Fred G. Dale Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fred G. Dale Planetarium clear sky chart from 2005-11-06 to 2026-09-14.

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	11% 7% 3 7% 4 4 4 3 5% 7% 44%	149220
	Night	9% 7% 3 7% 4 4 4 3 5% 7% 47%	142140
Transparency	Day	38% 6% 13% 25% 15% 2	152830
	Night	36% 5% 13% 26% 17% 2	146601
Seeing	Day	15% 11% 32% 28% 12% 2	18485
	Night	13% 11% 36% 31% 9%	28247

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	9% 13% 5% 16% 8% 5% 3 3 4 5% 29%	4812
	11th to 20th	Day	4 9% 4 10% 7% 6% 7% 4 6% 7% 35%	5310
	21st to end	Day	4 9% 4 12% 6% 5% 6% 4 5% 6% 39%	5812
February	1st to 10th	Day	10% 11% 5% 11% 7% 6% 5% 4 5% 6% 31%	1890
	11th to 20th	Day	9% 10% 4 8% 5% 4 5% 3 4 6% 42%	1912
	21st to end	Day	10% 9% 4 8% 4 4 5% 3 5% 7% 41%	1730
March	1st to 10th	Day	11% 8% 3 7% 4 3 3 2 4 5% 49%	4864
	11th to 20th	Day	10% 8% 2 5% 4 3 4 3 4 7% 51%	5090
	21st to end	Day	20% 11% 4 11% 5% 4 5% 4 5% 7% 25%	5583
April	1st to 10th	Day	14% 11% 3 7% 4 3 3 3 6% 7% 38%	2344
	11th to 20th	Day	17% 9% 3 6% 4 3 3 4 5% 7% 40%	2702
	21st to end	Day	20% 9% 2 6% 4 4 4 3 5% 6% 37%	2730
May	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 6% 7% 46%	7617
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 5% 7% 48%	7627
	21st to end	Day	19% 8% 3 7% 4 4 5% 4 5% 7% 35%	8607
June	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 5% 9% 48%	2940
	11th to 20th	Day	10% 6% 2 6% 4 4 4 5% 6% 8% 46%	3143
	21st to end	Day	8% 5% 2 5% 5% 4 5% 4 6% 9% 49%	3150
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 44%	8355
	11th to 20th	Day	7% 4 2 4 3 3 3 3 4 7% 60%	7874
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 7% 10% 44%	8337
August	1st to 10th	Day	9% 4 2 6% 4 4 4 3 6% 7% 51%	6014
	11th to 20th	Day	17% 7% 3 7% 5% 4 4 4 6% 7% 35%	5948
	21st to end	Day	7% 5% 2 6% 4 4 4 3 5% 8% 52%	6593
September	1st to 10th	Day	12% 5% 2 5% 4 4 3 3 5% 7% 50%	2398
	11th to 20th	Day	8% 5% 2 5% 4 3 4 3 6% 6% 53%	2233
	21st to end	Day	10% 4 2 4 3 3 3 3 5% 7% 57%	2200
October	1st to 10th	Day	14% 6% 2 5% 4 3 3 3 5% 5% 51%	2080
	11th to 20th	Day	10% 7% 2 4 3 3 3 3 5% 5% 55%	2000
	21st to end	Day	15% 7% 3 6% 3 4 3 2 4 6% 48%	2135
November	1st to 10th	Day	9% 7% 2 6% 5% 3 4 4 5% 6% 50%	1853
	11th to 20th	Day	9% 7% 2 7% 6% 5% 3 3 5% 8% 47%	1841
	21st to end	Day	11% 9% 3 6% 4 4 3 3 5% 7% 45%	1680
December	1st to 10th	Day	5% 5% 2 8% 6% 6% 5% 3 5% 7% 48%	2690
	11th to 20th	Day	8% 9% 4 8% 5% 5% 5% 4 6% 8% 37%	3088
	21st to end	Day	14% 10% 4 13% 5% 4 4 4 5% 6% 33%	4038
January	1st to 10th	Night	9% 11% 3 12% 8% 7% 6% 4 5% 6% 29%	8073
	11th to 20th	Night	3 8% 3 10% 7% 6% 7% 4 8% 6% 38%	7680
	21st to end	Night	4 9% 5% 11% 4 4 4 4 5% 6% 44%	8402
February	1st to 10th	Night	11% 10% 4 10% 6% 5% 4 3 5% 6% 38%	2730
	11th to 20th	Night	8% 11% 4 7% 4 4 4 4 5% 7% 42%	2615
