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 2026-09-14.

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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	148960
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 43%	142118
Transparency	Day	39% 4 12% 25% 17% 3	152549
	Night	40% 4 11% 23% 17% 4	146601
Seeing	Day	18% 11% 27% 24% 14% 5%	17461
	Night	17% 10% 29% 30% 12% 1	26811

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% 5% 2 5% 4 4 4 3 4 5% 49%	4661
	11th to 20th	Day	15% 7% 3 7% 3 3 2 2 3 4 50%	5142
	21st to end	Day	15% 5% 2 5% 4 4 4 3 5% 5% 48%	5780
February	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 6% 7% 42%	1890
	11th to 20th	Day	18% 7% 2 5% 4 4 3 3 5% 6% 42%	1975
	21st to end	Day	18% 8% 3 6% 4 4 4 2 5% 6% 40%	1730
March	1st to 10th	Day	19% 5% 2 5% 3 3 3 3 5% 7% 46%	4756
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 6% 51%	5090
	21st to end	Day	17% 6% 2 6% 4 3 4 3 5% 6% 44%	5815
April	1st to 10th	Day	14% 7% 2 5% 4 4 4 4 4 7% 45%	2520
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 8% 47%	2494
	21st to end	Day	16% 6% 3 5% 3 3 3 3 5% 8% 44%	2520
May	1st to 10th	Day	19% 7% 2 5% 4 4 4 3 5% 8% 38%	7274
	11th to 20th	Day	17% 6% 2 5% 4 4 4 4 6% 8% 40%	8221
	21st to end	Day	22% 6% 2 6% 3 4 4 3 5% 7% 39%	9045
June	1st to 10th	Day	10% 5% 1 6% 4 4 5% 4 7% 10% 44%	2940
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 7% 10% 45%	2940
	21st to end	Day	10% 4 2 4 3 3 4 4 7% 10% 48%	2940
July	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 4 7% 10% 43%	8280
	11th to 20th	Day	8% 6% 2 7% 5% 5% 6% 5% 10% 12% 34%	8280
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	8442
August	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 5% 8% 11% 34%	6014
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 4 7% 10% 41%	5538
	21st to end	Day	8% 4 2 5% 4 5% 6% 5% 7% 9% 46%	6080
September	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 5% 7% 47%	2520
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 49%	2436
	21st to end	Day	19% 9% 3 6% 3 3 5% 3 4 7% 38%	2145
October	1st to 10th	Day	15% 6% 1 5% 4 4 3 3 4 6% 48%	2000
	11th to 20th	Day	17% 5% 1 4 3 3 3 2 4 7% 51%	2000
	21st to end	Day	14% 5% 3 5% 4 3 3 3 4 5% 51%	2200
November	1st to 10th	Day	13% 5% 1 4 3 3 4 3 5% 7% 53%	1861
	11th to 20th	Day	12% 7% 2 4 3 4 4 4 6% 6% 49%	1890
	21st to end	Day	14% 6% 2 5% 4 4 4 3 4 6% 48%	1743
December	1st to 10th	Day	12% 5% 2 5% 4 4 3 3 5% 7% 48%	2680
	11th to 20th	Day	18% 8% 2 5% 3 3 3 3 4 6% 44%	3070
	21st to end	Day	13% 8% 2 6% 4 4 4 4 6% 7% 44%	4048
January	1st to 10th	Night	12% 7% 2 6% 4 4 5% 4 5% 6% 45%	8233
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 5% 51%	7802
	21st to end	Night	14% 7% 2 5% 3 3 4 3 5% 5% 48%	8401
February	1st to 10th	Night	16% 6% 3 5% 5% 4 4 4 5% 6% 41%	2730
	11th to 20th	Night	19% 7% 2 6% 3 3 4 3 4 6% 42%	2530
