GCAstro
Clear Sky Chart
History

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

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	12% 10% 4 10% 6% 5% 5% 4 6% 7% 30%	126526
	Night	11% 11% 5% 11% 7% 5% 5% 4 5% 6% 30%	123831
Transparency	Day	47% 9% 12% 19% 10% 2	130132
	Night	48% 9% 12% 18% 10% 2	128294
Seeing	Day	25% 20% 26% 17% 8% 3	16203
	Night	23% 20% 24% 23% 10% 1	25914

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	12% 10% 10% 17% 13% 7% 5% 3 4 4 17%	4056
	11th to 20th	Day	7% 14% 7% 24% 8% 9% 4 3 4 3 15%	4060
	21st to end	Day	11% 12% 7% 16% 8% 5% 5% 3 4 5% 23%	4466
February	1st to 10th	Day	12% 14% 6% 12% 9% 5% 4 4 6% 5% 23%	1800
	11th to 20th	Day	15% 10% 5% 16% 8% 5% 5% 4 6% 5% 22%	1753
	21st to end	Day	13% 14% 5% 13% 7% 5% 4 4 4 5% 26%	1682
March	1st to 10th	Day	12% 11% 8% 10% 4 5% 4 3 5% 6% 32%	4935
	11th to 20th	Day	10% 9% 5% 8% 4 4 4 4 6% 8% 37%	4980
	21st to end	Day	11% 11% 4 10% 7% 6% 6% 4 5% 5% 31%	5465
April	1st to 10th	Day	13% 14% 5% 9% 5% 5% 5% 4 5% 8% 27%	2328
	11th to 20th	Day	13% 11% 4 7% 5% 5% 4 4 5% 6% 35%	2374
	21st to end	Day	17% 11% 4 8% 6% 4 4 3 5% 7% 31%	2550
May	1st to 10th	Day	16% 11% 4 9% 6% 6% 6% 5% 7% 7% 24%	5862
	11th to 20th	Day	18% 9% 3 8% 5% 5% 6% 5% 8% 8% 24%	5861
	21st to end	Day	16% 10% 4 8% 5% 5% 6% 5% 7% 8% 27%	6768
June	1st to 10th	Day	13% 8% 3 7% 5% 5% 6% 4 8% 9% 33%	2800
	11th to 20th	Day	13% 8% 3 8% 6% 6% 5% 5% 8% 9% 30%	2800
	21st to end	Day	11% 7% 3 7% 5% 5% 6% 5% 6% 10% 34%	2800
July	1st to 10th	Day	7% 7% 3 8% 5% 5% 5% 4 7% 9% 40%	6092
	11th to 20th	Day	10% 7% 3 9% 6% 6% 7% 5% 8% 10% 29%	5900
	21st to end	Day	11% 10% 3 8% 5% 6% 6% 5% 7% 9% 31%	6424
August	1st to 10th	Day	11% 9% 3 9% 5% 6% 7% 5% 7% 9% 28%	4234
	11th to 20th	Day	10% 8% 3 8% 6% 6% 6% 5% 7% 9% 32%	4174
	21st to end	Day	8% 4 2 5% 4 6% 6% 5% 8% 10% 41%	4357
September	1st to 10th	Day	13% 7% 3 7% 5% 5% 4 3 5% 6% 43%	2400
	11th to 20th	Day	10% 8% 3 6% 4 5% 5% 5% 6% 8% 40%	2249
	21st to end	Day	18% 10% 3 7% 6% 4 5% 4 6% 7% 32%	2170
October	1st to 10th	Day	14% 7% 3 5% 4 4 5% 4 5% 8% 41%	2000
	11th to 20th	Day	14% 12% 3 8% 5% 4 3 3 4 6% 37%	2000
	21st to end	Day	14% 14% 4 8% 4 4 5% 3 4 6% 36%	2200
November	1st to 10th	Day	9% 11% 3 7% 5% 4 4 3 5% 5% 44%	2057
	11th to 20th	Day	10% 11% 6% 10% 6% 5% 5% 4 6% 6% 30%	1733
	21st to end	Day	12% 15% 6% 12% 6% 5% 5% 3 5% 5% 26%	1600
