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 2025-05-11.

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%	114169
	Night	9% 7% 3 7% 4 4 4 3 5% 7% 46%	114895
Transparency	Day	38% 6% 13% 25% 15% 2	117820
	Night	36% 6% 12% 26% 18% 2	119377
Seeing	Day	14% 11% 32% 28% 12% 2	16492
	Night	13% 10% 36% 31% 9% 1	26514

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% 17% 8% 6% 4 3 4 5% 25%	3702
	11th to 20th	Day	6% 10% 4 10% 6% 5% 7% 4 7% 6% 36%	4080
	21st to end	Day	5% 9% 4 12% 5% 4 5% 3 5% 6% 42%	4488
February	1st to 10th	Day	10% 11% 4 11% 7% 7% 5% 5% 4 5% 31%	1800
	11th to 20th	Day	9% 11% 5% 8% 5% 4 5% 3 4 6% 41%	1819
	21st to end	Day	10% 9% 4 9% 4 4 5% 3 5% 7% 40%	1650
March	1st to 10th	Day	11% 8% 3 7% 4 3 3 3 4 5% 49%	4758
	11th to 20th	Day	10% 8% 2 5% 3 3 4 3 4 7% 51%	4980
	21st to end	Day	20% 11% 4 11% 5% 4 5% 3 5% 7% 25%	5462
April	1st to 10th	Day	13% 10% 3 7% 4 3 3 3 7% 8% 39%	2233
	11th to 20th	Day	17% 9% 3 6% 4 3 3 3 4 7% 40%	2572
	21st to end	Day	20% 9% 2 6% 4 4 4 3 5% 6% 37%	2600
May	1st to 10th	Day	15% 9% 3 7% 5% 5% 4 4 6% 7% 36%	4212
	11th to 20th	Day	11% 7% 2 6% 4 4 4 4 5% 7% 48%	4083
	21st to end	Day	15% 7% 2 6% 4 4 4 4 5% 7% 41%	4594
June	1st to 10th	Day	8% 5% 1 6% 4 4 4 5% 5% 9% 49%	2660
	11th to 20th	Day	11% 6% 2 6% 4 4 4 5% 6% 8% 45%	2843
	21st to end	Day	7% 5% 2 4 5% 3 4 4 6% 9% 50%	2850
July	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 7% 10% 44%	4479
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 5% 7% 54%	4223
	21st to end	Day	4 4 2 4 4 4 4 4 7% 10% 53%	4465
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 7% 7% 47%	4104
	11th to 20th	Day	17% 8% 2 6% 4 3 4 4 5% 7% 40%	4041
	21st to end	Day	7% 5% 2 6% 3 4 4 3 5% 8% 53%	4532
September	1st to 10th	Day	13% 5% 2 5% 3 4 3 3 5% 6% 51%	2170
	11th to 20th	Day	8% 5% 2 4 4 3 4 3 5% 6% 55%	2090
	21st to end	Day	10% 4 2 4 3 3 3 3 5% 7% 57%	2090
October	1st to 10th	Day	14% 6% 2 5% 4 3 3 3 5% 5% 50%	1976
	11th to 20th	Day	10% 7% 2 4 3 2 3 3 5% 5% 56%	1900
	21st to end	Day	15% 7% 3 5% 4 3 3 2 4 6% 49%	2028
November	1st to 10th	Day	9% 7% 2 5% 5% 4 4 4 5% 6% 49%	1763
	11th to 20th	Day	9% 7% 2 7% 6% 5% 3 3 5% 8% 47%	1754
	21st to end	Day	11% 9% 3 6% 4 4 3 3 5% 7% 46%	1600
December	1st to 10th	Day	5% 5% 3 8% 6% 6% 4 3 5% 7% 49%	2610
	11th to 20th	Day	7% 9% 4 8% 5% 5% 5% 4 6% 8% 37%	3008
	21st to end	Day	14% 10% 4 14% 5% 4 4 4 5% 6% 32%	3950
January	1st to 10th	Night	10% 12% 4 14% 9% 7% 6% 4 5% 6% 24%	6225
	11th to 20th	Night	4 9% 3 9% 6% 5% 8% 4 8% 6% 38%	5900
	21st to end	Night	5% 8% 5% 11% 4 4 4 4 5% 6% 45%	6490
February	1st to 10th	Night	11% 10% 4 10% 6% 5% 4 3 5% 6% 39%	2600
	11th to 20th	Night	8% 11% 4 7% 5% 4 4 4 5% 6% 41%	2488
	21st to end	Night	9% 7% 3 7% 3 4 5% 3 5% 6% 47%	1980
March	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 51%	5103
