COG Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the COG Observing Site clear sky chart from 2005-11-06 to 2026-01-17.

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	19% 8% 3 7% 4 4 4 4 5% 7% 36%	126604
	Night	18% 9% 3 7% 4 4 4 4 5% 7% 35%	125109
Transparency	Day	48% 5% 11% 21% 13% 2	130111
	Night	48% 5% 12% 21% 13% 2	129690
Seeing	Day	24% 14% 30% 21% 9% 2	16910
	Night	23% 11% 31% 28% 7% 1	25550

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	14% 9% 4 8% 7% 5% 5% 4 5% 6% 34%	3677
	11th to 20th	Day	14% 8% 3 8% 4 3 4 3 4 6% 40%	3618
	21st to end	Day	11% 10% 4 8% 5% 4 4 3 5% 5% 42%	3960
February	1st to 10th	Day	17% 11% 2 7% 4 5% 4 4 5% 5% 35%	1762
	11th to 20th	Day	13% 10% 3 9% 5% 6% 5% 4 5% 6% 34%	1908
	21st to end	Day	15% 10% 3 8% 6% 4 5% 4 5% 5% 35%	1650
March	1st to 10th	Day	23% 12% 3 8% 5% 4 4 3 3 5% 29%	4476
	11th to 20th	Day	19% 11% 3 7% 5% 5% 4 3 4 5% 34%	4924
	21st to end	Day	27% 8% 3 6% 3 3 3 3 6% 6% 31%	5554
April	1st to 10th	Day	22% 11% 3 7% 4 4 3 3 4 5% 34%	2400
	11th to 20th	Day	20% 8% 2 5% 4 4 4 3 5% 7% 38%	2507
	21st to end	Day	24% 9% 2 7% 4 4 3 2 5% 5% 34%	2600
May	1st to 10th	Day	30% 9% 3 7% 5% 4 4 3 4 5% 24%	5886
	11th to 20th	Day	21% 8% 3 8% 5% 5% 5% 4 5% 6% 30%	5986
	21st to end	Day	19% 7% 2 6% 5% 5% 5% 5% 6% 7% 32%	6949
June	1st to 10th	Day	24% 7% 3 6% 4 5% 4 4 7% 7% 30%	2975
	11th to 20th	Day	18% 7% 2 6% 4 4 5% 4 6% 7% 38%	3000
	21st to end	Day	24% 7% 2 5% 4 4 5% 4 5% 7% 32%	2966
July	1st to 10th	Day	15% 7% 2 7% 5% 5% 6% 5% 7% 9% 31%	6360
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 7% 9% 37%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 3 6% 9% 42%	6926
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 4 6% 9% 34%	4270
	11th to 20th	Day	16% 9% 3 7% 5% 4 5% 4 6% 8% 34%	4175
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 7% 9% 44%	4553
September	1st to 10th	Day	12% 5% 3 5% 4 4 4 4 5% 7% 47%	2341
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 5% 7% 52%	2200
	21st to end	Day	17% 5% 2 6% 4 4 4 3 6% 8% 43%	2200
October	1st to 10th	Day	18% 7% 3 6% 4 4 5% 3 5% 6% 40%	2185
	11th to 20th	Day	20% 7% 3 5% 4 4 4 3 5% 7% 40%	1926
	21st to end	Day	24% 8% 3 7% 4 4 4 3 4 5% 34%	1980
November	1st to 10th	Day	17% 6% 2 6% 4 4 5% 4 5% 7% 40%	1851
	11th to 20th	Day	19% 6% 2 5% 4 4 4 3 5% 7% 40%	1890
	21st to end	Day	22% 7% 3 6% 4 3 3 2 4 6% 40%	1734
December	1st to 10th	Day	18% 9% 3 6% 4 4 4 4 6% 7% 35%	2366
	11th to 20th	Day	19% 8% 2 5% 3 3 3 4 5% 6% 42%	2688
	21st to end	Day	21% 11% 3 5% 5% 4 4 3 4 5% 34%	3801
January	1st to 10th	Night	12% 9% 3 9% 6% 5% 4 4 5% 6% 37%	6489
	11th to 20th	Night	13% 10% 4 10% 6% 6% 6% 5% 6% 7% 27%	6400
	21st to end	Night	11% 10% 4 8% 5% 5% 5% 4 5% 6% 37%	6996
February	1st to 10th	Night	15% 11% 4 7% 5% 5% 5% 4 6% 6% 32%	2635
	11th to 20th	Night	13% 10% 4 7% 5% 5% 4 4 4 6% 38%	2375