December	1st to 10th	Day	11% 12% 6% 15% 12% 7% 4 3 4 5% 22%	2600
	11th to 20th	Day	17% 14% 6% 13% 6% 4 4 3 4 5% 23%	2990
	21st to end	Day	12% 14% 8% 14% 7% 6% 5% 5% 6% 6% 18%	4006
January	1st to 10th	Night	8% 13% 8% 21% 14% 7% 4 3 4 4 15%	5889
	11th to 20th	Night	9% 13% 6% 21% 11% 7% 5% 3 4 4 18%	5900
	21st to end	Night	12% 13% 5% 14% 9% 6% 4 3 4 4 26%	6490
February	1st to 10th	Night	11% 11% 6% 13% 7% 5% 5% 4 6% 6% 27%	2600
	11th to 20th	Night	12% 12% 7% 14% 8% 6% 5% 5% 4 5% 22%	2533
	21st to end	Night	12% 14% 5% 14% 8% 5% 4 3 5% 5% 26%	1941
March	1st to 10th	Night	14% 11% 6% 9% 6% 5% 5% 4 5% 6% 31%	4918
	11th to 20th	Night	8% 10% 5% 11% 4 4 4 3 5% 7% 38%	4980
	21st to end	Night	10% 11% 5% 8% 5% 5% 4 4 5% 6% 37%	5467
April	1st to 10th	Night	12% 13% 6% 11% 6% 4 4 4 4 5% 31%	2071
	11th to 20th	Night	12% 10% 4 7% 5% 5% 4 4 5% 7% 36%	1975
	21st to end	Night	13% 10% 3 8% 5% 4 5% 3 5% 6% 36%	1865
May	1st to 10th	Night	21% 14% 3 9% 5% 5% 4 3 6% 7% 24%	4033
	11th to 20th	Night	17% 11% 4 9% 6% 5% 6% 4 5% 7% 26%	4032
	21st to end	Night	16% 14% 3 9% 5% 5% 4 4 6% 7% 27%	4084
June	1st to 10th	Night	9% 8% 3 7% 6% 5% 5% 5% 5% 7% 40%	1600
	11th to 20th	Night	10% 8% 3 8% 6% 5% 5% 5% 6% 10% 34%	1600
	21st to end	Night	9% 7% 3 7% 5% 4 5% 5% 10% 11% 36%	1600
July	1st to 10th	Night	8% 9% 4 7% 5% 5% 5% 4 6% 8% 38%	3861
	11th to 20th	Night	9% 9% 3 9% 5% 6% 6% 4 7% 10% 33%	4057
	21st to end	Night	11% 9% 3 7% 5% 5% 5% 5% 6% 8% 36%	4427
August	1st to 10th	Night	12% 10% 3 8% 5% 4 6% 4 7% 10% 32%	2915
	11th to 20th	Night	11% 9% 3 8% 5% 4 4 5% 6% 6% 39%	2875
	21st to end	Night	6% 5% 2 6% 5% 5% 4 4 5% 8% 48%	3506
September	1st to 10th	Night	12% 6% 3 7% 4 4 4 3 5% 7% 46%	2000
	11th to 20th	Night	9% 6% 2 5% 5% 5% 4 4 5% 7% 48%	2148
	21st to end	Night	15% 9% 3 7% 6% 5% 5% 4 5% 7% 35%	2228
October	1st to 10th	Night	14% 7% 3 6% 5% 4 4 3 5% 6% 44%	2400
	11th to 20th	Night	11% 9% 3 7% 5% 5% 5% 4 6% 7% 40%	2400
	21st to end	Night	15% 13% 3 7% 5% 4 4 3 5% 5% 37%	2640
November	1st to 10th	Night	9% 10% 4 7% 5% 5% 4 4 4 5% 43%	2461
	11th to 20th	Night	12% 12% 5% 11% 7% 5% 4 4 4 5% 30%	2818
	21st to end	Night	12% 13% 7% 12% 7% 5% 4 3 5% 5% 28%	2800
December	1st to 10th	Night	9% 11% 6% 20% 9% 7% 5% 4 4 4 22%	4580
	11th to 20th	Night	15% 11% 6% 16% 7% 5% 3 3 5% 6% 22%	5205
	21st to end	Night	11% 15% 8% 15% 7% 6% 5% 4 5% 6% 18%	6932

