Broken Tooth Rock Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Broken Tooth Rock Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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% 7% 2 6% 4 4 5% 4 5% 7% 37%	126274
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	123099
Transparency	Day	45% 5% 11% 22% 15% 2	129794
	Night	44% 5% 11% 22% 15% 2	127666
Seeing	Day	25% 16% 30% 18% 8% 2	16119
	Night	23% 15% 31% 24% 7% 1	24973

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	11% 7% 3 11% 6% 6% 5% 4 6% 6% 35%	3597
	11th to 20th	Day	12% 8% 3 7% 4 4 4 3 4 6% 42%	3570
	21st to end	Day	15% 10% 3 6% 4 4 4 3 5% 5% 42%	3960
February	1st to 10th	Day	16% 8% 2 7% 6% 6% 5% 4 6% 6% 34%	1818
	11th to 20th	Day	13% 7% 3 8% 5% 3 5% 4 6% 8% 38%	1948
	21st to end	Day	16% 12% 3 8% 4 4 3 4 5% 4 37%	1650
March	1st to 10th	Day	19% 12% 3 7% 5% 4 4 3 5% 5% 33%	4476
	11th to 20th	Day	15% 9% 3 6% 4 4 5% 3 6% 6% 39%	4722
	21st to end	Day	25% 9% 2 6% 4 3 4 3 5% 8% 32%	5847
April	1st to 10th	Day	18% 10% 3 7% 5% 5% 4 3 4 6% 36%	2400
	11th to 20th	Day	17% 6% 2 6% 5% 4 4 4 6% 6% 38%	2379
	21st to end	Day	21% 8% 2 7% 5% 4 4 4 4 6% 35%	2600
May	1st to 10th	Day	32% 9% 3 7% 4 4 4 3 4 6% 24%	5886
	11th to 20th	Day	20% 8% 3 9% 5% 5% 5% 4 5% 6% 30%	6122
	21st to end	Day	19% 6% 2 6% 5% 5% 5% 4 6% 8% 35%	6949
June	1st to 10th	Day	19% 7% 2 6% 5% 5% 4 4 7% 8% 33%	2800
	11th to 20th	Day	17% 6% 2 7% 5% 5% 4 4 6% 9% 36%	2800
	21st to end	Day	19% 6% 2 7% 5% 5% 5% 3 6% 8% 34%	2800
July	1st to 10th	Day	15% 6% 2 6% 5% 5% 6% 5% 7% 9% 33%	6360
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 4 8% 10% 38%	6360
	21st to end	Day	15% 6% 2 5% 4 4 4 3 6% 7% 43%	6926
August	1st to 10th	Day	14% 5% 3 8% 5% 5% 5% 5% 6% 8% 36%	4371
	11th to 20th	Day	14% 8% 2 7% 4 4 5% 5% 6% 7% 37%	4094
	21st to end	Day	8% 5% 2 5% 4 5% 6% 5% 8% 9% 43%	4312
September	1st to 10th	Day	12% 6% 1 5% 4 4 5% 4 6% 7% 47%	2400
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 4 6% 53%	2218
	21st to end	Day	16% 6% 2 5% 4 4 4 3 4 6% 45%	2200
October	1st to 10th	Day	23% 6% 2 5% 4 3 4 2 5% 6% 38%	2123
	11th to 20th	Day	16% 6% 2 4 3 3 4 4 4 8% 45%	1829
	21st to end	Day	22% 8% 2 6% 4 4 4 3 5% 5% 38%	1881
November	1st to 10th	Day	14% 7% 2 5% 4 4 3 3 4 5% 49%	1761
	11th to 20th	Day	17% 6% 2 5% 3 4 4 3 5% 6% 46%	1800
	21st to end	Day	20% 8% 2 5% 4 4 4 3 4 6% 41%	1769
December	1st to 10th	Day	15% 8% 3 5% 5% 4 4 4 6% 7% 39%	2600
	11th to 20th	Day	20% 6% 2 5% 4 4 3 4 5% 5% 42%	2986
	21st to end	Day	19% 10% 3 7% 4 4 4 3 5% 6% 36%	3960
January	1st to 10th	Night	13% 6% 3 7% 5% 5% 5% 4 5% 6% 41%	6349
	11th to 20th	Night	12% 8% 3 9% 6% 5% 5% 4 5% 7% 37%	6315
	21st to end	Night	15% 10% 4 7% 5% 5% 4 4 5% 5% 38%	6996
February	1st to 10th	Night	15% 10% 3 6% 5% 3 4 4 6% 6% 38%	2577
	11th to 20th	Night	12% 9% 2 6% 5% 5% 5% 4 6% 7% 39%	2338
