to 20th	Night	3 1 2 3 3 2 4 5% 7% 10% 61%	1620
	21st to end	Night	3 2 2 3 4 2 3 4 6% 11% 58%	1772
August	1st to 10th	Night	5% 4 2 4 5% 2 5% 6% 5% 9% 54%	1611
	11th to 20th	Night	6% 4 1 4 4 2 4 4 6% 10% 57%	1600
	21st to end	Night	7% 3 2 4 4 2 4 4 5% 6% 58%	1808
September	1st to 10th	Night	8% 4 2 3 2 2 3 3 5% 6% 61%	1886
	11th to 20th	Night	9% 3 1 3 2 2 4 4 6% 8% 59%	1980
	21st to end	Night	9% 5% 1 3 3 2 4 4 4 5% 60%	1980
October	1st to 10th	Night	14% 4 2 3 4 3 4 5% 5% 7% 49%	2086
	11th to 20th	Night	14% 5% 2 4 4 2 3 3 4 6% 56%	2160
	21st to end	Night	14% 5% 2 4 4 2 3 3 4 5% 53%	2376
November	1st to 10th	Night	7% 4 2 3 4 2 4 5% 5% 7% 58%	2398
	11th to 20th	Night	9% 6% 3 5% 4 2 3 3 5% 6% 53%	2497
	21st to end	Night	11% 6% 2 4 4 2 3 4 4 7% 52%	2660
December	1st to 10th	Night	9% 7% 3 6% 6% 3 4 5% 5% 7% 46%	2660
	11th to 20th	Night	9% 8% 3 6% 5% 2 4 3 4 8% 49%	3279
	21st to end	Night	18% 14% 6% 10% 5% 2 3 3 3 4 33%	4742

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	47% 2 16% 23% 11% 1	1708
	11th to 20th	Day	45% 3 14% 25% 13% 1	1634
	21st to end	Day	51% 3 15% 21% 9% 1	1782
February	1st to 10th	Day	50% 4 14% 20% 11% 1	1620
	11th to 20th	Day	45% 4 12% 23% 14% 3	1630
	21st to end	Day	42% 4 13% 25% 14% 3	1480
March	1st to 10th	Day	42% 3 13% 26% 14% 2	1935
	11th to 20th	Day	46% 3 10% 25% 14% 2	1980
	21st to end	Day	54% 4 15% 19% 7% 1	2178
April	1st to 10th	Day	42% 3 14% 25% 15% 1	2020
	11th to 20th	Day	45% 3 13% 23% 13% 3	2257
	21st to end	Day	47% 3 10% 26% 13% 1	2340
May	1st to 10th	Day	48% 4 12% 20% 14% 1	2340
	11th to 20th	Day	42% 3 15% 26% 13%	2306
	21st to end	Day	47% 4 16% 21% 11% 1	2562
June	1st to 10th	Day	31% 4 18% 32% 14% 2	2520
	11th to 20th	Day	34% 4 13% 31% 16% 2	2520
	21st to end	Day	31% 4 17% 29% 15% 4	2510
July	1st to 10th	Day	29% 3 17% 33% 17% 2	2276
	11th to 20th	Day	27% 4 13% 30% 21% 5%	2271
	21st to end	Day	28% 5% 18% 31% 18% 1	2405
August	1st to 10th	Day	34% 5% 20% 28% 11% 2	2314
	11th to 20th	Day	30% 4 15% 32% 18% 1	2301
	21st to end	Day	30% 4 12% 29% 20% 6%	2547
September	1st to 10th	Day	34% 3 9% 30% 18% 6%	2070
	11th to 20th	Day	32% 3 11% 34% 19% 1	1980
	21st to end	Day	29% 2 9% 33% 24% 1	1980
October	1st to 10th	Day	37% 3 11% 27% 21% 2	1854
	11th to 20th	Day	33% 3 9% 27% 23% 6%	1800
	21st to end	Day	39% 2 13% 22% 20% 4	1980
November	1st to 10th	Day	30% 2 14% 30% 21% 2	1678
	11th to 20th	Day	38% 3 12% 26% 20% 1	1680
	21st to end	Day	41% 2 11% 31% 14% 2	1520
December	1st to 10th	Day	38% 3 15% 29% 14% 2	1520
	11th to 20th	Day	44% 4 16% 29% 7%	1915
