Syzygy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Syzygy Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	10% 7% 3 7% 5% 5% 5% 5% 7% 8% 38%	131567
	Night	10% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	130278
Transparency	Day	40% 8% 13% 19% 15% 5%	135278
	Night	38% 8% 13% 20% 16% 5%	134637
Seeing	Day	20% 14% 24% 21% 14% 7%	16875
	Night	18% 12% 21% 28% 18% 3	27016

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	11% 7% 3 6% 3 4 4 3 6% 8% 45%	5258
	11th to 20th	Day	6% 5% 2 5% 4 4 5% 4 5% 5% 55%	5280
	21st to end	Day	11% 11% 5% 7% 4 4 4 3 5% 6% 41%	5780
February	1st to 10th	Day	12% 8% 3 7% 5% 4 5% 3 6% 7% 40%	1976
	11th to 20th	Day	14% 10% 4 8% 4 4 4 4 5% 6% 37%	2048
	21st to end	Day	13% 7% 4 9% 5% 5% 5% 4 5% 6% 39%	1730
March	1st to 10th	Day	14% 8% 3 8% 4 5% 4 3 5% 6% 38%	4944
	11th to 20th	Day	10% 7% 3 6% 3 4 4 3 5% 6% 49%	5090
	21st to end	Day	13% 8% 3 7% 4 4 4 4 5% 6% 43%	5586
April	1st to 10th	Day	10% 6% 2 6% 4 5% 6% 4 6% 9% 41%	2211
	11th to 20th	Day	13% 7% 3 6% 5% 3 4 3 6% 8% 42%	2393
	21st to end	Day	11% 7% 2 7% 4 4 5% 4 6% 7% 43%	2600
May	1st to 10th	Day	9% 8% 3 10% 6% 6% 6% 5% 7% 8% 30%	5862
	11th to 20th	Day	14% 8% 3 9% 6% 6% 7% 5% 7% 9% 25%	5861
	21st to end	Day	9% 7% 3 8% 6% 6% 6% 5% 7% 10% 35%	6433
June	1st to 10th	Day	9% 7% 2 7% 7% 7% 7% 5% 8% 9% 32%	2600
	11th to 20th	Day	6% 6% 3 8% 7% 7% 6% 6% 9% 10% 33%	2600
	21st to end	Day	7% 6% 2 7% 6% 6% 7% 6% 8% 10% 34%	2600
July	1st to 10th	Day	4 5% 3 8% 6% 7% 7% 5% 9% 10% 35%	5900
	11th to 20th	Day	6% 6% 3 9% 7% 7% 8% 6% 9% 11% 27%	5900
	21st to end	Day	9% 8% 4 10% 8% 8% 8% 6% 9% 9% 22%	6424
August	1st to 10th	Day	10% 8% 3 9% 7% 7% 7% 6% 9% 11% 23%	4234
	11th to 20th	Day	7% 5% 3 7% 6% 6% 8% 6% 9% 11% 31%	4174
	21st to end	Day	5% 5% 2 6% 5% 5% 6% 5% 6% 9% 45%	4330
September	1st to 10th	Day	9% 6% 3 8% 6% 6% 6% 5% 6% 8% 37%	2200
	11th to 20th	Day	13% 7% 2 7% 7% 6% 6% 5% 6% 10% 32%	2200
	21st to end	Day	12% 7% 2 7% 5% 6% 5% 6% 6% 7% 36%	2200
October	1st to 10th	Day	10% 7% 3 6% 4 4 4 4 5% 6% 48%	2163
	11th to 20th	Day	8% 6% 2 7% 3 3 4 3 4 6% 54%	2000
	21st to end	Day	11% 6% 3 5% 4 3 3 3 5% 5% 52%	2200
November	1st to 10th	Day	7% 7% 2 5% 4 3 3 3 4 7% 54%	1962
	11th to 20th	Day	9% 6% 2 6% 4 3 4 3 6% 6% 50%	1890
	21st to end	Day	12% 8% 3 6% 3 3 4 3 6% 6% 46%	1890
December	1st to 10th	Day	10% 7% 2 6% 4 5% 4 4 6% 6% 46%	3020
	11th to 20th	Day	11% 7% 4 7% 5% 4 5% 5% 7% 8% 38%	3463
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 5% 8% 38%	4565
January	1st to 10th	Night	12% 7% 3 8% 4 4 4 4 6% 7% 40%	7635
	11th to 20th	Night	8% 6% 2 7% 5% 4 4 3 6% 7% 48%	7680
	21st to end	Night	10% 10% 4 8% 4 4 4 3 5% 6% 42%	8401
February	1st to 10th	Night	11% 9% 3 7% 4 4 5% 4 6% 7% 40%	2641
	11th to 20th	Night	15% 9% 3 7% 5% 5% 4 4 5% 6% 37%	2458
	21st to end	Night	13% 8% 3 10% 6% 5% 5% 3 6% 6% 35%	2076
