CAS Starfield Observatory Dark Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CAS Starfield Observatory Dark Site clear sky chart from 2005-11-06 to 2024-11-01.

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	15% 9% 3 7% 5% 4 4 4 6% 7% 37%	101498
	Night	15% 10% 3 7% 4 4 4 3 5% 7% 38%	97414
Transparency	Day	45% 5% 10% 23% 14% 2	101307
	Night	45% 5% 10% 22% 15% 2	97275
Seeing	Day	20% 12% 27% 22% 13% 6%	15107
	Night	20% 10% 25% 27% 15% 2	24763

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	16% 18% 5% 9% 5% 4 4 4 5% 6% 24%	2836
	11th to 20th	Day	18% 14% 6% 13% 5% 5% 3 2 3 4 27%	2840
	21st to end	Day	28% 17% 7% 6% 4 3 3 2 4 4 21%	3124
February	1st to 10th	Day	20% 13% 4 9% 4 4 4 2 5% 5% 31%	1808
	11th to 20th	Day	18% 12% 4 8% 6% 5% 4 3 5% 5% 32%	1848
	21st to end	Day	20% 12% 4 8% 4 4 4 3 5% 5% 31%	1570
March	1st to 10th	Day	23% 15% 4 7% 4 3 3 3 4 5% 30%	3176
	11th to 20th	Day	14% 7% 3 6% 5% 3 4 4 5% 7% 42%	3480
	21st to end	Day	17% 9% 2 7% 5% 4 4 4 5% 6% 37%	3815
April	1st to 10th	Day	16% 10% 3 6% 4 5% 3 4 6% 7% 37%	2209
	11th to 20th	Day	18% 8% 3 6% 4 4 4 3 4 8% 39%	2312
	21st to end	Day	17% 8% 2 7% 5% 4 5% 4 6% 6% 36%	2470
May	1st to 10th	Day	21% 10% 3 7% 5% 4 4 4 6% 7% 29%	4082
	11th to 20th	Day	20% 8% 3 6% 4 4 4 4 5% 7% 35%	4081
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 8% 9% 34%	4476
June	1st to 10th	Day	12% 5% 2 7% 5% 4 6% 5% 6% 9% 40%	2470
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 6% 9% 42%	2510
	21st to end	Day	8% 6% 3 6% 6% 6% 6% 5% 6% 8% 39%	2660
July	1st to 10th	Day	12% 7% 3 6% 4 4 5% 4 6% 10% 39%	4381
	11th to 20th	Day	8% 4 2 5% 5% 5% 6% 5% 7% 10% 43%	4120
	21st to end	Day	13% 10% 4 9% 6% 6% 6% 5% 7% 8% 27%	4466
August	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 4 8% 11% 43%	4104
	11th to 20th	Day	7% 6% 2 7% 5% 5% 6% 4 8% 11% 39%	4044
	21st to end	Day	6% 4 2 4 4 4 5% 4 6% 9% 53%	4222
September	1st to 10th	Day	15% 5% 2 5% 4 4 5% 4 4 7% 46%	2142
	11th to 20th	Day	9% 6% 2 6% 4 4 4 4 5% 7% 49%	2090
	21st to end	Day	16% 6% 2 6% 3 3 4 3 5% 6% 45%	2090
October	1st to 10th	Day	12% 4 2 5% 4 4 3 3 5% 7% 52%	2090
	11th to 20th	Day	14% 6% 2 6% 4 4 3 3 5% 6% 47%	1920
	21st to end	Day	20% 8% 3 5% 3 3 4 3 4 6% 42%	1983
November	1st to 10th	Day	11% 7% 1 4 3 3 3 3 4 6% 53%	1672
	11th to 20th	Day	15% 11% 4 6% 5% 3 3 3 5% 6% 40%	1710
	21st to end	Day	18% 9% 4 7% 4 3 2 3 4 6% 39%	1710
December	1st to 10th	Day	23% 14% 3 8% 5% 4 4 3 4 4 28%	1710
	11th to 20th	Day	17% 13% 5% 8% 5% 5% 4 3 5% 4 32%	2153
	21st to end	Day	20% 18% 5% 10% 5% 6% 4 3 4 5% 20%	3124
January	1st to 10th	Night	15% 18% 6% 11% 6% 5% 4 4 4 5% 23%	4109
	11th to 20th	Night	18% 16% 6% 13% 5% 4 3 2 4 4 25%	4120
	21st to end	Night	26% 15% 5% 7% 4 3 3 2 3 5% 26%	4532
February	1st to 10th	Night	22% 14% 4 9% 4 3 3 3 5% 4 29%	2371
	11th to 20th	Night	19% 12% 4 8% 6% 4 3 3 4 5% 32%	2218
