Windy Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windy Hill Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 9% 3 8% 5% 4 5% 4 6% 7% 35%	131290
	Night	16% 10% 3 7% 4 4 4 3 5% 7% 36%	129900
Transparency	Day	48% 5% 11% 20% 13% 3	134975
	Night	47% 5% 11% 21% 14% 3	134285
Seeing	Day	23% 14% 23% 21% 13% 5%	16771
	Night	22% 11% 21% 29% 16% 2	26920

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	17% 17% 6% 9% 5% 3 3 3 4 5% 28%	5258
	11th to 20th	Day	13% 17% 8% 10% 6% 3 3 2 2 3 32%	5280
	21st to end	Day	16% 15% 4 11% 4 3 4 2 3 4 33%	5780
February	1st to 10th	Day	20% 13% 4 7% 4 4 5% 4 4 5% 31%	1913
	11th to 20th	Day	24% 12% 5% 9% 3 4 3 3 3 5% 29%	2048
	21st to end	Day	23% 12% 3 7% 4 3 3 2 4 6% 33%	1730
March	1st to 10th	Day	17% 15% 4 7% 4 3 4 3 4 5% 34%	4883
	11th to 20th	Day	12% 9% 3 6% 3 4 3 3 4 7% 45%	4980
	21st to end	Day	17% 9% 2 7% 4 4 4 4 5% 6% 39%	5465
April	1st to 10th	Day	15% 8% 3 6% 4 4 4 4 7% 7% 38%	2200
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 5% 6% 42%	2418
	21st to end	Day	17% 8% 2 7% 5% 4 5% 3 5% 6% 38%	2600
May	1st to 10th	Day	18% 9% 3 9% 6% 6% 5% 5% 6% 8% 25%	5862
	11th to 20th	Day	19% 9% 3 9% 5% 6% 6% 5% 6% 7% 25%	5861
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 5% 7% 9% 30%	6433
June	1st to 10th	Day	13% 6% 2 7% 4 5% 5% 5% 7% 9% 36%	2600
	11th to 20th	Day	9% 6% 2 9% 6% 6% 6% 6% 8% 9% 33%	2600
	21st to end	Day	11% 6% 2 6% 5% 4 6% 5% 7% 10% 37%	2600
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 6% 9% 11% 38%	5900
	11th to 20th	Day	13% 7% 2 7% 5% 5% 6% 5% 8% 10% 31%	5900
	21st to end	Day	17% 8% 3 9% 7% 6% 6% 5% 6% 9% 25%	6424
August	1st to 10th	Day	13% 7% 3 8% 6% 6% 7% 5% 8% 11% 28%	4234
	11th to 20th	Day	12% 6% 2 8% 5% 6% 6% 4 7% 11% 34%	4174
	21st to end	Day	6% 3 2 4 4 4 4 4 6% 8% 55%	4427
September	1st to 10th	Day	12% 5% 2 7% 4 5% 6% 4 6% 8% 42%	2200
	11th to 20th	Day	14% 7% 2 7% 4 5% 5% 4 6% 7% 41%	2200
	21st to end	Day	15% 6% 3 6% 4 5% 5% 4 6% 8% 38%	2200
October	1st to 10th	Day	14% 5% 2 5% 4 3 3 2 5% 6% 51%	2130
	11th to 20th	Day	15% 6% 2 5% 3 4 3 3 5% 5% 50%	2000
	21st to end	Day	19% 7% 2 6% 3 3 3 3 3 6% 45%	2200
November	1st to 10th	Day	13% 6% 2 6% 3 3 3 3 5% 6% 50%	1962
	11th to 20th	Day	12% 10% 2 6% 4 4 5% 3 5% 7% 43%	1890
	21st to end	Day	15% 10% 4 6% 4 4 3 3 5% 6% 40%	1890
December	1st to 10th	Day	16% 13% 3 11% 4 4 4 3 4 5% 34%	3020
	11th to 20th	Day	19% 15% 4 8% 4 4 4 3 5% 6% 27%	3463
	21st to end	Day	14% 13% 6% 7% 5% 5% 6% 4 6% 7% 27%	4565
January	1st to 10th	Night	19% 16% 5% 10% 4 3 3 3 4 5% 28%	7635
	11th to 20th	Night	16% 12% 4 10% 4 3 3 2 3 4 38%	7680
	21st to end	Night	19% 11% 5% 9% 5% 3 3 3 4 5% 34%	8401
February	1st to 10th	Night	18% 12% 3 7% 3 4 4 3 5% 6% 35%	2704
	11th to 20th	Night	22% 13% 5% 7% 5% 4 4 3 3 5% 30%	2458
	21st to end	Night	22% 12% 3 8% 4 3 3 3 4 6% 31%	2076