	21st to end	Night	14% 12% 3 7% 5% 4 4 3 5% 6% 37%	1980
March	1st to 10th	Night	26% 11% 3 7% 5% 4 4 3 4 4 29%	5364
	11th to 20th	Night	17% 11% 3 7% 4 5% 5% 4 5% 7% 32%	5034
	21st to end	Night	25% 10% 3 6% 4 5% 4 3 5% 7% 29%	5362
April	1st to 10th	Night	22% 12% 3 6% 4 3 4 2 4 5% 34%	2000
	11th to 20th	Night	19% 7% 2 6% 5% 5% 4 4 5% 6% 35%	1842
	21st to end	Night	25% 9% 2 7% 4 3 3 3 5% 5% 34%	1800
May	1st to 10th	Night	30% 11% 3 7% 4 3 3 3 5% 6% 25%	4054
	11th to 20th	Night	21% 8% 3 8% 5% 4 5% 4 6% 7% 30%	3951
	21st to end	Night	17% 7% 3 8% 5% 4 5% 3 6% 7% 35%	3926
June	1st to 10th	Night	22% 9% 2 6% 4 4 5% 3 5% 8% 32%	1420
	11th to 20th	Night	16% 7% 2 6% 5% 3 4 4 6% 8% 39%	1400
	21st to end	Night	25% 7% 2 6% 5% 3 3 5% 6% 8% 31%	1429
July	1st to 10th	Night	12% 7% 3 7% 4 5% 6% 5% 7% 9% 35%	3600
	11th to 20th	Night	17% 7% 2 7% 5% 5% 4 4 5% 8% 36%	3600
	21st to end	Night	11% 4 2 5% 4 4 5% 4 7% 9% 46%	3926
August	1st to 10th	Night	19% 8% 3 7% 5% 5% 5% 4 6% 6% 31%	2845
	11th to 20th	Night	16% 8% 3 8% 5% 4 5% 4 5% 8% 34%	2872
	21st to end	Night	12% 6% 2 6% 5% 4 4 4 5% 7% 45%	3341
September	1st to 10th	Night	13% 5% 2 5% 3 4 4 3 5% 7% 49%	2058
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 6% 52%	2200
	21st to end	Night	17% 7% 2 5% 3 4 4 3 5% 6% 45%	2200
October	1st to 10th	Night	19% 6% 2 6% 4 5% 4 3 6% 5% 41%	2215
	11th to 20th	Night	18% 6% 2 5% 4 5% 4 3 5% 7% 40%	2470
	21st to end	Night	25% 8% 2 6% 4 4 4 3 5% 6% 34%	2860
November	1st to 10th	Night	19% 6% 2 6% 4 4 4 3 5% 5% 41%	2667
	11th to 20th	Night	17% 6% 2 6% 4 4 4 4 4 7% 41%	2730
	21st to end	Night	18% 7% 2 6% 4 4 4 4 5% 7% 37%	2882
December	1st to 10th	Night	17% 10% 3 7% 4 4 5% 4 6% 7% 33%	5042
	11th to 20th	Night	23% 8% 2 5% 4 3 4 3 4 5% 38%	5741
	21st to end	Night	21% 11% 3 6% 3 3 4 3 5% 6% 36%	7403

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	50% 5% 10% 23% 11% 1	3678
	11th to 20th	Day	44% 5% 10% 28% 10% 2	3648
	21st to end	Day	45% 4 10% 17% 18% 6%	3960
February	1st to 10th	Day	49% 6% 10% 19% 14% 3	1762
	11th to 20th	Day	50% 6% 12% 17% 13% 3	1917
	21st to end	Day	50% 5% 11% 18% 11% 5%	1650
March	1st to 10th	Day	60% 4 6% 10% 14% 6%	4479
	11th to 20th	Day	51% 6% 13% 18% 10% 2	4924
	21st to end	Day	53% 3 5% 18% 17% 4	5554
April	1st to 10th	Day	53% 4 8% 17% 14% 5%	2400
	11th to 20th	Day	46% 5% 9% 23% 15% 2	2507
	21st to end	Day	51% 5% 9% 19% 13% 4	2600
May	1st to 10th	Day	61% 7% 10% 15% 7% 1	5897
	11th to 20th	Day	53% 7% 13% 17% 7% 3	5990
	21st to end	Day	47% 7% 13% 19% 12% 1	6949
June	1st to 10th	Day	51% 7% 12% 20% 9% 1	2975
	11th to 20th	Day	44% 6% 11% 21% 15% 2	3000
	21st to end	Day	51% 7% 12% 19% 10% 1	2966
July	1st to 10th	Day	49% 4 15% 22% 10%	6290
	11th to 20th	Day	44% 4 14% 24% 13% 1	6285
	21st to end	Day	40% 4 12% 28% 15% 1	6728
August	1st to 10th	Day	47% 6% 14% 26% 7%	4256
