Tamiment
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tamiment clear sky chart from 2005-11-06 to 2026-02-01.

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% 3 7% 4 4 4 4 5% 7% 37%	127703
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	124333
Transparency	Day	29% 38% 7% 15% 9% 1	131205
	Night	29% 37% 8% 14% 10% 2	128918
Seeing	Day	25% 17% 28% 19% 9% 3	16398
	Night	22% 15% 28% 25% 9% 1	25713

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	12% 10% 3 8% 5% 5% 4 3 4 6% 39%	3677
	11th to 20th	Day	14% 11% 4 11% 6% 5% 4 3 4 5% 33%	3650
	21st to end	Day	17% 8% 3 7% 5% 5% 4 4 4 5% 38%	4532
February	1st to 10th	Day	16% 9% 3 8% 5% 5% 5% 3 6% 6% 34%	1800
	11th to 20th	Day	13% 9% 3 8% 4 5% 5% 4 5% 5% 38%	1889
	21st to end	Day	18% 10% 3 7% 5% 3 4 3 5% 6% 36%	1650
March	1st to 10th	Day	21% 9% 2 6% 4 3 3 3 4 5% 39%	4476
	11th to 20th	Day	12% 8% 3 8% 5% 4 4 3 5% 7% 41%	4929
	21st to end	Day	18% 8% 2 6% 4 4 4 3 5% 6% 40%	5825
April	1st to 10th	Day	18% 10% 3 7% 4 4 4 4 5% 6% 36%	2400
	11th to 20th	Day	18% 7% 3 6% 4 5% 4 3 6% 6% 39%	2374
	21st to end	Day	17% 6% 2 6% 5% 4 5% 4 6% 6% 38%	2400
May	1st to 10th	Day	29% 7% 2 6% 4 4 4 3 5% 7% 29%	6007
	11th to 20th	Day	23% 9% 3 7% 5% 5% 4 3 5% 6% 31%	6318
	21st to end	Day	21% 6% 2 5% 4 4 5% 4 6% 8% 33%	6949
June	1st to 10th	Day	15% 5% 2 7% 5% 5% 5% 5% 6% 8% 36%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 7% 9% 33%	2800
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 5% 6% 9% 38%	2800
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 5% 6% 10% 36%	6360
	11th to 20th	Day	11% 6% 3 7% 5% 5% 6% 6% 8% 11% 32%	6360
	21st to end	Day	13% 7% 3 6% 4 4 4 4 6% 8% 41%	6926
August	1st to 10th	Day	22% 5% 3 6% 5% 5% 5% 4 6% 7% 32%	4274
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 9% 39%	3883
	21st to end	Day	15% 5% 2 4 3 3 4 3 6% 9% 45%	4312
September	1st to 10th	Day	16% 6% 2 5% 3 4 5% 4 5% 8% 41%	2400
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 4 7% 50%	2360
	21st to end	Day	19% 7% 3 7% 4 4 4 4 5% 6% 38%	2200
October	1st to 10th	Day	20% 7% 2 5% 4 3 4 3 4 7% 41%	2010
	11th to 20th	Day	15% 6% 3 6% 4 4 4 3 5% 7% 44%	2000
	21st to end	Day	21% 6% 2 6% 3 4 4 3 6% 6% 39%	2090
November	1st to 10th	Day	14% 6% 3 5% 3 3 3 3 4 5% 51%	1851
	11th to 20th	Day	18% 7% 2 5% 4 3 4 4 6% 6% 43%	1890
	21st to end	Day	17% 8% 3 6% 4 5% 5% 4 6% 6% 37%	1717
December	1st to 10th	Day	16% 8% 3 7% 4 4 5% 4 4 5% 39%	2680
	11th to 20th	Day	22% 9% 2 5% 3 4 3 3 5% 5% 38%	3066
	21st to end	Day	18% 10% 4 9% 5% 4 4 3 5% 6% 31%	4048
January	1st to 10th	Night	13% 7% 3 9% 6% 5% 5% 3 5% 6% 38%	6489
	11th to 20th	Night	12% 9% 4 10% 6% 5% 4 3 5% 6% 36%	6455
	21st to end	Night	16% 8% 2 7% 5% 4 4 4 5% 6% 39%	6662
February	1st to 10th	Night	16% 9% 3 6% 5% 4 5% 3 5% 6% 39%	2601
	11th to 20th	Night	15% 8% 3 7% 4 5% 5% 4 6% 6% 38%	2396
