The Star Deck Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Star Deck Observatory clear sky chart from 2005-11-06 to 2025-01-22.

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% 9% 3 7% 5% 4 4 4 5% 7% 36%	104982
	Night	15% 10% 4 8% 4 4 4 3 5% 6% 37%	104398
Transparency	Day	46% 3 11% 24% 14% 2	108637
	Night	47% 3 10% 23% 15% 2	108883
Seeing	Day	17% 8% 25% 25% 17% 7%	15793
	Night	19% 7% 27% 31% 15% 2	25705

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	15% 16% 9% 11% 5% 4 4 3 5% 5% 22%	2607
	11th to 20th	Day	16% 13% 6% 14% 6% 7% 7% 5% 4 4 17%	2610
	21st to end	Day	26% 20% 9% 10% 4 3 2 2 3 5% 15%	2959
February	1st to 10th	Day	18% 14% 6% 11% 6% 4 4 3 4 5% 26%	1881
	11th to 20th	Day	15% 13% 4 11% 5% 5% 4 4 4 6% 29%	1848
	21st to end	Day	18% 15% 5% 10% 4 3 3 3 3 5% 31%	1570
March	1st to 10th	Day	21% 19% 5% 8% 4 3 3 3 3 5% 27%	3120
	11th to 20th	Day	14% 8% 3 7% 4 4 4 3 5% 8% 41%	3383
	21st to end	Day	16% 10% 4 9% 6% 5% 5% 3 5% 7% 30%	3939
April	1st to 10th	Day	15% 11% 3 8% 5% 5% 4 4 5% 7% 34%	2280
	11th to 20th	Day	17% 8% 4 6% 4 5% 4 4 5% 7% 37%	2254
	21st to end	Day	19% 8% 3 7% 4 4 4 4 5% 7% 34%	2459
May	1st to 10th	Day	19% 10% 3 8% 5% 5% 5% 4 6% 7% 28%	4082
	11th to 20th	Day	19% 7% 3 6% 5% 4 4 4 5% 7% 37%	4081
	21st to end	Day	9% 7% 3 6% 5% 5% 6% 5% 7% 10% 37%	4525
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 5% 8% 41%	2660
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 4 7% 10% 44%	2660
	21st to end	Day	9% 6% 2 6% 5% 4 6% 4 7% 9% 42%	2660
July	1st to 10th	Day	13% 6% 2 7% 4 4 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	7% 4 1 5% 4 4 4 4 8% 11% 46%	4440
	21st to end	Day	9% 7% 2 7% 5% 6% 6% 5% 7% 9% 35%	4629
August	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 45%	4104
	11th to 20th	Day	9% 5% 2 6% 4 4 6% 4 8% 12% 40%	3901
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 9% 53%	4180
September	1st to 10th	Day	11% 6% 2 6% 3 4 4 4 5% 7% 46%	2233
	11th to 20th	Day	10% 5% 2 5% 3 4 4 5% 6% 8% 48%	2090
	21st to end	Day	15% 6% 2 5% 4 4 3 3 5% 7% 47%	2090
October	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2090
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 6% 6% 44%	1948
	21st to end	Day	20% 9% 3 7% 4 5% 3 4 4 6% 35%	1881
November	1st to 10th	Day	9% 6% 2 5% 4 4 3 3 5% 7% 50%	1762
	11th to 20th	Day	16% 12% 4 7% 4 3 5% 3 6% 6% 35%	1800
	21st to end	Day	20% 11% 4 7% 5% 4 4 3 4 5% 33%	1800
December	1st to 10th	Day	16% 12% 4 7% 5% 3 3 2 3 5% 40%	2930
	11th to 20th	Day	17% 15% 6% 10% 5% 5% 5% 3 5% 5% 25%	3104
	21st to end	Day	26% 19% 8% 10% 4 5% 4 3 4 5% 14%	3982
January	1st to 10th	Night	18% 16% 7% 12% 5% 4 4 3 5% 5% 22%	4568
	11th to 20th	Night	15% 19% 7% 13% 6% 6% 5% 3 4 4 19%	4580
	21st to end	Night	26% 18% 7% 8% 4 3 3 3 3 4 21%	4710
February	1st to 10th	Night	21% 12% 5% 9% 6% 4 3 3 4 5% 28%	2295