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 7% 52%	4980
	21st to end	Night	15% 9% 3 7% 5% 5% 5% 4 5% 7% 35%	5467
April	1st to 10th	Night	13% 8% 2 7% 4 4 4 3 6% 6% 43%	2164
	11th to 20th	Night	12% 8% 3 6% 4 3 4 4 3 7% 45%	1779
	21st to end	Night	17% 7% 2 6% 3 3 5% 3 5% 6% 42%	1800
May	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 5% 6% 47%	2913
	11th to 20th	Night	9% 5% 2 5% 3 4 4 4 6% 8% 50%	2828
	21st to end	Night	15% 6% 2 5% 3 4 4 3 5% 8% 44%	2968
June	1st to 10th	Night	7% 3 1 4 4 3 3 3 7% 9% 55%	1520
	11th to 20th	Night	8% 5% 2 4 3 3 3 3 5% 8% 55%	1330
	21st to end	Night	4 4 1 5% 3 4 3 3 6% 8% 59%	1330
July	1st to 10th	Night	7% 5% 2 5% 4 4 3 3 5% 7% 55%	2472
	11th to 20th	Night	5% 3 1 4 3 3 4 3 5% 8% 63%	2743
	21st to end	Night	2 3 1 4 3 3 4 4 7% 11% 58%	3098
August	1st to 10th	Night	6% 4 1 4 4 3 5% 4 6% 10% 51%	2836
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 6% 8% 49%	2798
	21st to end	Night	5% 4 2 5% 4 5% 4 4 5% 8% 55%	3135
September	1st to 10th	Night	8% 4 1 4 3 3 4 3 4 7% 59%	2007
	11th to 20th	Night	7% 4 2 4 2 3 4 3 5% 7% 59%	2090
	21st to end	Night	9% 5% 1 4 3 3 3 3 3 6% 61%	2090
October	1st to 10th	Night	14% 5% 2 4 3 3 4 3 4 5% 52%	2183
	11th to 20th	Night	12% 6% 2 4 3 3 3 2 4 4 56%	2280
	21st to end	Night	14% 6% 2 5% 3 3 3 2 4 6% 53%	2567
November	1st to 10th	Night	8% 5% 2 5% 3 3 3 4 5% 7% 55%	2535
	11th to 20th	Night	9% 6% 2 7% 4 3 3 3 6% 6% 50%	2645
	21st to end	Night	8% 8% 3 6% 4 3 3 3 5% 7% 49%	2800
December	1st to 10th	Night	4 7% 3 8% 4 4 4 4 5% 6% 50%	4580
	11th to 20th	Night	9% 8% 3 9% 5% 4 5% 4 6% 7% 41%	5199
	21st to end	Night	13% 13% 4 12% 6% 4 4 3 4 5% 32%	6972

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	61% 5% 12% 17% 5%	3703
	11th to 20th	Day	47% 4 14% 19% 15% 2	4080
	21st to end	Day	42% 6% 15% 23% 12% 2	4488
February	1st to 10th	Day	53% 7% 13% 16% 10% 1	1800
	11th to 20th	Day	45% 4 13% 22% 14% 3	1828
	21st to end	Day	44% 6% 11% 24% 12% 3	1650
March	1st to 10th	Day	37% 5% 8% 29% 18% 3	4758
	11th to 20th	Day	33% 5% 8% 29% 21% 3	4980
	21st to end	Day	57% 8% 14% 16% 4 2	5464
April	1st to 10th	Day	42% 7% 13% 25% 12% 2	2233
	11th to 20th	Day	43% 6% 12% 22% 14% 3	2572
	21st to end	Day	47% 6% 11% 24% 12%	2600
May	1st to 10th	Day	44% 8% 12% 20% 15% 1	4250
	11th to 20th	Day	34% 8% 13% 27% 17% 2	4087
	21st to end	Day	41% 9% 12% 24% 14% 1	4607
June	1st to 10th	Day	30% 9% 17% 30% 12% 2	2660
	11th to 20th	Day	35% 8% 13% 25% 17% 3	2843
	21st to end	Day	29% 8% 16% 32% 12% 4	2850
July	1st to 10th	Day	36% 7% 15% 29% 13% 1	4410
	11th to 20th	Day	29% 6% 13% 32% 18% 3	4150
	21st to end	Day	24% 7% 17% 26% 22% 3	4325
August	1st to 10th	Day	33% 8% 13% 29% 15% 2	4090
	11th to 20th	Day	44% 5% 12% 27% 12% 1	4080
	21st to end	Day	30% 7% 15% 24% 20% 4	4531