	21st to end	Night	17% 8% 3 6% 5% 4 5% 3 5% 6% 39%	1980
March	1st to 10th	Night	18% 9% 3 6% 4 4 3 3 4 5% 39%	5364
	11th to 20th	Night	13% 7% 3 6% 3 4 4 3 5% 6% 46%	5025
	21st to end	Night	16% 8% 3 7% 4 4 4 3 5% 5% 41%	5106
April	1st to 10th	Night	18% 10% 2 7% 6% 4 4 4 5% 7% 34%	2000
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 6% 7% 38%	1975
	21st to end	Night	19% 7% 2 5% 4 2 4 4 5% 6% 41%	2000
May	1st to 10th	Night	31% 8% 2 7% 3 4 4 3 5% 6% 27%	3974
	11th to 20th	Night	24% 9% 3 6% 4 4 4 3 4 5% 34%	3575
	21st to end	Night	22% 6% 2 6% 4 4 5% 4 6% 8% 35%	3926
June	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 5% 8% 43%	1600
	11th to 20th	Night	16% 6% 1 5% 5% 3 5% 5% 6% 8% 41%	1600
	21st to end	Night	13% 4 2 5% 4 4 4 3 7% 9% 45%	1600
July	1st to 10th	Night	12% 6% 2 7% 4 5% 5% 4 6% 9% 40%	3600
	11th to 20th	Night	14% 8% 2 6% 4 5% 5% 5% 7% 9% 34%	3600
	21st to end	Night	14% 5% 1 4 3 3 4 4 5% 7% 49%	3926
August	1st to 10th	Night	22% 5% 2 6% 4 4 5% 3 6% 8% 35%	2872
	11th to 20th	Night	15% 7% 2 6% 4 4 5% 3 5% 6% 43%	3131
	21st to end	Night	14% 4 2 4 3 3 3 3 5% 8% 51%	3586
September	1st to 10th	Night	16% 5% 2 5% 3 4 4 3 4 8% 47%	2000
	11th to 20th	Night	12% 6% 2 4 4 3 3 3 4 7% 52%	2040
	21st to end	Night	18% 6% 3 6% 4 3 3 3 5% 6% 43%	2200
October	1st to 10th	Night	22% 5% 2 5% 3 3 3 2 4 6% 44%	2385
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 5% 5% 48%	2400
	21st to end	Night	21% 7% 2 4 3 3 3 2 5% 7% 43%	2750
November	1st to 10th	Night	13% 6% 2 4 3 3 4 4 5% 6% 49%	2667
	11th to 20th	Night	18% 6% 3 6% 3 3 4 3 5% 5% 44%	2730
	21st to end	Night	14% 7% 2 6% 4 4 4 4 6% 7% 42%	2900
December	1st to 10th	Night	13% 7% 3 7% 4 4 4 3 5% 7% 42%	4720
	11th to 20th	Night	23% 8% 2 6% 4 3 4 3 4 5% 38%	5348
	21st to end	Night	19% 13% 4 7% 4 4 3 3 4 5% 34%	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	36% 26% 7% 18% 12% 2	3678
	11th to 20th	Day	41% 26% 5% 15% 9% 4	3680
	21st to end	Day	37% 28% 7% 16% 11% 1	4532
February	1st to 10th	Day	19% 62% 6% 8% 4	1800
	11th to 20th	Day	18% 61% 3 10% 8% 1	1898
	21st to end	Day	18% 62% 5% 9% 5% 1	1650
March	1st to 10th	Day	36% 29% 5% 14% 9% 6%	4479
	11th to 20th	Day	28% 30% 12% 22% 4 4	4929
	21st to end	Day	32% 29% 8% 7% 19% 5%	5825
April	1st to 10th	Day	22% 62% 3 7% 5% 1	2400
	11th to 20th	Day	18% 62% 4 8% 8%	2374
	21st to end	Day	18% 63% 4 9% 5% 1	2400
May	1st to 10th	Day	42% 30% 11% 11% 5%	6018
	11th to 20th	Day	42% 29% 7% 15% 7%	6322
	21st to end	Day	36% 30% 8% 17% 9% 1	6949
June	1st to 10th	Day	18% 63% 4 8% 7%	2800
	11th to 20th	Day	17% 63% 5% 9% 7%	2800
	21st to end	Day	16% 61% 5% 10% 7%	2800
July	1st to 10th	Day	33% 29% 9% 17% 12% 1	6290
	11th to 20th	Day	36% 26% 14% 17% 7%	6285
	21st to end	Day	32% 25% 8% 18% 16% 1	6728
August	1st to 10th	Day	36% 36% 9% 14% 5%	4260