	21st to end	Night	14% 9% 3 6% 5% 4 4 4 5% 7% 39%	1980
March	1st to 10th	Night	22% 8% 2 6% 4 4 4 4 5% 6% 35%	5364
	11th to 20th	Night	15% 9% 4 8% 5% 4 4 4 5% 7% 37%	5223
	21st to end	Night	24% 8% 2 5% 3 3 4 4 6% 6% 34%	5082
April	1st to 10th	Night	20% 8% 2 7% 4 4 4 4 5% 6% 36%	2000
	11th to 20th	Night	19% 8% 2 5% 4 4 4 3 5% 5% 40%	1969
	21st to end	Night	21% 7% 3 6% 3 4 3 3 5% 6% 39%	1800
May	1st to 10th	Night	34% 9% 2 6% 4 3 3 3 5% 6% 24%	4054
	11th to 20th	Night	17% 8% 4 8% 6% 6% 5% 4 6% 6% 30%	3814
	21st to end	Night	21% 7% 3 8% 5% 4 5% 4 5% 7% 33%	3926
June	1st to 10th	Night	19% 9% 2 7% 4 4 4 3 5% 6% 37%	1600
	11th to 20th	Night	19% 6% 3 5% 4 5% 5% 3 6% 7% 39%	1600
	21st to end	Night	18% 7% 2 5% 4 4 5% 3 6% 8% 39%	1600
July	1st to 10th	Night	13% 5% 3 6% 4 4 6% 3 6% 9% 42%	3600
	11th to 20th	Night	12% 6% 2 6% 4 4 5% 4 7% 9% 40%	3600
	21st to end	Night	11% 5% 2 5% 3 3 4 3 5% 7% 51%	3926
August	1st to 10th	Night	20% 7% 2 7% 5% 4 4 4 5% 6% 36%	2746
	11th to 20th	Night	12% 8% 3 7% 4 5% 4 3 6% 8% 39%	2937
	21st to end	Night	9% 3 1 4 3 3 4 3 5% 8% 56%	3586
September	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 5% 7% 52%	2000
	11th to 20th	Night	14% 5% 2 4 3 3 2 2 5% 6% 54%	2180
	21st to end	Night	15% 5% 2 5% 4 4 3 3 5% 6% 48%	2200
October	1st to 10th	Night	21% 6% 2 6% 4 3 4 3 4 6% 41%	2124
	11th to 20th	Night	16% 5% 2 4 4 4 5% 4 5% 7% 45%	2345
	21st to end	Night	22% 8% 2 5% 4 4 4 4 4 6% 37%	2717
November	1st to 10th	Night	14% 6% 3 5% 4 4 4 3 4 5% 47%	2537
	11th to 20th	Night	17% 7% 2 5% 4 3 4 3 5% 5% 46%	2600
	21st to end	Night	15% 8% 2 6% 4 4 3 3 6% 6% 43%	2630
December	1st to 10th	Night	16% 11% 3 7% 4 4 4 3 5% 6% 37%	4580
	11th to 20th	Night	22% 8% 3 6% 3 3 3 3 4 5% 40%	5208
	21st to end	Night	19% 10% 3 6% 4 3 4 3 5% 5% 38%	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	47% 4 8% 27% 14% 1	3598
	11th to 20th	Day	42% 4 12% 23% 16% 3	3600
	21st to end	Day	43% 4 8% 21% 20% 4	3960
February	1st to 10th	Day	49% 5% 12% 20% 11% 2	1818
	11th to 20th	Day	44% 7% 11% 20% 15% 3	1957
	21st to end	Day	50% 4 9% 19% 15% 3	1650
March	1st to 10th	Day	54% 3 9% 14% 12% 9%	4479
	11th to 20th	Day	44% 5% 14% 22% 13% 2	4722
	21st to end	Day	51% 3 5% 16% 18% 6%	5847
April	1st to 10th	Day	49% 4 8% 19% 16% 4	2400
	11th to 20th	Day	44% 6% 11% 19% 18% 3	2379
	21st to end	Day	50% 5% 8% 17% 17% 4	2600
May	1st to 10th	Day	62% 6% 10% 13% 9% 1	5897
	11th to 20th	Day	54% 6% 13% 16% 10% 1	6126
	21st to end	Day	47% 6% 12% 21% 13% 1	6949
June	1st to 10th	Day	47% 7% 11% 22% 12% 1	2800
	11th to 20th	Day	44% 7% 11% 20% 16% 3	2800
	21st to end	Day	46% 7% 12% 20% 13% 1	2800
July	1st to 10th	Day	47% 5% 12% 19% 16% 1	6290
	11th to 20th	Day	40% 5% 16% 24% 14% 1	6285
	21st to end	Day	40% 4 11% 27% 17% 1	6728
August	1st to 10th	Day	45% 6% 15% 22% 11% 1	4357
	11th to 20th	Day	44% 5% 14% 26% 10% 1	4133