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	62% 11% 9% 11% 5% 1	4057
	11th to 20th	Day	66% 11% 7% 9% 7% 1	4060
	21st to end	Day	56% 10% 10% 14% 9% 1	4466
February	1st to 10th	Day	58% 9% 10% 15% 8%	1800
	11th to 20th	Day	60% 7% 11% 12% 9%	1762
	21st to end	Day	58% 7% 10% 13% 10% 2	1682
March	1st to 10th	Day	49% 8% 8% 21% 10% 4	4935
	11th to 20th	Day	39% 10% 11% 26% 11% 3	4980
	21st to end	Day	49% 9% 9% 18% 14% 1	5465
April	1st to 10th	Day	53% 9% 10% 16% 11% 3	2328
	11th to 20th	Day	46% 9% 11% 16% 15% 3	2374
	21st to end	Day	51% 7% 11% 20% 9% 1	2550
May	1st to 10th	Day	53% 11% 10% 17% 7% 2	5897
	11th to 20th	Day	50% 11% 12% 22% 4 1	5865
	21st to end	Day	47% 12% 14% 17% 9% 1	6783
June	1st to 10th	Day	44% 10% 13% 20% 12% 1	2800
	11th to 20th	Day	46% 11% 13% 20% 9% 1	2800
	21st to end	Day	42% 11% 14% 22% 10% 1	2800
July	1st to 10th	Day	39% 9% 12% 26% 13% 2	6027
	11th to 20th	Day	45% 11% 16% 21% 7%	5831
	21st to end	Day	45% 9% 11% 22% 13% 1	6240
August	1st to 10th	Day	46% 12% 16% 20% 6%	4221
	11th to 20th	Day	44% 9% 13% 26% 9%	4213
	21st to end	Day	33% 10% 16% 26% 14% 1	4357
September	1st to 10th	Day	41% 7% 10% 20% 19% 3	2400
	11th to 20th	Day	39% 8% 11% 24% 15% 2	2249
	21st to end	Day	49% 8% 13% 17% 13% 1	2170
October	1st to 10th	Day	32% 7% 9% 25% 22% 6%	3260
	11th to 20th	Day	38% 6% 11% 21% 13% 10%	3240
	21st to end	Day	32% 7% 10% 27% 22% 2	3538
November	1st to 10th	Day	40% 6% 10% 22% 17% 5%	2057
	11th to 20th	Day	49% 8% 11% 15% 15% 3	1733
	21st to end	Day	56% 8% 11% 14% 10% 1	1600
December	1st to 10th	Day	59% 11% 14% 14% 3 1	2600
	11th to 20th	Day	58% 8% 12% 15% 6% 1	2986
	21st to end	Day	57% 12% 12% 12% 7%	4006
January	1st to 10th	Night	66% 10% 7% 10% 6% 1	5889
	11th to 20th	Night	61% 9% 12% 9% 7% 2	5900
	21st to end	Night	52% 12% 12% 15% 8% 1	6490
February	1st to 10th	Night	53% 10% 11% 15% 10% 1	2600
	11th to 20th	Night	61% 8% 10% 12% 9%	2535
	21st to end	Night	56% 9% 8% 15% 9% 3	1941
March	1st to 10th	Night	48% 9% 10% 22% 9% 2	4918
	11th to 20th	Night	40% 10% 11% 27% 9% 4	4980
	21st to end	Night	42% 8% 14% 21% 13% 2	5467
April	1st to 10th	Night	53% 8% 10% 15% 12% 2	2071
	11th to 20th	Night	44% 10% 12% 18% 13% 3	1975
	21st to end	Night	45% 8% 12% 23% 10% 2	1865
May	1st to 10th	Night	56% 11% 11% 12% 8% 2	4060
	11th to 20th	Night	51% 12% 11% 20% 6%	4033
	21st to end	Night	50% 13% 13% 17% 5% 3	4091
June	1st to 10th	Night	39% 10% 12% 24% 13% 1	1600
	11th to 20th	Night	42% 11% 14% 23% 10% 1	1600
	21st to end	Night	36% 12% 16% 22% 13% 1	1600
July	1st to 10th	Night	41% 10% 14% 24% 10% 1	3818
	11th to 20th	Night	44% 11% 17% 20% 7% 1	4006
	21st to end	Night	41% 9% 12% 23% 14%	4297
August	1st to 10th	Night	43% 10% 20% 19% 6%	2904
	11th to 20th	Night	42% 8% 12% 24% 14%	2902
	21st to end	Night	32% 8% 14% 33% 11% 1	3506
September	1st to 10th	Night	37% 8% 14% 19% 19% 3	2000
	11th to 20th	Night	33% 8% 13% 26% 18% 2	2148
	21st to end	Night	45% 9% 13% 18% 14%	2228
October	1st to 10th	Night	34% 6% 11% 26% 21% 2	3920
	11th to 20th	Night	36% 7% 10% 21% 16% 9%	3899
	21st to end	Night	34% 6% 8% 27% 21% 4	4252
November	1st to 10th	Night	41% 8% 10% 19% 17% 6%	2461
	11th to 20th	Night	52% 7% 9% 16% 13% 3	2818
	21st to end	Night	56% 8% 10% 15% 10% 2	2800
December	1st to 10th	Night	59% 11% 12% 14% 4 1	4580
	11th to 20th	Night	60% 7% 13% 14% 6%	5208
	21st to end	Night	62% 10% 11% 10% 6%	6932