March	1st to 10th	Night	15% 11% 4 9% 4 4 4 4 5% 6% 34%	5128
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 48%	5090
	21st to end	Night	13% 8% 3 6% 5% 4 4 3 5% 7% 42%	5588
April	1st to 10th	Night	11% 8% 3 5% 5% 4 4 4 6% 8% 43%	2212
	11th to 20th	Night	9% 8% 2 7% 4 4 4 4 6% 7% 46%	1969
	21st to end	Night	9% 7% 3 7% 4 3 5% 3 5% 7% 46%	1800
May	1st to 10th	Night	10% 9% 3 9% 4 5% 5% 5% 8% 11% 32%	4033
	11th to 20th	Night	14% 11% 4 10% 5% 5% 5% 4 6% 8% 26%	4032
	21st to end	Night	9% 8% 4 8% 6% 5% 5% 4 7% 10% 34%	4421
June	1st to 10th	Night	7% 7% 3 7% 5% 6% 7% 5% 8% 13% 33%	1800
	11th to 20th	Night	5% 7% 2 5% 6% 5% 5% 5% 9% 13% 39%	1800
	21st to end	Night	5% 5% 2 6% 5% 6% 5% 5% 8% 10% 42%	1800
July	1st to 10th	Night	5% 6% 3 8% 6% 7% 6% 5% 8% 11% 34%	4058
	11th to 20th	Night	6% 5% 3 8% 7% 7% 7% 6% 10% 12% 30%	4057
	21st to end	Night	8% 9% 3 11% 7% 7% 7% 5% 7% 10% 26%	4427
August	1st to 10th	Night	7% 7% 2 8% 6% 7% 7% 5% 9% 11% 30%	2915
	11th to 20th	Night	7% 7% 3 10% 7% 6% 6% 6% 7% 10% 30%	2875
	21st to end	Night	5% 5% 2 6% 4 4 5% 5% 7% 10% 47%	3530
September	1st to 10th	Night	9% 5% 2 7% 6% 6% 5% 5% 7% 9% 39%	2200
	11th to 20th	Night	12% 8% 3 9% 5% 5% 5% 5% 6% 7% 36%	2200
	21st to end	Night	10% 7% 3 8% 5% 4 5% 3 6% 7% 43%	2200
October	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 5% 7% 46%	2235
	11th to 20th	Night	9% 6% 2 7% 4 4 4 3 6% 6% 48%	2400
	21st to end	Night	9% 7% 3 6% 4 2 4 3 5% 5% 52%	2640
November	1st to 10th	Night	10% 6% 2 5% 4 5% 4 4 6% 6% 48%	2552
	11th to 20th	Night	10% 7% 3 6% 4 5% 3 3 6% 7% 48%	2730
	21st to end	Night	11% 7% 2 7% 4 5% 4 4 5% 6% 44%	2730
December	1st to 10th	Night	11% 6% 3 6% 4 5% 5% 4 5% 7% 44%	4380
	11th to 20th	Night	13% 9% 4 10% 6% 5% 5% 3 6% 6% 33%	4952
	21st to end	Night	13% 9% 3 8% 5% 4 5% 4 6% 7% 36%	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	35% 3 11% 24% 24% 3	5259
	11th to 20th	Day	27% 6% 7% 13% 35% 13%	5280
	21st to end	Day	41% 5% 8% 20% 18% 9%	5780
February	1st to 10th	Day	41% 5% 12% 21% 18% 3	1976
	11th to 20th	Day	45% 5% 10% 18% 17% 4	2058
	21st to end	Day	44% 6% 10% 15% 18% 6%	1730
March	1st to 10th	Day	43% 6% 8% 14% 20% 9%	4944
	11th to 20th	Day	33% 5% 7% 17% 20% 18%	5090
	21st to end	Day	40% 5% 10% 26% 15% 4	5586
April	1st to 10th	Day	36% 8% 13% 21% 18% 4	2211
	11th to 20th	Day	39% 6% 9% 20% 20% 5%	2393
	21st to end	Day	38% 7% 13% 25% 14% 2	2600
May	1st to 10th	Day	48% 9% 14% 19% 9% 1	5897
	11th to 20th	Day	51% 10% 16% 15% 8%	5865
	21st to end	Day	41% 10% 13% 19% 15% 2	6447
June	1st to 10th	Day	44% 13% 18% 17% 8% 1	2600
	11th to 20th	Day	41% 12% 18% 17% 10% 2	2600
	21st to end	Day	41% 12% 16% 18% 11% 2	2600
July	1st to 10th	Day	40% 11% 18% 22% 9%	5835
	11th to 20th	Day	46% 12% 25% 13% 3	5831
	21st to end	Day	53% 10% 18% 12% 6%	6240
August	1st to 10th	Day	49% 10% 18% 19% 3	4221
	11th to 20th	Day	41% 9% 23% 16% 9% 2	4213
