Joseph D. Tatem Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joseph D. Tatem Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	16% 6% 2 6% 4 4 4 4 5% 7% 41%	126916
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	123436
Transparency	Day	41% 5% 11% 23% 17% 3	130457
	Night	40% 4 11% 23% 18% 4	127982
Seeing	Day	22% 16% 29% 21% 10% 3	16393
	Night	20% 14% 31% 26% 9% 1	25614

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	14% 7% 2 4 3 3 4 3 4 5% 50%	3597
	11th to 20th	Day	14% 7% 3 9% 4 4 3 3 3 5% 44%	3632
	21st to end	Day	19% 9% 3 6% 4 3 3 2 4 5% 42%	4466
February	1st to 10th	Day	15% 8% 2 7% 4 4 5% 4 5% 7% 39%	1800
	11th to 20th	Day	18% 8% 2 6% 4 3 5% 4 5% 6% 41%	1938
	21st to end	Day	17% 10% 3 5% 4 4 3 3 4 6% 41%	1650
March	1st to 10th	Day	22% 6% 2 4 3 3 3 3 3 5% 47%	4476
	11th to 20th	Day	11% 7% 2 6% 4 5% 4 3 5% 6% 46%	4906
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 6% 44%	5846
April	1st to 10th	Day	18% 8% 3 6% 3 4 4 3 5% 6% 40%	2400
	11th to 20th	Day	15% 7% 2 5% 4 4 4 5% 6% 7% 42%	2374
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 7% 40%	2400
May	1st to 10th	Day	25% 8% 2 6% 4 5% 4 4 5% 7% 30%	5766
	11th to 20th	Day	25% 7% 3 6% 4 4 3 3 5% 6% 33%	6308
	21st to end	Day	20% 6% 2 6% 4 4 4 3 6% 8% 37%	6949
June	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 7% 9% 40%	2800
	11th to 20th	Day	14% 5% 2 6% 6% 4 5% 4 7% 8% 39%	2800
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 6% 8% 43%	2800
July	1st to 10th	Day	10% 5% 2 7% 6% 6% 6% 5% 7% 10% 37%	6360
	11th to 20th	Day	12% 6% 3 7% 6% 6% 6% 5% 8% 11% 31%	6360
	21st to end	Day	11% 6% 2 7% 6% 5% 5% 4 6% 9% 38%	6882
August	1st to 10th	Day	19% 6% 3 7% 5% 6% 5% 4 6% 8% 32%	4178
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 4 7% 9% 38%	3850
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 7% 8% 46%	4312
September	1st to 10th	Day	14% 5% 2 6% 5% 4 4 3 5% 7% 45%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 8% 49%	2345
	21st to end	Day	19% 7% 2 7% 4 4 5% 3 4 6% 38%	2200
October	1st to 10th	Day	18% 5% 2 5% 4 3 3 3 4 6% 46%	2040
	11th to 20th	Day	17% 5% 2 4 3 3 3 2 4 6% 50%	2000
	21st to end	Day	17% 5% 2 6% 3 3 3 3 4 6% 47%	2106
November	1st to 10th	Day	12% 5% 2 5% 4 4 3 2 5% 6% 53%	1851
	11th to 20th	Day	15% 5% 2 6% 4 3 3 3 5% 6% 47%	1863
	21st to end	Day	16% 5% 2 5% 3 4 4 4 5% 6% 47%	1715
December	1st to 10th	Day	13% 7% 2 5% 3 3 4 3 6% 8% 46%	2600
	11th to 20th	Day	21% 10% 3 4 3 2 3 2 3 4 46%	2986
	21st to end	Day	15% 6% 2 5% 4 5% 5% 4 6% 8% 41%	3960
January	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 4 6% 51%	6349
	11th to 20th	Night	15% 8% 2 6% 3 3 3 2 3 5% 50%	6251
	21st to end	Night	16% 8% 2 5% 4 4 4 3 4 6% 42%	6490
February	1st to 10th	Night	17% 9% 3 5% 5% 3 3 3 5% 6% 41%	2600
	11th to 20th	Night	19% 8% 2 6% 4 3 3 3 5% 5% 42%	2347