	11th to 20th	Night	14% 12% 5% 11% 5% 3 4 3 5% 6% 32%	2218
	21st to end	Night	19% 14% 4 9% 4 5% 4 3 4 5% 30%	1884
March	1st to 10th	Night	20% 13% 3 7% 4 4 3 3 4 5% 34%	3733
	11th to 20th	Night	13% 7% 3 6% 4 3 4 3 5% 6% 46%	3576
	21st to end	Night	17% 8% 3 7% 5% 4 4 3 5% 6% 37%	3707
April	1st to 10th	Night	14% 10% 2 7% 4 4 5% 3 5% 6% 38%	1900
	11th to 20th	Night	16% 8% 3 6% 5% 4 3 3 5% 8% 40%	1875
	21st to end	Night	18% 7% 2 7% 4 6% 4 3 6% 6% 36%	1720
May	1st to 10th	Night	18% 11% 4 8% 5% 5% 5% 4 6% 7% 27%	2813
	11th to 20th	Night	19% 7% 2 7% 4 3 4 3 5% 7% 41%	2812
	21st to end	Night	10% 5% 2 7% 4 4 5% 4 6% 9% 44%	3028
June	1st to 10th	Night	9% 7% 2 5% 4 4 4 3 5% 7% 48%	1520
	11th to 20th	Night	9% 6% 2 6% 3 4 4 4 6% 10% 46%	1520
	21st to end	Night	7% 3 2 5% 6% 5% 4 6% 8% 11% 44%	1520
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 48%	2530
	11th to 20th	Night	6% 3 1 4 3 3 4 4 6% 8% 56%	2530
	21st to end	Night	11% 6% 2 6% 5% 5% 4 5% 6% 10% 39%	2929
August	1st to 10th	Night	8% 6% 2 6% 4 4 4 3 5% 8% 50%	2825
	11th to 20th	Night	6% 6% 3 7% 5% 5% 5% 4 6% 9% 44%	2911
	21st to end	Night	4 4 2 6% 4 4 5% 4 6% 8% 54%	3476
September	1st to 10th	Night	11% 6% 2 5% 4 3 4 4 5% 8% 49%	1926
	11th to 20th	Night	8% 4 1 5% 4 3 4 4 5% 7% 55%	2090
	21st to end	Night	12% 5% 2 6% 4 4 4 3 6% 6% 48%	2090
October	1st to 10th	Night	9% 4 2 5% 4 3 4 3 4 7% 55%	2090
	11th to 20th	Night	16% 7% 2 5% 4 3 4 2 5% 6% 46%	2228
	21st to end	Night	18% 8% 2 6% 4 4 4 4 4 6% 40%	2717
November	1st to 10th	Night	13% 8% 3 5% 4 3 3 3 5% 5% 48%	2536
	11th to 20th	Night	14% 10% 3 8% 4 4 4 2 5% 7% 39%	2600
	21st to end	Night	19% 13% 4 9% 5% 4 3 3 4 6% 31%	2600
December	1st to 10th	Night	14% 11% 4 8% 5% 3 3 3 4 5% 38%	4250
	11th to 20th	Night	19% 15% 6% 11% 5% 4 4 3 4 5% 26%	5095
	21st to end	Night	26% 19% 6% 10% 5% 4 4 3 4 4 16%	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	65% 1 6% 20% 7%	2608
	11th to 20th	Day	72% 2 10% 11% 5%	2610
	21st to end	Day	75% 2 6% 14% 3 1	2959
February	1st to 10th	Day	62% 2 8% 16% 11% 1	1881
	11th to 20th	Day	57% 3 9% 20% 10% 1	1858
	21st to end	Day	58% 1 9% 22% 8% 2	1570
March	1st to 10th	Day	62% 2 6% 18% 10% 1	3120
	11th to 20th	Day	44% 2 10% 29% 14% 1	3383
	21st to end	Day	55% 2 13% 23% 6% 1	3939
April	1st to 10th	Day	51% 3 11% 21% 14% 1	2280
	11th to 20th	Day	48% 3 11% 25% 11% 3	2254
	21st to end	Day	51% 3 11% 22% 11% 2	2459
May	1st to 10th	Day	56% 2 12% 23% 7%	4120
	11th to 20th	Day	48% 3 13% 25% 11% 1	4085
	21st to end	Day	42% 4 11% 24% 18% 2	4538
June	1st to 10th	Day	40% 5% 13% 25% 16% 3	2660
	11th to 20th	Day	34% 4 12% 27% 20% 2	2660
	21st to end	Day	38% 5% 16% 20% 16% 4	2660
July	1st to 10th	Day	42% 4 11% 23% 17% 2	4371
	11th to 20th	Day	30% 5% 15% 30% 19% 1	4365
	21st to end	Day	42% 7% 15% 18% 14% 3	4482
August	1st to 10th	Day	32% 6% 17% 30% 14% 1	4091
