JAT Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the JAT Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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 6% 8% 40%	126670
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	123045
Transparency	Day	27% 38% 7% 15% 11% 2	130198
	Night	27% 37% 7% 15% 12% 3	127604
Seeing	Day	22% 13% 28% 23% 11% 3	16252
	Night	20% 11% 29% 29% 10% 1	25443

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	13% 6% 2 4 4 3 3 4 5% 6% 52%	3597
	11th to 20th	Day	13% 8% 3 8% 4 4 5% 3 4 6% 42%	3595
	21st to end	Day	19% 7% 3 6% 5% 3 4 3 4 5% 41%	4466
February	1st to 10th	Day	16% 7% 3 6% 5% 4 4 3 5% 7% 39%	1800
	11th to 20th	Day	17% 8% 2 6% 5% 3 4 4 5% 6% 40%	1924
	21st to end	Day	18% 8% 3 6% 4 4 4 4 5% 6% 39%	1650
March	1st to 10th	Day	21% 5% 2 5% 3 3 4 3 4 5% 45%	4476
	11th to 20th	Day	12% 6% 2 5% 4 5% 4 4 5% 7% 44%	4924
	21st to end	Day	19% 5% 2 5% 4 4 4 3 5% 6% 43%	5832
April	1st to 10th	Day	17% 7% 3 6% 5% 4 4 3 6% 7% 39%	2400
	11th to 20th	Day	15% 7% 2 5% 4 4 5% 4 6% 8% 40%	2374
	21st to end	Day	15% 6% 2 7% 4 4 4 4 6% 8% 40%	2400
May	1st to 10th	Day	26% 7% 2 6% 4 4 4 3 5% 8% 29%	5823
	11th to 20th	Day	24% 9% 3 6% 4 4 4 3 4 6% 33%	6318
	21st to end	Day	20% 6% 2 6% 4 4 4 4 6% 8% 36%	6949
June	1st to 10th	Day	13% 6% 2 6% 5% 4 4 4 8% 9% 38%	2800
	11th to 20th	Day	13% 6% 2 5% 5% 5% 5% 4 7% 9% 39%	2800
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 10% 42%	2800
July	1st to 10th	Day	9% 5% 2 7% 6% 5% 6% 4 7% 11% 37%	6360
	11th to 20th	Day	11% 7% 3 8% 5% 5% 6% 5% 8% 10% 31%	6360
	21st to end	Day	11% 7% 2 7% 5% 4 5% 4 6% 9% 39%	6893
August	1st to 10th	Day	18% 6% 2 7% 5% 5% 5% 4 6% 8% 34%	4205
	11th to 20th	Day	13% 5% 2 6% 3 4 5% 4 7% 9% 41%	3850
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 9% 47%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 45%	2400
	11th to 20th	Day	13% 5% 1 4 3 4 3 3 6% 7% 51%	2350
	21st to end	Day	18% 7% 3 7% 5% 4 4 3 5% 7% 37%	2200
October	1st to 10th	Day	17% 6% 2 5% 3 4 4 3 5% 6% 46%	2033
	11th to 20th	Day	16% 5% 2 4 3 3 4 3 4 7% 48%	1980
	21st to end	Day	18% 6% 2 5% 3 2 3 3 5% 5% 46%	1997
November	1st to 10th	Day	12% 6% 2 4 3 3 3 2 5% 6% 53%	1761
	11th to 20th	Day	16% 7% 2 4 4 3 3 3 5% 5% 49%	1800
	21st to end	Day	16% 5% 2 5% 4 2 3 4 5% 7% 47%	1695
December	1st to 10th	Day	14% 7% 2 5% 4 3 3 3 6% 7% 45%	2600
	11th to 20th	Day	21% 8% 2 4 3 3 3 2 4 5% 44%	2986
	21st to end	Day	16% 6% 2 5% 4 4 5% 4 6% 8% 40%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 6% 50%	6349
	11th to 20th	Night	15% 7% 3 6% 3 3 3 3 4 5% 49%	6287
	21st to end	Night	18% 8% 2 5% 4 3 4 3 4 6% 42%	6490
February	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 6% 6% 41%	2600
	11th to 20th	Night	18% 8% 2 6% 4 4 4 3 5% 5% 42%	2360
	21st to end	Night	18% 7% 3 5% 4 3 3 3 5% 5% 43%	1980
