Wilson Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilson Hill Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	17% 7% 3 6% 4 4 5% 4 6% 7% 37%	126805
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 39%	123898
Transparency	Day	45% 4 10% 22% 16% 3	130312
	Night	44% 3 11% 23% 16% 3	128478
Seeing	Day	26% 14% 30% 19% 8% 3	16317
	Night	23% 13% 30% 25% 8% 1	25217

By Month

Percentage Cloud Cover NE (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	12% 7% 3 9% 6% 6% 6% 4 6% 6% 37%	3597
	11th to 20th	Day	12% 7% 3 8% 3 3 4 4 4 6% 43%	3570
	21st to end	Day	14% 10% 3 7% 4 3 3 3 5% 6% 43%	3960
February	1st to 10th	Day	17% 8% 2 7% 5% 6% 5% 3 7% 6% 35%	1822
	11th to 20th	Day	12% 8% 3 7% 5% 4 5% 4 6% 8% 38%	1948
	21st to end	Day	16% 10% 4 8% 5% 4 4 3 4 4 37%	1650
March	1st to 10th	Day	19% 11% 3 8% 4 4 4 3 4 5% 34%	4476
	11th to 20th	Day	15% 10% 2 6% 4 4 4 4 6% 6% 39%	4740
	21st to end	Day	24% 9% 2 6% 4 3 4 3 5% 7% 32%	5827
April	1st to 10th	Day	18% 10% 2 7% 5% 4 4 3 5% 6% 36%	2400
	11th to 20th	Day	17% 7% 2 6% 4 5% 4 4 7% 7% 38%	2384
	21st to end	Day	20% 8% 3 7% 5% 4 4 3 5% 6% 36%	2600
May	1st to 10th	Day	31% 9% 3 6% 4 4 4 4 5% 5% 24%	5886
	11th to 20th	Day	19% 7% 3 9% 5% 5% 5% 4 5% 7% 30%	6087
	21st to end	Day	19% 6% 2 6% 5% 4 5% 4 6% 7% 35%	6949
June	1st to 10th	Day	18% 7% 3 6% 5% 5% 4 4 7% 8% 33%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 6% 8% 37%	2800
	21st to end	Day	18% 6% 2 6% 5% 5% 5% 4 6% 8% 35%	2800
July	1st to 10th	Day	15% 7% 2 7% 5% 5% 6% 5% 6% 10% 33%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 11% 37%	6360
	21st to end	Day	14% 7% 2 6% 3 3 4 4 6% 7% 42%	6926
August	1st to 10th	Day	13% 6% 2 7% 5% 5% 6% 5% 6% 8% 35%	4349
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 7% 37%	4114
	21st to end	Day	7% 4 2 6% 4 4 6% 5% 8% 10% 42%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	13% 6% 3 4 3 3 3 3 4 5% 53%	2214
	21st to end	Day	16% 5% 2 6% 4 4 4 4 4 7% 44%	2200
October	1st to 10th	Day	22% 6% 3 5% 5% 3 4 3 4 5% 41%	2200
	11th to 20th	Day	16% 6% 2 4 4 3 4 3 4 7% 45%	1927
	21st to end	Day	22% 7% 2 5% 4 4 4 3 4 5% 39%	1980
November	1st to 10th	Day	14% 6% 2 6% 4 3 3 3 4 5% 49%	1851
	11th to 20th	Day	17% 5% 2 4 3 4 4 3 4 5% 47%	1890
	21st to end	Day	19% 6% 2 6% 4 4 4 3 5% 6% 40%	1864
December	1st to 10th	Day	15% 8% 2 6% 4 4 4 4 6% 7% 40%	2616
	11th to 20th	Day	21% 5% 2 5% 4 3 3 4 4 5% 43%	2986
	21st to end	Day	19% 11% 3 7% 4 4 5% 3 4 5% 36%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 5% 5% 3 6% 7% 43%	6349
	11th to 20th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 6% 38%	6315
	21st to end	Night	15% 10% 4 7% 5% 5% 4 3 5% 6% 38%	6996
February	1st to 10th	Night	14% 10% 4 6% 4 4 4 4 5% 6% 38%	2575
	11th to 20th	Night	12% 9% 2 7% 5% 5% 5% 4 6% 6% 39%	2338
	21st to end	Night	15% 9% 2 6% 4 5% 4 4 5% 6% 40%	1980