	11th to 20th	Day	26% 36% 10% 21% 6%	3920
	21st to end	Day	21% 36% 8% 25% 9% 1	4312
September	1st to 10th	Day	17% 58% 7% 11% 7%	2400
	11th to 20th	Day	16% 57% 4 10% 11% 1	2360
	21st to end	Day	22% 57% 6% 8% 7% 1	2200
October	1st to 10th	Day	26% 36% 3 12% 18% 3	3270
	11th to 20th	Day	19% 35% 4 20% 18% 4	3240
	21st to end	Day	13% 35% 13% 21% 16% 2	3354
November	1st to 10th	Day	20% 53% 6% 12% 8% 2	1851
	11th to 20th	Day	16% 54% 4 12% 11% 2	1890
	21st to end	Day	21% 56% 6% 12% 4 1	1717
December	1st to 10th	Day	32% 34% 8% 18% 6% 1	2680
	11th to 20th	Day	35% 31% 8% 16% 9% 1	3066
	21st to end	Day	42% 26% 5% 14% 11% 1	4048
January	1st to 10th	Night	36% 26% 8% 16% 12% 2	6489
	11th to 20th	Night	35% 27% 10% 14% 10% 3	6500
	21st to end	Night	36% 28% 8% 12% 13% 3	6662
February	1st to 10th	Night	17% 62% 7% 10% 3 1	2601
	11th to 20th	Night	18% 60% 4 9% 7% 1	2398
	21st to end	Night	18% 62% 5% 8% 6% 2	1980
March	1st to 10th	Night	36% 30% 7% 12% 10% 5%	5374
	11th to 20th	Night	26% 30% 13% 19% 9% 4	5026
	21st to end	Night	35% 29% 4 11% 18% 3	5106
April	1st to 10th	Night	22% 62% 3 8% 5% 1	2000
	11th to 20th	Night	16% 62% 5% 9% 7%	1975
	21st to end	Night	16% 62% 4 10% 7% 1	2000
May	1st to 10th	Night	44% 29% 9% 12% 5% 1	3980
	11th to 20th	Night	42% 30% 9% 11% 8% 1	3576
	21st to end	Night	37% 30% 9% 16% 7% 1	3927
June	1st to 10th	Night	16% 62% 3 10% 9%	1600
	11th to 20th	Night	16% 63% 6% 9% 6%	1600
	21st to end	Night	15% 61% 5% 11% 7% 1	1600
July	1st to 10th	Night	33% 28% 11% 14% 13% 1	3560
	11th to 20th	Night	37% 27% 12% 16% 8%	3552
	21st to end	Night	27% 25% 9% 22% 16% 1	3809
August	1st to 10th	Night	36% 37% 10% 12% 4 1	2862
	11th to 20th	Night	28% 36% 9% 21% 6%	3160
	21st to end	Night	18% 36% 7% 27% 12%	3586
September	1st to 10th	Night	15% 57% 6% 13% 9% 1	2000
	11th to 20th	Night	14% 57% 4 11% 12% 2	2040
	21st to end	Night	21% 57% 5% 9% 7% 1	2200
October	1st to 10th	Night	27% 35% 7% 16% 14% 1	3905
	11th to 20th	Night	16% 35% 6% 19% 20% 3	3899
	21st to end	Night	17% 35% 10% 22% 14% 1	4436
November	1st to 10th	Night	18% 55% 6% 12% 8% 1	2667
	11th to 20th	Night	17% 54% 5% 11% 12% 1	2730
	21st to end	Night	19% 55% 7% 12% 6% 1	2900
December	1st to 10th	Night	27% 34% 9% 24% 5% 1	4720
	11th to 20th	Night	36% 31% 7% 13% 11% 1	5348
	21st to end	Night	41% 27% 7% 12% 11% 2	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	35% 25% 26% 11% 2	220
	11th to 20th	Day	43% 25% 25% 6% 1	220
	21st to end	Day	30% 33% 26% 8% 3 1	353
February	1st to 10th	Day	32% 31% 28% 8% 2	300
	11th to 20th	Day	37% 26% 27% 8% 1	437
	21st to end	Day	34% 24% 28% 11% 2	414
March	1st to 10th	Day	33% 21% 30% 13% 3	500
	11th to 20th	Day	29% 25% 28% 14% 3 1	500
	21st to end	Day	34% 23% 27% 11% 4 1	550
April	1st to 10th	Day	33% 24% 26% 13% 4	500
	11th to 20th	Day	30% 24% 25% 13% 7% 1	492
	21st to end	Day	27% 21% 33% 13% 4 2	500
