TGCO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the TGCO clear sky chart from 2005-11-06 to 2025-10-19.

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% 10% 4 8% 5% 5% 4 4 5% 7% 31%	126893
	Night	17% 10% 4 9% 5% 4 4 3 5% 6% 33%	122795
Transparency	Day	50% 7% 11% 19% 11% 1	130411
	Night	50% 7% 12% 19% 11% 1	127364
Seeing	Day	25% 17% 27% 18% 9% 3	16048
	Night	23% 16% 28% 24% 9% 1	25229

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	13% 14% 10% 19% 6% 4 3 2 3 4 21%	3597
	11th to 20th	Day	13% 15% 8% 18% 7% 5% 4 3 4 4 19%	3588
	21st to end	Day	18% 15% 6% 8% 5% 4 4 3 4 5% 29%	4466
February	1st to 10th	Day	16% 13% 4 11% 7% 6% 5% 5% 5% 5% 24%	1800
	11th to 20th	Day	14% 13% 6% 9% 6% 5% 4 3 5% 7% 27%	1843
	21st to end	Day	19% 12% 3 11% 7% 3 4 3 5% 6% 29%	1650
March	1st to 10th	Day	20% 14% 4 9% 5% 4 3 2 3 4 31%	4522
	11th to 20th	Day	12% 12% 4 7% 5% 4 3 4 5% 6% 38%	4980
	21st to end	Day	17% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	5757
April	1st to 10th	Day	19% 13% 3 8% 4 4 3 3 5% 6% 32%	2400
	11th to 20th	Day	17% 11% 3 7% 4 5% 5% 3 5% 6% 35%	2374
	21st to end	Day	17% 10% 3 8% 4 4 4 3 5% 6% 36%	2400
May	1st to 10th	Day	27% 9% 3 7% 5% 5% 4 3 5% 6% 26%	6101
	11th to 20th	Day	20% 11% 4 8% 5% 5% 5% 3 5% 6% 27%	6318
	21st to end	Day	21% 8% 3 6% 4 5% 5% 5% 6% 8% 30%	6949
June	1st to 10th	Day	16% 7% 2 6% 6% 5% 5% 5% 7% 9% 33%	2800
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 5% 6% 8% 32%	2800
	21st to end	Day	14% 6% 3 7% 5% 5% 5% 4 7% 9% 35%	2800
July	1st to 10th	Day	9% 6% 3 8% 6% 6% 6% 5% 7% 9% 35%	6360
	11th to 20th	Day	10% 6% 3 8% 5% 5% 7% 5% 9% 12% 31%	6360
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 4 7% 8% 39%	6909
August	1st to 10th	Day	19% 8% 3 8% 5% 5% 4 4 6% 8% 30%	4234
	11th to 20th	Day	16% 7% 3 7% 4 5% 6% 4 6% 9% 34%	3956
	21st to end	Day	15% 7% 2 6% 3 4 4 4 6% 9% 40%	4312
September	1st to 10th	Day	15% 7% 2 6% 4 6% 5% 3 5% 7% 40%	2400
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 6% 8% 45%	2390
	21st to end	Day	19% 8% 3 7% 4 4 4 4 7% 8% 34%	2159
October	1st to 10th	Day	17% 7% 2 7% 3 4 5% 3 6% 7% 38%	2000
	11th to 20th	Day	17% 9% 2 7% 5% 5% 4 4 6% 6% 36%	1980
	21st to end	Day	21% 12% 3 7% 4 4 4 3 5% 6% 30%	2011
November	1st to 10th	Day	14% 9% 3 7% 5% 3 4 2 5% 5% 42%	1761
	11th to 20th	Day	16% 11% 3 9% 5% 4 3 3 5% 6% 34%	1770
	21st to end	Day	20% 13% 4 6% 6% 4 5% 3 5% 6% 29%	1600
December	1st to 10th	Day	16% 15% 6% 12% 6% 5% 4 4 5% 5% 23%	2600
	11th to 20th	Day	20% 16% 6% 9% 4 3 3 2 4 4 29%	2986
	21st to end	Day	18% 15% 6% 12% 5% 5% 4 3 4 6% 22%	3960
January	1st to 10th	Night	13% 12% 6% 19% 8% 5% 4 3 4 5% 22%	6349
	11th to 20th	Night	11% 14% 8% 17% 6% 4 4 3 4 5% 22%	6294
	21st to end	Night	17% 11% 5% 11% 7% 6% 6% 4 5% 5% 23%	6490
February	1st to 10th	Night	16% 12% 3 10% 7% 5% 4 3 6% 6% 28%	2600