March	1st to 10th	Night	20% 13% 3 6% 3 3 3 2 3 5% 38%	5015
	11th to 20th	Night	15% 6% 3 4 3 2 3 3 5% 7% 50%	4980
	21st to end	Night	13% 8% 2 6% 5% 5% 5% 4 6% 7% 41%	5467
April	1st to 10th	Night	16% 9% 2 6% 4 4 4 4 6% 7% 39%	2200
	11th to 20th	Night	15% 8% 2 6% 4 4 4 4 4 7% 42%	1945
	21st to end	Night	15% 7% 3 6% 5% 5% 4 4 5% 7% 39%	1800
May	1st to 10th	Night	20% 9% 3 8% 5% 4 5% 4 6% 8% 27%	4033
	11th to 20th	Night	18% 9% 3 9% 6% 5% 5% 4 5% 8% 27%	4032
	21st to end	Night	19% 9% 3 8% 5% 4 5% 3 6% 7% 31%	4421
June	1st to 10th	Night	13% 6% 2 6% 5% 4 6% 3 8% 10% 38%	1800
	11th to 20th	Night	7% 6% 3 6% 6% 6% 5% 5% 7% 10% 40%	1800
	21st to end	Night	11% 5% 2 6% 5% 6% 4 5% 7% 11% 38%	1800
July	1st to 10th	Night	11% 6% 2 7% 5% 4 6% 5% 9% 11% 34%	4058
	11th to 20th	Night	13% 8% 3 6% 5% 5% 6% 5% 7% 12% 31%	4057
	21st to end	Night	15% 11% 4 11% 7% 5% 5% 5% 6% 8% 24%	4427
August	1st to 10th	Night	13% 7% 3 7% 5% 4 6% 5% 7% 11% 32%	2915
	11th to 20th	Night	14% 6% 2 7% 4 6% 5% 4 7% 8% 36%	2875
	21st to end	Night	7% 3 1 4 3 4 4 4 7% 9% 53%	3433
September	1st to 10th	Night	13% 7% 2 6% 5% 4 4 3 6% 7% 44%	2200
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 5% 8% 45%	2200
	21st to end	Night	15% 6% 3 6% 4 4 5% 3 5% 6% 44%	2200
October	1st to 10th	Night	15% 5% 2 5% 3 3 3 3 4 6% 50%	2269
	11th to 20th	Night	16% 7% 2 4 3 3 3 4 5% 4 50%	2400
	21st to end	Night	17% 7% 2 5% 3 3 3 3 4 6% 46%	2640
November	1st to 10th	Night	14% 7% 3 4 3 3 3 3 5% 5% 50%	2554
	11th to 20th	Night	16% 9% 3 6% 3 3 3 3 4 6% 44%	2730
	21st to end	Night	15% 10% 2 5% 4 3 3 3 4 5% 45%	2730
December	1st to 10th	Night	18% 12% 5% 8% 5% 4 4 2 3 4 35%	4380
	11th to 20th	Night	20% 13% 6% 10% 4 4 3 3 4 5% 28%	4952
	21st to end	Night	15% 15% 5% 8% 4 4 4 3 6% 7% 28%	6633

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	60% 2 8% 18% 10% 1	5259
	11th to 20th	Day	59% 2 3 13% 19% 3	5280
	21st to end	Day	57% 2 7% 21% 12% 2	5780
February	1st to 10th	Day	56% 4 6% 18% 14% 1	1913
	11th to 20th	Day	59% 4 6% 16% 14% 2	2058
	21st to end	Day	54% 3 7% 16% 16% 4	1730
March	1st to 10th	Day	53% 4 5% 15% 17% 7%	4883
	11th to 20th	Day	41% 3 5% 20% 24% 6%	4980
	21st to end	Day	46% 3 7% 28% 14% 2	5465
April	1st to 10th	Day	45% 5% 11% 23% 14% 3	2200
	11th to 20th	Day	43% 4 8% 21% 19% 4	2418
	21st to end	Day	47% 5% 12% 23% 11% 2	2600
May	1st to 10th	Day	57% 6% 12% 18% 7% 1	5897
	11th to 20th	Day	58% 6% 13% 19% 4 1	5865
	21st to end	Day	48% 6% 13% 20% 11% 2	6447
June	1st to 10th	Day	43% 9% 14% 21% 12% 1	2600
	11th to 20th	Day	44% 10% 14% 18% 12% 2	2600
	21st to end	Day	41% 9% 14% 20% 14% 1	2600
July	1st to 10th	Day	38% 9% 20% 21% 12% 1	5835
	11th to 20th	Day	48% 7% 21% 19% 4 1	5831
	21st to end	Day	56% 6% 13% 15% 9%	6240
August	1st to 10th	Day	49% 8% 19% 19% 6%	4221
	11th to 20th	Day	44% 7% 18% 21% 9% 1	4213