	11th to 20th	Day	48% 5% 16% 23% 8%	4214
	21st to end	Day	37% 5% 13% 27% 17% 1	4553
September	1st to 10th	Day	35% 7% 13% 28% 17% 1	2341
	11th to 20th	Day	32% 5% 10% 30% 21% 2	2200
	21st to end	Day	39% 5% 12% 27% 16% 1	2200
October	1st to 10th	Day	53% 3 9% 22% 12% 1	3562
	11th to 20th	Day	37% 3 12% 29% 18% 1	3116
	21st to end	Day	40% 3 14% 22% 19% 2	3180
November	1st to 10th	Day	44% 6% 12% 22% 15% 1	1851
	11th to 20th	Day	42% 6% 12% 21% 16% 3	1890
	21st to end	Day	47% 4 15% 23% 10% 1	1734
December	1st to 10th	Day	49% 3 15% 20% 13% 1	2366
	11th to 20th	Day	44% 4 12% 17% 16% 7%	2688
	21st to end	Day	53% 4 9% 18% 13% 3	3801
January	1st to 10th	Night	48% 4 13% 23% 11% 1	6489
	11th to 20th	Night	54% 6% 13% 18% 7% 2	6445
	21st to end	Night	47% 5% 8% 20% 16% 5%	6996
February	1st to 10th	Night	50% 6% 11% 19% 12% 3	2635
	11th to 20th	Night	47% 5% 11% 19% 15% 3	2377
	21st to end	Night	48% 6% 9% 20% 14% 4	1980
March	1st to 10th	Night	58% 4 6% 12% 13% 7%	5374
	11th to 20th	Night	50% 6% 17% 17% 7% 2	5035
	21st to end	Night	54% 4 6% 16% 18% 2	5362
April	1st to 10th	Night	54% 4 7% 17% 15% 3	2000
	11th to 20th	Night	47% 5% 10% 19% 16% 1	1842
	21st to end	Night	52% 5% 11% 16% 14% 3	1800
May	1st to 10th	Night	59% 6% 13% 13% 8% 1	4060
	11th to 20th	Night	52% 8% 13% 19% 8% 1	3952
	21st to end	Night	47% 6% 15% 20% 11% 1	3927
June	1st to 10th	Night	51% 6% 13% 20% 9% 1	1420
	11th to 20th	Night	43% 6% 15% 22% 13% 2	1400
	21st to end	Night	49% 8% 13% 18% 10% 2	1429
July	1st to 10th	Night	45% 4 17% 24% 10%	3560
	11th to 20th	Night	48% 3 12% 26% 10%	3552
	21st to end	Night	36% 4 17% 31% 10% 1	3809
August	1st to 10th	Night	52% 6% 13% 22% 6%	2835
	11th to 20th	Night	48% 5% 17% 19% 11%	2899
	21st to end	Night	39% 4 14% 25% 16% 2	3341
September	1st to 10th	Night	34% 6% 14% 30% 15%	2058
	11th to 20th	Night	34% 5% 9% 32% 19% 1	2200
	21st to end	Night	39% 6% 11% 28% 14% 2	2200
October	1st to 10th	Night	47% 4 11% 24% 13% 1	3618
	11th to 20th	Night	38% 4 11% 29% 18% 1	4019
	21st to end	Night	42% 2 12% 27% 15% 2	4610
November	1st to 10th	Night	44% 5% 12% 24% 13% 1	2667
	11th to 20th	Night	43% 6% 11% 23% 16% 2	2730
	21st to end	Night	46% 5% 15% 20% 12% 1	2882
December	1st to 10th	Night	52% 3 10% 26% 9% 1	5042
	11th to 20th	Night	49% 4 9% 18% 18% 2	5742
	21st to end	Night	50% 5% 11% 17% 16% 1	7403

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	29% 19% 28% 19% 6%	220
	11th to 20th	Day	30% 19% 30% 17% 3 1	212
	21st to end	Day	23% 20% 32% 20% 5%	220
February	1st to 10th	Day	30% 16% 30% 20% 5%	362
	11th to 20th	Day	27% 19% 36% 17% 2	476
	21st to end	Day	24% 20% 32% 19% 5%	414
March	1st to 10th	Day	30% 18% 32% 16% 4	500
	11th to 20th	Day	26% 21% 32% 16% 4 1	500
	21st to end	Day	29% 21% 27% 15% 6% 2	550
April	1st to 10th	Day	36% 19% 26% 12% 6% 1	500
	11th to 20th	Day	33% 22% 26% 13% 5% 1	492