March	1st to 10th	Night	19% 8% 2 5% 3 3 3 3 4 6% 44%	5364
	11th to 20th	Night	12% 9% 3 5% 3 4 3 3 4 5% 50%	5033
	21st to end	Night	16% 8% 3 6% 4 4 4 4 4 5% 43%	5100
April	1st to 10th	Night	18% 6% 3 6% 5% 4 4 4 5% 6% 40%	2000
	11th to 20th	Night	16% 7% 2 7% 4 4 4 4 4 6% 41%	1975
	21st to end	Night	17% 6% 2 5% 3 4 4 3 5% 6% 45%	2000
May	1st to 10th	Night	23% 9% 3 7% 4 4 4 4 6% 8% 27%	4151
	11th to 20th	Night	23% 8% 3 8% 4 3 4 3 5% 6% 33%	3580
	21st to end	Night	21% 7% 2 6% 3 4 3 3 5% 9% 36%	3926
June	1st to 10th	Night	15% 5% 2 6% 5% 3 4 3 6% 7% 43%	1600
	11th to 20th	Night	13% 5% 3 6% 5% 5% 4 5% 5% 8% 41%	1600
	21st to end	Night	10% 5% 2 5% 4 5% 6% 5% 7% 10% 41%	1600
July	1st to 10th	Night	8% 6% 2 8% 5% 6% 6% 4 8% 9% 37%	3600
	11th to 20th	Night	13% 8% 3 7% 5% 5% 6% 4 7% 10% 32%	3600
	21st to end	Night	11% 6% 2 7% 4 4 5% 4 6% 8% 43%	3942
August	1st to 10th	Night	22% 6% 2 6% 4 4 5% 4 6% 8% 33%	2915
	11th to 20th	Night	13% 7% 3 6% 5% 4 5% 4 5% 7% 41%	3193
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 8% 50%	3586
September	1st to 10th	Night	14% 7% 2 5% 5% 3 4 3 5% 6% 46%	2000
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 5% 6% 52%	2050
	21st to end	Night	19% 6% 2 6% 4 3 3 3 6% 7% 40%	2200
October	1st to 10th	Night	18% 7% 2 4 4 4 4 3 5% 5% 45%	2364
	11th to 20th	Night	15% 4 2 5% 3 3 3 3 4 7% 51%	2377
	21st to end	Night	18% 7% 2 5% 3 3 3 3 5% 6% 46%	2599
November	1st to 10th	Night	11% 5% 2 5% 4 3 2 3 6% 6% 54%	2537
	11th to 20th	Night	17% 5% 2 5% 4 3 3 3 4 5% 48%	2600
	21st to end	Night	14% 6% 2 5% 5% 5% 3 4 5% 7% 45%	2703
December	1st to 10th	Night	14% 7% 2 6% 3 3 3 3 5% 7% 47%	4580
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 5% 6% 41%	5208
	21st to end	Night	20% 7% 3 6% 3 4 3 3 5% 5% 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	25% 26% 5% 21% 19% 3	3598
	11th to 20th	Day	32% 28% 4 18% 13% 5%	3625
	21st to end	Day	36% 27% 8% 16% 12% 1	4466
February	1st to 10th	Day	19% 63% 6% 8% 3 1	1800
	11th to 20th	Day	18% 60% 2 9% 8% 2	1933
	21st to end	Day	17% 62% 4 9% 6% 1	1650
March	1st to 10th	Day	32% 29% 5% 15% 12% 7%	4479
	11th to 20th	Day	28% 28% 6% 23% 10% 4	4924
	21st to end	Day	31% 28% 7% 10% 19% 5%	5832
April	1st to 10th	Day	20% 62% 3 7% 6% 2	2400
	11th to 20th	Day	16% 63% 5% 8% 7%	2374
	21st to end	Day	16% 62% 4 9% 8% 1	2400
May	1st to 10th	Day	42% 28% 11% 13% 6%	5834
	11th to 20th	Day	41% 28% 6% 17% 8%	6322
	21st to end	Day	36% 29% 6% 18% 11% 1	6949
June	1st to 10th	Day	17% 62% 4 10% 7% 1	2800
	11th to 20th	Day	16% 62% 4 9% 7% 1	2800
	21st to end	Day	15% 62% 5% 9% 9% 1	2800
July	1st to 10th	Day	33% 28% 11% 14% 12% 1	6290
	11th to 20th	Day	39% 26% 14% 14% 7%	6285
	21st to end	Day	31% 26% 7% 17% 19% 1	6695
August	1st to 10th	Day	35% 36% 8% 15% 5%	4192
	11th to 20th	Day	25% 36% 8% 22% 9%	3886