March	1st to 10th	Night	22% 9% 2 5% 4 4 4 3 5% 7% 35%	5364
	11th to 20th	Night	15% 9% 3 8% 4 4 4 3 5% 7% 37%	5219
	21st to end	Night	25% 8% 2 5% 3 3 4 4 6% 7% 33%	5100
April	1st to 10th	Night	19% 8% 3 7% 4 5% 3 4 5% 6% 36%	2000
	11th to 20th	Night	19% 8% 2 5% 4 4 4 3 6% 5% 39%	1965
	21st to end	Night	21% 6% 3 6% 4 4 4 3 5% 6% 40%	1800
May	1st to 10th	Night	34% 9% 3 6% 3 3 3 2 5% 6% 24%	4054
	11th to 20th	Night	15% 8% 3 9% 5% 6% 6% 5% 6% 7% 29%	3835
	21st to end	Night	20% 8% 3 7% 5% 4 4 4 6% 7% 32%	3926
June	1st to 10th	Night	18% 9% 3 6% 4 3 4 3 5% 7% 37%	1600
	11th to 20th	Night	19% 6% 2 5% 4 3 5% 4 5% 8% 38%	1600
	21st to end	Night	18% 7% 2 5% 5% 4 4 4 5% 8% 39%	1600
July	1st to 10th	Night	12% 6% 2 6% 5% 5% 5% 3 6% 9% 41%	3600
	11th to 20th	Night	11% 7% 3 6% 5% 4 5% 4 7% 9% 40%	3600
	21st to end	Night	11% 5% 2 5% 4 3 4 3 6% 7% 50%	3926
August	1st to 10th	Night	20% 6% 2 7% 5% 4 5% 4 5% 7% 35%	2766
	11th to 20th	Night	12% 8% 3 8% 4 5% 5% 4 6% 8% 37%	2930
	21st to end	Night	9% 4 1 4 3 3 4 3 5% 9% 55%	3586
September	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 5% 7% 51%	2000
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 4 6% 54%	2185
	21st to end	Night	16% 5% 2 6% 3 4 3 3 4 6% 48%	2200
October	1st to 10th	Night	20% 6% 2 6% 4 4 3 3 4 5% 42%	2200
	11th to 20th	Night	16% 5% 2 5% 4 3 5% 4 5% 7% 45%	2468
	21st to end	Night	22% 7% 2 5% 4 4 4 4 5% 5% 39%	2860
November	1st to 10th	Night	15% 7% 2 6% 3 3 5% 3 5% 6% 46%	2667
	11th to 20th	Night	16% 6% 2 5% 4 4 3 3 4 6% 47%	2730
	21st to end	Night	15% 8% 2 5% 4 4 3 4 5% 6% 43%	2751
December	1st to 10th	Night	16% 10% 3 7% 4 4 4 3 5% 6% 38%	4609
	11th to 20th	Night	22% 8% 3 5% 3 3 3 3 4 4 41%	5208
	21st to end	Night	18% 10% 3 7% 4 4 4 3 5% 5% 38%	6996

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	46% 4 7% 26% 16% 1	3598
	11th to 20th	Day	42% 2 11% 23% 18% 3	3600
	21st to end	Day	44% 3 7% 22% 20% 4	3960
February	1st to 10th	Day	49% 3 11% 22% 12% 3	1822
	11th to 20th	Day	45% 4 10% 21% 16% 4	1957
	21st to end	Day	50% 2 9% 20% 15% 3	1650
March	1st to 10th	Day	54% 2 8% 15% 11% 10%	4479
	11th to 20th	Day	47% 2 12% 24% 13% 2	4740
	21st to end	Day	52% 3 5% 16% 18% 7%	5827
April	1st to 10th	Day	49% 3 7% 19% 17% 4	2400
	11th to 20th	Day	45% 3 9% 21% 18% 3	2384
	21st to end	Day	51% 3 7% 18% 18% 4	2600
May	1st to 10th	Day	63% 3 10% 14% 9% 1	5897
	11th to 20th	Day	54% 4 13% 18% 10% 2	6091
	21st to end	Day	48% 3 11% 23% 14% 1	6949
June	1st to 10th	Day	48% 4 11% 24% 13% 1	2800
	11th to 20th	Day	45% 4 11% 21% 16% 3	2800
	21st to end	Day	47% 4 11% 23% 14% 1	2800
July	1st to 10th	Day	46% 6% 11% 20% 16% 1	6290
	11th to 20th	Day	38% 7% 15% 24% 14% 1	6285
	21st to end	Day	39% 4 11% 27% 18% 1	6728
August	1st to 10th	Day	45% 5% 15% 23% 12% 1	4335
	11th to 20th	Day	44% 5% 13% 26% 11% 1	4153
