TBM Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the TBM Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	14% 6% 2 5% 4 4 4 4 6% 8% 44%	113782
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	115141
Transparency	Day	37% 5% 12% 25% 17% 4	117414
	Night	39% 5% 11% 24% 18% 4	119649
Seeing	Day	18% 11% 27% 24% 14% 6%	15652
	Night	17% 9% 29% 30% 13% 1	25185

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	14% 4 2 4 3 3 3 3 4 6% 53%	3597
	11th to 20th	Day	15% 8% 3 8% 4 3 3 3 3 4 47%	3950
	21st to end	Day	16% 6% 2 6% 4 4 4 3 4 5% 45%	4466
February	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 41%	1800
	11th to 20th	Day	19% 7% 2 5% 4 4 3 3 5% 6% 43%	1878
	21st to end	Day	18% 8% 3 6% 4 4 4 2 5% 6% 40%	1650
March	1st to 10th	Day	19% 5% 2 4 3 3 4 3 5% 7% 46%	4653
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 6% 51%	4980
	21st to end	Day	17% 6% 2 6% 4 3 4 3 5% 6% 44%	5689
April	1st to 10th	Day	14% 7% 2 6% 4 5% 4 4 4 7% 44%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 8% 47%	2374
	21st to end	Day	16% 6% 2 5% 3 3 3 3 6% 9% 44%	2400
May	1st to 10th	Day	24% 8% 3 5% 3 4 4 4 6% 8% 32%	4033
	11th to 20th	Day	21% 7% 2 7% 5% 5% 4 4 6% 8% 31%	4398
	21st to end	Day	11% 4 2 5% 4 4 4 4 6% 10% 46%	4823
June	1st to 10th	Day	10% 5% 1 6% 4 4 5% 4 7% 10% 43%	2660
	11th to 20th	Day	10% 4 2 5% 4 4 5% 4 7% 10% 44%	2660
	21st to end	Day	10% 4 2 4 3 3 5% 4 7% 11% 48%	2660
July	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	4440
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 5% 8% 11% 40%	4440
	21st to end	Day	9% 6% 3 7% 5% 6% 6% 5% 8% 10% 36%	4513
August	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 8% 11% 37%	4104
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 44%	3765
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 9% 49%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 5% 6% 46%	2280
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 50%	2280
	21st to end	Day	19% 9% 3 6% 3 3 5% 3 4 7% 38%	2038
October	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 4 6% 47%	1900
	11th to 20th	Day	16% 5% 1 4 3 3 3 2 4 7% 52%	1900
	21st to end	Day	14% 6% 2 5% 4 3 3 3 4 5% 51%	2090
November	1st to 10th	Day	13% 5% 1 5% 3 3 4 3 5% 6% 53%	1771
	11th to 20th	Day	12% 7% 2 4 3 4 4 4 6% 6% 49%	1800
	21st to end	Day	14% 6% 2 5% 4 4 4 3 4 6% 48%	1660
December	1st to 10th	Day	12% 5% 2 5% 4 4 3 3 5% 7% 49%	2600
	11th to 20th	Day	19% 8% 2 5% 3 3 3 3 4 6% 44%	2990
	21st to end	Day	13% 8% 2 6% 4 4 4 4 6% 7% 44%	3960
January	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 4 5% 49%	6349
	11th to 20th	Night	13% 7% 2 5% 3 3 4 3 4 5% 51%	5994
	21st to end	Night	16% 7% 3 5% 3 3 3 3 4 5% 47%	6490
February	1st to 10th	Night	17% 6% 3 5% 5% 4 4 4 5% 6% 41%	2600
	11th to 20th	Night	20% 7% 2 6% 3 3 3 3 4 6% 42%	2407
	21st to end	Night	17% 7% 1 5% 5% 4 4 2 4 6% 44%	1980