May	1st to 10th	Day	34% 18% 28% 12% 7% 1	503
	11th to 20th	Day	25% 20% 29% 17% 8% 2	527
	21st to end	Day	21% 12% 30% 22% 13% 2	573
June	1st to 10th	Day	21% 14% 30% 22% 11% 2	530
	11th to 20th	Day	22% 13% 29% 24% 11% 1	530
	21st to end	Day	16% 13% 32% 27% 9% 2	530
July	1st to 10th	Day	14% 8% 30% 27% 17% 5%	525
	11th to 20th	Day	12% 6% 27% 31% 20% 3	524
	21st to end	Day	15% 8% 33% 28% 12% 3	576
August	1st to 10th	Day	14% 6% 28% 32% 15% 6%	526
	11th to 20th	Day	15% 7% 23% 34% 16% 6%	519
	21st to end	Day	14% 10% 31% 23% 14% 7%	583
September	1st to 10th	Day	19% 12% 25% 24% 14% 6%	530
	11th to 20th	Day	13% 8% 29% 24% 16% 10%	530
	21st to end	Day	24% 9% 28% 24% 11% 4	530
October	1st to 10th	Day	26% 10% 25% 20% 14% 6%	530
	11th to 20th	Day	22% 17% 30% 21% 8% 2	530
	21st to end	Day	27% 16% 31% 18% 7% 1	522
November	1st to 10th	Day	23% 18% 28% 20% 9% 2	425
	11th to 20th	Day	28% 20% 29% 18% 5% 1	430
	21st to end	Day	30% 24% 30% 14% 2	257
December	1st to 10th	Day	30% 26% 30% 9% 5%	220
	11th to 20th	Day	39% 23% 25% 11% 2	220
	21st to end	Day	32% 19% 33% 12% 4	242
January	1st to 10th	Night	24% 23% 35% 16% 3	840
	11th to 20th	Night	29% 20% 30% 19% 2	840
	21st to end	Night	21% 25% 34% 18% 2	924
February	1st to 10th	Night	26% 20% 35% 18% 1	801
	11th to 20th	Night	27% 24% 36% 12%	783
	21st to end	Night	29% 20% 32% 16% 2	660
March	1st to 10th	Night	26% 22% 32% 17% 3	800
	11th to 20th	Night	25% 22% 30% 19% 4	800
	21st to end	Night	22% 24% 34% 15% 5%	880
April	1st to 10th	Night	25% 22% 31% 20% 3	800
	11th to 20th	Night	24% 18% 31% 20% 6%	791
	21st to end	Night	24% 17% 30% 24% 5%	800
May	1st to 10th	Night	27% 13% 29% 24% 6%	572
	11th to 20th	Night	21% 11% 32% 28% 8% 1	394
	21st to end	Night	17% 7% 26% 37% 12%	433
June	1st to 10th	Night	19% 8% 22% 37% 15%	400
	11th to 20th	Night	17% 5% 32% 35% 12%	400
	21st to end	Night	15% 5% 24% 44% 12% 1	400
July	1st to 10th	Night	15% 4 21% 42% 17% 2	390
	11th to 20th	Night	12% 4 21% 44% 18% 2	388
	21st to end	Night	13% 5% 22% 43% 17% 1	438
August	1st to 10th	Night	13% 2 19% 40% 24% 2	567
	11th to 20th	Night	16% 5% 17% 36% 25% 1	748
	21st to end	Night	16% 5% 21% 34% 22% 2	880
September	1st to 10th	Night	18% 5% 24% 32% 18% 2	800
	11th to 20th	Night	16% 4 23% 31% 20% 5%	800
	21st to end	Night	21% 7% 23% 31% 16% 2	800
October	1st to 10th	Night	25% 10% 21% 26% 17% 1	800
	11th to 20th	Night	21% 13% 27% 30% 8%	800
	21st to end	Night	26% 14% 30% 23% 7%	880
November	1st to 10th	Night	19% 17% 29% 26% 9% 1	820
	11th to 20th	Night	27% 18% 26% 23% 6% 1	840
	21st to end	Night	22% 20% 30% 23% 5%	840
December	1st to 10th	Night	26% 18% 32% 19% 5%	840
	11th to 20th	Night	27% 19% 31% 21% 3	840
	21st to end	Night	27% 18% 32% 20% 3	924

Page generated at 2026-01-29 01:21:40. Version ac07

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