	21st to end	Night	18% 8% 2 6% 3 4 3 3 4 6% 43%	1980
March	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 5% 46%	5364
	11th to 20th	Night	12% 10% 3 5% 4 3 3 2 4 5% 48%	5035
	21st to end	Night	16% 7% 2 6% 4 4 5% 3 4 5% 43%	5084
April	1st to 10th	Night	17% 8% 3 7% 5% 4 3 3 5% 6% 40%	2000
	11th to 20th	Night	15% 8% 3 6% 5% 4 3 3 5% 6% 43%	1975
	21st to end	Night	17% 6% 2 5% 3 3 5% 3 5% 6% 44%	2000
May	1st to 10th	Night	21% 8% 3 8% 5% 4 4 4 6% 9% 29%	4158
	11th to 20th	Night	23% 9% 3 6% 4 4 3 3 5% 6% 33%	3626
	21st to end	Night	21% 7% 3 6% 4 3 4 3 5% 8% 37%	3926
June	1st to 10th	Night	14% 6% 2 5% 5% 4 5% 4 4 9% 43%	1600
	11th to 20th	Night	12% 7% 2 6% 5% 5% 5% 4 6% 9% 41%	1600
	21st to end	Night	10% 5% 1 5% 5% 4 5% 4 8% 11% 41%	1600
July	1st to 10th	Night	10% 5% 3 7% 6% 6% 6% 5% 6% 9% 37%	3600
	11th to 20th	Night	12% 8% 3 8% 5% 5% 5% 4 7% 10% 32%	3600
	21st to end	Night	12% 6% 2 7% 4 4 4 4 6% 8% 42%	3965
August	1st to 10th	Night	21% 6% 3 6% 4 5% 4 4 6% 8% 33%	2938
	11th to 20th	Night	13% 7% 2 6% 5% 4 4 4 5% 7% 44%	3197
	21st to end	Night	10% 5% 1 5% 4 3 5% 3 5% 8% 51%	3586
September	1st to 10th	Night	14% 5% 1 7% 5% 4 3 3 5% 7% 46%	2000
	11th to 20th	Night	11% 5% 1 5% 3 3 4 3 5% 7% 53%	2055
	21st to end	Night	21% 6% 3 5% 4 3 4 3 6% 7% 40%	2200
October	1st to 10th	Night	17% 7% 2 5% 3 4 3 3 5% 6% 46%	2355
	11th to 20th	Night	17% 4 1 4 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	17% 6% 1 5% 3 3 3 2 4 6% 48%	2732
November	1st to 10th	Night	11% 5% 2 6% 3 3 3 3 5% 6% 53%	2667
	11th to 20th	Night	16% 6% 2 5% 4 3 4 3 4 4 49%	2692
	21st to end	Night	13% 5% 2 5% 5% 4 4 4 6% 6% 46%	2680
December	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 49%	4580
	11th to 20th	Night	23% 7% 2 5% 3 3 3 2 5% 7% 41%	5208
	21st to end	Night	18% 7% 3 5% 4 4 4 3 5% 6% 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	36% 3 8% 26% 24% 3	3598
	11th to 20th	Day	43% 4 7% 23% 17% 5%	3662
	21st to end	Day	46% 2 10% 25% 13% 2	4466
February	1st to 10th	Day	44% 5% 12% 26% 12% 2	1800
	11th to 20th	Day	44% 5% 9% 23% 16% 3	1947
	21st to end	Day	46% 3 9% 22% 15% 5%	1650
March	1st to 10th	Day	41% 3 8% 24% 16% 7%	4479
	11th to 20th	Day	38% 4 8% 28% 17% 5%	4906
	21st to end	Day	43% 3 8% 17% 23% 6%	5846
April	1st to 10th	Day	44% 5% 10% 20% 16% 5%	2400
	11th to 20th	Day	40% 5% 12% 23% 17% 4	2374
	21st to end	Day	41% 5% 9% 21% 20% 4	2400
May	1st to 10th	Day	53% 5% 13% 18% 9% 1	5777
	11th to 20th	Day	52% 4 9% 22% 12% 1	6312
	21st to end	Day	45% 5% 10% 23% 15% 1	6949
June	1st to 10th	Day	41% 6% 13% 23% 15% 2	2800
	11th to 20th	Day	41% 7% 12% 25% 12% 3	2800
	21st to end	Day	38% 7% 13% 23% 17% 2	2800
July	1st to 10th	Day	41% 10% 15% 19% 14% 2	6290
	11th to 20th	Day	45% 8% 17% 20% 9% 1	6285
	21st to end	Day	41% 7% 10% 21% 20% 1	6684
August	1st to 10th	Day	49% 6% 13% 21% 9% 1	4165
	11th to 20th	Day	40% 6% 14% 25% 13% 1	3886