	21st to end	Night	24% 14% 4 7% 4 3 4 3 4 4 29%	1884
March	1st to 10th	Night	21% 13% 3 7% 4 4 4 3 4 5% 32%	3674
	11th to 20th	Night	15% 8% 2 6% 3 4 3 3 5% 7% 45%	3480
	21st to end	Night	15% 9% 3 7% 5% 5% 4 3 5% 6% 39%	3817
April	1st to 10th	Night	14% 8% 2 6% 6% 4 5% 5% 5% 8% 38%	1986
	11th to 20th	Night	15% 8% 2 6% 5% 4 4 4 5% 6% 42%	1815
	21st to end	Night	16% 7% 3 6% 4 4 4 3 6% 7% 38%	1710
May	1st to 10th	Night	20% 11% 3 8% 5% 4 4 4 6% 6% 27%	2813
	11th to 20th	Night	20% 6% 2 5% 3 3 5% 4 6% 9% 37%	2812
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 6% 10% 45%	3079
June	1st to 10th	Night	7% 5% 2 4 4 5% 4 5% 8% 10% 47%	1710
	11th to 20th	Night	7% 4 2 7% 3 4 5% 5% 8% 9% 46%	1661
	21st to end	Night	7% 5% 1 3 5% 5% 5% 5% 8% 10% 46%	1520
July	1st to 10th	Night	8% 6% 2 6% 4 4 4 4 6% 8% 47%	2579
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 7% 9% 49%	2840
	21st to end	Night	11% 9% 3 8% 5% 4 5% 4 6% 9% 35%	3087
August	1st to 10th	Night	8% 6% 3 6% 3 3 4 4 7% 10% 46%	2825
	11th to 20th	Night	7% 7% 3 8% 4 5% 4 5% 6% 9% 43%	2785
	21st to end	Night	4 3 1 4 3 3 4 3 5% 10% 59%	3431
September	1st to 10th	Night	12% 5% 2 4 4 3 4 3 5% 7% 51%	2019
	11th to 20th	Night	8% 6% 2 5% 3 3 3 3 6% 7% 53%	2090
	21st to end	Night	14% 6% 2 5% 3 4 4 3 4 6% 49%	2090
October	1st to 10th	Night	10% 5% 1 5% 3 3 3 3 5% 8% 55%	2090
	11th to 20th	Night	14% 6% 2 5% 3 4 4 4 5% 7% 46%	2258
	21st to end	Night	18% 9% 2 6% 4 3 3 3 4 6% 42%	2612
November	1st to 10th	Night	14% 6% 1 5% 4 4 3 3 4 5% 50%	2413
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 3 4 6% 41%	2470
	21st to end	Night	20% 10% 3 5% 5% 3 3 3 4 5% 37%	2470
December	1st to 10th	Night	23% 12% 4 6% 4 3 4 3 4 6% 29%	2470
	11th to 20th	Night	18% 12% 6% 9% 4 4 3 3 3 5% 33%	3042
	21st to end	Night	19% 20% 5% 10% 5% 5% 5% 3 4 5% 18%	4532

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	60% 4 11% 15% 9% 1	2837
	11th to 20th	Day	64% 2 6% 19% 7% 1	2840
	21st to end	Day	67% 2 8% 19% 4 1	3124
February	1st to 10th	Day	57% 3 8% 20% 10% 2	1808
	11th to 20th	Day	55% 3 8% 20% 11% 2	1858
	21st to end	Day	55% 4 9% 20% 10% 2	1570
March	1st to 10th	Day	57% 3 8% 18% 10% 4	3176
	11th to 20th	Day	41% 5% 11% 26% 17% 1	3480
	21st to end	Day	47% 5% 10% 25% 12% 1	3815
April	1st to 10th	Day	46% 5% 9% 24% 13% 2	2209
	11th to 20th	Day	45% 5% 8% 23% 16% 4	2312
	21st to end	Day	47% 6% 11% 23% 11% 2	2470
May	1st to 10th	Day	52% 7% 11% 20% 9% 1	4120
	11th to 20th	Day	48% 7% 11% 22% 13% 1	4085
	21st to end	Day	40% 9% 13% 19% 16% 2	4489
June	1st to 10th	Day	40% 8% 10% 25% 17% 1	2470
	11th to 20th	Day	38% 7% 14% 22% 18% 2	2510
	21st to end	Day	39% 8% 13% 21% 15% 4	2660
July	1st to 10th	Day	41% 6% 10% 22% 19% 1	4313
	11th to 20th	Day	33% 6% 11% 31% 16% 1	4051
	21st to end	Day	53% 7% 11% 16% 12% 2	4324
August	1st to 10th	Day	33% 8% 13% 34% 11% 1	4091