	21st to end	Day	19% 35% 6% 22% 17% 1	4312
September	1st to 10th	Day	16% 57% 6% 11% 8% 1	2400
	11th to 20th	Day	16% 57% 4 11% 9% 2	2350
	21st to end	Day	25% 57% 4 8% 6%	2200
October	1st to 10th	Day	23% 36% 6% 9% 21% 5%	3306
	11th to 20th	Day	12% 36% 6% 16% 24% 6%	3220
	21st to end	Day	11% 36% 9% 18% 23% 4	3274
November	1st to 10th	Day	16% 56% 6% 10% 9% 2	1761
	11th to 20th	Day	14% 56% 4 11% 13% 2	1800
	21st to end	Day	16% 56% 7% 15% 5% 2	1695
December	1st to 10th	Day	26% 35% 10% 19% 8% 2	2600
	11th to 20th	Day	31% 32% 6% 21% 9% 2	2986
	21st to end	Day	32% 27% 5% 19% 14% 2	3960
January	1st to 10th	Night	26% 26% 5% 19% 19% 5%	6349
	11th to 20th	Night	28% 26% 8% 18% 14% 6%	6332
	21st to end	Night	34% 27% 6% 15% 14% 3	6490
February	1st to 10th	Night	18% 62% 6% 11% 3 1	2600
	11th to 20th	Night	17% 60% 2 8% 10% 2	2362
	21st to end	Night	17% 62% 4 8% 7% 3	1980
March	1st to 10th	Night	33% 28% 4 16% 8% 9%	5374
	11th to 20th	Night	26% 28% 11% 20% 11% 3	5034
	21st to end	Night	33% 28% 4 11% 19% 4	5100
April	1st to 10th	Night	19% 62% 2 8% 7% 1	2000
	11th to 20th	Night	16% 62% 5% 8% 8% 1	1975
	21st to end	Night	16% 61% 4 9% 9% 1	2000
May	1st to 10th	Night	42% 29% 9% 12% 5% 2	4157
	11th to 20th	Night	42% 28% 8% 13% 8%	3581
	21st to end	Night	36% 29% 8% 18% 8% 1	3927
June	1st to 10th	Night	16% 62% 3 9% 10%	1600
	11th to 20th	Night	16% 63% 4 10% 6%	1600
	21st to end	Night	15% 61% 6% 11% 7% 1	1600
July	1st to 10th	Night	33% 28% 13% 15% 10% 1	3560
	11th to 20th	Night	40% 26% 13% 14% 6%	3552
	21st to end	Night	30% 25% 9% 18% 17% 1	3825
August	1st to 10th	Night	37% 36% 12% 10% 5%	2904
	11th to 20th	Night	27% 36% 8% 21% 8%	3223
	21st to end	Night	19% 35% 5% 24% 16% 1	3586
September	1st to 10th	Night	17% 58% 6% 11% 8% 1	2000
	11th to 20th	Night	16% 57% 4 10% 11% 2	2050
	21st to end	Night	24% 57% 4 8% 7% 1	2200
October	1st to 10th	Night	26% 36% 6% 15% 16% 2	3871
	11th to 20th	Night	12% 35% 5% 21% 23% 4	3876
	21st to end	Night	14% 36% 7% 23% 18% 3	4272
November	1st to 10th	Night	16% 56% 6% 11% 8% 3	2537
	11th to 20th	Night	16% 56% 3 10% 11% 3	2600
	21st to end	Night	15% 57% 7% 13% 7% 1	2703
December	1st to 10th	Night	25% 35% 7% 23% 8% 1	4580
	11th to 20th	Night	32% 32% 6% 17% 10% 3	5208
	21st to end	Night	36% 26% 6% 15% 14% 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	30% 23% 31% 12% 4 1	200
	11th to 20th	Day	37% 22% 31% 9% 1	207
	21st to end	Day	27% 23% 31% 15% 4 1	330
February	1st to 10th	Day	30% 22% 34% 10% 3	300
	11th to 20th	Day	33% 20% 32% 12% 2	472
	21st to end	Day	30% 20% 32% 14% 3	414
March	1st to 10th	Day	29% 20% 30% 16% 4	500
	11th to 20th	Day	26% 21% 33% 14% 4 1	500
	21st to end	Day	29% 23% 28% 15% 5% 1	550
April	1st to 10th	Day	31% 20% 28% 15% 5% 1	500
	11th to 20th	Day	27% 20% 30% 15% 6% 2	492
	21st to end	Day	27% 16% 34% 17% 5% 2	500
