Bhills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bhills Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	127005
	Night	8% 6% 2 6% 4 4 5% 4 7% 9% 43%	124018
Transparency	Day	36% 6% 16% 21% 14% 6%	130753
	Night	37% 6% 15% 20% 16% 6%	128338
Seeing	Day	9% 4 17% 25% 25% 20%	16911
	Night	10% 3 17% 34% 28% 8%	26952

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	9% 5% 2 6% 4 3 4 3 4 7% 54%	4056
	11th to 20th	Day	19% 11% 4 7% 4 4 4 3 4 6% 34%	4060
	21st to end	Day	11% 5% 2 5% 3 4 4 3 6% 9% 49%	4760
February	1st to 10th	Day	12% 6% 3 7% 5% 4 4 4 6% 7% 43%	2000
	11th to 20th	Day	9% 4 2 5% 4 4 4 3 4 6% 53%	1948
	21st to end	Day	9% 6% 2 6% 5% 3 4 4 6% 7% 49%	1686
March	1st to 10th	Day	8% 6% 2 7% 5% 5% 4 4 6% 10% 44%	4935
	11th to 20th	Day	6% 4 1 5% 3 3 3 3 6% 9% 58%	4980
	21st to end	Day	9% 5% 2 5% 4 4 3 4 6% 9% 50%	5465
April	1st to 10th	Day	7% 4 1 5% 4 4 5% 4 6% 9% 51%	2200
	11th to 20th	Day	8% 5% 2 6% 5% 5% 4 5% 5% 7% 48%	2176
	21st to end	Day	4 3 1 4 4 4 4 4 7% 10% 55%	2460
May	1st to 10th	Day	3 3 2 5% 4 5% 5% 5% 7% 11% 51%	5862
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 5% 6% 9% 46%	5861
	21st to end	Day	3 3 2 4 4 4 5% 4 7% 12% 52%	6433
June	1st to 10th	Day	9% 7% 3 8% 6% 6% 6% 6% 8% 11% 29%	2600
	11th to 20th	Day	6% 5% 2 7% 7% 7% 8% 6% 10% 12% 30%	2600
	21st to end	Day	7% 7% 3 8% 6% 6% 8% 6% 9% 10% 30%	2600
July	1st to 10th	Day	5% 6% 2 8% 7% 8% 9% 8% 11% 16% 19%	5900
	11th to 20th	Day	4 4 2 7% 6% 7% 8% 8% 11% 16% 29%	5900
	21st to end	Day	4 4 2 7% 7% 7% 8% 7% 10% 15% 28%	6424
August	1st to 10th	Day	8% 5% 2 7% 6% 6% 7% 6% 8% 13% 30%	4234
	11th to 20th	Day	5% 4 2 7% 4 6% 7% 6% 9% 13% 36%	3892
	21st to end	Day	8% 6% 3 8% 7% 7% 7% 6% 9% 12% 25%	3982
September	1st to 10th	Day	8% 7% 3 10% 8% 7% 8% 6% 9% 11% 22%	2200
	11th to 20th	Day	9% 7% 3 10% 7% 7% 8% 6% 7% 9% 27%	2200
	21st to end	Day	8% 7% 3 9% 7% 5% 5% 6% 7% 9% 35%	2200
October	1st to 10th	Day	12% 7% 3 8% 5% 6% 5% 4 6% 7% 39%	2200
	11th to 20th	Day	5% 4 2 6% 4 4 4 3 5% 8% 55%	2033
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 8% 51%	2200
November	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 5% 8% 52%	2057
	11th to 20th	Day	7% 6% 2 6% 5% 4 3 4 5% 6% 51%	2089
	21st to end	Day	10% 4 2 6% 4 4 5% 4 6% 8% 48%	1890
December	1st to 10th	Day	5% 4 2 5% 3 4 4 3 6% 7% 57%	3020
	11th to 20th	Day	9% 6% 3 6% 4 4 4 3 6% 8% 47%	3436
	21st to end	Day	12% 7% 2 7% 4 4 4 4 5% 7% 43%	4466
January	1st to 10th	Night	9% 6% 2 4 3 3 3 2 5% 8% 55%	5889
	11th to 20th	Night	20% 10% 3 7% 4 4 5% 4 6% 7% 30%	5900
	21st to end	Night	15% 6% 2 6% 4 4 4 4 6% 9% 41%	6191
February	1st to 10th	Night	12% 6% 2 6% 5% 5% 4 4 5% 7% 45%	2400
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 6% 7% 48%	2338
	21st to end	Night	8% 7% 2 6% 4 4 4 5% 6% 6% 47%	1941