	21st to end	Day	59% 2 11% 20% 7% 1	2920
January	1st to 10th	Night	42% 3 14% 23% 17% 1	2458
	11th to 20th	Night	44% 3 14% 28% 10%	2366
	21st to end	Night	44% 3 15% 25% 12% 2	2574
February	1st to 10th	Night	48% 2 13% 21% 13% 2	2340
	11th to 20th	Night	41% 3 14% 24% 14% 3	2242
	21st to end	Night	37% 3 11% 27% 19% 4	1776
March	1st to 10th	Night	38% 3 11% 29% 15% 3	2023
	11th to 20th	Night	41% 3 9% 26% 18% 3	1980
	21st to end	Night	50% 4 15% 20% 10% 1	2178
April	1st to 10th	Night	37% 5% 10% 27% 17% 3	1940
	11th to 20th	Night	39% 3 11% 27% 17% 2	1651
	21st to end	Night	40% 4 13% 24% 18% 1	1620
May	1st to 10th	Night	43% 4 14% 23% 15% 2	1620
	11th to 20th	Night	38% 3 15% 25% 18%	1594
	21st to end	Night	41% 4 19% 20% 13% 3	1710
June	1st to 10th	Night	27% 5% 14% 35% 17% 3	1440
	11th to 20th	Night	28% 5% 15% 27% 20% 5%	1440
	21st to end	Night	22% 4 15% 34% 20% 4	1444
July	1st to 10th	Night	27% 3 14% 31% 20% 4	1575
	11th to 20th	Night	20% 4 18% 32% 21% 6%	1566
	21st to end	Night	20% 3 17% 38% 20% 2	1673
August	1st to 10th	Night	28% 5% 19% 33% 15% 1	1601
	11th to 20th	Night	25% 3 14% 34% 22% 2	1600
	21st to end	Night	27% 3 12% 27% 22% 9%	1808
September	1st to 10th	Night	25% 3 12% 31% 23% 7%	1886
	11th to 20th	Night	25% 3 11% 37% 20% 4	1980
	21st to end	Night	28% 2 7% 31% 27% 4	1980
October	1st to 10th	Night	35% 3 10% 26% 23% 3	2086
	11th to 20th	Night	31% 3 7% 25% 27% 6%	2160
	21st to end	Night	35% 2 11% 24% 25% 3	2376
November	1st to 10th	Night	27% 2 15% 27% 25% 4	2398
	11th to 20th	Night	34% 2 12% 30% 19% 3	2497
	21st to end	Night	35% 2 12% 28% 20% 2	2660
December	1st to 10th	Night	38% 4 13% 30% 14% 1	2660
	11th to 20th	Night	36% 3 14% 36% 11%	3281
	21st to end	Night	57% 2 10% 22% 8% 1	4742

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	20% 9% 34% 26% 9% 2	270
	11th to 20th	Day	20% 12% 34% 27% 6% 1	245
	21st to end	Day	17% 9% 43% 22% 8% 1	264
February	1st to 10th	Day	16% 18% 37% 25% 4	240
	11th to 20th	Day	17% 13% 38% 23% 8% 1	240
	21st to end	Day	16% 18% 39% 20% 6% 1	198
March	1st to 10th	Day	14% 12% 43% 25% 5% 2	240
	11th to 20th	Day	29% 6% 33% 23% 9% 1	240
	21st to end	Day	30% 13% 36% 16% 4 1	264
April	1st to 10th	Day	20% 13% 38% 25% 5%	280
	11th to 20th	Day	21% 12% 37% 21% 9%	535
	21st to end	Day	25% 11% 30% 23% 9% 2	600
May	1st to 10th	Day	22% 7% 32% 27% 9% 2	603
	11th to 20th	Day	20% 7% 30% 27% 14% 3	625
	21st to end	Day	19% 6% 26% 30% 17% 3	679
June	1st to 10th	Day	8% 3 23% 33% 27% 7%	630
	11th to 20th	Day	10% 3 26% 36% 19% 5%	630