	11th to 20th	Day	36% 6% 11% 31% 15% 1	4083
	21st to end	Day	27% 7% 13% 30% 20% 3	4221
September	1st to 10th	Day	37% 6% 11% 21% 19% 6%	2142
	11th to 20th	Day	34% 5% 10% 23% 24% 3	2090
	21st to end	Day	39% 4 9% 25% 21% 2	2090
October	1st to 10th	Day	33% 5% 10% 24% 25% 3	2090
	11th to 20th	Day	38% 3 9% 26% 21% 3	1920
	21st to end	Day	45% 3 10% 23% 17% 2	1983
November	1st to 10th	Day	34% 4 8% 29% 22% 4	1672
	11th to 20th	Day	46% 3 7% 21% 19% 3	1710
	21st to end	Day	47% 3 9% 22% 17% 1	1710
December	1st to 10th	Day	60% 3 10% 17% 9% 1	1710
	11th to 20th	Day	56% 4 7% 19% 13% 1	2150
	21st to end	Day	66% 5% 8% 13% 7% 1	3124
January	1st to 10th	Night	62% 5% 7% 16% 10% 1	4109
	11th to 20th	Night	64% 3 9% 16% 8% 1	4120
	21st to end	Night	63% 3 8% 18% 8% 1	4532
February	1st to 10th	Night	59% 3 10% 19% 8% 1	2371
	11th to 20th	Night	56% 3 7% 21% 12% 1	2219
	21st to end	Night	59% 3 10% 15% 10% 3	1884
March	1st to 10th	Night	54% 4 6% 23% 11% 2	3674
	11th to 20th	Night	38% 4 11% 28% 18% 1	3480
	21st to end	Night	45% 6% 8% 26% 14% 1	3817
April	1st to 10th	Night	44% 5% 10% 23% 16% 2	1986
	11th to 20th	Night	42% 5% 9% 19% 19% 6%	1815
	21st to end	Night	45% 5% 11% 23% 13% 3	1710
May	1st to 10th	Night	54% 9% 9% 19% 7% 2	2840
	11th to 20th	Night	43% 8% 14% 24% 10% 1	2813
	21st to end	Night	32% 8% 13% 25% 18% 4	3087
June	1st to 10th	Night	30% 8% 14% 27% 18% 4	1710
	11th to 20th	Night	30% 8% 14% 23% 21% 3	1661
	21st to end	Night	31% 9% 15% 23% 18% 4	1520
July	1st to 10th	Night	35% 7% 12% 22% 18% 5%	2538
	11th to 20th	Night	30% 7% 12% 31% 18% 2	2786
	21st to end	Night	45% 6% 12% 21% 13% 3	2988
August	1st to 10th	Night	33% 6% 14% 32% 13% 2	2814
	11th to 20th	Night	36% 8% 9% 29% 18% 1	2812
	21st to end	Night	20% 6% 15% 33% 22% 4	3431
September	1st to 10th	Night	33% 5% 12% 24% 20% 7%	2019
	11th to 20th	Night	29% 5% 11% 24% 27% 4	2090
	21st to end	Night	37% 4 8% 22% 25% 3	2090
October	1st to 10th	Night	29% 4 10% 27% 25% 4	2090
	11th to 20th	Night	37% 4 9% 24% 22% 3	2258
	21st to end	Night	44% 3 7% 22% 19% 4	2612
November	1st to 10th	Night	36% 3 8% 24% 24% 4	2413
	11th to 20th	Night	45% 3 8% 19% 20% 4	2470
	21st to end	Night	49% 3 9% 23% 14% 2	2470
December	1st to 10th	Night	56% 4 10% 19% 10% 1	2470
	11th to 20th	Night	54% 3 9% 22% 12% 1	3044
	21st to end	Night	67% 4 8% 13% 6% 1	4532

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	29% 20% 34% 13% 4	270
	11th to 20th	Day	36% 24% 31% 8% 1	270
	21st to end	Day	36% 22% 29% 11% 2	297
February	1st to 10th	Day	31% 26% 30% 11% 1	270
	11th to 20th	Day	36% 27% 27% 8% 2	270
	21st to end	Day	37% 16% 40% 5% 2	225
March	1st to 10th	Day	33% 21% 34% 11% 2	300
	11th to 20th	Day	27% 23% 32% 14% 4	460
	21st to end	Day	31% 21% 28% 15% 5%	506
April	1st to 10th	Day	29% 17% 28% 17% 9% 1	460
	11th to 20th	Day	26% 19% 26% 20% 6% 3	452
	21st to end	Day	24% 17% 35% 16% 6% 2	460