September	1st to 10th	Day	34% 6% 11% 28% 19% 3	2170
	11th to 20th	Day	29% 6% 12% 33% 18% 1	2090
	21st to end	Day	28% 5% 10% 31% 23% 2	2090
October	1st to 10th	Day	22% 3 6% 23% 41% 4	3288
	11th to 20th	Day	23% 4 17% 29% 21% 7%	3140
	21st to end	Day	35% 4 11% 28% 19% 3	3321
November	1st to 10th	Day	34% 6% 15% 24% 19% 2	1763
	11th to 20th	Day	37% 6% 14% 25% 16% 2	1754
	21st to end	Day	40% 4 14% 26% 15% 2	1600
December	1st to 10th	Day	35% 5% 18% 28% 14% 2	2610
	11th to 20th	Day	44% 5% 19% 22% 9%	3005
	21st to end	Day	52% 6% 11% 22% 8% 1	3950
January	1st to 10th	Night	59% 7% 10% 15% 10%	6225
	11th to 20th	Night	43% 5% 14% 21% 16% 1	5900
	21st to end	Night	40% 5% 12% 24% 17% 1	6490
February	1st to 10th	Night	47% 5% 13% 22% 12% 2	2600
	11th to 20th	Night	42% 6% 14% 21% 14% 4	2489
	21st to end	Night	37% 5% 13% 26% 16% 4	1980
March	1st to 10th	Night	34% 4 8% 29% 23% 2	5103
	11th to 20th	Night	32% 4 7% 28% 24% 5%	4980
	21st to end	Night	46% 9% 15% 22% 6% 1	5467
April	1st to 10th	Night	40% 7% 11% 25% 16% 1	2164
	11th to 20th	Night	41% 5% 9% 26% 16% 3	1779
	21st to end	Night	41% 8% 11% 23% 15% 2	1800
May	1st to 10th	Night	36% 7% 12% 22% 21% 2	2940
	11th to 20th	Night	29% 9% 13% 25% 21% 3	2829
	21st to end	Night	37% 8% 13% 24% 16% 3	2976
June	1st to 10th	Night	24% 9% 14% 34% 15% 3	1520
	11th to 20th	Night	28% 6% 15% 26% 21% 4	1330
	21st to end	Night	23% 7% 16% 33% 18% 3	1330
July	1st to 10th	Night	30% 4 17% 34% 13% 3	2432
	11th to 20th	Night	21% 5% 14% 36% 20% 4	2693
	21st to end	Night	19% 8% 18% 30% 22% 3	2999
August	1st to 10th	Night	27% 7% 15% 32% 16% 2	2826
	11th to 20th	Night	34% 6% 15% 27% 17% 2	2825
	21st to end	Night	28% 6% 15% 24% 21% 6%	3135
September	1st to 10th	Night	25% 6% 10% 30% 23% 5%	2007
	11th to 20th	Night	24% 6% 12% 33% 22% 3	2090
	21st to end	Night	26% 5% 9% 31% 26% 4	2090
October	1st to 10th	Night	21% 3 6% 17% 49% 4	3638
	11th to 20th	Night	24% 3 10% 30% 28% 5%	3779
	21st to end	Night	34% 3 10% 33% 18% 1	4228
November	1st to 10th	Night	28% 5% 16% 25% 21% 4	2535
	11th to 20th	Night	33% 5% 13% 25% 19% 4	2645
	21st to end	Night	34% 6% 13% 28% 18% 2	2800
December	1st to 10th	Night	35% 5% 15% 31% 14% 1	4580
	11th to 20th	Night	42% 6% 14% 28% 9%	5201
	21st to end	Night	55% 4 10% 22% 8%	6972

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% 24% 9% 1	300
	11th to 20th	Day	17% 16% 36% 24% 6% 1	300
	21st to end	Day	12% 12% 45% 25% 5% 1	330
February	1st to 10th	Day	18% 14% 45% 21% 2	300
	11th to 20th	Day	15% 14% 42% 23% 6% 1	300
	21st to end	Day	17% 19% 41% 18% 5%	249
March	1st to 10th	Day	15% 17% 44% 18% 4 1	300
	11th to 20th	Day	24% 12% 38% 19% 8%	300
	21st to end	Day	29% 13% 36% 15% 6% 1	330
April	1st to 10th	Day	22% 17% 40% 16% 4	333
	11th to 20th	Day	22% 18% 33% 22% 6%	690
	21st to end	Day	25% 17% 28% 21% 8% 1	700