	21st to end	Day	8% 2 32% 36% 18% 3	630
July	1st to 10th	Day	10% 1 14% 33% 30% 11%	621
	11th to 20th	Day	7% 1 20% 39% 25% 8%	618
	21st to end	Day	7% 1 17% 41% 30% 4	684
August	1st to 10th	Day	7% 1 17% 42% 27% 5%	626
	11th to 20th	Day	8% 2 16% 42% 28% 4	615
	21st to end	Day	11% 2 18% 37% 26% 5%	666
September	1st to 10th	Day	17% 2 23% 36% 19% 4	360
	11th to 20th	Day	7% 2 29% 36% 21% 6%	270
	21st to end	Day	14% 6% 32% 31% 15% 2	270
October	1st to 10th	Day	23% 4 23% 28% 16% 7%	270
	11th to 20th	Day	13% 14% 38% 24% 9% 1	270
	21st to end	Day	20% 11% 34% 21% 12% 2	297
November	1st to 10th	Day	17% 9% 36% 26% 9% 3	270
	11th to 20th	Day	14% 12% 41% 25% 7% 1	270
	21st to end	Day	24% 9% 30% 27% 10% 1	270
December	1st to 10th	Day	10% 9% 38% 33% 9% 1	270
	11th to 20th	Day	19% 6% 36% 29% 10% 1	270
	21st to end	Day	21% 11% 40% 21% 6%	297
January	1st to 10th	Night	12% 11% 33% 35% 8% 1	950
	11th to 20th	Night	16% 10% 35% 32% 7% 1	910
	21st to end	Night	17% 7% 39% 31% 6% 1	990
February	1st to 10th	Night	15% 10% 38% 31% 5%	900
	11th to 20th	Night	14% 9% 40% 31% 5%	847
	21st to end	Night	16% 9% 40% 30% 5% 1	592
March	1st to 10th	Night	16% 12% 35% 28% 9%	583
	11th to 20th	Night	20% 6% 35% 31% 7%	540
	21st to end	Night	23% 8% 36% 27% 6%	594
April	1st to 10th	Night	15% 14% 39% 27% 6%	540
	11th to 20th	Night	17% 11% 38% 30% 4 1	533
	21st to end	Night	20% 8% 33% 30% 8%	540
May	1st to 10th	Night	17% 9% 33% 32% 9%	540
	11th to 20th	Night	16% 8% 30% 34% 11% 1	531
	21st to end	Night	16% 5% 28% 35% 14% 1	584
June	1st to 10th	Night	8% 3 21% 47% 20% 1	540
	11th to 20th	Night	10% 3 28% 43% 15% 1	540
	21st to end	Night	5% 2 31% 45% 16%	540
July	1st to 10th	Night	8% 1 15% 44% 30% 2	525
	11th to 20th	Night	3 2 17% 52% 25% 2	522
	21st to end	Night	4 1 12% 56% 27% 1	591
August	1st to 10th	Night	6% 2 16% 49% 25% 2	537
	11th to 20th	Night	7% 2 16% 51% 24% 1	534
	21st to end	Night	9% 1 19% 43% 24% 3	594
September	1st to 10th	Night	11% 2 24% 40% 20% 2	540
	11th to 20th	Night	10% 1 29% 41% 18% 2	540
	21st to end	Night	12% 4 32% 34% 17% 1	540
October	1st to 10th	Night	18% 5% 29% 34% 12% 2	646
	11th to 20th	Night	17% 8% 35% 31% 9%	720
	21st to end	Night	19% 8% 29% 35% 10%	792
November	1st to 10th	Night	9% 9% 33% 37% 10% 1	918
	11th to 20th	Night	13% 9% 39% 32% 6% 1	950
	21st to end	Night	16% 10% 33% 32% 9% 1	950
December	1st to 10th	Night	14% 10% 38% 31% 7%	950
	11th to 20th	Night	16% 7% 32% 37% 7%	950
	21st to end	Night	20% 9% 33% 32% 5%	1045

Page generated at 2024-01-09 15:24:28. Version ac07

Copyright 2024 Attilla Danko