	21st to end	Day	28% 6% 19% 35% 11% 1	4427
September	1st to 10th	Day	40% 7% 14% 21% 15% 4	2200
	11th to 20th	Day	44% 6% 9% 24% 16% 2	2200
	21st to end	Day	44% 6% 12% 19% 15% 4	2200
October	1st to 10th	Day	30% 3 7% 21% 31% 7%	3468
	11th to 20th	Day	29% 2 8% 26% 17% 18%	3240
	21st to end	Day	32% 2 5% 32% 16% 13%	3538
November	1st to 10th	Day	37% 3 9% 20% 27% 5%	1962
	11th to 20th	Day	41% 4 7% 22% 23% 5%	1890
	21st to end	Day	46% 4 6% 23% 18% 3	1890
December	1st to 10th	Day	54% 3 7% 17% 18% 2	3020
	11th to 20th	Day	58% 3 9% 17% 12% 2	3460
	21st to end	Day	56% 4 12% 19% 9% 1	4565
January	1st to 10th	Night	60% 3 8% 18% 9% 2	7636
	11th to 20th	Night	53% 3 4 15% 21% 4	7680
	21st to end	Night	55% 3 6% 20% 14% 2	8401
February	1st to 10th	Night	51% 3 10% 19% 15% 1	2704
	11th to 20th	Night	58% 4 7% 17% 11% 3	2459
	21st to end	Night	56% 4 7% 18% 12% 4	2076
March	1st to 10th	Night	51% 3 6% 20% 14% 6%	5015
	11th to 20th	Night	36% 3 8% 20% 23% 9%	4980
	21st to end	Night	43% 4 10% 26% 13% 2	5467
April	1st to 10th	Night	45% 5% 12% 22% 14% 3	2200
	11th to 20th	Night	43% 4 8% 22% 17% 5%	1945
	21st to end	Night	45% 5% 12% 25% 12% 1	1800
May	1st to 10th	Night	54% 7% 14% 17% 8% 1	4060
	11th to 20th	Night	55% 7% 14% 20% 4	4033
	21st to end	Night	50% 7% 14% 19% 9% 1	4429
June	1st to 10th	Night	40% 8% 17% 21% 13% 1	1800
	11th to 20th	Night	38% 9% 19% 23% 11% 1	1800
	21st to end	Night	39% 10% 15% 21% 12% 3	1800
July	1st to 10th	Night	42% 7% 19% 23% 9% 1	4015
	11th to 20th	Night	46% 8% 23% 18% 4 2	4006
	21st to end	Night	56% 7% 14% 14% 8%	4297
August	1st to 10th	Night	46% 6% 22% 20% 5%	2904
	11th to 20th	Night	44% 6% 18% 23% 9%	2902
	21st to end	Night	26% 5% 18% 38% 12% 1	3433
September	1st to 10th	Night	39% 6% 15% 20% 17% 3	2200
	11th to 20th	Night	39% 6% 13% 23% 18% 1	2200
	21st to end	Night	41% 6% 11% 24% 17% 2	2200
October	1st to 10th	Night	36% 3 7% 21% 26% 7%	3711
	11th to 20th	Night	31% 3 7% 22% 21% 16%	3899
	21st to end	Night	28% 2 5% 31% 21% 13%	4252
November	1st to 10th	Night	38% 4 9% 21% 24% 5%	2554
	11th to 20th	Night	42% 3 7% 20% 22% 6%	2730
	21st to end	Night	43% 3 6% 25% 19% 3	2730
December	1st to 10th	Night	54% 3 6% 20% 15% 1	4380
	11th to 20th	Night	60% 3 8% 16% 11% 2	4954
	21st to end	Night	55% 4 11% 17% 12% 1	6633

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	35% 29% 26% 8% 2	330
	11th to 20th	Day	45% 25% 22% 9%	330
	21st to end	Day	30% 35% 23% 10% 1	363
February	1st to 10th	Day	29% 27% 32% 9% 3	330
	11th to 20th	Day	51% 22% 20% 6% 1	330
	21st to end	Day	42% 24% 24% 9% 1	273
March	1st to 10th	Day	28% 23% 26% 16% 5% 1	471
	11th to 20th	Day	30% 25% 27% 14% 3 1	500
	21st to end	Day	31% 21% 27% 15% 5%	550
April	1st to 10th	Day	27% 24% 23% 17% 7% 2	500
	11th to 20th	Day	23% 20% 28% 19% 8% 2	492
	21st to end	Day	26% 14% 28% 19% 8% 4	500