	11th to 20th	Day	34% 6% 13% 32% 15%	3940
	21st to end	Day	29% 4 13% 31% 18% 4	4179
September	1st to 10th	Day	37% 4 10% 26% 18% 5%	2233
	11th to 20th	Day	34% 4 10% 26% 22% 4	2090
	21st to end	Day	37% 3 9% 26% 24% 1	2090
October	1st to 10th	Day	24% 3 9% 32% 31% 2	3480
	11th to 20th	Day	33% 3 9% 28% 25% 2	3216
	21st to end	Day	42% 2 10% 24% 21% 1	3081
November	1st to 10th	Day	35% 2 11% 30% 18% 4	1762
	11th to 20th	Day	50% 2 9% 22% 14% 4	1800
	21st to end	Day	54% 2 9% 23% 11% 1	1800
December	1st to 10th	Day	50% 2 9% 28% 11%	2930
	11th to 20th	Day	62% 3 9% 18% 7%	3101
	21st to end	Day	74% 2 8% 11% 5%	3982
January	1st to 10th	Night	65% 2 9% 16% 8%	4568
	11th to 20th	Night	71% 1 8% 14% 6% 1	4580
	21st to end	Night	69% 1 7% 15% 7%	4710
February	1st to 10th	Night	60% 2 10% 17% 9% 1	2295
	11th to 20th	Night	54% 3 10% 20% 13% 1	2219
	21st to end	Night	58% 2 9% 18% 10% 2	1884
March	1st to 10th	Night	54% 2 7% 25% 11% 1	3733
	11th to 20th	Night	41% 2 10% 26% 19% 2	3576
	21st to end	Night	49% 2 10% 29% 8% 1	3707
April	1st to 10th	Night	47% 2 11% 24% 14% 2	1900
	11th to 20th	Night	45% 2 8% 22% 17% 5%	1875
	21st to end	Night	48% 3 9% 22% 13% 4	1720
May	1st to 10th	Night	56% 3 10% 22% 7% 1	2840
	11th to 20th	Night	45% 2 13% 28% 11% 1	2813
	21st to end	Night	37% 3 14% 26% 16% 3	3036
June	1st to 10th	Night	36% 3 11% 26% 21% 3	1520
	11th to 20th	Night	35% 3 11% 25% 22% 4	1520
	21st to end	Night	31% 8% 14% 26% 18% 4	1520
July	1st to 10th	Night	35% 5% 11% 27% 17% 5%	2490
	11th to 20th	Night	25% 4 15% 31% 23% 2	2482
	21st to end	Night	40% 5% 15% 22% 15% 4	2836
August	1st to 10th	Night	34% 4 13% 34% 14% 1	2814
	11th to 20th	Night	35% 5% 13% 28% 18% 1	2938
	21st to end	Night	28% 5% 13% 29% 22% 3	3476
September	1st to 10th	Night	34% 3 11% 25% 21% 7%	1926
	11th to 20th	Night	28% 3 9% 29% 26% 4	2090
	21st to end	Night	37% 3 9% 25% 23% 3	2090
October	1st to 10th	Night	21% 3 7% 27% 40% 2	3480
	11th to 20th	Night	29% 3 8% 26% 34% 2	3699
	21st to end	Night	35% 2 7% 27% 24% 4	4467
November	1st to 10th	Night	38% 2 9% 25% 22% 3	2536
	11th to 20th	Night	46% 2 8% 23% 17% 4	2600
	21st to end	Night	56% 2 9% 22% 10%	2600
December	1st to 10th	Night	48% 3 11% 26% 12%	4250
	11th to 20th	Night	62% 2 9% 17% 9% 1	5097
	21st to end	Night	74% 2 8% 11% 4	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	23% 16% 31% 23% 6% 1	300
	11th to 20th	Day	23% 17% 38% 17% 5% 1	300
	21st to end	Day	31% 14% 29% 22% 5%	300
February	1st to 10th	Day	28% 10% 36% 21% 4	270
	11th to 20th	Day	22% 18% 33% 20% 7%	270
	21st to end	Day	28% 9% 38% 21% 3	225
March	1st to 10th	Day	23% 16% 36% 21% 4	270
	11th to 20th	Day	26% 13% 34% 19% 9%	402
	21st to end	Day	29% 12% 32% 18% 7% 1	506
April	1st to 10th	Day	18% 13% 32% 25% 9% 2	460
	11th to 20th	Day	22% 12% 29% 24% 10% 3	452
	21st to end	Day	24% 11% 36% 19% 7% 2	460