	21st to end	Night	8% 7% 3 6% 3 4 5% 4 5% 6% 48%	2076
March	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 51%	5217
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 7% 52%	5090
	21st to end	Night	15% 9% 3 7% 5% 5% 5% 4 5% 7% 36%	5588
April	1st to 10th	Night	14% 8% 2 7% 4 4 4 3 6% 6% 43%	2273
	11th to 20th	Night	12% 8% 3 6% 4 3 4 4 3 7% 46%	1869
	21st to end	Night	17% 7% 2 6% 3 3 5% 3 5% 6% 42%	1890
May	1st to 10th	Night	10% 4 2 5% 4 3 4 4 5% 7% 52%	5266
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 7% 53%	5265
	21st to end	Night	16% 7% 2 6% 4 4 4 3 5% 8% 41%	5559
June	1st to 10th	Night	7% 4 1 4 4 3 3 3 6% 9% 55%	1680
	11th to 20th	Night	7% 5% 2 4 3 3 3 3 5% 8% 57%	1470
	21st to end	Night	6% 4 1 5% 3 4 4 3 6% 8% 56%	1470
July	1st to 10th	Night	7% 5% 2 6% 5% 4 4 4 6% 9% 48%	4614
	11th to 20th	Night	7% 4 1 4 2 2 3 2 4 7% 63%	5096
	21st to end	Night	6% 5% 2 6% 4 4 4 4 6% 11% 48%	5770
August	1st to 10th	Night	6% 4 1 4 4 3 4 3 6% 9% 55%	4156
	11th to 20th	Night	13% 6% 2 5% 3 3 4 4 7% 9% 44%	4118
	21st to end	Night	5% 3 2 5% 4 4 4 4 6% 9% 53%	4555
September	1st to 10th	Night	8% 4 1 4 3 3 4 3 4 7% 58%	2218
	11th to 20th	Night	7% 3 2 4 3 3 4 3 5% 7% 58%	2233
	21st to end	Night	8% 5% 1 5% 3 3 3 3 3 6% 60%	2200
October	1st to 10th	Night	13% 5% 2 4 3 3 4 3 4 5% 53%	2298
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 5% 4 55%	2400
	21st to end	Night	14% 6% 2 5% 3 3 3 2 4 6% 53%	2702
November	1st to 10th	Night	8% 5% 2 5% 2 3 3 4 5% 7% 55%	2665
	11th to 20th	Night	9% 6% 3 6% 4 3 4 4 6% 6% 50%	2777
	21st to end	Night	9% 8% 3 6% 4 3 3 3 5% 7% 49%	2940
December	1st to 10th	Night	4 7% 3 8% 4 4 4 4 5% 6% 49%	4720
	11th to 20th	Night	9% 8% 3 9% 5% 4 5% 4 6% 7% 41%	5339
	21st to end	Night	13% 13% 4 12% 6% 4 4 3 4 5% 32%	7126

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	58% 4 12% 16% 9% 1	4813
	11th to 20th	Day	47% 4 13% 19% 15% 2	5310
	21st to end	Day	44% 6% 16% 21% 12% 1	5812
February	1st to 10th	Day	53% 7% 14% 16% 10% 1	1890
	11th to 20th	Day	44% 4 12% 23% 14% 3	1921
	21st to end	Day	42% 6% 12% 25% 13% 3	1730
March	1st to 10th	Day	37% 4 8% 29% 18% 3	4864
	11th to 20th	Day	34% 5% 8% 29% 21% 3	5090
	21st to end	Day	57% 7% 14% 16% 4 2	5585
April	1st to 10th	Day	43% 6% 13% 25% 11% 1	2344
	11th to 20th	Day	43% 6% 12% 23% 14% 3	2702
	21st to end	Day	47% 6% 11% 24% 13%	2730
May	1st to 10th	Day	35% 6% 9% 28% 21% 1	7655
	11th to 20th	Day	36% 6% 10% 24% 18% 7%	7631
	21st to end	Day	47% 6% 10% 22% 14% 1	8621
June	1st to 10th	Day	31% 8% 17% 29% 12% 2	2940
	11th to 20th	Day	34% 7% 13% 25% 18% 3	3143
	21st to end	Day	31% 8% 15% 30% 13% 3	3150
July	1st to 10th	Day	35% 6% 16% 30% 12% 1	8286
	11th to 20th	Day	26% 4 13% 29% 22% 6%	7801
	21st to end	Day	35% 6% 18% 26% 14% 1	8155
August	1st to 10th	Day	32% 6% 12% 33% 15% 2	6000
	11th to 20th	Day	47% 6% 15% 23% 9%	5987
