J. Stephen Schmidt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the J. Stephen Schmidt Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	14% 7% 2 6% 4 4 5% 4 6% 8% 40%	131124
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	130063
Transparency	Day	40% 6% 11% 20% 18% 5%	134911
	Night	39% 6% 10% 20% 20% 6%	134345
Seeing	Day	20% 11% 22% 23% 16% 8%	16546
	Night	18% 9% 22% 29% 19% 3	26648

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	21% 11% 3 5% 3 3 3 2 3 4 42%	5258
	11th to 20th	Day	11% 7% 3 5% 3 3 2 2 3 4 57%	5280
	21st to end	Day	24% 10% 3 6% 3 3 3 3 4 5% 37%	6001
February	1st to 10th	Day	17% 9% 2 5% 4 5% 4 4 5% 6% 39%	2100
	11th to 20th	Day	24% 6% 2 5% 3 3 3 4 5% 6% 39%	2048
	21st to end	Day	21% 8% 3 6% 4 4 4 2 4 5% 41%	1730
March	1st to 10th	Day	22% 9% 3 7% 4 4 4 3 5% 5% 34%	4628
	11th to 20th	Day	12% 6% 2 5% 3 4 4 3 4 7% 50%	4980
	21st to end	Day	16% 6% 2 7% 4 4 4 3 4 5% 44%	5465
April	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 6% 6% 43%	2272
	11th to 20th	Day	12% 6% 3 6% 5% 4 4 3 5% 6% 46%	2374
	21st to end	Day	11% 5% 1 5% 4 5% 4 4 6% 9% 46%	2560
May	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 9% 35%	5862
	11th to 20th	Day	18% 8% 3 8% 5% 6% 6% 4 7% 8% 28%	5861
	21st to end	Day	14% 7% 2 7% 4 5% 5% 4 6% 8% 38%	6433
June	1st to 10th	Day	10% 7% 2 7% 6% 5% 6% 5% 8% 10% 33%	2600
	11th to 20th	Day	5% 4 2 6% 6% 6% 6% 6% 9% 12% 39%	2600
	21st to end	Day	7% 4 2 6% 5% 5% 7% 6% 9% 10% 38%	2600
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 7% 6% 9% 12% 35%	5900
	11th to 20th	Day	6% 5% 2 6% 6% 6% 7% 6% 10% 13% 33%	5900
	21st to end	Day	9% 8% 3 9% 7% 7% 8% 5% 8% 10% 25%	6424
August	1st to 10th	Day	6% 6% 2 7% 5% 5% 6% 6% 8% 11% 38%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 11% 41%	3917
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 6% 8% 48%	4312
September	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 5% 9% 39%	2215
	11th to 20th	Day	13% 7% 2 6% 4 5% 5% 3 5% 8% 41%	2200
	21st to end	Day	13% 5% 2 6% 4 4 5% 3 5% 7% 46%	2200
October	1st to 10th	Day	11% 5% 2 5% 3 4 4 3 4 5% 55%	2200
	11th to 20th	Day	12% 5% 2 3 3 3 3 3 4 7% 57%	2090
	21st to end	Day	15% 5% 2 5% 3 2 3 2 4 5% 54%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 2 4 5% 55%	1852
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 5% 5% 53%	1890
	21st to end	Day	15% 6% 2 5% 5% 4 4 3 6% 6% 46%	1890
December	1st to 10th	Day	21% 6% 2 4 3 3 3 3 5% 6% 44%	3020
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 3 3 4 40%	3463
	21st to end	Day	21% 7% 3 5% 4 3 4 3 6% 6% 38%	4565
January	1st to 10th	Night	18% 7% 3 6% 3 3 3 3 5% 6% 43%	7635
	11th to 20th	Night	14% 8% 3 7% 3 3 3 2 4 5% 49%	7680
	21st to end	Night	22% 11% 3 6% 3 3 3 2 3 4 42%	8164
February	1st to 10th	Night	16% 7% 2 7% 4 3 3 3 5% 7% 43%	2520
	11th to 20th	Night	22% 7% 2 6% 4 4 3 3 4 4 41%	2458