	21st to end	Day	34% 10% 13% 32% 10% 1	4330
September	1st to 10th	Day	41% 11% 13% 17% 15% 3	2200
	11th to 20th	Day	46% 10% 12% 18% 13% 2	2200
	21st to end	Day	44% 9% 14% 16% 14% 4	2200
October	1st to 10th	Day	28% 6% 7% 20% 27% 12%	3527
	11th to 20th	Day	22% 4 7% 21% 24% 21%	3240
	21st to end	Day	28% 3 8% 25% 21% 15%	3538
November	1st to 10th	Day	31% 4 11% 20% 26% 8%	1962
	11th to 20th	Day	33% 5% 8% 22% 25% 7%	1890
	21st to end	Day	36% 5% 8% 24% 22% 5%	1890
December	1st to 10th	Day	38% 5% 6% 19% 26% 6%	3020
	11th to 20th	Day	40% 5% 12% 19% 20% 5%	3460
	21st to end	Day	40% 5% 12% 25% 14% 3	4565
January	1st to 10th	Night	35% 4 14% 25% 17% 4	7636
	11th to 20th	Night	30% 5% 7% 18% 29% 10%	7680
	21st to end	Night	40% 3 9% 19% 20% 8%	8401
February	1st to 10th	Night	40% 6% 13% 20% 18% 3	2641
	11th to 20th	Night	44% 5% 12% 18% 16% 4	2459
	21st to end	Night	46% 7% 11% 15% 17% 5%	2076
March	1st to 10th	Night	44% 8% 9% 13% 19% 7%	5128
	11th to 20th	Night	30% 7% 8% 21% 20% 15%	5090
	21st to end	Night	42% 6% 12% 24% 12% 5%	5588
April	1st to 10th	Night	35% 7% 15% 21% 18% 3	2212
	11th to 20th	Night	34% 7% 11% 23% 20% 4	1969
	21st to end	Night	36% 9% 14% 24% 15% 2	1800
May	1st to 10th	Night	43% 11% 15% 24% 7% 1	4060
	11th to 20th	Night	49% 11% 17% 17% 6%	4033
	21st to end	Night	40% 11% 18% 21% 10% 1	4429
June	1st to 10th	Night	39% 14% 19% 19% 8% 1	1800
	11th to 20th	Night	33% 14% 23% 22% 9% 1	1800
	21st to end	Night	33% 13% 19% 20% 13% 2	1800
July	1st to 10th	Night	40% 11% 19% 23% 6%	4015
	11th to 20th	Night	41% 15% 28% 13% 3 1	4006
	21st to end	Night	49% 12% 20% 13% 5%	4297
August	1st to 10th	Night	43% 12% 26% 15% 4	2904
	11th to 20th	Night	44% 10% 23% 15% 8%	2902
	21st to end	Night	31% 9% 17% 34% 8% 1	3530
September	1st to 10th	Night	39% 11% 15% 19% 13% 3	2200
	11th to 20th	Night	45% 9% 14% 16% 15% 1	2200
	21st to end	Night	39% 10% 12% 21% 16% 3	2200
October	1st to 10th	Night	30% 5% 10% 20% 24% 10%	3651
	11th to 20th	Night	24% 5% 6% 20% 24% 20%	3899
	21st to end	Night	30% 4 6% 23% 22% 14%	4252
November	1st to 10th	Night	34% 7% 10% 20% 21% 7%	2552
	11th to 20th	Night	34% 5% 9% 20% 24% 8%	2730
	21st to end	Night	37% 6% 8% 23% 21% 6%	2730
December	1st to 10th	Night	36% 7% 11% 20% 23% 4	4380
	11th to 20th	Night	45% 8% 9% 18% 17% 3	4954
	21st to end	Night	42% 5% 14% 20% 17% 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	38% 26% 28% 6% 2	330
	11th to 20th	Day	40% 22% 23% 13% 2	330
	21st to end	Day	33% 31% 27% 8% 2	363
February	1st to 10th	Day	28% 28% 31% 12% 1	330
	11th to 20th	Day	45% 29% 20% 6%	330
	21st to end	Day	39% 29% 23% 8% 1	273
March	1st to 10th	Day	27% 21% 30% 17% 5%	487
	11th to 20th	Day	29% 24% 27% 15% 4 1	540
	21st to end	Day	27% 24% 27% 16% 5%	594
April	1st to 10th	Day	25% 21% 26% 19% 8% 1	504
	11th to 20th	Day	22% 22% 25% 20% 10% 3	492
	21st to end	Day	20% 16% 28% 20% 12% 4	500