	21st to end	Night	16% 8% 2 5% 5% 4 4 2 4 6% 44%	2076
March	1st to 10th	Night	17% 6% 2 5% 3 3 3 2 4 5% 48%	5311
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 50%	5090
	21st to end	Night	14% 7% 2 6% 4 4 4 4 5% 5% 45%	5357
April	1st to 10th	Night	15% 7% 3 5% 4 4 4 3 4 7% 42%	2100
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 7% 46%	2075
	21st to end	Night	17% 5% 2 5% 4 4 3 4 6% 8% 44%	2100
May	1st to 10th	Night	16% 7% 3 7% 4 5% 5% 4 7% 7% 34%	5622
	11th to 20th	Night	17% 7% 2 6% 4 4 5% 3 7% 8% 37%	4639
	21st to end	Night	23% 7% 3 7% 4 4 4 3 5% 7% 35%	5108
June	1st to 10th	Night	10% 5% 2 5% 4 5% 4 4 6% 9% 45%	1680
	11th to 20th	Night	9% 6% 3 7% 6% 5% 5% 4 7% 9% 40%	1680
	21st to end	Night	10% 5% 2 6% 5% 5% 6% 4 8% 10% 40%	1680
July	1st to 10th	Night	10% 6% 3 8% 5% 5% 5% 4 6% 8% 39%	4678
	11th to 20th	Night	10% 8% 3 8% 5% 5% 6% 4 7% 11% 34%	4677
	21st to end	Night	10% 6% 3 7% 5% 5% 5% 4 7% 9% 39%	5616
August	1st to 10th	Night	14% 7% 2 9% 6% 6% 6% 4 7% 9% 29%	4135
	11th to 20th	Night	12% 7% 3 8% 5% 4 5% 4 8% 9% 35%	4462
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 6% 8% 44%	5045
September	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 5% 7% 46%	2100
	11th to 20th	Night	10% 6% 2 6% 3 3 4 3 5% 7% 50%	2031
	21st to end	Night	19% 7% 3 6% 4 4 4 3 5% 7% 38%	2249
October	1st to 10th	Night	16% 7% 2 6% 4 3 4 2 4 5% 46%	2400
	11th to 20th	Night	15% 6% 2 5% 3 2 3 2 4 6% 52%	2400
	21st to end	Night	16% 5% 1 5% 3 3 4 3 4 6% 49%	2640
November	1st to 10th	Night	11% 4 2 6% 3 3 3 3 5% 5% 53%	2654
	11th to 20th	Night	15% 6% 2 6% 4 3 4 2 4 5% 48%	2730
	21st to end	Night	12% 5% 1 4 4 4 4 4 6% 6% 48%	2872
December	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 51%	4720
	11th to 20th	Night	20% 6% 2 5% 4 4 4 3 5% 7% 39%	5345
	21st to end	Night	15% 7% 2 6% 4 4 4 4 6% 7% 41%	7150

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	38% 3 7% 28% 21% 3	4662
	11th to 20th	Day	41% 2 6% 22% 22% 7%	5142
	21st to end	Day	39% 3 8% 34% 13% 3	5780
February	1st to 10th	Day	41% 5% 13% 25% 14% 2	1890
	11th to 20th	Day	43% 3 9% 25% 17% 2	1984
	21st to end	Day	45% 4 11% 22% 15% 4	1730
March	1st to 10th	Day	39% 4 8% 23% 19% 7%	4756
	11th to 20th	Day	35% 3 9% 27% 20% 6%	5090
	21st to end	Day	42% 4 9% 19% 21% 6%	5815
April	1st to 10th	Day	39% 5% 12% 20% 18% 5%	2520
	11th to 20th	Day	34% 6% 11% 24% 19% 6%	2494
	21st to end	Day	38% 4 12% 23% 19% 4	2520
May	1st to 10th	Day	45% 4 13% 22% 15% 1	7306
	11th to 20th	Day	42% 5% 11% 28% 14% 1	8225
	21st to end	Day	46% 5% 9% 22% 16% 2	9061
June	1st to 10th	Day	34% 8% 14% 25% 17% 2	2940
	11th to 20th	Day	33% 8% 15% 26% 16% 3	2940
	21st to end	Day	29% 8% 15% 26% 20% 1	2940
July	1st to 10th	Day	37% 5% 20% 21% 14% 3	8210
	11th to 20th	Day	41% 6% 17% 26% 10% 1	8205
	21st to end	Day	38% 5% 12% 25% 19% 1	8256
August	1st to 10th	Day	43% 6% 20% 23% 6%	6001