March	1st to 10th	Night	17% 6% 2 5% 3 3 3 2 4 5% 48%	5195
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 50%	4980
	21st to end	Night	14% 7% 2 6% 4 4 4 4 5% 5% 45%	5241
April	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 4 7% 42%	2000
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 5% 6% 45%	1975
	21st to end	Night	17% 5% 2 5% 4 4 3 4 6% 8% 44%	2000
May	1st to 10th	Night	19% 8% 3 7% 5% 5% 4 4 6% 6% 32%	3098
	11th to 20th	Night	21% 8% 3 6% 5% 4 5% 4 6% 7% 32%	2501
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	2732
June	1st to 10th	Night	11% 5% 2 5% 4 5% 4 4 6% 9% 44%	1520
	11th to 20th	Night	8% 5% 2 7% 6% 4 5% 4 6% 9% 42%	1520
	21st to end	Night	10% 5% 2 5% 5% 5% 6% 4 8% 10% 41%	1520
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 7% 4 8% 10% 41%	2520
	11th to 20th	Night	10% 6% 2 6% 4 5% 5% 4 7% 11% 39%	2520
	21st to end	Night	12% 8% 3 7% 5% 6% 5% 4 6% 8% 35%	3022
August	1st to 10th	Night	17% 6% 2 7% 5% 5% 5% 4 7% 9% 33%	2825
	11th to 20th	Night	11% 6% 3 8% 4 4 5% 4 7% 9% 37%	3032
	21st to end	Night	9% 5% 2 7% 4 4 5% 4 6% 8% 49%	3476
September	1st to 10th	Night	14% 5% 2 6% 4 4 4 4 5% 7% 46%	1900
	11th to 20th	Night	9% 6% 2 6% 3 3 4 3 5% 7% 52%	1900
	21st to end	Night	18% 6% 3 6% 4 4 4 3 5% 7% 39%	2136
October	1st to 10th	Night	16% 8% 2 6% 4 3 4 3 4 5% 45%	2280
	11th to 20th	Night	14% 5% 2 5% 3 2 3 2 4 6% 52%	2280
	21st to end	Night	16% 5% 1 5% 3 3 4 3 4 6% 50%	2508
November	1st to 10th	Night	11% 5% 2 6% 3 3 3 3 5% 5% 53%	2524
	11th to 20th	Night	15% 7% 2 5% 4 3 4 2 4 5% 48%	2600
	21st to end	Night	12% 5% 1 4 4 4 4 4 6% 6% 48%	2735
December	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 5% 7% 51%	4580
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 7% 39%	5205
	21st to end	Night	15% 7% 3 7% 4 4 4 4 6% 7% 41%	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	33% 3 6% 28% 26% 4	3598
	11th to 20th	Day	43% 3 7% 22% 19% 7%	3950
	21st to end	Day	41% 4 9% 29% 14% 3	4466
February	1st to 10th	Day	41% 6% 13% 25% 14% 2	1800
	11th to 20th	Day	43% 3 9% 26% 17% 2	1887
	21st to end	Day	44% 4 11% 21% 15% 4	1650
March	1st to 10th	Day	38% 4 8% 24% 19% 7%	4653
	11th to 20th	Day	35% 3 9% 27% 21% 6%	4980
	21st to end	Day	42% 4 8% 18% 21% 6%	5689
April	1st to 10th	Day	40% 5% 12% 20% 17% 6%	2400
	11th to 20th	Day	35% 6% 11% 23% 19% 6%	2374
	21st to end	Day	38% 4 12% 22% 20% 4	2400
May	1st to 10th	Day	49% 7% 11% 20% 10% 2	4068
	11th to 20th	Day	48% 8% 14% 21% 8% 1	4402
	21st to end	Day	32% 8% 13% 27% 18% 2	4837
June	1st to 10th	Day	34% 9% 15% 25% 15% 2	2660
	11th to 20th	Day	33% 9% 14% 27% 16% 2	2660
	21st to end	Day	29% 9% 15% 26% 20% 2	2660
July	1st to 10th	Day	34% 7% 23% 19% 13% 5%	4370
	11th to 20th	Day	36% 6% 19% 28% 9% 2	4365
	21st to end	Day	41% 7% 14% 21% 15% 1	4369
August	1st to 10th	Day	41% 6% 18% 27% 8%	4091