May	1st to 10th	Day	18% 14% 34% 22% 10% 2	703
	11th to 20th	Day	17% 13% 30% 24% 13% 3	676
	21st to end	Day	17% 10% 32% 28% 10% 3	734
June	1st to 10th	Day	7% 8% 27% 35% 19% 4	680
	11th to 20th	Day	12% 6% 36% 31% 12% 3	680
	21st to end	Day	9% 9% 33% 30% 17% 2	680
July	1st to 10th	Day	7% 4 19% 40% 23% 7%	671
	11th to 20th	Day	7% 4 26% 40% 19% 5%	668
	21st to end	Day	5% 4 24% 42% 21% 3	739
August	1st to 10th	Day	8% 4 23% 41% 20% 3	676
	11th to 20th	Day	12% 5% 25% 35% 20% 3	665
	21st to end	Day	8% 3 28% 38% 19% 3	748
September	1st to 10th	Day	15% 4 27% 37% 13% 3	380
	11th to 20th	Day	7% 5% 38% 31% 16% 3	300
	21st to end	Day	14% 12% 32% 28% 11% 3	300
October	1st to 10th	Day	20% 11% 28% 25% 13% 4	300
	11th to 20th	Day	16% 14% 40% 20% 8% 2	300
	21st to end	Day	18% 17% 38% 20% 6% 1	330
November	1st to 10th	Day	18% 14% 33% 24% 7% 3	300
	11th to 20th	Day	15% 17% 42% 22% 4	300
	21st to end	Day	21% 18% 27% 24% 8% 1	300
December	1st to 10th	Day	12% 15% 42% 24% 6% 1	300
	11th to 20th	Day	14% 17% 38% 25% 6%	300
	21st to end	Day	20% 17% 41% 17% 5%	330
January	1st to 10th	Night	13% 15% 38% 28% 6% 1	1000
	11th to 20th	Night	11% 17% 38% 28% 5% 1	1000
	21st to end	Night	14% 12% 43% 24% 6%	1100
February	1st to 10th	Night	16% 13% 42% 25% 4	1000
	11th to 20th	Night	13% 15% 42% 25% 5%	934
	21st to end	Night	12% 12% 44% 26% 6% 1	660
March	1st to 10th	Night	14% 12% 42% 27% 5%	685
	11th to 20th	Night	17% 12% 40% 25% 6%	600
	21st to end	Night	20% 12% 38% 23% 5%	660
April	1st to 10th	Night	19% 15% 38% 25% 3	600
	11th to 20th	Night	18% 15% 38% 25% 5%	593
	21st to end	Night	20% 12% 34% 25% 8% 1	600
May	1st to 10th	Night	16% 9% 34% 32% 9%	600
	11th to 20th	Night	14% 10% 32% 33% 10% 1	563
	21st to end	Night	17% 8% 33% 31% 11%	617
June	1st to 10th	Night	7% 6% 27% 43% 15% 1	570
	11th to 20th	Night	9% 6% 34% 39% 11%	570
	21st to end	Night	5% 5% 29% 46% 15%	570
July	1st to 10th	Night	5% 3 23% 41% 27%	555
	11th to 20th	Night	3 2 26% 49% 20%	552
	21st to end	Night	3 3 20% 54% 20%	624
August	1st to 10th	Night	5% 4 24% 45% 21%	567
	11th to 20th	Night	9% 5% 24% 43% 19% 1	564
	21st to end	Night	7% 4 29% 41% 18% 2	627
September	1st to 10th	Night	9% 4 28% 41% 16% 2	570
	11th to 20th	Night	9% 5% 33% 38% 13% 2	570
	21st to end	Night	10% 8% 34% 32% 15%	570
October	1st to 10th	Night	17% 8% 32% 32% 9% 1	663
	11th to 20th	Night	15% 14% 37% 29% 6% 1	760
	21st to end	Night	18% 12% 34% 26% 10%	895
November	1st to 10th	Night	10% 11% 41% 31% 6% 1	975
	11th to 20th	Night	12% 13% 42% 27% 6%	1000
	21st to end	Night	12% 13% 41% 26% 9%	1000
December	1st to 10th	Night	11% 13% 43% 27% 5%	1000
	11th to 20th	Night	16% 12% 38% 28% 6% 1	1000
	21st to end	Night	18% 13% 38% 25% 5%	1100

Page generated at 2025-05-08 08:33:57. Version ac07

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