May	1st to 10th	Day	17% 21% 26% 20% 10% 5%	503
	11th to 20th	Day	20% 13% 23% 23% 14% 7%	527
	21st to end	Day	16% 9% 25% 21% 17% 11%	573
June	1st to 10th	Day	13% 8% 26% 26% 18% 9%	530
	11th to 20th	Day	9% 6% 25% 24% 24% 13%	530
	21st to end	Day	10% 8% 24% 27% 18% 13%	530
July	1st to 10th	Day	8% 3 17% 26% 28% 17%	525
	11th to 20th	Day	7% 4 15% 28% 25% 21%	524
	21st to end	Day	9% 4 12% 29% 28% 17%	576
August	1st to 10th	Day	19% 6% 19% 22% 18% 17%	526
	11th to 20th	Day	10% 4 20% 27% 23% 16%	519
	21st to end	Day	10% 4 18% 25% 27% 16%	583
September	1st to 10th	Day	9% 5% 20% 29% 25% 13%	530
	11th to 20th	Day	15% 6% 18% 22% 27% 11%	530
	21st to end	Day	15% 8% 23% 28% 22% 5%	530
October	1st to 10th	Day	15% 10% 22% 28% 18% 6%	530
	11th to 20th	Day	15% 16% 24% 26% 15% 4	530
	21st to end	Day	22% 15% 24% 25% 12% 2	583
November	1st to 10th	Day	20% 14% 29% 25% 9% 2	440
	11th to 20th	Day	26% 22% 30% 17% 4 1	330
	21st to end	Day	27% 20% 31% 15% 5% 1	330
December	1st to 10th	Day	31% 28% 29% 11% 2	330
	11th to 20th	Day	27% 19% 32% 16% 5% 1	330
	21st to end	Day	29% 22% 32% 12% 4 1	363
January	1st to 10th	Night	28% 19% 25% 22% 5%	840
	11th to 20th	Night	24% 21% 27% 23% 4	840
	21st to end	Night	23% 20% 31% 24% 2	924
February	1st to 10th	Night	21% 18% 29% 28% 4	840
	11th to 20th	Night	29% 24% 26% 20% 1	823
	21st to end	Night	25% 23% 29% 20% 3	692
March	1st to 10th	Night	20% 19% 28% 29% 3	840
	11th to 20th	Night	23% 16% 31% 23% 7% 1	840
	21st to end	Night	21% 18% 30% 24% 7%	924
April	1st to 10th	Night	21% 20% 24% 27% 7%	805
	11th to 20th	Night	18% 17% 26% 26% 11% 1	693
	21st to end	Night	15% 13% 21% 35% 17% 1	600
May	1st to 10th	Night	15% 10% 24% 32% 18% 1	600
	11th to 20th	Night	16% 7% 21% 30% 22% 2	591
	21st to end	Night	12% 4 18% 26% 32% 7%	650
June	1st to 10th	Night	11% 5% 14% 34% 30% 6%	600
	11th to 20th	Night	8% 3 15% 32% 33% 10%	600
	21st to end	Night	6% 3 11% 33% 41% 6%	600
July	1st to 10th	Night	9% 2 10% 26% 46% 7%	585
	11th to 20th	Night	8% 3 9% 21% 47% 12%	582
	21st to end	Night	8% 2 7% 26% 46% 11%	657
August	1st to 10th	Night	11% 1 13% 25% 40% 10%	597
	11th to 20th	Night	12% 3 13% 25% 38% 9%	594
	21st to end	Night	11% 1 10% 28% 42% 9%	715
September	1st to 10th	Night	9% 3 15% 29% 37% 6%	800
	11th to 20th	Night	15% 4 16% 25% 33% 6%	800
	21st to end	Night	17% 6% 16% 33% 26% 4	800
October	1st to 10th	Night	15% 9% 19% 26% 24% 7%	800
	11th to 20th	Night	16% 12% 22% 31% 18% 2	800
	21st to end	Night	18% 13% 18% 34% 15% 2	880
November	1st to 10th	Night	17% 9% 24% 32% 15% 3	820
	11th to 20th	Night	20% 13% 23% 33% 11% 1	840
	21st to end	Night	22% 15% 21% 30% 11% 1	840
December	1st to 10th	Night	26% 18% 23% 27% 6%	840
	11th to 20th	Night	21% 15% 26% 31% 7% 1	840
	21st to end	Night	25% 16% 25% 29% 5%	924

Page generated at 2026-03-30 02:18:39. Version ac07

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