	11th to 20th	Day	36% 5% 13% 31% 14% 1	3802
	21st to end	Day	32% 5% 13% 29% 18% 3	4180
September	1st to 10th	Day	37% 6% 12% 21% 19% 4	2280
	11th to 20th	Day	34% 5% 11% 25% 23% 3	2280
	21st to end	Day	47% 5% 8% 19% 18% 3	2038
October	1st to 10th	Day	36% 3 7% 21% 27% 5%	3160
	11th to 20th	Day	25% 2 6% 29% 29% 9%	3140
	21st to end	Day	23% 3 8% 32% 26% 8%	3428
November	1st to 10th	Day	32% 4 12% 28% 20% 4	1771
	11th to 20th	Day	35% 5% 11% 24% 22% 3	1800
	21st to end	Day	37% 4 9% 30% 17% 2	1660
December	1st to 10th	Day	36% 4 16% 28% 13% 4	2600
	11th to 20th	Day	44% 2 11% 30% 10% 2	2986
	21st to end	Day	39% 4 15% 23% 18% 1	3960
January	1st to 10th	Night	39% 3 6% 24% 23% 5%	6349
	11th to 20th	Night	36% 3 8% 26% 20% 7%	5994
	21st to end	Night	41% 3 8% 28% 19% 2	6490
February	1st to 10th	Night	43% 5% 13% 23% 15% 1	2600
	11th to 20th	Night	44% 3 8% 22% 19% 4	2409
	21st to end	Night	43% 4 10% 21% 18% 5%	1980
March	1st to 10th	Night	40% 4 7% 25% 15% 10%	5195
	11th to 20th	Night	33% 4 12% 25% 18% 6%	4980
	21st to end	Night	41% 3 8% 20% 22% 5%	5241
April	1st to 10th	Night	43% 5% 9% 19% 20% 5%	2000
	11th to 20th	Night	39% 5% 10% 21% 19% 5%	1975
	21st to end	Night	38% 5% 10% 23% 19% 5%	2000
May	1st to 10th	Night	50% 7% 10% 21% 10% 2	3128
	11th to 20th	Night	51% 8% 13% 18% 10% 1	2502
	21st to end	Night	35% 8% 16% 21% 15% 4	2739
June	1st to 10th	Night	36% 7% 13% 24% 19% 1	1520
	11th to 20th	Night	36% 8% 14% 25% 15% 2	1520
	21st to end	Night	35% 8% 14% 23% 16% 3	1520
July	1st to 10th	Night	37% 10% 21% 17% 12% 2	2480
	11th to 20th	Night	39% 5% 21% 25% 9% 1	2472
	21st to end	Night	46% 6% 13% 21% 14%	2926
August	1st to 10th	Night	47% 6% 18% 20% 9%	2814
	11th to 20th	Night	42% 6% 12% 29% 12% 1	3061
	21st to end	Night	33% 5% 10% 28% 21% 2	3476
September	1st to 10th	Night	38% 7% 11% 21% 18% 5%	1900
	11th to 20th	Night	31% 7% 8% 26% 23% 4	1900
	21st to end	Night	45% 6% 8% 20% 18% 3	2136
October	1st to 10th	Night	42% 3 8% 22% 20% 4	3800
	11th to 20th	Night	24% 2 12% 21% 32% 8%	3779
	21st to end	Night	23% 3 10% 31% 26% 7%	4120
November	1st to 10th	Night	33% 5% 10% 27% 20% 6%	2524
	11th to 20th	Night	40% 4 9% 22% 21% 6%	2600
	21st to end	Night	35% 5% 12% 25% 20% 3	2735
December	1st to 10th	Night	34% 3 13% 29% 18% 3	4580
	11th to 20th	Night	45% 3 12% 20% 15% 4	5208
	21st to end	Night	42% 5% 12% 22% 18% 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	25% 21% 34% 15% 5% 1	200
	11th to 20th	Day	28% 22% 32% 14% 4	277
	21st to end	Day	24% 17% 42% 13% 4 1	330
February	1st to 10th	Day	26% 22% 33% 13% 6%	300
	11th to 20th	Day	29% 20% 38% 11% 2	426
	21st to end	Day	27% 18% 34% 17% 3	414
March	1st to 10th	Day	22% 21% 32% 20% 4	500
	11th to 20th	Day	22% 18% 34% 19% 6% 1	500
	21st to end	Day	21% 19% 33% 20% 6% 1	550
April	1st to 10th	Day	24% 16% 35% 18% 5% 2	500
	11th to 20th	Day	20% 16% 29% 21% 12% 2	492
	21st to end	Day	22% 12% 31% 25% 10% 1	500