	11th to 20th	Night	15% 12% 5% 11% 6% 4 5% 4 5% 6% 27%	2443
	21st to end	Night	17% 12% 4 10% 6% 4 4 3 5% 5% 30%	1980
March	1st to 10th	Night	18% 12% 5% 8% 6% 3 4 3 4 5% 32%	5304
	11th to 20th	Night	13% 11% 5% 8% 5% 4 4 3 5% 5% 37%	4979
	21st to end	Night	17% 12% 3 9% 6% 4 4 3 4 5% 33%	5174
April	1st to 10th	Night	19% 13% 4 9% 6% 4 4 3 5% 5% 29%	2000
	11th to 20th	Night	17% 10% 3 7% 4 4 3 4 5% 7% 36%	1975
	21st to end	Night	18% 8% 3 7% 4 3 4 4 5% 6% 38%	2000
May	1st to 10th	Night	30% 9% 3 7% 4 4 4 3 4 6% 27%	3865
	11th to 20th	Night	20% 8% 3 7% 6% 4 4 4 4 6% 33%	3575
	21st to end	Night	22% 11% 3 7% 5% 4 4 4 5% 6% 31%	3926
June	1st to 10th	Night	16% 8% 2 6% 4 5% 4 4 6% 8% 38%	1600
	11th to 20th	Night	15% 7% 2 6% 5% 5% 3 3 6% 8% 40%	1600
	21st to end	Night	13% 4 2 6% 4 4 4 4 7% 8% 43%	1600
July	1st to 10th	Night	12% 7% 2 8% 5% 5% 5% 3 5% 8% 39%	3600
	11th to 20th	Night	14% 6% 3 7% 5% 5% 4 4 7% 9% 34%	3600
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 7% 47%	3926
August	1st to 10th	Night	20% 9% 3 7% 4 5% 4 4 7% 6% 31%	2915
	11th to 20th	Night	17% 7% 3 6% 5% 4 4 3 5% 6% 40%	3057
	21st to end	Night	13% 7% 2 6% 4 4 4 3 4 7% 46%	3586
September	1st to 10th	Night	15% 7% 2 5% 4 4 5% 4 6% 6% 44%	2000
	11th to 20th	Night	12% 6% 2 5% 3 4 3 3 5% 7% 50%	2009
	21st to end	Night	17% 8% 3 6% 4 4 3 3 5% 6% 39%	2238
October	1st to 10th	Night	19% 8% 3 5% 4 4 3 3 5% 7% 40%	2400
	11th to 20th	Night	18% 8% 3 6% 4 3 3 3 5% 5% 41%	2377
	21st to end	Night	22% 10% 2 6% 4 4 4 4 5% 6% 33%	2585
November	1st to 10th	Night	12% 10% 3 7% 4 4 3 3 5% 6% 42%	2537
	11th to 20th	Night	19% 11% 3 7% 3 3 4 3 5% 6% 35%	2627
	21st to end	Night	15% 12% 3 7% 5% 5% 4 4 5% 7% 33%	2800
December	1st to 10th	Night	15% 13% 7% 12% 6% 5% 4 3 5% 6% 25%	4580
	11th to 20th	Night	23% 14% 5% 7% 4 3 3 2 3 5% 31%	5208
	21st to end	Night	20% 16% 6% 10% 6% 4 4 3 4 5% 24%	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	67% 4 8% 17% 3 1	3598
	11th to 20th	Day	67% 6% 8% 11% 7% 1	3618
	21st to end	Day	58% 5% 9% 19% 9%	4466
February	1st to 10th	Day	59% 8% 11% 14% 9% 1	1800
	11th to 20th	Day	56% 6% 10% 19% 9%	1852
	21st to end	Day	56% 5% 10% 19% 8% 2	1650
March	1st to 10th	Day	58% 4 9% 15% 9% 3	4525
	11th to 20th	Day	44% 8% 16% 22% 7% 3	4980
	21st to end	Day	50% 7% 9% 12% 19% 3	5757
April	1st to 10th	Day	53% 6% 7% 21% 10% 3	2400
	11th to 20th	Day	49% 6% 11% 19% 13% 2	2374
	21st to end	Day	48% 6% 9% 20% 13% 3	2400
May	1st to 10th	Day	58% 8% 10% 14% 9% 1	6112
	11th to 20th	Day	55% 8% 8% 18% 10% 1	6322
	21st to end	Day	50% 8% 11% 18% 11% 1	6949
June	1st to 10th	Day	46% 9% 13% 19% 13%	2800
	11th to 20th	Day	47% 8% 12% 21% 10% 1	2800
	21st to end	Day	44% 8% 14% 23% 11%	2800
July	1st to 10th	Day	43% 9% 13% 23% 11%	6290
	11th to 20th	Day	41% 11% 16% 23% 10%	6285
	21st to end	Day	41% 8% 13% 21% 17% 1	6711
August	1st to 10th	Day	51% 8% 13% 18% 9%	4221