	21st to end	Day	35% 5% 12% 29% 17% 2	4312
September	1st to 10th	Day	35% 7% 12% 26% 19% 2	2400
	11th to 20th	Day	34% 5% 8% 26% 24% 2	2218
	21st to end	Day	40% 5% 9% 23% 20% 3	2200
October	1st to 10th	Day	45% 5% 7% 23% 18% 2	3513
	11th to 20th	Day	34% 4 9% 29% 21% 4	3019
	21st to end	Day	36% 4 8% 31% 19% 4	3081
November	1st to 10th	Day	39% 3 13% 26% 17% 2	1761
	11th to 20th	Day	39% 5% 10% 22% 22% 3	1800
	21st to end	Day	45% 4 15% 21% 13% 2	1769
December	1st to 10th	Day	43% 4 14% 28% 9% 1	2600
	11th to 20th	Day	43% 4 11% 19% 19% 4	2986
	21st to end	Day	50% 5% 8% 18% 13% 6%	3960
January	1st to 10th	Night	41% 4 13% 28% 12% 2	6349
	11th to 20th	Night	46% 5% 12% 22% 12% 3	6360
	21st to end	Night	48% 4 8% 17% 17% 5%	6996
February	1st to 10th	Night	45% 6% 12% 21% 13% 2	2577
	11th to 20th	Night	43% 6% 12% 21% 16% 3	2340
	21st to end	Night	44% 5% 10% 24% 13% 4	1980
March	1st to 10th	Night	50% 3 8% 16% 16% 6%	5374
	11th to 20th	Night	46% 7% 14% 18% 13% 2	5224
	21st to end	Night	49% 4 6% 14% 22% 6%	5082
April	1st to 10th	Night	48% 5% 11% 18% 17% 2	2000
	11th to 20th	Night	46% 4 9% 20% 19% 2	1969
	21st to end	Night	46% 5% 10% 18% 17% 4	1800
May	1st to 10th	Night	61% 6% 11% 13% 8% 1	4060
	11th to 20th	Night	51% 8% 14% 16% 10% 1	3815
	21st to end	Night	50% 7% 14% 17% 11% 1	3927
June	1st to 10th	Night	47% 6% 11% 23% 12% 1	1600
	11th to 20th	Night	44% 8% 13% 20% 13% 2	1600
	21st to end	Night	42% 9% 13% 21% 14% 2	1600
July	1st to 10th	Night	40% 5% 14% 26% 14% 1	3560
	11th to 20th	Night	40% 6% 16% 20% 17% 1	3552
	21st to end	Night	34% 4 13% 33% 15% 1	3809
August	1st to 10th	Night	49% 6% 12% 20% 12% 1	2736
	11th to 20th	Night	43% 7% 15% 24% 10% 1	2964
	21st to end	Night	27% 5% 15% 33% 19% 1	3586
September	1st to 10th	Night	33% 6% 12% 28% 20% 1	2000
	11th to 20th	Night	32% 4 9% 28% 24% 2	2180
	21st to end	Night	38% 5% 10% 28% 17% 2	2200
October	1st to 10th	Night	41% 5% 15% 23% 14% 2	3514
	11th to 20th	Night	30% 4 11% 27% 25% 4	3894
	21st to end	Night	37% 4 10% 33% 15% 2	4467
November	1st to 10th	Night	40% 5% 10% 23% 20% 3	2537
	11th to 20th	Night	41% 4 9% 23% 20% 3	2600
	21st to end	Night	42% 5% 14% 25% 13% 1	2630
December	1st to 10th	Night	48% 4 10% 28% 9%	4580
	11th to 20th	Night	47% 3 13% 14% 21% 2	5208
	21st to end	Night	48% 4 7% 22% 15% 3	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	37% 20% 28% 12% 5%	200
	11th to 20th	Day	32% 24% 33% 10% 1 2	200
	21st to end	Day	28% 23% 33% 12% 4 1	220
February	1st to 10th	Day	34% 26% 27% 12% 2	356
	11th to 20th	Day	34% 27% 27% 11% 1	496
	21st to end	Day	30% 24% 30% 14% 3	414
March	1st to 10th	Day	33% 20% 31% 13% 3	500
	11th to 20th	Day	33% 23% 29% 11% 4 1	500
	21st to end	Day	33% 21% 29% 11% 4 2	550
April	1st to 10th	Day	35% 20% 29% 10% 5% 2	500
	11th to 20th	Day	34% 22% 26% 12% 5% 1	492
	21st to end	Day	30% 19% 30% 14% 6% 1	500