	21st to end	Day	33% 6% 12% 27% 19% 2	4312
September	1st to 10th	Day	39% 6% 11% 20% 21% 3	2400
	11th to 20th	Day	35% 4 10% 24% 24% 3	2345
	21st to end	Day	48% 4 9% 18% 20% 2	2200
October	1st to 10th	Day	36% 3 9% 19% 28% 5%	3326
	11th to 20th	Day	26% 2 9% 24% 30% 8%	3240
	21st to end	Day	26% 3 12% 26% 28% 5%	3383
November	1st to 10th	Day	33% 3 13% 27% 21% 3	1851
	11th to 20th	Day	37% 4 9% 24% 23% 3	1863
	21st to end	Day	38% 4 11% 31% 13% 2	1715
December	1st to 10th	Day	37% 3 16% 26% 15% 2	2600
	11th to 20th	Day	44% 3 10% 30% 10% 3	2986
	21st to end	Day	41% 5% 12% 22% 18% 2	3960
January	1st to 10th	Night	36% 3 8% 25% 23% 7%	6349
	11th to 20th	Night	39% 2 9% 24% 18% 7%	6296
	21st to end	Night	44% 3 8% 24% 19% 2	6490
February	1st to 10th	Night	45% 3 13% 24% 13% 2	2600
	11th to 20th	Night	45% 3 8% 21% 18% 5%	2349
	21st to end	Night	44% 3 8% 22% 17% 6%	1980
March	1st to 10th	Night	43% 3 8% 24% 13% 10%	5374
	11th to 20th	Night	39% 4 12% 25% 15% 5%	5036
	21st to end	Night	44% 3 7% 16% 24% 5%	5084
April	1st to 10th	Night	45% 4 9% 19% 18% 5%	2000
	11th to 20th	Night	43% 3 9% 20% 20% 4	1975
	21st to end	Night	40% 5% 9% 20% 21% 5%	2000
May	1st to 10th	Night	52% 6% 12% 17% 10% 2	4164
	11th to 20th	Night	52% 5% 10% 18% 13% 1	3627
	21st to end	Night	46% 5% 12% 21% 14% 2	3927
June	1st to 10th	Night	39% 7% 12% 20% 20% 2	1600
	11th to 20th	Night	41% 7% 13% 26% 13% 2	1600
	21st to end	Night	34% 8% 16% 24% 16% 2	1600
July	1st to 10th	Night	43% 9% 15% 20% 11% 1	3560
	11th to 20th	Night	45% 8% 19% 20% 7% 1	3552
	21st to end	Night	38% 6% 14% 23% 18% 1	3848
August	1st to 10th	Night	49% 7% 17% 17% 9% 1	2927
	11th to 20th	Night	39% 5% 12% 30% 12% 1	3227
	21st to end	Night	33% 5% 10% 29% 21% 2	3586
September	1st to 10th	Night	38% 5% 12% 20% 21% 3	2000
	11th to 20th	Night	31% 5% 11% 25% 24% 4	2055
	21st to end	Night	44% 5% 9% 22% 18% 2	2200
October	1st to 10th	Night	37% 4 10% 24% 22% 3	3849
	11th to 20th	Night	25% 3 10% 26% 30% 6%	3899
	21st to end	Night	26% 3 10% 31% 25% 4	4405
November	1st to 10th	Night	32% 5% 11% 25% 22% 5%	2667
	11th to 20th	Night	39% 4 8% 22% 21% 6%	2692
	21st to end	Night	37% 5% 11% 27% 18% 2	2680
December	1st to 10th	Night	37% 3 12% 30% 16% 3	4580
	11th to 20th	Night	45% 3 10% 24% 14% 4	5208
	21st to end	Night	44% 3 11% 20% 18% 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	28% 27% 31% 11% 4 1	200
	11th to 20th	Day	35% 27% 26% 11% 1	213
	21st to end	Day	28% 30% 26% 12% 3 1	330
February	1st to 10th	Day	29% 30% 28% 11% 2	300
	11th to 20th	Day	34% 26% 29% 9% 1	486
	21st to end	Day	30% 25% 33% 11% 2	414
March	1st to 10th	Day	29% 24% 29% 15% 3	500
	11th to 20th	Day	26% 26% 31% 11% 3 2	500
	21st to end	Day	29% 23% 30% 13% 4 1	550
April	1st to 10th	Day	29% 26% 29% 10% 4 1	500
	11th to 20th	Day	24% 22% 33% 14% 6% 1	492
	21st to end	Day	27% 19% 33% 15% 5% 1	500