	11th to 20th	Day	44% 9% 15% 24% 7%	3995
	21st to end	Day	39% 7% 12% 31% 10% 1	4312
September	1st to 10th	Day	43% 7% 11% 23% 15% 1	2400
	11th to 20th	Day	37% 6% 12% 23% 21% 1	2390
	21st to end	Day	46% 7% 12% 21% 13% 1	2159
October	1st to 10th	Day	45% 6% 10% 22% 16% 1	3260
	11th to 20th	Day	42% 6% 8% 25% 15% 4	3220
	21st to end	Day	37% 5% 17% 27% 13% 1	3288
November	1st to 10th	Day	43% 7% 12% 22% 14% 2	1761
	11th to 20th	Day	51% 6% 12% 19% 11% 2	1770
	21st to end	Day	55% 7% 13% 18% 6% 1	1600
December	1st to 10th	Day	62% 7% 12% 14% 4	2600
	11th to 20th	Day	59% 5% 12% 14% 9% 1	2986
	21st to end	Day	63% 6% 11% 12% 8%	3960
January	1st to 10th	Night	63% 6% 10% 14% 6% 1	6349
	11th to 20th	Night	62% 7% 12% 12% 6% 1	6339
	21st to end	Night	60% 6% 9% 12% 11% 1	6490
February	1st to 10th	Night	56% 6% 13% 17% 8% 1	2600
	11th to 20th	Night	56% 7% 11% 16% 9% 1	2445
	21st to end	Night	55% 5% 9% 19% 9% 2	1980
March	1st to 10th	Night	55% 4 13% 15% 12% 1	5314
	11th to 20th	Night	46% 8% 18% 19% 7% 4	4980
	21st to end	Night	51% 8% 8% 17% 15% 2	5174
April	1st to 10th	Night	56% 6% 9% 17% 11% 2	2000
	11th to 20th	Night	48% 7% 10% 19% 15%	1975
	21st to end	Night	45% 6% 10% 21% 14% 3	2000
May	1st to 10th	Night	58% 6% 8% 17% 8% 1	3871
	11th to 20th	Night	50% 10% 12% 17% 10% 1	3576
	21st to end	Night	52% 8% 11% 18% 10% 1	3927
June	1st to 10th	Night	42% 7% 11% 24% 15% 1	1600
	11th to 20th	Night	42% 7% 16% 25% 10% 1	1600
	21st to end	Night	36% 10% 16% 25% 13% 1	1600
July	1st to 10th	Night	42% 7% 15% 20% 15% 1	3560
	11th to 20th	Night	43% 9% 15% 22% 11%	3552
	21st to end	Night	36% 7% 12% 26% 18% 1	3809
August	1st to 10th	Night	51% 9% 14% 19% 8% 1	2904
	11th to 20th	Night	43% 7% 15% 25% 10%	3084
	21st to end	Night	38% 6% 12% 31% 12% 1	3586
September	1st to 10th	Night	40% 6% 13% 25% 15% 1	2000
	11th to 20th	Night	33% 6% 11% 27% 21% 1	2009
	21st to end	Night	45% 6% 10% 21% 17% 1	2238
October	1st to 10th	Night	47% 7% 10% 21% 14% 1	3920
	11th to 20th	Night	38% 5% 13% 21% 18% 4	3876
	21st to end	Night	38% 6% 16% 26% 14% 1	4258
November	1st to 10th	Night	42% 5% 12% 22% 17% 2	2537
	11th to 20th	Night	49% 6% 13% 19% 12% 2	2627
	21st to end	Night	49% 7% 15% 21% 8%	2800
December	1st to 10th	Night	59% 7% 12% 20% 3	4580
	11th to 20th	Night	57% 5% 12% 16% 11% 1	5208
	21st to end	Night	63% 6% 9% 14% 8% 1	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	33% 27% 29% 9% 2 1	200
	11th to 20th	Day	40% 29% 22% 7% 1	203
	21st to end	Day	28% 30% 29% 10% 3	330
February	1st to 10th	Day	34% 32% 26% 7%	300
	11th to 20th	Day	38% 28% 27% 5% 1	390
	21st to end	Day	36% 25% 28% 9% 2	414
March	1st to 10th	Day	35% 18% 31% 13% 3	500
	11th to 20th	Day	30% 27% 26% 13% 4	500
	21st to end	Day	33% 27% 26% 10% 3 2	550
April	1st to 10th	Day	36% 23% 25% 12% 3 1	500
	11th to 20th	Day	33% 25% 24% 10% 6% 2	492