March	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	4918
	11th to 20th	Night	5% 5% 2 4 3 3 4 4 5% 10% 56%	4980
	21st to end	Night	9% 6% 3 5% 4 4 4 3 5% 7% 50%	5467
April	1st to 10th	Night	8% 5% 2 4 4 3 4 4 7% 8% 51%	2200
	11th to 20th	Night	6% 5% 2 5% 3 4 4 3 6% 8% 53%	2173
	21st to end	Night	6% 3 2 4 4 4 3 3 5% 9% 57%	1954
May	1st to 10th	Night	4 3 1 4 4 4 5% 4 6% 10% 53%	4033
	11th to 20th	Night	8% 4 2 5% 3 4 4 3 6% 9% 51%	4032
	21st to end	Night	3 3 1 4 3 4 4 4 7% 11% 57%	4421
June	1st to 10th	Night	9% 6% 2 8% 5% 5% 6% 5% 7% 10% 39%	1800
	11th to 20th	Night	5% 6% 2 7% 5% 6% 6% 5% 9% 14% 34%	1800
	21st to end	Night	6% 6% 2 7% 6% 5% 6% 5% 8% 13% 36%	1800
July	1st to 10th	Night	6% 6% 3 10% 8% 7% 9% 7% 10% 15% 19%	4058
	11th to 20th	Night	2 5% 2 8% 6% 6% 8% 6% 11% 15% 30%	4057
	21st to end	Night	4 6% 3 10% 8% 7% 8% 7% 11% 12% 25%	4427
August	1st to 10th	Night	6% 5% 2 7% 6% 5% 6% 6% 10% 12% 33%	2915
	11th to 20th	Night	4 3 2 6% 4 5% 5% 5% 8% 16% 41%	3138
	21st to end	Night	6% 7% 3 9% 6% 6% 7% 6% 9% 12% 28%	3881
September	1st to 10th	Night	6% 6% 3 10% 7% 6% 8% 6% 8% 12% 27%	2200
	11th to 20th	Night	9% 8% 3 8% 6% 7% 6% 6% 8% 10% 29%	2200
	21st to end	Night	9% 6% 3 8% 5% 6% 5% 4 7% 10% 37%	2200
October	1st to 10th	Night	9% 7% 2 7% 5% 5% 4 5% 7% 8% 42%	2200
	11th to 20th	Night	6% 4 2 4 4 4 4 4 5% 8% 56%	2365
	21st to end	Night	7% 5% 2 6% 3 3 5% 3 6% 8% 50%	2640
November	1st to 10th	Night	8% 5% 1 5% 4 3 4 4 6% 7% 52%	2461
	11th to 20th	Night	8% 6% 3 6% 4 4 3 3 5% 7% 51%	2525
	21st to end	Night	8% 7% 2 6% 4 4 4 4 6% 8% 48%	2730
December	1st to 10th	Night	7% 5% 2 5% 3 4 3 3 5% 7% 57%	4380
	11th to 20th	Night	9% 7% 2 6% 4 3 4 3 5% 7% 50%	4914
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 4 6% 7% 38%	6490

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	32% 2 8% 15% 28% 15%	4057
	11th to 20th	Day	53% 2 4 15% 17% 9%	4060
	21st to end	Day	34% 2 9% 27% 19% 9%	4760
February	1st to 10th	Day	40% 3 9% 16% 20% 12%	2000
	11th to 20th	Day	33% 3 8% 20% 25% 11%	1958
	21st to end	Day	35% 4 9% 19% 22% 12%	1686
March	1st to 10th	Day	38% 2 12% 20% 21% 8%	4935
	11th to 20th	Day	25% 2 11% 23% 22% 17%	4980
	21st to end	Day	32% 3 8% 29% 19% 8%	5465
April	1st to 10th	Day	30% 4 13% 21% 22% 10%	2200
	11th to 20th	Day	35% 4 10% 17% 24% 10%	2176
	21st to end	Day	24% 5% 12% 26% 25% 9%	2460
May	1st to 10th	Day	27% 6% 21% 29% 14% 4	5897
	11th to 20th	Day	35% 4 16% 33% 11% 2	5865
	21st to end	Day	25% 4 17% 34% 17% 2	6447
June	1st to 10th	Day	47% 7% 19% 20% 7%	2600
	11th to 20th	Day	42% 10% 22% 23% 4	2600
	21st to end	Day	45% 10% 24% 17% 4 1	2600
July	1st to 10th	Day	45% 16% 32% 6% 1	5835
	11th to 20th	Day	37% 16% 36% 10% 1	5831
	21st to end	Day	38% 14% 27% 19% 1	6240
August	1st to 10th	Day	41% 10% 31% 14% 3	4221
	11th to 20th	Day	35% 10% 27% 23% 5%	3929