May	1st to 10th	Day	32% 20% 26% 15% 6% 1	503
	11th to 20th	Day	25% 15% 34% 19% 6% 2	527
	21st to end	Day	22% 12% 23% 28% 13% 2	573
June	1st to 10th	Day	16% 14% 29% 27% 12% 2	530
	11th to 20th	Day	19% 11% 32% 25% 11% 3	530
	21st to end	Day	12% 10% 32% 30% 13% 3	530
July	1st to 10th	Day	13% 6% 26% 30% 19% 6%	525
	11th to 20th	Day	10% 3 28% 34% 21% 4	524
	21st to end	Day	11% 8% 29% 31% 16% 5%	576
August	1st to 10th	Day	13% 5% 29% 32% 15% 6%	526
	11th to 20th	Day	15% 5% 20% 35% 19% 6%	519
	21st to end	Day	13% 8% 27% 27% 18% 7%	583
September	1st to 10th	Day	17% 8% 28% 24% 15% 7%	530
	11th to 20th	Day	15% 7% 27% 29% 14% 8%	530
	21st to end	Day	25% 11% 28% 22% 11% 5%	530
October	1st to 10th	Day	23% 10% 25% 22% 13% 7%	530
	11th to 20th	Day	22% 18% 25% 20% 11% 3	530
	21st to end	Day	19% 18% 34% 17% 10% 1	531
November	1st to 10th	Day	17% 16% 35% 20% 10% 2	425
	11th to 20th	Day	24% 21% 31% 18% 6%	421
	21st to end	Day	23% 21% 30% 19% 7% 1	315
December	1st to 10th	Day	26% 28% 32% 10% 5% 1	200
	11th to 20th	Day	34% 16% 36% 13% 2 1	200
	21st to end	Day	28% 17% 32% 16% 6% 1	220
January	1st to 10th	Night	22% 22% 36% 18% 3	800
	11th to 20th	Night	26% 20% 31% 20% 3	800
	21st to end	Night	19% 20% 37% 21% 2	880
February	1st to 10th	Night	22% 20% 36% 21% 2	800
	11th to 20th	Night	27% 25% 32% 14% 1	783
	21st to end	Night	25% 23% 31% 18% 2	660
March	1st to 10th	Night	23% 22% 33% 20% 4	800
	11th to 20th	Night	23% 22% 32% 16% 6%	800
	21st to end	Night	22% 15% 40% 17% 4	880
April	1st to 10th	Night	23% 23% 33% 16% 5%	800
	11th to 20th	Night	21% 15% 37% 22% 4	791
	21st to end	Night	21% 14% 35% 25% 5%	800
May	1st to 10th	Night	23% 12% 36% 23% 6%	652
	11th to 20th	Night	21% 11% 38% 25% 4	419
	21st to end	Night	21% 5% 25% 38% 10% 1	433
June	1st to 10th	Night	19% 5% 30% 36% 10% 1	400
	11th to 20th	Night	14% 6% 30% 39% 12%	400
	21st to end	Night	11% 3 32% 43% 11%	400
July	1st to 10th	Night	10% 3 28% 43% 15% 1	390
	11th to 20th	Night	10% 3 22% 45% 18% 2	388
	21st to end	Night	11% 4 26% 41% 18%	452
August	1st to 10th	Night	12% 4 17% 44% 22% 1	607
	11th to 20th	Night	14% 4 17% 42% 20% 2	790
	21st to end	Night	13% 3 21% 39% 21% 2	880
September	1st to 10th	Night	16% 5% 27% 34% 15% 3	800
	11th to 20th	Night	15% 5% 21% 34% 22% 4	800
	21st to end	Night	25% 6% 23% 31% 15% 1	800
October	1st to 10th	Night	21% 8% 26% 27% 16% 3	800
	11th to 20th	Night	18% 13% 28% 29% 12% 1	800
	21st to end	Night	19% 12% 36% 23% 9%	880
November	1st to 10th	Night	15% 13% 35% 26% 10% 1	820
	11th to 20th	Night	21% 17% 30% 25% 8%	829
	21st to end	Night	18% 18% 36% 20% 8% 1	800
December	1st to 10th	Night	25% 17% 32% 19% 7% 1	800
	11th to 20th	Night	21% 19% 33% 22% 6%	800
	21st to end	Night	22% 16% 35% 25% 2	880

Page generated at 2025-11-15 00:55:55. Version ac07

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