	21st to end	Day	32% 6% 18% 26% 16% 3	6592
September	1st to 10th	Day	34% 6% 12% 28% 18% 3	2398
	11th to 20th	Day	30% 6% 12% 33% 17% 1	2233
	21st to end	Day	28% 5% 10% 31% 23% 3	2200
October	1st to 10th	Day	22% 3 6% 23% 41% 4	3392
	11th to 20th	Day	24% 4 17% 28% 21% 7%	3240
	21st to end	Day	35% 4 11% 28% 19% 3	3428
November	1st to 10th	Day	34% 6% 15% 24% 19% 2	1853
	11th to 20th	Day	36% 6% 15% 25% 16% 2	1841
	21st to end	Day	41% 4 13% 26% 15% 2	1680
December	1st to 10th	Day	36% 5% 18% 27% 13% 2	2690
	11th to 20th	Day	44% 5% 19% 22% 10%	3085
	21st to end	Day	52% 6% 11% 22% 8% 1	4038
January	1st to 10th	Night	54% 6% 12% 17% 10% 1	8074
	11th to 20th	Night	42% 5% 13% 22% 16% 1	7680
	21st to end	Night	41% 5% 13% 23% 17% 1	8402
February	1st to 10th	Night	47% 5% 13% 22% 12% 1	2730
	11th to 20th	Night	41% 6% 13% 22% 14% 4	2616
	21st to end	Night	36% 5% 13% 26% 17% 4	2076
March	1st to 10th	Night	34% 4 9% 29% 22% 2	5217
	11th to 20th	Night	32% 4 7% 28% 24% 5%	5090
	21st to end	Night	46% 9% 15% 22% 7% 1	5588
April	1st to 10th	Night	41% 7% 11% 25% 15% 1	2273
	11th to 20th	Night	40% 5% 9% 26% 17% 3	1869
	21st to end	Night	41% 8% 11% 23% 15% 2	1890
May	1st to 10th	Night	30% 5% 9% 28% 26% 2	5293
	11th to 20th	Night	31% 6% 10% 21% 24% 7%	5266
	21st to end	Night	42% 6% 12% 25% 13% 2	5570
June	1st to 10th	Night	25% 8% 15% 34% 15% 3	1680
	11th to 20th	Night	27% 6% 15% 26% 22% 5%	1470
	21st to end	Night	26% 6% 15% 31% 18% 3	1470
July	1st to 10th	Night	33% 4 17% 32% 11% 2	4576
	11th to 20th	Night	23% 4 12% 32% 24% 4	5049
	21st to end	Night	30% 6% 22% 27% 13% 2	5641
August	1st to 10th	Night	25% 6% 15% 33% 18% 3	4146
	11th to 20th	Night	36% 7% 19% 24% 12% 1	4145
	21st to end	Night	28% 6% 18% 29% 16% 4	4555
September	1st to 10th	Night	26% 6% 12% 29% 22% 5%	2218
	11th to 20th	Night	25% 6% 13% 33% 21% 2	2233
	21st to end	Night	27% 5% 9% 30% 26% 4	2200
October	1st to 10th	Night	21% 3 6% 18% 48% 4	3753
	11th to 20th	Night	25% 3 10% 30% 27% 5%	3899
	21st to end	Night	35% 3 10% 33% 18% 1	4363
November	1st to 10th	Night	28% 5% 16% 26% 21% 4	2665
	11th to 20th	Night	33% 5% 14% 26% 19% 4	2777
	21st to end	Night	35% 5% 13% 28% 17% 2	2940
December	1st to 10th	Night	35% 5% 15% 31% 14% 1	4720
	11th to 20th	Night	42% 6% 14% 28% 10%	5341
	21st to end	Night	55% 4 10% 22% 8%	7126

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	17% 14% 35% 25% 8% 1	330
	11th to 20th	Day	18% 17% 37% 22% 6% 1	330
	21st to end	Day	12% 13% 45% 24% 5% 1	363
February	1st to 10th	Day	17% 16% 45% 20% 2	330
	11th to 20th	Day	15% 14% 41% 24% 5% 1	330
	21st to end	Day	16% 21% 42% 17% 4	273
March	1st to 10th	Day	16% 17% 44% 18% 4 1	330
	11th to 20th	Day	24% 14% 35% 19% 8%	330
	21st to end	Day	28% 13% 37% 14% 6% 1	363
April	1st to 10th	Day	24% 17% 40% 16% 4	364
	11th to 20th	Day	21% 18% 33% 21% 6%	740