	21st to end	Day	46% 11% 23% 17% 2	3982
September	1st to 10th	Day	52% 11% 19% 13% 5%	2200
	11th to 20th	Day	51% 7% 19% 17% 6%	2200
	21st to end	Day	44% 7% 15% 18% 12% 4	2200
October	1st to 10th	Day	54% 8% 12% 13% 8% 6%	3590
	11th to 20th	Day	22% 4 5% 23% 36% 10%	3286
	21st to end	Day	26% 4 6% 24% 32% 8%	3538
November	1st to 10th	Day	31% 4 10% 23% 18% 14%	2057
	11th to 20th	Day	35% 3 8% 20% 22% 12%	2089
	21st to end	Day	34% 4 8% 23% 22% 9%	1890
December	1st to 10th	Day	27% 3 15% 24% 22% 10%	3020
	11th to 20th	Day	35% 4 9% 23% 18% 11%	3433
	21st to end	Day	41% 4 9% 22% 20% 5%	4466
January	1st to 10th	Night	29% 2 7% 17% 31% 14%	5889
	11th to 20th	Night	54% 3 6% 16% 17% 5%	5900
	21st to end	Night	42% 2 10% 20% 17% 9%	6191
February	1st to 10th	Night	39% 4 8% 17% 19% 12%	2400
	11th to 20th	Night	36% 3 10% 20% 21% 11%	2339
	21st to end	Night	36% 4 10% 19% 22% 9%	1941
March	1st to 10th	Night	42% 2 9% 17% 19% 11%	4918
	11th to 20th	Night	26% 2 13% 21% 20% 17%	4980
	21st to end	Night	34% 2 9% 27% 19% 9%	5467
April	1st to 10th	Night	30% 5% 15% 23% 19% 7%	2200
	11th to 20th	Night	29% 4 10% 27% 21% 9%	2173
	21st to end	Night	25% 3 13% 29% 22% 7%	1954
May	1st to 10th	Night	27% 5% 21% 31% 12% 3	4060
	11th to 20th	Night	31% 3 18% 30% 16% 1	4033
	21st to end	Night	22% 5% 17% 38% 17% 1	4429
June	1st to 10th	Night	40% 7% 23% 23% 6%	1800
	11th to 20th	Night	36% 9% 31% 21% 2	1800
	21st to end	Night	37% 12% 26% 20% 5%	1800
July	1st to 10th	Night	49% 16% 28% 6% 1	4015
	11th to 20th	Night	38% 15% 38% 9%	4006
	21st to end	Night	46% 12% 25% 16% 1	4297
August	1st to 10th	Night	39% 11% 35% 12% 3	2904
	11th to 20th	Night	29% 10% 34% 23% 4	3166
	21st to end	Night	45% 13% 25% 15% 2	3881
September	1st to 10th	Night	47% 12% 24% 13% 4	2200
	11th to 20th	Night	47% 11% 21% 17% 4 1	2200
	21st to end	Night	41% 7% 18% 20% 13% 2	2200
October	1st to 10th	Night	54% 8% 12% 14% 7% 5%	3590
	11th to 20th	Night	23% 4 6% 22% 38% 8%	3851
	21st to end	Night	28% 4 6% 20% 34% 7%	4252
November	1st to 10th	Night	31% 4 10% 24% 20% 12%	2461
	11th to 20th	Night	34% 3 7% 22% 21% 13%	2525
	21st to end	Night	34% 4 8% 23% 20% 10%	2730
December	1st to 10th	Night	29% 3 12% 25% 18% 14%	4380
	11th to 20th	Night	35% 5% 10% 21% 18% 11%	4916
	21st to end	Night	45% 4 10% 21% 16% 5%	6490

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	11% 6% 43% 27% 11% 2	300
	11th to 20th	Day	9% 12% 31% 31% 14% 3	300
	21st to end	Day	14% 12% 43% 25% 6%	330
February	1st to 10th	Day	11% 8% 31% 30% 16% 4	300
	11th to 20th	Day	11% 9% 31% 33% 12% 4	300
	21st to end	Day	9% 7% 31% 31% 20% 2	285
March	1st to 10th	Day	7% 7% 37% 33% 15% 1	500
	11th to 20th	Day	11% 7% 29% 30% 20% 3	500
	21st to end	Day	8% 7% 25% 37% 19% 4	550
April	1st to 10th	Day	11% 7% 25% 31% 23% 3	500
	11th to 20th	Day	7% 9% 25% 29% 26% 3	492