	21st to end	Day	31% 19% 30% 14% 5% 1	500
May	1st to 10th	Day	33% 16% 29% 17% 4 1	503
	11th to 20th	Day	24% 17% 33% 17% 7% 2	527
	21st to end	Day	23% 16% 35% 18% 8%	573
June	1st to 10th	Day	25% 15% 29% 21% 9% 2	705
	11th to 20th	Day	21% 15% 26% 24% 13% 2	730
	21st to end	Day	26% 11% 28% 24% 9% 2	696
July	1st to 10th	Day	18% 11% 33% 27% 9% 2	525
	11th to 20th	Day	16% 8% 36% 24% 12% 4	524
	21st to end	Day	16% 8% 36% 27% 10% 3	576
August	1st to 10th	Day	13% 6% 31% 32% 15% 2	526
	11th to 20th	Day	16% 9% 26% 28% 15% 6%	519
	21st to end	Day	16% 10% 31% 26% 14% 3	583
September	1st to 10th	Day	14% 12% 30% 26% 15% 4	530
	11th to 20th	Day	18% 9% 25% 22% 15% 10%	530
	21st to end	Day	18% 13% 27% 23% 14% 6%	530
October	1st to 10th	Day	22% 14% 23% 23% 15% 4	522
	11th to 20th	Day	27% 12% 27% 19% 12% 3	420
	21st to end	Day	32% 12% 27% 20% 8%	462
November	1st to 10th	Day	27% 13% 28% 22% 8% 2	425
	11th to 20th	Day	27% 15% 30% 21% 7% 1	430
	21st to end	Day	29% 15% 25% 23% 7%	430
December	1st to 10th	Day	28% 14% 33% 17% 8%	236
	11th to 20th	Day	33% 20% 31% 14% 2	220
	21st to end	Day	32% 16% 26% 19% 6%	242
January	1st to 10th	Night	21% 15% 35% 27% 3	840
	11th to 20th	Night	24% 14% 33% 24% 5%	825
	21st to end	Night	17% 15% 37% 26% 4	880
February	1st to 10th	Night	23% 15% 31% 28% 4	800
	11th to 20th	Night	21% 17% 38% 21% 3	783
	21st to end	Night	21% 14% 34% 28% 3	660
March	1st to 10th	Night	26% 15% 35% 23% 2	800
	11th to 20th	Night	24% 14% 33% 24% 6%	800
	21st to end	Night	25% 16% 30% 23% 6%	880
April	1st to 10th	Night	31% 15% 29% 22% 3	800
	11th to 20th	Night	23% 15% 35% 23% 5%	658
	21st to end	Night	31% 12% 31% 22% 4	600
May	1st to 10th	Night	27% 11% 29% 29% 4	600
	11th to 20th	Night	23% 10% 37% 24% 5% 1	562
	21st to end	Night	20% 8% 32% 36% 3	433
June	1st to 10th	Night	25% 7% 34% 28% 7%	400
	11th to 20th	Night	18% 7% 28% 36% 11%	400
	21st to end	Night	25% 5% 26% 38% 6% 1	400
July	1st to 10th	Night	19% 2 30% 39% 10%	390
	11th to 20th	Night	18% 4 24% 39% 14% 1	388
	21st to end	Night	14% 3 25% 42% 15%	438
August	1st to 10th	Night	15% 3 26% 41% 14%	553
	11th to 20th	Night	16% 6% 22% 42% 14% 1	592
	21st to end	Night	15% 6% 26% 37% 15% 2	726
September	1st to 10th	Night	16% 4 30% 36% 14% 1	800
	11th to 20th	Night	17% 6% 24% 32% 18% 3	800
	21st to end	Night	21% 6% 29% 30% 13% 2	800
October	1st to 10th	Night	22% 7% 23% 34% 15% 1	800
	11th to 20th	Night	21% 9% 29% 29% 10% 2	800
	21st to end	Night	30% 10% 28% 27% 6%	880
November	1st to 10th	Night	25% 9% 31% 26% 8% 1	820
	11th to 20th	Night	23% 11% 31% 26% 8% 1	840
	21st to end	Night	23% 14% 30% 28% 5%	920
December	1st to 10th	Night	28% 10% 33% 23% 5%	950
	11th to 20th	Night	25% 14% 34% 24% 4	950
	21st to end	Night	28% 11% 32% 23% 4	982

Page generated at 2026-01-14 01:14:24. Version ac07

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