	21st to end	Day	28% 21% 31% 15% 4 2	500
May	1st to 10th	Day	34% 16% 30% 11% 8% 2	503
	11th to 20th	Day	25% 17% 28% 20% 7% 2	527
	21st to end	Day	23% 11% 28% 22% 13% 3	573
June	1st to 10th	Day	19% 17% 30% 22% 10% 2	530
	11th to 20th	Day	22% 15% 26% 23% 14% 1	530
	21st to end	Day	17% 15% 32% 25% 9% 2	530
July	1st to 10th	Day	14% 9% 29% 28% 16% 4	525
	11th to 20th	Day	11% 8% 29% 29% 19% 4	524
	21st to end	Day	14% 11% 34% 26% 11% 4	576
August	1st to 10th	Day	13% 10% 27% 25% 19% 7%	526
	11th to 20th	Day	18% 8% 24% 28% 17% 6%	519
	21st to end	Day	14% 11% 28% 24% 14% 8%	583
September	1st to 10th	Day	18% 12% 23% 22% 18% 6%	530
	11th to 20th	Day	13% 7% 25% 25% 18% 11%	530
	21st to end	Day	26% 11% 27% 20% 13% 3	530
October	1st to 10th	Day	25% 12% 23% 20% 15% 5%	530
	11th to 20th	Day	25% 18% 27% 21% 8% 1	522
	21st to end	Day	30% 16% 27% 20% 6% 1	496
November	1st to 10th	Day	26% 19% 23% 21% 8% 3	395
	11th to 20th	Day	29% 22% 29% 16% 5% 1	370
	21st to end	Day	38% 21% 33% 7% 3	200
December	1st to 10th	Day	32% 30% 24% 10% 3 1	200
	11th to 20th	Day	36% 25% 26% 10% 4	200
	21st to end	Day	32% 22% 33% 11% 2	220
January	1st to 10th	Night	25% 27% 32% 14% 2	800
	11th to 20th	Night	27% 22% 30% 18% 3	800
	21st to end	Night	21% 26% 35% 16% 2	880
February	1st to 10th	Night	28% 20% 33% 18% 1	800
	11th to 20th	Night	29% 26% 32% 11% 1	783
	21st to end	Night	30% 21% 33% 13% 3	660
March	1st to 10th	Night	29% 22% 30% 17% 2	800
	11th to 20th	Night	26% 23% 30% 17% 4	800
	21st to end	Night	24% 23% 31% 16% 5%	880
April	1st to 10th	Night	27% 21% 31% 19% 3	800
	11th to 20th	Night	25% 20% 32% 17% 6% 1	791
	21st to end	Night	24% 17% 31% 23% 5%	800
May	1st to 10th	Night	28% 14% 28% 22% 8%	528
	11th to 20th	Night	18% 12% 32% 30% 7% 1	394
	21st to end	Night	16% 8% 29% 33% 14%	433
June	1st to 10th	Night	18% 8% 22% 40% 13%	400
	11th to 20th	Night	17% 9% 24% 39% 12%	400
	21st to end	Night	12% 7% 25% 43% 13%	400
July	1st to 10th	Night	13% 5% 21% 41% 19% 1	390
	11th to 20th	Night	13% 4 20% 45% 18% 1	388
	21st to end	Night	14% 6% 21% 40% 19%	438
August	1st to 10th	Night	15% 2 19% 41% 20% 2	597
	11th to 20th	Night	14% 4 15% 38% 26% 3	698
	21st to end	Night	15% 7% 18% 35% 24% 1	880
September	1st to 10th	Night	19% 6% 22% 33% 18% 2	800
	11th to 20th	Night	14% 6% 21% 31% 24% 4	800
	21st to end	Night	21% 9% 23% 31% 15% 2	800
October	1st to 10th	Night	23% 11% 23% 24% 19% 1	800
	11th to 20th	Night	23% 14% 27% 29% 7% 1	793
	21st to end	Night	29% 14% 29% 22% 6%	836
November	1st to 10th	Night	19% 15% 33% 24% 8%	780
	11th to 20th	Night	26% 18% 30% 19% 6% 1	800
	21st to end	Night	23% 19% 33% 20% 4 1	800
December	1st to 10th	Night	29% 20% 31% 16% 4	800
	11th to 20th	Night	26% 22% 29% 21% 3	800
	21st to end	Night	28% 20% 32% 17% 3	880

Page generated at 2025-10-16 01:50:27. Version ac07

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