May	1st to 10th	Day	25% 22% 23% 16% 9% 4	503
	11th to 20th	Day	22% 12% 26% 23% 14% 4	527
	21st to end	Day	20% 10% 21% 22% 17% 11%	573
June	1st to 10th	Day	16% 8% 24% 27% 18% 7%	530
	11th to 20th	Day	12% 6% 23% 28% 21% 8%	530
	21st to end	Day	16% 7% 23% 28% 19% 8%	530
July	1st to 10th	Day	15% 4 18% 28% 23% 12%	525
	11th to 20th	Day	13% 5% 14% 34% 23% 11%	524
	21st to end	Day	16% 4 17% 28% 23% 12%	576
August	1st to 10th	Day	17% 4 21% 24% 20% 14%	526
	11th to 20th	Day	15% 4 19% 26% 25% 10%	519
	21st to end	Day	12% 3 14% 30% 27% 15%	583
September	1st to 10th	Day	12% 4 19% 28% 26% 11%	530
	11th to 20th	Day	16% 5% 15% 27% 26% 11%	530
	21st to end	Day	21% 8% 20% 29% 17% 6%	530
October	1st to 10th	Day	18% 6% 23% 29% 17% 6%	530
	11th to 20th	Day	20% 16% 22% 25% 15% 3	530
	21st to end	Day	25% 11% 25% 24% 11% 3	583
November	1st to 10th	Day	18% 17% 27% 24% 10% 3	440
	11th to 20th	Day	24% 23% 25% 20% 7%	330
	21st to end	Day	30% 18% 30% 16% 5%	330
December	1st to 10th	Day	33% 24% 30% 11% 2	330
	11th to 20th	Day	31% 20% 29% 16% 4	330
	21st to end	Day	29% 24% 29% 13% 4	363
January	1st to 10th	Night	28% 22% 25% 23% 3	840
	11th to 20th	Night	32% 18% 25% 20% 4	840
	21st to end	Night	26% 20% 31% 21% 2	924
February	1st to 10th	Night	27% 15% 30% 26% 2	840
	11th to 20th	Night	31% 23% 29% 16% 1	823
	21st to end	Night	30% 22% 27% 19% 2	692
March	1st to 10th	Night	26% 16% 29% 25% 4	809
	11th to 20th	Night	29% 16% 26% 22% 6% 1	800
	21st to end	Night	22% 16% 29% 27% 7%	880
April	1st to 10th	Night	24% 16% 25% 27% 8%	800
	11th to 20th	Night	20% 15% 26% 28% 10% 1	707
	21st to end	Night	21% 9% 25% 35% 8% 1	600
May	1st to 10th	Night	21% 11% 24% 30% 14% 1	600
	11th to 20th	Night	19% 6% 19% 34% 20% 2	591
	21st to end	Night	16% 3 15% 32% 30% 5%	650
June	1st to 10th	Night	15% 4 13% 38% 27% 4	600
	11th to 20th	Night	11% 3 13% 38% 30% 5%	600
	21st to end	Night	13% 2 14% 37% 31% 3	600
July	1st to 10th	Night	15% 2 8% 33% 37% 4	585
	11th to 20th	Night	15% 2 7% 30% 40% 6%	582
	21st to end	Night	14% 2 9% 33% 33% 9%	657
August	1st to 10th	Night	16% 2 9% 31% 39% 4	597
	11th to 20th	Night	16% 4 7% 35% 34% 5%	594
	21st to end	Night	13% 2 9% 34% 35% 7%	725
September	1st to 10th	Night	15% 3 13% 31% 34% 5%	800
	11th to 20th	Night	15% 3 14% 30% 33% 5%	800
	21st to end	Night	19% 4 17% 33% 24% 3	800
October	1st to 10th	Night	19% 5% 17% 30% 25% 5%	800
	11th to 20th	Night	20% 8% 23% 33% 14% 2	800
	21st to end	Night	22% 10% 21% 34% 12% 2	880
November	1st to 10th	Night	20% 7% 23% 33% 15% 2	820
	11th to 20th	Night	23% 12% 23% 32% 9% 1	840
	21st to end	Night	23% 14% 24% 29% 10% 1	840
December	1st to 10th	Night	30% 14% 26% 24% 6%	840
	11th to 20th	Night	29% 13% 25% 27% 5% 1	840
	21st to end	Night	29% 15% 25% 27% 4	924

Page generated at 2026-02-28 01:40:20. Version ac07

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