Joseph P. Strogen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joseph P. Strogen Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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	12% 8% 3 6% 4 4 4 3 5% 7% 44%	113582
	Night	13% 8% 3 6% 4 4 3 3 4 6% 45%	114899
Transparency	Day	39% 5% 11% 25% 17% 4	117228
	Night	39% 5% 10% 25% 18% 4	119395
Seeing	Day	18% 12% 29% 25% 13% 3	16083
	Night	18% 12% 31% 27% 11% 1	26077

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% 12% 3 9% 4 4 3 3 4 5% 37%	3955
	11th to 20th	Day	10% 12% 4 7% 4 5% 4 3 4 5% 42%	4080
	21st to end	Day	16% 11% 4 7% 3 3 3 3 5% 5% 39%	4488
February	1st to 10th	Day	15% 11% 4 10% 5% 4 4 3 5% 5% 34%	1800
	11th to 20th	Day	15% 11% 3 7% 5% 4 4 4 4 5% 39%	1807
	21st to end	Day	15% 10% 3 5% 4 4 4 4 5% 6% 40%	1655
March	1st to 10th	Day	15% 10% 3 5% 3 3 3 3 4 5% 48%	4936
	11th to 20th	Day	11% 7% 2 4 3 3 3 3 5% 7% 53%	4980
	21st to end	Day	13% 10% 3 8% 5% 5% 5% 4 6% 6% 35%	5499
April	1st to 10th	Day	13% 8% 2 6% 5% 4 4 3 4 7% 43%	2400
	11th to 20th	Day	17% 9% 2 6% 3 4 5% 3 5% 6% 40%	2254
	21st to end	Day	21% 9% 3 7% 4 3 3 4 5% 6% 35%	2280
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	3974
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 5% 7% 39%	4135
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 5% 8% 42%	4824
June	1st to 10th	Day	8% 6% 2 5% 4 5% 5% 5% 6% 9% 45%	2660
	11th to 20th	Day	8% 4 2 5% 5% 4 5% 4 6% 9% 48%	2660
	21st to end	Day	6% 3 2 4 4 4 5% 4 7% 9% 52%	2660
July	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 40%	4421
	11th to 20th	Day	5% 4 2 6% 4 5% 5% 5% 6% 10% 48%	4120
	21st to end	Day	7% 4 2 6% 5% 4 6% 5% 9% 11% 40%	4465
August	1st to 10th	Day	5% 3 1 5% 4 4 5% 4 6% 9% 53%	4104
	11th to 20th	Day	10% 5% 2 6% 5% 5% 4 4 5% 9% 45%	3893
	21st to end	Day	7% 5% 2 4 3 4 4 4 6% 8% 53%	4180
September	1st to 10th	Day	11% 6% 1 4 5% 4 4 3 5% 7% 50%	2280
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 5% 6% 50%	2280
	21st to end	Day	8% 4 2 5% 4 4 4 3 5% 7% 53%	1981
October	1st to 10th	Day	12% 4 2 6% 3 2 3 3 4 5% 57%	1900
	11th to 20th	Day	15% 6% 2 4 2 2 2 3 3 4 56%	1900
	21st to end	Day	18% 9% 2 5% 3 2 3 3 4 5% 47%	2090
November	1st to 10th	Day	13% 6% 1 4 4 3 3 3 3 5% 54%	1926
	11th to 20th	Day	14% 8% 2 5% 4 2 4 2 4 6% 49%	1795
	21st to end	Day	16% 10% 3 5% 2 3 2 4 4 6% 45%	1600
December	1st to 10th	Day	10% 9% 2 6% 3 3 3 3 4 5% 51%	2610
	11th to 20th	Day	11% 11% 4 6% 3 3 3 2 4 5% 47%	3008
	21st to end	Day	20% 21% 5% 9% 3 3 4 2 3 4 25%	3982
January	1st to 10th	Night	14% 11% 5% 13% 6% 5% 4 2 3 4 32%	6007
	11th to 20th	Night	11% 8% 3 13% 5% 6% 5% 3 3 4 39%	5900
	21st to end	Night	17% 11% 5% 7% 3 3 3 2 4 4 41%	6490
February	1st to 10th	Night	15% 12% 4 8% 4 3 3 3 5% 6% 37%	2600
	11th to 20th	Night	16% 10% 3 7% 4 3 4 3 4 5% 41%	2478
	21st to end	Night	14% 10% 3 7% 5% 4 3 2 4 6% 42%	1980
March	1st to 10th	Night	17% 12% 2 5% 4 3 3 3 4 4 44%	4929
	11th to 20th	Night	12% 8% 2 4 3 3 3 3 5% 7% 50%	4980
	21st to end	Night	14% 10% 3 7% 5% 5% 4 4 5% 7% 37%	5429