May	1st to 10th	Day	32% 14% 29% 17% 6% 2	503
	11th to 20th	Day	24% 12% 33% 21% 8% 2	527
	21st to end	Day	22% 7% 26% 27% 14% 4	573
June	1st to 10th	Day	18% 10% 28% 26% 15% 3	530
	11th to 20th	Day	19% 12% 27% 29% 11% 3	530
	21st to end	Day	16% 8% 30% 31% 12% 3	530
July	1st to 10th	Day	13% 5% 22% 33% 20% 7%	525
	11th to 20th	Day	10% 4 23% 34% 24% 5%	524
	21st to end	Day	13% 4 30% 33% 15% 5%	576
August	1st to 10th	Day	13% 4 25% 34% 18% 6%	526
	11th to 20th	Day	14% 4 18% 36% 22% 7%	519
	21st to end	Day	15% 5% 26% 29% 16% 9%	583
September	1st to 10th	Day	17% 8% 24% 26% 18% 7%	530
	11th to 20th	Day	16% 4 24% 30% 16% 9%	530
	21st to end	Day	25% 9% 24% 25% 12% 6%	530
October	1st to 10th	Day	24% 5% 26% 23% 14% 9%	530
	11th to 20th	Day	19% 13% 31% 23% 11% 3	522
	21st to end	Day	21% 13% 31% 21% 12% 2	489
November	1st to 10th	Day	20% 11% 32% 22% 12% 3	395
	11th to 20th	Day	24% 16% 33% 21% 6% 1	400
	21st to end	Day	24% 18% 36% 16% 6%	295
December	1st to 10th	Day	26% 24% 32% 12% 7%	200
	11th to 20th	Day	35% 20% 29% 12% 5%	200
	21st to end	Day	29% 15% 32% 16% 8%	220
January	1st to 10th	Night	22% 19% 34% 21% 4	800
	11th to 20th	Night	24% 15% 33% 23% 4	800
	21st to end	Night	20% 17% 38% 24% 2	880
February	1st to 10th	Night	22% 15% 34% 27% 2	800
	11th to 20th	Night	27% 21% 33% 18% 1	783
	21st to end	Night	24% 17% 37% 20% 3	660
March	1st to 10th	Night	24% 18% 34% 20% 4	800
	11th to 20th	Night	23% 18% 34% 20% 5% 1	800
	21st to end	Night	24% 12% 38% 21% 5%	880
April	1st to 10th	Night	22% 19% 37% 18% 5%	800
	11th to 20th	Night	21% 14% 36% 23% 6%	791
	21st to end	Night	20% 13% 33% 28% 6%	800
May	1st to 10th	Night	24% 12% 31% 25% 7%	645
	11th to 20th	Night	23% 10% 33% 27% 7% 1	399
	21st to end	Night	18% 6% 22% 39% 14% 1	433
June	1st to 10th	Night	18% 5% 29% 35% 13%	400
	11th to 20th	Night	15% 4 26% 41% 14%	400
	21st to end	Night	11% 4 25% 48% 13% 1	400
July	1st to 10th	Night	12% 3 22% 42% 20% 1	390
	11th to 20th	Night	10% 2 21% 43% 22% 1	388
	21st to end	Night	10% 2 24% 43% 19% 1	442
August	1st to 10th	Night	15% 1 13% 44% 25% 2	597
	11th to 20th	Night	15% 2 16% 39% 26% 2	786
	21st to end	Night	16% 2 16% 38% 25% 3	880
September	1st to 10th	Night	17% 3 24% 34% 19% 3	800
	11th to 20th	Night	15% 3 20% 37% 20% 5%	800
	21st to end	Night	24% 4 20% 32% 20% 1	800
October	1st to 10th	Night	21% 7% 23% 27% 19% 4	800
	11th to 20th	Night	18% 9% 26% 33% 13% 1	793
	21st to end	Night	22% 10% 31% 27% 10% 1	836
November	1st to 10th	Night	15% 9% 34% 30% 11% 1	780
	11th to 20th	Night	21% 16% 28% 27% 8% 1	800
	21st to end	Night	18% 15% 32% 28% 7% 1	800
December	1st to 10th	Night	26% 14% 33% 21% 6% 1	800
	11th to 20th	Night	22% 14% 34% 25% 5%	800
	21st to end	Night	23% 14% 31% 29% 3	880

Page generated at 2025-10-16 01:50:25. Version ac07

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