	21st to end	Day	25% 16% 29% 21% 8% 1	750
May	1st to 10th	Day	17% 14% 36% 22% 10% 1	753
	11th to 20th	Day	18% 14% 29% 23% 13% 3	775
	21st to end	Day	18% 10% 32% 27% 11% 2	844
June	1st to 10th	Day	9% 8% 27% 34% 18% 4	780
	11th to 20th	Day	11% 8% 36% 30% 12% 3	780
	21st to end	Day	9% 9% 35% 29% 15% 2	780
July	1st to 10th	Day	7% 4 20% 39% 22% 7%	771
	11th to 20th	Day	7% 4 25% 40% 19% 6%	768
	21st to end	Day	7% 4 25% 41% 20% 3	849
August	1st to 10th	Day	9% 4 24% 41% 18% 3	776
	11th to 20th	Day	12% 5% 25% 35% 20% 3	765
	21st to end	Day	9% 3 28% 38% 19% 3	858
September	1st to 10th	Day	14% 5% 28% 36% 14% 3	448
	11th to 20th	Day	8% 6% 39% 29% 15% 3	339
	21st to end	Day	13% 12% 30% 28% 13% 4	330
October	1st to 10th	Day	19% 11% 29% 25% 13% 3	330
	11th to 20th	Day	18% 13% 41% 18% 7% 2	330
	21st to end	Day	20% 18% 36% 20% 6% 1	363
November	1st to 10th	Day	18% 15% 34% 24% 7% 3	330
	11th to 20th	Day	14% 17% 43% 22% 5%	330
	21st to end	Day	23% 18% 27% 22% 8% 1	330
December	1st to 10th	Day	12% 15% 42% 24% 6% 1	330
	11th to 20th	Day	15% 18% 37% 25% 5%	330
	21st to end	Day	20% 16% 42% 17% 5%	363
January	1st to 10th	Night	13% 14% 39% 28% 6% 1	1050
	11th to 20th	Night	11% 18% 39% 27% 5% 1	1050
	21st to end	Night	14% 12% 44% 24% 6%	1155
February	1st to 10th	Night	16% 12% 42% 26% 4	1050
	11th to 20th	Night	13% 14% 43% 25% 5%	981
	21st to end	Night	12% 12% 45% 25% 6% 1	692
March	1st to 10th	Night	14% 12% 42% 26% 5%	719
	11th to 20th	Night	17% 12% 41% 25% 6%	630
	21st to end	Night	19% 13% 39% 23% 5%	693
April	1st to 10th	Night	19% 15% 39% 24% 3	630
	11th to 20th	Night	17% 15% 38% 24% 5%	623
	21st to end	Night	21% 12% 35% 25% 8%	630
May	1st to 10th	Night	15% 9% 35% 33% 8%	630
	11th to 20th	Night	15% 10% 32% 33% 10%	621
	21st to end	Night	19% 7% 31% 31% 11%	683
June	1st to 10th	Night	8% 7% 27% 43% 15% 1	630
	11th to 20th	Night	9% 7% 34% 39% 11%	630
	21st to end	Night	8% 5% 30% 43% 14%	630
July	1st to 10th	Night	6% 4 23% 41% 26%	615
	11th to 20th	Night	3 2 25% 49% 20%	612
	21st to end	Night	4 3 21% 52% 19%	690
August	1st to 10th	Night	6% 4 25% 45% 20%	627
	11th to 20th	Night	9% 5% 24% 42% 20% 1	624
	21st to end	Night	7% 3 28% 44% 17% 2	693
September	1st to 10th	Night	9% 5% 28% 41% 16% 1	630
	11th to 20th	Night	9% 5% 33% 38% 13% 2	609
	21st to end	Night	10% 8% 34% 33% 15% 1	600
October	1st to 10th	Night	17% 8% 33% 31% 10% 1	698
	11th to 20th	Night	15% 14% 37% 28% 5% 1	800
	21st to end	Night	18% 13% 33% 26% 10%	942
November	1st to 10th	Night	10% 11% 42% 30% 6% 1	1025
	11th to 20th	Night	12% 13% 42% 27% 5%	1050
	21st to end	Night	12% 13% 40% 26% 9%	1050
December	1st to 10th	Night	11% 13% 43% 27% 5%	1050
	11th to 20th	Night	16% 13% 38% 27% 6%	1050
	21st to end	Night	18% 13% 38% 25% 5%	1155

Page generated at 2026-09-11 03:11:54. Version ac07

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