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	40% 31% 20% 6% 3	300
	11th to 20th	Day	45% 29% 22% 3 1	300
	21st to end	Day	40% 32% 22% 6% 1	330
February	1st to 10th	Day	41% 31% 21% 6% 1	300
	11th to 20th	Day	53% 24% 21% 2 1	300
	21st to end	Day	39% 33% 21% 6% 1	331
March	1st to 10th	Day	35% 26% 27% 10% 2	500
	11th to 20th	Day	38% 26% 22% 10% 3 1	500
	21st to end	Day	34% 27% 25% 11% 3	550
April	1st to 10th	Day	31% 28% 26% 11% 3	500
	11th to 20th	Day	26% 25% 27% 13% 6% 2	492
	21st to end	Day	30% 26% 26% 12% 5% 1	500
May	1st to 10th	Day	29% 22% 28% 13% 4 3	503
	11th to 20th	Day	26% 21% 29% 16% 6% 3	527
	21st to end	Day	19% 16% 29% 19% 12% 5%	573
June	1st to 10th	Day	15% 19% 30% 21% 11% 3	530
	11th to 20th	Day	17% 17% 28% 23% 11% 4	530
	21st to end	Day	13% 18% 28% 24% 14% 3	530
July	1st to 10th	Day	12% 10% 33% 21% 17% 8%	525
	11th to 20th	Day	10% 12% 30% 28% 13% 6%	524
	21st to end	Day	15% 11% 28% 27% 14% 6%	576
August	1st to 10th	Day	13% 13% 28% 25% 14% 7%	526
	11th to 20th	Day	14% 11% 25% 26% 13% 10%	519
	21st to end	Day	11% 10% 30% 23% 17% 9%	583
September	1st to 10th	Day	16% 12% 23% 28% 14% 9%	530
	11th to 20th	Day	10% 8% 30% 25% 19% 7%	530
	21st to end	Day	26% 15% 26% 19% 10% 3	530
October	1st to 10th	Day	17% 15% 25% 25% 13% 4	530
	11th to 20th	Day	23% 22% 27% 19% 8% 2	530
	21st to end	Day	27% 25% 26% 15% 7% 1	583
November	1st to 10th	Day	23% 22% 29% 17% 7% 2	535
	11th to 20th	Day	32% 27% 28% 8% 4	256
	21st to end	Day	37% 27% 28% 7% 3 1	200
December	1st to 10th	Day	37% 33% 21% 7% 2 2	200
	11th to 20th	Day	41% 27% 23% 9% 2	200
	21st to end	Day	35% 29% 23% 10% 3	230
January	1st to 10th	Night	33% 34% 22% 9% 2	800
	11th to 20th	Night	34% 30% 24% 11% 1	800
	21st to end	Night	35% 30% 24% 10% 1	880
February	1st to 10th	Night	30% 27% 26% 16% 1	800
	11th to 20th	Night	36% 33% 23% 7%	783
	21st to end	Night	31% 32% 27% 10% 1	660
March	1st to 10th	Night	29% 25% 28% 17% 2	800
	11th to 20th	Night	31% 28% 21% 15% 5% 1	800
	21st to end	Night	26% 26% 29% 14% 4 1	880
April	1st to 10th	Night	24% 27% 30% 15% 3	800
	11th to 20th	Night	22% 20% 28% 21% 8% 1	791
	21st to end	Night	24% 18% 27% 24% 7%	665
May	1st to 10th	Night	24% 17% 24% 26% 8% 1	600
	11th to 20th	Night	21% 16% 25% 27% 10% 1	591
	21st to end	Night	13% 11% 22% 29% 22% 2	507
June	1st to 10th	Night	10% 13% 24% 37% 16% 1	400
	11th to 20th	Night	13% 12% 23% 32% 19% 1	400
	21st to end	Night	10% 8% 28% 38% 16% 1	400
July	1st to 10th	Night	9% 9% 21% 37% 20% 4	492
	11th to 20th	Night	8% 9% 20% 34% 25% 4	582
	21st to end	Night	10% 9% 20% 36% 23% 2	657
August	1st to 10th	Night	11% 6% 19% 39% 24% 2	597
	11th to 20th	Night	11% 8% 17% 39% 22% 4	594
	21st to end	Night	11% 6% 19% 38% 23% 2	826
September	1st to 10th	Night	15% 7% 22% 33% 19% 4	800
	11th to 20th	Night	11% 8% 21% 33% 25% 3	800
	21st to end	Night	20% 13% 21% 29% 16% 2	800
October	1st to 10th	Night	16% 14% 22% 28% 17% 3	800
	11th to 20th	Night	20% 17% 26% 27% 9% 1	800
	21st to end	Night	26% 20% 25% 22% 7% 1	880
November	1st to 10th	Night	19% 18% 26% 28% 9% 1	820
	11th to 20th	Night	30% 21% 24% 19% 5% 1	829
	21st to end	Night	28% 25% 26% 16% 4 1	800
December	1st to 10th	Night	32% 28% 23% 12% 4	800
	11th to 20th	Night	32% 28% 23% 15% 2	800
	21st to end	Night	29% 27% 26% 16% 1	880

Page generated at 2025-11-15 00:55:59. Version ac07

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