	21st to end	Day	4 3 19% 30% 28% 15%	500
May	1st to 10th	Day	4 5% 20% 28% 31% 14%	503
	11th to 20th	Day	9% 2 16% 27% 26% 20%	527
	21st to end	Day	7% 2 10% 23% 30% 29%	573
June	1st to 10th	Day	12% 2 7% 24% 28% 26%	530
	11th to 20th	Day	7% 2 8% 19% 29% 35%	530
	21st to end	Day	5% 2 3 18% 26% 46%	530
July	1st to 10th	Day	7% 3 3 12% 23% 53%	525
	11th to 20th	Day	5% 4 10% 26% 54%	524
	21st to end	Day	7% 1 4 13% 27% 48%	576
August	1st to 10th	Day	11% 1 3 13% 25% 46%	526
	11th to 20th	Day	9% 2 2 9% 27% 51%	519
	21st to end	Day	11% 2 4 13% 31% 39%	583
September	1st to 10th	Day	12% 1 6% 17% 27% 36%	530
	11th to 20th	Day	8% 4 5% 20% 37% 26%	530
	21st to end	Day	11% 2 6% 20% 36% 26%	530
October	1st to 10th	Day	13% 3 15% 32% 28% 10%	530
	11th to 20th	Day	5% 4 16% 30% 34% 12%	530
	21st to end	Day	6% 4 16% 39% 28% 7%	583
November	1st to 10th	Day	8% 4 21% 33% 26% 9%	535
	11th to 20th	Day	11% 4 22% 34% 22% 6%	529
	21st to end	Day	9% 5% 28% 34% 18% 4	330
December	1st to 10th	Day	10% 6% 35% 25% 17% 6%	330
	11th to 20th	Day	12% 6% 30% 31% 20% 2	321
	21st to end	Day	10% 5% 27% 38% 16% 4	330
January	1st to 10th	Night	13% 6% 31% 36% 13% 1	800
	11th to 20th	Night	12% 7% 25% 42% 14% 1	800
	21st to end	Night	19% 6% 31% 36% 8%	880
February	1st to 10th	Night	15% 6% 26% 38% 16%	800
	11th to 20th	Night	13% 5% 29% 40% 11% 1	783
	21st to end	Night	12% 4 29% 45% 9%	660
March	1st to 10th	Night	9% 7% 31% 41% 13% 1	800
	11th to 20th	Night	10% 6% 26% 42% 16% 1	800
	21st to end	Night	8% 6% 28% 43% 14% 1	880
April	1st to 10th	Night	9% 5% 28% 42% 16% 1	800
	11th to 20th	Night	8% 5% 27% 42% 17% 1	791
	21st to end	Night	7% 3 19% 45% 25% 1	735
May	1st to 10th	Night	7% 1 12% 46% 31% 3	600
	11th to 20th	Night	7% 2 11% 40% 35% 5%	591
	21st to end	Night	6% 2 6% 34% 44% 8%	650
June	1st to 10th	Night	11% 1 6% 30% 44% 8%	600
	11th to 20th	Night	8% 2 4 20% 44% 23%	600
	21st to end	Night	8% 1 2 16% 45% 28%	600
July	1st to 10th	Night	8% 2 2 11% 43% 34%	585
	11th to 20th	Night	5% 1 3 8% 46% 37%	582
	21st to end	Night	4 1 4 9% 53% 29%	657
August	1st to 10th	Night	8% 1 4 11% 48% 29%	597
	11th to 20th	Night	8% 1 2 7% 46% 37%	594
	21st to end	Night	12% 2 4 19% 44% 20%	838
September	1st to 10th	Night	9% 2 6% 20% 47% 17%	800
	11th to 20th	Night	12% 3 5% 28% 41% 12%	800
	21st to end	Night	10% 2 8% 28% 43% 10%	800
October	1st to 10th	Night	13% 2 13% 39% 30% 3	800
	11th to 20th	Night	6% 2 13% 41% 35% 3	800
	21st to end	Night	10% 3 16% 41% 27% 3	880
November	1st to 10th	Night	10% 3 19% 40% 25% 3	820
	11th to 20th	Night	11% 4 20% 43% 21% 2	840
	21st to end	Night	10% 4 26% 42% 18% 1	840
December	1st to 10th	Night	11% 4 22% 40% 21% 1	840
	11th to 20th	Night	12% 4 23% 44% 18% 1	829
	21st to end	Night	12% 5% 25% 40% 18% 1	880

Page generated at 2025-12-15 01:13:53. Version ac07

Copyright © 2025 Attilla Danko.