May	1st to 10th	Day	35% 18% 26% 15% 4 1	503
	11th to 20th	Day	23% 17% 36% 17% 6% 1	527
	21st to end	Day	19% 16% 35% 22% 7% 1	573
June	1st to 10th	Day	20% 15% 38% 16% 9% 2	530
	11th to 20th	Day	19% 15% 35% 22% 9% 2	530
	21st to end	Day	21% 14% 34% 21% 7% 3	530
July	1st to 10th	Day	19% 9% 31% 27% 10% 3	525
	11th to 20th	Day	16% 7% 33% 30% 12% 3	524
	21st to end	Day	15% 10% 33% 30% 9% 2	576
August	1st to 10th	Day	13% 9% 31% 29% 15% 4	526
	11th to 20th	Day	17% 8% 28% 27% 15% 5%	519
	21st to end	Day	14% 11% 33% 23% 15% 4	583
September	1st to 10th	Day	18% 11% 27% 21% 15% 7%	530
	11th to 20th	Day	18% 11% 25% 19% 16% 10%	530
	21st to end	Day	19% 11% 32% 21% 13% 5%	530
October	1st to 10th	Day	29% 12% 24% 20% 12% 5%	502
	11th to 20th	Day	23% 15% 28% 21% 10% 2	440
	21st to end	Day	31% 13% 29% 20% 6% 1	429
November	1st to 10th	Day	23% 19% 26% 21% 9% 1	395
	11th to 20th	Day	33% 17% 25% 20% 5% 1	400
	21st to end	Day	33% 19% 25% 16% 6% 1	369
December	1st to 10th	Day	26% 28% 31% 11% 5% 1	200
	11th to 20th	Day	38% 19% 30% 12% 2	200
	21st to end	Day	32% 17% 30% 16% 5% 1	220
January	1st to 10th	Night	24% 23% 32% 18% 3	800
	11th to 20th	Night	28% 20% 31% 19% 3	800
	21st to end	Night	21% 20% 37% 18% 3	880
February	1st to 10th	Night	25% 19% 35% 19% 2	800
	11th to 20th	Night	25% 23% 35% 16% 2	783
	21st to end	Night	26% 22% 33% 18% 2	660
March	1st to 10th	Night	28% 20% 33% 18% 2	800
	11th to 20th	Night	26% 20% 29% 20% 4	800
	21st to end	Night	27% 18% 32% 19% 5%	880
April	1st to 10th	Night	28% 18% 31% 18% 4	800
	11th to 20th	Night	26% 19% 32% 18% 5%	785
	21st to end	Night	27% 19% 30% 21% 4	600
May	1st to 10th	Night	28% 13% 33% 21% 5%	600
	11th to 20th	Night	21% 14% 32% 28% 4	503
	21st to end	Night	20% 9% 34% 29% 8%	433
June	1st to 10th	Night	19% 11% 32% 28% 9% 1	400
	11th to 20th	Night	20% 9% 30% 32% 8% 1	400
	21st to end	Night	19% 6% 32% 35% 8%	400
July	1st to 10th	Night	16% 7% 29% 38% 8% 1	390
	11th to 20th	Night	14% 5% 25% 40% 14% 2	388
	21st to end	Night	13% 6% 29% 41% 11%	438
August	1st to 10th	Night	16% 4 25% 40% 15%	493
	11th to 20th	Night	14% 8% 27% 34% 15% 2	631
	21st to end	Night	15% 5% 25% 36% 17% 1	880
September	1st to 10th	Night	15% 7% 31% 31% 16% 1	800
	11th to 20th	Night	16% 7% 26% 30% 19% 2	800
	21st to end	Night	18% 8% 29% 30% 14% 2	800
October	1st to 10th	Night	26% 10% 23% 27% 12% 1	773
	11th to 20th	Night	21% 11% 32% 25% 11% 1	760
	21st to end	Night	28% 16% 27% 22% 6%	836
November	1st to 10th	Night	20% 15% 31% 26% 7% 1	780
	11th to 20th	Night	24% 19% 31% 19% 7% 1	800
	21st to end	Night	27% 17% 30% 21% 5% 1	800
December	1st to 10th	Night	25% 20% 34% 17% 4	800
	11th to 20th	Night	28% 19% 33% 18% 3	800
	21st to end	Night	26% 20% 30% 20% 3	880

Page generated at 2025-10-01 01:39:04. Version ac07

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