	21st to end	Day	33% 6% 12% 30% 18% 2	4312
September	1st to 10th	Day	36% 4 11% 27% 20% 2	2400
	11th to 20th	Day	35% 3 7% 27% 25% 3	2214
	21st to end	Day	41% 3 8% 24% 21% 3	2200
October	1st to 10th	Day	43% 5% 8% 22% 19% 2	3590
	11th to 20th	Day	33% 3 8% 30% 22% 3	3104
	21st to end	Day	35% 3 7% 31% 20% 3	3180
November	1st to 10th	Day	39% 2 12% 27% 18% 2	1851
	11th to 20th	Day	40% 3 8% 23% 22% 3	1890
	21st to end	Day	46% 2 14% 22% 13% 2	1864
December	1st to 10th	Day	42% 4 14% 29% 10% 1	2616
	11th to 20th	Day	44% 3 11% 19% 20% 4	2986
	21st to end	Day	52% 2 8% 19% 14% 5%	3960
January	1st to 10th	Night	39% 3 11% 31% 13% 2	6349
	11th to 20th	Night	46% 4 11% 22% 14% 3	6360
	21st to end	Night	49% 2 9% 18% 17% 5%	6996
February	1st to 10th	Night	46% 4 12% 22% 14% 2	2575
	11th to 20th	Night	44% 3 10% 22% 17% 3	2340
	21st to end	Night	45% 3 9% 24% 15% 4	1980
March	1st to 10th	Night	50% 2 8% 17% 16% 7%	5374
	11th to 20th	Night	49% 3 15% 19% 13% 2	5220
	21st to end	Night	50% 3 4 14% 22% 6%	5100
April	1st to 10th	Night	49% 2 9% 19% 18% 3	2000
	11th to 20th	Night	47% 2 8% 21% 19% 2	1965
	21st to end	Night	47% 2 10% 20% 18% 4	1800
May	1st to 10th	Night	63% 2 11% 14% 8% 1	4060
	11th to 20th	Night	54% 4 14% 17% 10% 1	3836
	21st to end	Night	52% 3 14% 18% 11% 1	3927
June	1st to 10th	Night	48% 3 10% 25% 12% 1	1600
	11th to 20th	Night	44% 6% 13% 21% 13% 2	1600
	21st to end	Night	44% 5% 13% 23% 14% 2	1600
July	1st to 10th	Night	40% 5% 13% 26% 14% 1	3560
	11th to 20th	Night	40% 6% 17% 20% 17% 1	3552
	21st to end	Night	34% 5% 12% 33% 16% 1	3809
August	1st to 10th	Night	49% 4 13% 21% 12% 1	2756
	11th to 20th	Night	44% 6% 14% 24% 11% 1	2957
	21st to end	Night	26% 5% 16% 32% 20% 1	3586
September	1st to 10th	Night	34% 4 12% 28% 21% 1	2000
	11th to 20th	Night	33% 3 9% 28% 25% 2	2185
	21st to end	Night	39% 3 9% 28% 18% 2	2200
October	1st to 10th	Night	40% 4 14% 24% 15% 2	3590
	11th to 20th	Night	29% 2 11% 28% 26% 4	4030
	21st to end	Night	36% 4 10% 33% 15% 2	4610
November	1st to 10th	Night	40% 4 9% 24% 20% 3	2667
	11th to 20th	Night	41% 2 9% 24% 21% 3	2730
	21st to end	Night	42% 3 14% 26% 14% 1	2751
December	1st to 10th	Night	46% 4 10% 28% 11%	4609
	11th to 20th	Night	47% 3 12% 15% 21% 2	5208
	21st to end	Night	49% 3 7% 23% 15% 3	6996

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	38% 18% 27% 15% 4	200
	11th to 20th	Day	32% 21% 32% 12% 2 2	200
	21st to end	Day	30% 20% 34% 12% 4 1	220
February	1st to 10th	Day	32% 22% 30% 14% 2	362
	11th to 20th	Day	36% 24% 28% 11% 2	496
	21st to end	Day	31% 21% 29% 15% 4	414
March	1st to 10th	Day	33% 17% 32% 15% 3	500
	11th to 20th	Day	33% 22% 28% 12% 4 1	500
	21st to end	Day	35% 17% 30% 12% 5% 2	550
April	1st to 10th	Day	35% 19% 27% 12% 5% 2	500
	11th to 20th	Day	35% 18% 29% 12% 5% 2	492
	21st to end	Day	31% 17% 30% 14% 7% 2	500