	21st to end	Night	21% 7% 2 6% 4 4 4 3 5% 5% 39%	2076
March	1st to 10th	Night	22% 9% 3 7% 4 4 4 3 5% 6% 34%	5270
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 6% 7% 50%	4980
	21st to end	Night	15% 7% 2 6% 4 4 3 3 5% 5% 44%	5467
April	1st to 10th	Night	12% 4 2 5% 4 3 4 5% 6% 8% 47%	2144
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 6% 6% 50%	1975
	21st to end	Night	9% 6% 3 6% 5% 4 4 3 6% 7% 49%	1838
May	1st to 10th	Night	12% 7% 3 8% 5% 5% 5% 4 7% 9% 35%	4033
	11th to 20th	Night	20% 8% 3 7% 5% 5% 5% 4 6% 7% 29%	4032
	21st to end	Night	12% 6% 3 6% 4 4 5% 4 7% 9% 40%	4421
June	1st to 10th	Night	8% 6% 2 7% 6% 6% 7% 6% 8% 10% 36%	1800
	11th to 20th	Night	5% 5% 2 5% 5% 4 5% 6% 7% 13% 42%	1800
	21st to end	Night	6% 5% 2 6% 5% 5% 6% 4 9% 12% 42%	1800
July	1st to 10th	Night	8% 6% 3 8% 6% 6% 6% 5% 8% 11% 34%	4058
	11th to 20th	Night	5% 5% 2 8% 6% 6% 8% 5% 9% 12% 34%	4057
	21st to end	Night	10% 8% 3 10% 7% 5% 6% 5% 7% 9% 30%	4427
August	1st to 10th	Night	7% 5% 3 7% 5% 5% 6% 4 8% 13% 38%	2915
	11th to 20th	Night	9% 7% 2 7% 4 5% 5% 3 5% 8% 45%	3127
	21st to end	Night	8% 5% 2 5% 4 4 5% 3 6% 8% 51%	3586
September	1st to 10th	Night	8% 6% 3 7% 6% 5% 6% 4 6% 9% 40%	2163
	11th to 20th	Night	13% 5% 2 6% 5% 4 4 4 6% 8% 43%	2200
	21st to end	Night	12% 5% 2 5% 3 3 4 5% 5% 7% 47%	2200
October	1st to 10th	Night	12% 5% 1 5% 3 3 3 3 5% 6% 56%	2200
	11th to 20th	Night	9% 5% 2 5% 4 3 3 2 4 5% 58%	2309
	21st to end	Night	13% 5% 2 5% 3 2 3 3 4 5% 55%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 55%	2663
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 4 6% 53%	2730
	21st to end	Night	16% 6% 2 5% 4 3 4 4 5% 6% 45%	2730
December	1st to 10th	Night	19% 7% 2 5% 3 3 3 3 4 5% 47%	4380
	11th to 20th	Night	19% 8% 2 6% 3 3 4 2 4 5% 43%	4952
	21st to end	Night	21% 7% 3 6% 3 3 3 3 4 6% 40%	6633

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	48% 3 5% 22% 18% 4	5259
	11th to 20th	Day	33% 3 5% 16% 32% 11%	5280
	21st to end	Day	51% 3 4 16% 18% 8%	6001
February	1st to 10th	Day	46% 3 7% 19% 20% 4	2100
	11th to 20th	Day	47% 4 7% 19% 18% 6%	2058
	21st to end	Day	48% 3 7% 15% 21% 7%	1730
March	1st to 10th	Day	51% 4 7% 11% 19% 8%	4628
	11th to 20th	Day	35% 3 6% 14% 25% 16%	4980
	21st to end	Day	43% 4 7% 23% 20% 4	5465
April	1st to 10th	Day	40% 5% 9% 21% 19% 5%	2272
	11th to 20th	Day	38% 4 8% 18% 23% 8%	2374
	21st to end	Day	34% 6% 12% 26% 18% 3	2560
May	1st to 10th	Day	44% 8% 13% 19% 14% 2	5897
	11th to 20th	Day	52% 9% 14% 17% 8% 1	5865
	21st to end	Day	42% 8% 10% 21% 17% 2	6447
June	1st to 10th	Day	43% 9% 16% 21% 10% 2	2600
	11th to 20th	Day	34% 9% 19% 25% 11% 1	2600
	21st to end	Day	36% 12% 17% 21% 12% 3	2600
July	1st to 10th	Day	37% 11% 20% 24% 7% 1	5835
	11th to 20th	Day	36% 13% 28% 18% 4 1	5831
	21st to end	Day	50% 11% 18% 17% 4	6240
August	1st to 10th	Day	35% 9% 14% 31% 10%	4221