May	1st to 10th	Day	23% 15% 31% 20% 8% 4	503
	11th to 20th	Day	16% 9% 33% 26% 12% 2	487
	21st to end	Day	16% 6% 21% 28% 19% 10%	529
June	1st to 10th	Day	12% 5% 28% 27% 20% 8%	490
	11th to 20th	Day	14% 6% 21% 30% 21% 8%	490
	21st to end	Day	10% 3 23% 35% 21% 7%	490
July	1st to 10th	Day	9% 3 16% 35% 24% 13%	485
	11th to 20th	Day	5% 1 16% 32% 31% 14%	484
	21st to end	Day	10% 2 18% 31% 30% 8%	532
August	1st to 10th	Day	12% 2 17% 30% 27% 12%	486
	11th to 20th	Day	11% 2 17% 29% 28% 11%	479
	21st to end	Day	9% 4 17% 32% 21% 17%	539
September	1st to 10th	Day	14% 5% 20% 24% 21% 15%	490
	11th to 20th	Day	11% 3 23% 30% 21% 12%	490
	21st to end	Day	25% 8% 24% 26% 13% 5%	490
October	1st to 10th	Day	19% 8% 22% 26% 16% 9%	490
	11th to 20th	Day	16% 12% 26% 24% 16% 5%	490
	21st to end	Day	17% 10% 33% 25% 12% 3	539
November	1st to 10th	Day	16% 10% 31% 26% 14% 4	400
	11th to 20th	Day	16% 17% 32% 24% 11% 1	400
	21st to end	Day	22% 16% 29% 22% 10% 2	260
December	1st to 10th	Day	18% 20% 40% 16% 6% 2	200
	11th to 20th	Day	24% 23% 32% 15% 7%	200
	21st to end	Day	24% 15% 29% 21% 9% 1	220
January	1st to 10th	Night	21% 18% 37% 21% 3	800
	11th to 20th	Night	20% 14% 35% 27% 4	800
	21st to end	Night	17% 16% 39% 25% 2	880
February	1st to 10th	Night	21% 12% 38% 27% 2	800
	11th to 20th	Night	25% 19% 37% 17% 1	783
	21st to end	Night	22% 16% 36% 22% 3	660
March	1st to 10th	Night	18% 16% 39% 23% 5%	800
	11th to 20th	Night	20% 15% 37% 22% 6% 1	800
	21st to end	Night	19% 12% 38% 26% 4	880
April	1st to 10th	Night	21% 15% 39% 20% 5%	800
	11th to 20th	Night	16% 12% 35% 30% 7%	791
	21st to end	Night	20% 9% 34% 29% 8%	800
May	1st to 10th	Night	18% 9% 32% 31% 10%	733
	11th to 20th	Night	15% 6% 32% 35% 10% 1	376
	21st to end	Night	12% 3 23% 41% 20% 1	411
June	1st to 10th	Night	13% 4 24% 40% 18% 1	380
	11th to 20th	Night	9% 3 23% 46% 18% 1	380
	21st to end	Night	10% 2 22% 46% 18% 1	380
July	1st to 10th	Night	11% 2 17% 41% 28% 2	370
	11th to 20th	Night	8% 1 15% 43% 31% 2	368
	21st to end	Night	8% 1 14% 46% 28% 3	585
August	1st to 10th	Night	10% 1 14% 43% 30% 2	567
	11th to 20th	Night	12% 3 16% 36% 30% 3	709
	21st to end	Night	10% 2 14% 36% 32% 5%	836
September	1st to 10th	Night	16% 1 15% 38% 26% 4	760
	11th to 20th	Night	12% 2 17% 35% 30% 4	760
	21st to end	Night	20% 4 22% 33% 19% 2	760
October	1st to 10th	Night	21% 5% 18% 32% 19% 4	760
	11th to 20th	Night	17% 8% 26% 35% 15% 1	760
	21st to end	Night	18% 7% 32% 30% 13% 1	836
November	1st to 10th	Night	14% 6% 33% 31% 13% 2	780
	11th to 20th	Night	18% 12% 32% 30% 8% 1	800
	21st to end	Night	16% 15% 32% 28% 8% 1	800
December	1st to 10th	Night	22% 14% 34% 21% 8% 1	800
	11th to 20th	Night	21% 12% 36% 25% 6%	800
	21st to end	Night	19% 11% 34% 32% 4	880

Page generated at 2025-05-08 09:34:01. Version ac07

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