May	1st to 10th	Day	37% 14% 27% 16% 5% 1	503
	11th to 20th	Day	24% 15% 35% 19% 6% 1	527
	21st to end	Day	21% 13% 35% 23% 7% 2	573
June	1st to 10th	Day	21% 13% 37% 17% 10% 2	530
	11th to 20th	Day	18% 14% 35% 22% 9% 2	530
	21st to end	Day	20% 12% 35% 21% 8% 3	530
July	1st to 10th	Day	19% 8% 30% 26% 12% 4	525
	11th to 20th	Day	15% 5% 34% 30% 12% 4	524
	21st to end	Day	13% 10% 32% 32% 10% 3	576
August	1st to 10th	Day	14% 7% 30% 29% 16% 4	526
	11th to 20th	Day	18% 6% 28% 27% 16% 6%	519
	21st to end	Day	14% 9% 32% 25% 15% 5%	583
September	1st to 10th	Day	19% 11% 26% 22% 15% 7%	530
	11th to 20th	Day	20% 9% 25% 19% 16% 10%	530
	21st to end	Day	19% 10% 33% 21% 12% 5%	530
October	1st to 10th	Day	30% 11% 22% 21% 12% 5%	530
	11th to 20th	Day	25% 13% 30% 19% 11% 2	472
	21st to end	Day	30% 13% 29% 21% 6% 1	462
November	1st to 10th	Day	24% 18% 27% 20% 10% 2	425
	11th to 20th	Day	33% 16% 27% 18% 6% 1	430
	21st to end	Day	32% 19% 26% 17% 6% 1	404
December	1st to 10th	Day	28% 24% 30% 12% 5%	204
	11th to 20th	Day	38% 19% 29% 12% 3	200
	21st to end	Day	33% 15% 28% 19% 4 1	220
January	1st to 10th	Night	25% 21% 32% 19% 3	800
	11th to 20th	Night	30% 18% 29% 21% 3	800
	21st to end	Night	22% 18% 37% 19% 3	880
February	1st to 10th	Night	25% 17% 34% 22% 3	800
	11th to 20th	Night	25% 21% 35% 17% 2	783
	21st to end	Night	26% 20% 32% 20% 2	660
March	1st to 10th	Night	29% 18% 32% 20% 2	800
	11th to 20th	Night	26% 19% 31% 21% 4	800
	21st to end	Night	28% 17% 33% 18% 5%	880
April	1st to 10th	Night	29% 17% 30% 19% 5%	800
	11th to 20th	Night	27% 18% 31% 19% 5%	781
	21st to end	Night	27% 17% 32% 20% 4 1	600
May	1st to 10th	Night	29% 12% 32% 21% 6%	600
	11th to 20th	Night	22% 13% 32% 27% 6%	511
	21st to end	Night	22% 6% 33% 31% 8%	433
June	1st to 10th	Night	20% 9% 32% 30% 9% 1	400
	11th to 20th	Night	20% 7% 30% 34% 9% 1	400
	21st to end	Night	19% 5% 31% 36% 9%	400
July	1st to 10th	Night	17% 5% 28% 38% 11% 1	390
	11th to 20th	Night	14% 3 23% 44% 14% 1	388
	21st to end	Night	14% 4 28% 42% 12%	438
August	1st to 10th	Night	16% 3 26% 39% 16%	500
	11th to 20th	Night	15% 6% 26% 36% 15% 2	624
	21st to end	Night	15% 5% 23% 36% 19% 1	880
September	1st to 10th	Night	16% 6% 30% 30% 16% 1	800
	11th to 20th	Night	17% 6% 24% 30% 20% 3	800
	21st to end	Night	20% 7% 27% 30% 14% 2	800
October	1st to 10th	Night	26% 8% 24% 28% 13% 1	800
	11th to 20th	Night	21% 9% 31% 26% 12% 1	800
	21st to end	Night	27% 14% 28% 24% 6% 1	880
November	1st to 10th	Night	21% 13% 31% 26% 8% 1	820
	11th to 20th	Night	24% 17% 29% 22% 8% 1	840
	21st to end	Night	27% 15% 30% 23% 5% 1	840
December	1st to 10th	Night	26% 18% 33% 19% 4	809
	11th to 20th	Night	27% 18% 32% 19% 3	800
	21st to end	Night	27% 18% 31% 21% 3	880

Page generated at 2025-11-30 01:00:51. Version ac07

Copyright © 2025 Attilla Danko.