	11th to 20th	Day	34% 8% 20% 22% 13% 2	3955
	21st to end	Day	33% 8% 12% 24% 20% 3	4311
September	1st to 10th	Day	40% 8% 13% 17% 18% 4	2215
	11th to 20th	Day	41% 8% 12% 19% 19% 2	2200
	21st to end	Day	37% 6% 13% 17% 20% 6%	2200
October	1st to 10th	Day	26% 4 6% 14% 38% 12%	3590
	11th to 20th	Day	20% 3 6% 16% 29% 27%	3382
	21st to end	Day	29% 3 5% 23% 28% 12%	3538
November	1st to 10th	Day	34% 3 7% 17% 31% 9%	1852
	11th to 20th	Day	33% 4 8% 18% 27% 10%	1890
	21st to end	Day	39% 4 7% 18% 26% 7%	1890
December	1st to 10th	Day	42% 3 6% 17% 26% 6%	3020
	11th to 20th	Day	50% 3 7% 17% 19% 6%	3460
	21st to end	Day	45% 4 9% 26% 14% 2	4565
January	1st to 10th	Night	42% 4 9% 19% 21% 4	7636
	11th to 20th	Night	39% 3 5% 19% 27% 8%	7680
	21st to end	Night	48% 2 4 16% 21% 8%	8164
February	1st to 10th	Night	42% 4 8% 19% 21% 6%	2520
	11th to 20th	Night	47% 4 5% 18% 19% 6%	2459
	21st to end	Night	46% 4 7% 17% 18% 8%	2076
March	1st to 10th	Night	50% 4 8% 15% 18% 6%	5270
	11th to 20th	Night	31% 4 8% 15% 27% 15%	4980
	21st to end	Night	41% 4 6% 22% 19% 7%	5467
April	1st to 10th	Night	35% 5% 11% 19% 24% 6%	2144
	11th to 20th	Night	35% 4 7% 22% 22% 9%	1975
	21st to end	Night	35% 5% 11% 26% 19% 4	1838
May	1st to 10th	Night	43% 9% 16% 20% 11% 1	4060
	11th to 20th	Night	51% 7% 16% 17% 8%	4033
	21st to end	Night	38% 9% 12% 24% 16% 2	4429
June	1st to 10th	Night	39% 12% 15% 21% 11% 2	1800
	11th to 20th	Night	31% 10% 22% 24% 13% 1	1800
	21st to end	Night	33% 11% 17% 22% 13% 4	1800
July	1st to 10th	Night	41% 11% 17% 23% 7%	4015
	11th to 20th	Night	38% 15% 24% 19% 4 1	4006
	21st to end	Night	48% 11% 18% 17% 5% 1	4297
August	1st to 10th	Night	35% 10% 17% 30% 8%	2904
	11th to 20th	Night	36% 8% 16% 24% 15% 1	3155
	21st to end	Night	30% 8% 13% 22% 25% 3	3586
September	1st to 10th	Night	39% 9% 14% 17% 16% 5%	2163
	11th to 20th	Night	37% 8% 13% 21% 19% 2	2200
	21st to end	Night	34% 8% 11% 19% 19% 8%	2200
October	1st to 10th	Night	24% 5% 9% 15% 37% 11%	3590
	11th to 20th	Night	18% 2 5% 18% 33% 23%	3756
	21st to end	Night	25% 3 4 24% 28% 16%	4252
November	1st to 10th	Night	33% 4 8% 18% 26% 11%	2663
	11th to 20th	Night	33% 4 5% 19% 28% 11%	2730
	21st to end	Night	38% 4 7% 20% 24% 6%	2730
December	1st to 10th	Night	40% 3 5% 18% 27% 7%	4380
	11th to 20th	Night	44% 4 8% 16% 23% 5%	4954
	21st to end	Night	46% 4 7% 28% 14% 2	6633

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	36% 20% 32% 11% 2	330
	11th to 20th	Day	34% 25% 23% 14% 4	330
	21st to end	Day	34% 22% 31% 10% 2 1	363
February	1st to 10th	Day	27% 21% 38% 12% 3	330
	11th to 20th	Day	42% 25% 23% 10% 1	330
	21st to end	Day	36% 21% 33% 8% 3	273
March	1st to 10th	Day	33% 16% 31% 15% 5%	356
	11th to 20th	Day	29% 18% 26% 20% 6%	500
	21st to end	Day	25% 17% 35% 17% 6%	550
April	1st to 10th	Day	28% 11% 29% 22% 8% 2	500
	11th to 20th	Day	18% 15% 33% 22% 12% 1	492