	11th to 20th	Day	39% 4 17% 28% 11% 1	5575
	21st to end	Day	34% 4 16% 28% 17% 2	6080
September	1st to 10th	Day	36% 6% 13% 22% 18% 4	2520
	11th to 20th	Day	35% 4 10% 26% 22% 3	2436
	21st to end	Day	48% 5% 9% 19% 17% 3	2145
October	1st to 10th	Day	36% 3 7% 21% 28% 5%	3260
	11th to 20th	Day	25% 2 6% 29% 29% 9%	3240
	21st to end	Day	23% 3 8% 32% 26% 8%	3538
November	1st to 10th	Day	31% 4 12% 28% 20% 3	1861
	11th to 20th	Day	35% 5% 11% 24% 22% 4	1890
	21st to end	Day	38% 4 9% 30% 17% 2	1743
December	1st to 10th	Day	36% 4 16% 28% 12% 4	2680
	11th to 20th	Day	43% 3 11% 30% 10% 2	3066
	21st to end	Day	40% 4 15% 23% 17% 1	4048
January	1st to 10th	Night	40% 2 10% 25% 18% 4	8234
	11th to 20th	Night	37% 2 7% 24% 21% 8%	7802
	21st to end	Night	39% 2 7% 29% 19% 3	8401
February	1st to 10th	Night	43% 5% 12% 23% 16% 2	2730
	11th to 20th	Night	44% 3 8% 22% 19% 4	2532
	21st to end	Night	43% 4 10% 22% 17% 5%	2076
March	1st to 10th	Night	40% 4 7% 24% 15% 9%	5311
	11th to 20th	Night	33% 4 12% 26% 18% 6%	5090
	21st to end	Night	42% 3 9% 20% 22% 5%	5357
April	1st to 10th	Night	42% 5% 10% 19% 20% 5%	2100
	11th to 20th	Night	38% 4 11% 22% 20% 5%	2075
	21st to end	Night	38% 5% 11% 23% 18% 5%	2100
May	1st to 10th	Night	49% 4 12% 22% 11% 2	5652
	11th to 20th	Night	45% 5% 13% 24% 13% 1	4640
	21st to end	Night	51% 5% 10% 18% 14% 2	5115
June	1st to 10th	Night	37% 7% 12% 23% 20% 2	1680
	11th to 20th	Night	39% 8% 14% 24% 15% 2	1680
	21st to end	Night	36% 8% 14% 23% 17% 3	1680
July	1st to 10th	Night	43% 7% 15% 20% 14% 1	4640
	11th to 20th	Night	46% 5% 17% 22% 10% 1	4632
	21st to end	Night	42% 4 14% 23% 16%	5489
August	1st to 10th	Night	51% 6% 18% 18% 7%	4124
	11th to 20th	Night	44% 5% 15% 26% 9% 1	4491
	21st to end	Night	38% 4 11% 27% 18% 1	5045
September	1st to 10th	Night	38% 6% 11% 22% 18% 5%	2100
	11th to 20th	Night	33% 6% 9% 26% 22% 4	2031
	21st to end	Night	46% 6% 9% 19% 17% 3	2249
October	1st to 10th	Night	42% 3 8% 22% 21% 4	3920
	11th to 20th	Night	25% 2 12% 21% 31% 8%	3899
	21st to end	Night	24% 2 10% 31% 26% 7%	4252
November	1st to 10th	Night	32% 5% 10% 27% 21% 6%	2654
	11th to 20th	Night	40% 4 8% 22% 21% 6%	2730
	21st to end	Night	36% 5% 12% 25% 20% 3	2872
December	1st to 10th	Night	34% 3 13% 29% 18% 3	4720
	11th to 20th	Night	45% 3 12% 20% 15% 4	5348
	21st to end	Night	42% 5% 12% 22% 18% 1	7150

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% 20% 34% 15% 5% 1	220
	11th to 20th	Day	29% 24% 30% 13% 4	305
	21st to end	Day	24% 20% 40% 12% 4 1	363
February	1st to 10th	Day	26% 22% 34% 12% 5%	330
	11th to 20th	Day	30% 20% 37% 11% 2	463
	21st to end	Day	27% 18% 33% 17% 3	446
March	1st to 10th	Day	23% 20% 33% 20% 4	540
	11th to 20th	Day	23% 18% 33% 19% 5% 1	540
	21st to end	Day	21% 19% 34% 19% 6% 1	594
April	1st to 10th	Day	24% 15% 34% 19% 6% 2	540
	11th to 20th	Day	19% 16% 30% 21% 11% 2	532