May	1st to 10th	Day	23% 10% 29% 26% 10% 3	463
	11th to 20th	Day	21% 6% 27% 24% 15% 7%	487
	21st to end	Day	14% 4 24% 25% 21% 12%	552
June	1st to 10th	Day	13% 5% 23% 31% 22% 7%	680
	11th to 20th	Day	12% 4 20% 34% 23% 8%	680
	21st to end	Day	11% 5% 24% 29% 22% 9%	680
July	1st to 10th	Day	13% 2 13% 26% 26% 20%	671
	11th to 20th	Day	9% 3 16% 33% 27% 13%	668
	21st to end	Day	11% 1 13% 30% 30% 14%	630
August	1st to 10th	Day	10% 2 10% 26% 32% 19%	486
	11th to 20th	Day	14% 4 14% 28% 28% 13%	479
	21st to end	Day	8% 2 10% 28% 34% 17%	539
September	1st to 10th	Day	11% 3 14% 27% 32% 13%	490
	11th to 20th	Day	9% 3 12% 27% 31% 18%	490
	21st to end	Day	18% 6% 20% 22% 22% 11%	490
October	1st to 10th	Day	12% 6% 20% 34% 19% 9%	490
	11th to 20th	Day	13% 12% 34% 25% 13% 3	443
	21st to end	Day	27% 11% 37% 20% 5% 1	330
November	1st to 10th	Day	14% 14% 31% 25% 14% 3	300
	11th to 20th	Day	23% 15% 30% 20% 11% 2	300
	21st to end	Day	25% 9% 38% 23% 5% 1	300
December	1st to 10th	Day	20% 15% 41% 18% 5% 1	300
	11th to 20th	Day	22% 14% 35% 23% 4 2	300
	21st to end	Day	29% 17% 31% 18% 5% 1	330
January	1st to 10th	Night	23% 13% 33% 25% 5%	1000
	11th to 20th	Night	26% 11% 38% 21% 4	1000
	21st to end	Night	25% 11% 34% 27% 4	940
February	1st to 10th	Night	28% 10% 33% 26% 3	760
	11th to 20th	Night	21% 13% 37% 27% 3	743
	21st to end	Night	26% 11% 37% 24% 2	628
March	1st to 10th	Night	23% 12% 33% 26% 6%	760
	11th to 20th	Night	20% 11% 37% 25% 6% 1	760
	21st to end	Night	26% 7% 32% 29% 6% 1	777
April	1st to 10th	Night	18% 10% 36% 27% 8% 1	570
	11th to 20th	Night	18% 9% 34% 30% 9% 1	563
	21st to end	Night	22% 6% 39% 25% 8% 1	570
May	1st to 10th	Night	20% 7% 31% 29% 12%	570
	11th to 20th	Night	21% 3 21% 34% 18% 2	561
	21st to end	Night	11% 2 20% 39% 26% 2	617
June	1st to 10th	Night	11% 4 19% 47% 18% 1	570
	11th to 20th	Night	11% 4 17% 44% 22% 2	570
	21st to end	Night	7% 2 22% 38% 29% 2	570
July	1st to 10th	Night	11% 3 8% 36% 36% 7%	555
	11th to 20th	Night	8% 3 12% 38% 34% 5%	552
	21st to end	Night	10% 3 10% 42% 33% 3	624
August	1st to 10th	Night	7% 3 9% 41% 35% 5%	567
	11th to 20th	Night	7% 2 11% 43% 31% 6%	564
	21st to end	Night	7% 2 11% 36% 39% 5%	627
September	1st to 10th	Night	12% 2 11% 37% 32% 5%	596
	11th to 20th	Night	9% 3 13% 35% 35% 6%	760
	21st to end	Night	15% 4 19% 34% 25% 3	760
October	1st to 10th	Night	11% 3 24% 35% 22% 5%	760
	11th to 20th	Night	19% 8% 29% 29% 15% 1	760
	21st to end	Night	21% 8% 35% 27% 7% 1	836
November	1st to 10th	Night	17% 7% 32% 30% 13% 1	780
	11th to 20th	Night	19% 13% 33% 28% 7% 1	800
	21st to end	Night	26% 10% 30% 28% 7%	800
December	1st to 10th	Night	23% 9% 38% 24% 6%	800
	11th to 20th	Night	26% 8% 33% 25% 7% 1	935
	21st to end	Night	31% 12% 30% 22% 4	1100

Page generated at 2025-01-19 03:06:47. Version ac07

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