	21st to end	Day	19% 11% 27% 25% 14% 3	500
May	1st to 10th	Day	20% 12% 28% 24% 12% 5%	503
	11th to 20th	Day	16% 10% 22% 28% 16% 9%	527
	21st to end	Day	17% 8% 17% 31% 18% 10%	573
June	1st to 10th	Day	13% 4 23% 28% 22% 10%	530
	11th to 20th	Day	8% 5% 18% 27% 24% 18%	530
	21st to end	Day	10% 4 14% 31% 25% 16%	530
July	1st to 10th	Day	13% 2 9% 28% 26% 22%	525
	11th to 20th	Day	8% 2 9% 21% 36% 23%	524
	21st to end	Day	9% 3 9% 27% 30% 23%	576
August	1st to 10th	Day	13% 6% 12% 26% 24% 19%	526
	11th to 20th	Day	13% 4 11% 25% 30% 16%	519
	21st to end	Day	15% 3 11% 23% 29% 19%	583
September	1st to 10th	Day	9% 7% 16% 30% 25% 13%	530
	11th to 20th	Day	15% 6% 13% 25% 29% 12%	530
	21st to end	Day	14% 5% 19% 27% 26% 9%	530
October	1st to 10th	Day	15% 6% 21% 34% 16% 7%	530
	11th to 20th	Day	15% 10% 21% 29% 20% 4	530
	21st to end	Day	20% 9% 26% 25% 15% 3	583
November	1st to 10th	Day	20% 12% 28% 25% 10% 4	330
	11th to 20th	Day	20% 18% 34% 19% 7% 1	330
	21st to end	Day	21% 16% 36% 20% 7% 1	330
December	1st to 10th	Day	34% 22% 26% 15% 3 1	330
	11th to 20th	Day	29% 15% 32% 21% 3	330
	21st to end	Day	33% 18% 28% 16% 4	363
January	1st to 10th	Night	28% 17% 28% 23% 5%	840
	11th to 20th	Night	25% 16% 29% 23% 6%	840
	21st to end	Night	24% 16% 33% 25% 3	924
February	1st to 10th	Night	22% 15% 30% 28% 6%	840
	11th to 20th	Night	29% 17% 30% 22% 2	823
	21st to end	Night	27% 15% 34% 22% 2	692
March	1st to 10th	Night	21% 17% 29% 28% 5%	809
	11th to 20th	Night	19% 15% 32% 25% 8% 1	800
	21st to end	Night	21% 14% 29% 29% 7%	880
April	1st to 10th	Night	16% 14% 29% 29% 11%	744
	11th to 20th	Night	17% 11% 28% 31% 13% 1	593
	21st to end	Night	12% 9% 25% 36% 18% 1	600
May	1st to 10th	Night	11% 9% 23% 34% 22% 1	600
	11th to 20th	Night	17% 5% 20% 31% 22% 4	591
	21st to end	Night	13% 4 14% 33% 31% 5%	650
June	1st to 10th	Night	10% 2 13% 38% 34% 5%	600
	11th to 20th	Night	7% 3 13% 32% 34% 13%	600
	21st to end	Night	6% 1 9% 37% 40% 8%	600
July	1st to 10th	Night	10% 2 8% 23% 44% 13%	585
	11th to 20th	Night	9% 2 7% 23% 45% 14%	582
	21st to end	Night	9% 2 7% 25% 45% 12%	657
August	1st to 10th	Night	10% 3 8% 28% 42% 10%	597
	11th to 20th	Night	12% 3 10% 24% 41% 10%	594
	21st to end	Night	14% 3 10% 26% 37% 10%	660
September	1st to 10th	Night	11% 4 11% 30% 35% 9%	763
	11th to 20th	Night	15% 3 14% 29% 36% 3	800
	21st to end	Night	14% 2 16% 35% 30% 2	800
October	1st to 10th	Night	14% 5% 21% 33% 23% 6%	800
	11th to 20th	Night	12% 9% 24% 32% 21% 2	800
	21st to end	Night	17% 9% 21% 35% 16% 2	880
November	1st to 10th	Night	16% 9% 23% 33% 18% 1	820
	11th to 20th	Night	16% 13% 26% 31% 13% 1	840
	21st to end	Night	21% 13% 25% 27% 12% 1	840
December	1st to 10th	Night	24% 15% 28% 25% 7%	840
	11th to 20th	Night	25% 13% 26% 28% 8% 1	840
	21st to end	Night	31% 11% 26% 26% 5%	924

Page generated at 2026-02-28 01:40:23. Version ac07

Copyright © 2026 Attilla Danko.