	21st to end	Day	22% 11% 30% 25% 10% 1	540
May	1st to 10th	Day	22% 16% 32% 20% 7% 3	543
	11th to 20th	Day	17% 10% 32% 26% 12% 2	567
	21st to end	Day	21% 7% 21% 25% 17% 8%	617
June	1st to 10th	Day	11% 6% 28% 27% 20% 7%	570
	11th to 20th	Day	15% 7% 24% 28% 19% 7%	570
	21st to end	Day	11% 3 23% 34% 21% 7%	570
July	1st to 10th	Day	10% 3 16% 35% 25% 13%	565
	11th to 20th	Day	5% 1 17% 32% 31% 14%	564
	21st to end	Day	10% 2 18% 32% 29% 9%	620
August	1st to 10th	Day	13% 2 18% 30% 25% 11%	566
	11th to 20th	Day	12% 2 17% 31% 27% 10%	559
	21st to end	Day	9% 4 20% 32% 20% 15%	627
September	1st to 10th	Day	14% 5% 24% 25% 20% 14%	570
	11th to 20th	Day	13% 3 21% 31% 21% 11%	542
	21st to end	Day	25% 7% 25% 26% 12% 5%	530
October	1st to 10th	Day	18% 7% 21% 27% 16% 10%	530
	11th to 20th	Day	18% 13% 26% 23% 15% 5%	530
	21st to end	Day	17% 12% 32% 24% 12% 3	583
November	1st to 10th	Day	17% 11% 30% 25% 13% 3	430
	11th to 20th	Day	18% 17% 31% 23% 10%	430
	21st to end	Day	23% 16% 29% 22% 9% 2	283
December	1st to 10th	Day	19% 20% 40% 15% 5% 1	220
	11th to 20th	Day	24% 24% 31% 15% 6%	220
	21st to end	Day	25% 16% 29% 20% 8% 1	242
January	1st to 10th	Night	20% 18% 37% 21% 3	840
	11th to 20th	Night	21% 14% 35% 26% 4	840
	21st to end	Night	18% 17% 39% 24% 2	924
February	1st to 10th	Night	21% 12% 37% 28% 2	840
	11th to 20th	Night	24% 19% 38% 18% 1	823
	21st to end	Night	23% 16% 36% 22% 3	692
March	1st to 10th	Night	19% 16% 39% 23% 4	840
	11th to 20th	Night	20% 16% 36% 22% 6% 1	840
	21st to end	Night	19% 13% 38% 25% 4	924
April	1st to 10th	Night	20% 15% 39% 20% 5%	840
	11th to 20th	Night	15% 12% 36% 30% 7%	831
	21st to end	Night	19% 9% 34% 30% 8%	840
May	1st to 10th	Night	18% 10% 33% 30% 9%	770
	11th to 20th	Night	16% 6% 33% 35% 10%	414
	21st to end	Night	15% 3 24% 39% 18% 1	455
June	1st to 10th	Night	13% 4 25% 41% 16% 1	420
	11th to 20th	Night	11% 4 23% 44% 17% 1	420
	21st to end	Night	10% 3 22% 46% 19% 1	420
July	1st to 10th	Night	12% 2 16% 42% 27% 1	410
	11th to 20th	Night	9% 1 15% 43% 30% 2	408
	21st to end	Night	9% 1 14% 45% 28% 3	647
August	1st to 10th	Night	11% 1 14% 43% 30% 2	627
	11th to 20th	Night	13% 3 15% 36% 30% 3	785
	21st to end	Night	11% 2 16% 36% 30% 5%	924
September	1st to 10th	Night	15% 1 16% 39% 25% 4	840
	11th to 20th	Night	13% 2 16% 36% 29% 4	813
	21st to end	Night	21% 4 21% 33% 19% 2	800
October	1st to 10th	Night	20% 5% 18% 32% 20% 5%	800
	11th to 20th	Night	18% 8% 25% 34% 14% 1	800
	21st to end	Night	18% 8% 32% 29% 12% 1	880
November	1st to 10th	Night	15% 7% 33% 31% 12% 2	820
	11th to 20th	Night	18% 13% 32% 30% 7% 1	840
	21st to end	Night	16% 15% 32% 28% 8% 1	840
December	1st to 10th	Night	22% 14% 34% 21% 8% 1	840
	11th to 20th	Night	20% 13% 37% 24% 6%	840
	21st to end	Night	20% 12% 34% 31% 4	924

Page generated at 2026-09-11 04:11:52. Version ac07

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