May	1st to 10th	Day	24% 17% 31% 17% 8% 3	463
	11th to 20th	Day	21% 12% 30% 23% 9% 5%	487
	21st to end	Day	14% 10% 26% 25% 18% 7%	529
June	1st to 10th	Day	14% 9% 24% 29% 18% 7%	490
	11th to 20th	Day	12% 6% 24% 34% 16% 8%	530
	21st to end	Day	11% 7% 29% 27% 18% 7%	680
July	1st to 10th	Day	15% 6% 21% 21% 20% 17%	639
	11th to 20th	Day	10% 5% 24% 28% 20% 13%	484
	21st to end	Day	12% 3 20% 32% 20% 12%	532
August	1st to 10th	Day	14% 3 17% 30% 22% 12%	486
	11th to 20th	Day	11% 5% 19% 36% 19% 10%	479
	21st to end	Day	10% 4 16% 32% 25% 13%	539
September	1st to 10th	Day	15% 4 16% 30% 25% 10%	490
	11th to 20th	Day	13% 3 17% 28% 28% 12%	490
	21st to end	Day	21% 9% 21% 23% 20% 7%	490
October	1st to 10th	Day	12% 7% 26% 32% 18% 6%	490
	11th to 20th	Day	17% 16% 29% 21% 14% 3	490
	21st to end	Day	24% 11% 36% 22% 6% 1	432
November	1st to 10th	Day	17% 14% 34% 24% 8% 3	270
	11th to 20th	Day	24% 21% 26% 21% 5% 2	270
	21st to end	Day	26% 16% 38% 15% 5% 1	270
December	1st to 10th	Day	33% 17% 33% 11% 6%	270
	11th to 20th	Day	27% 20% 33% 17% 4	270
	21st to end	Day	32% 21% 35% 9% 3	297
January	1st to 10th	Night	26% 21% 30% 18% 5%	760
	11th to 20th	Night	33% 18% 31% 16% 2	760
	21st to end	Night	28% 18% 31% 21% 2	836
February	1st to 10th	Night	31% 15% 30% 21% 3	760
	11th to 20th	Night	29% 20% 31% 19% 1	743
	21st to end	Night	35% 15% 32% 16% 1	628
March	1st to 10th	Night	27% 16% 31% 22% 4	760
	11th to 20th	Night	26% 17% 30% 21% 6%	760
	21st to end	Night	24% 14% 31% 25% 6%	836
April	1st to 10th	Night	18% 15% 32% 28% 6%	656
	11th to 20th	Night	19% 13% 31% 25% 11% 1	563
	21st to end	Night	21% 8% 37% 25% 8% 1	570
May	1st to 10th	Night	20% 12% 29% 26% 11% 2	570
	11th to 20th	Night	20% 5% 26% 30% 18% 1	561
	21st to end	Night	11% 4 23% 32% 26% 5%	617
June	1st to 10th	Night	10% 2 21% 41% 24% 2	570
	11th to 20th	Night	8% 4 17% 42% 26% 3	570
	21st to end	Night	10% 2 24% 36% 24% 4	570
July	1st to 10th	Night	14% 2 16% 29% 30% 10%	555
	11th to 20th	Night	6% 3 15% 34% 38% 4	552
	21st to end	Night	11% 2 14% 37% 33% 4	624
August	1st to 10th	Night	10% 1 11% 34% 39% 4	567
	11th to 20th	Night	9% 2 15% 37% 32% 5%	564
	21st to end	Night	7% 1 14% 32% 38% 7%	627
September	1st to 10th	Night	13% 3 13% 34% 32% 4	689
	11th to 20th	Night	10% 4 17% 31% 33% 6%	760
	21st to end	Night	16% 6% 19% 32% 24% 2	760
October	1st to 10th	Night	13% 7% 19% 33% 23% 4	760
	11th to 20th	Night	19% 8% 26% 29% 16% 1	760
	21st to end	Night	23% 11% 29% 24% 11% 1	836
November	1st to 10th	Night	17% 10% 26% 28% 17% 2	743
	11th to 20th	Night	21% 14% 29% 26% 10% 1	760
	21st to end	Night	26% 12% 26% 28% 8% 1	760
December	1st to 10th	Night	30% 15% 27% 23% 4	760
	11th to 20th	Night	26% 14% 30% 24% 5% 1	760
	21st to end	Night	31% 17% 27% 21% 4	836

Page generated at 2024-10-29 11:20:51. Version ac07

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