April	1st to 10th	Night	10% 7% 3 5% 4 4 4 3 5% 7% 50%	2000
	11th to 20th	Night	14% 9% 2 5% 3 3 3 3 4 7% 47%	1881
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 7% 39%	1900
May	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 7% 41%	2924
	11th to 20th	Night	14% 8% 3 6% 4 4 3 3 5% 7% 43%	2765
	21st to end	Night	12% 6% 2 5% 5% 5% 4 4 6% 8% 43%	2743
June	1st to 10th	Night	7% 4 1 5% 3 4 6% 4 7% 8% 49%	1520
	11th to 20th	Night	6% 4 1 3 3 4 4 5% 7% 11% 52%	1520
	21st to end	Night	4 3 1 2 2 4 4 3 7% 10% 60%	1520
July	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 46%	2545
	11th to 20th	Night	5% 3 1 5% 3 3 3 4 5% 10% 57%	2850
	21st to end	Night	6% 4 2 5% 4 4 4 4 7% 11% 50%	3098
August	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 8% 62%	2836
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 6% 6% 50%	2938
	21st to end	Night	5% 3 2 3 3 3 4 3 5% 9% 62%	3476
September	1st to 10th	Night	12% 5% 1 4 2 2 3 2 4 7% 56%	1900
	11th to 20th	Night	10% 4 2 4 3 3 4 4 5% 8% 53%	1900
	21st to end	Night	10% 5% 1 5% 3 3 3 3 5% 7% 56%	2174
October	1st to 10th	Night	8% 4 2 4 2 3 3 3 4 7% 61%	2280
	11th to 20th	Night	15% 5% 1 4 3 3 3 2 4 5% 53%	2280
	21st to end	Night	18% 6% 2 4 3 3 2 3 4 4 51%	2508
November	1st to 10th	Night	12% 5% 2 4 3 3 3 2 4 5% 57%	2370
	11th to 20th	Night	14% 7% 2 6% 4 3 3 3 4 5% 51%	2603
	21st to end	Night	16% 9% 3 5% 4 3 2 2 4 4 48%	2800
December	1st to 10th	Night	10% 9% 3 6% 4 3 3 3 4 6% 49%	4580
	11th to 20th	Night	16% 9% 3 7% 3 3 4 3 4 5% 44%	5199
	21st to end	Night	23% 18% 5% 8% 4 4 3 3 4 4 26%	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	51% 3 10% 21% 14% 1	3956
	11th to 20th	Day	45% 3 8% 24% 16% 4	4080
	21st to end	Day	48% 3 8% 16% 16% 9%	4488
February	1st to 10th	Day	53% 4 9% 20% 12% 1	1800
	11th to 20th	Day	49% 3 11% 18% 15% 4	1816
	21st to end	Day	46% 4 7% 23% 16% 5%	1655
March	1st to 10th	Day	40% 3 7% 22% 20% 9%	4936
	11th to 20th	Day	32% 4 8% 24% 24% 9%	4980
	21st to end	Day	48% 6% 13% 26% 8%	5501
April	1st to 10th	Day	41% 6% 10% 23% 19% 2	2400
	11th to 20th	Day	44% 5% 12% 21% 15% 3	2254
	21st to end	Day	50% 5% 10% 22% 12% 1	2280
May	1st to 10th	Day	46% 9% 11% 20% 13% 1	4010
	11th to 20th	Day	42% 9% 11% 23% 14% 1	4139
	21st to end	Day	39% 8% 9% 26% 17% 1	4837
June	1st to 10th	Day	34% 9% 15% 25% 15% 2	2660
	11th to 20th	Day	31% 8% 12% 28% 17% 3	2660
	21st to end	Day	26% 8% 16% 29% 17% 4	2660
July	1st to 10th	Day	40% 7% 12% 24% 14% 2	4352
	11th to 20th	Day	30% 7% 13% 30% 17% 2	4051
	21st to end	Day	34% 9% 21% 26% 9% 1	4325
August	1st to 10th	Day	26% 7% 18% 27% 21% 1	4090
	11th to 20th	Day	37% 6% 12% 29% 15% 1	3932
	21st to end	Day	28% 6% 13% 35% 14% 4	4179
September	1st to 10th	Day	33% 7% 10% 25% 18% 6%	2280
	11th to 20th	Day	34% 5% 10% 24% 25% 2	2280
	21st to end	Day	31% 6% 10% 26% 23% 4	1981
October	1st to 10th	Day	18% 3 5% 19% 41% 14%	3160
	11th to 20th	Day	22% 3 4 24% 36% 11%	3140
	21st to end	Day	32% 3 7% 28% 27% 3	3428
November	1st to 10th	Day	35% 4 10% 24% 22% 5%	1926
	11th to 20th	Day	38% 3 7% 24% 22% 6%	1795
	21st to end	Day	41% 4 8% 29% 17% 1	1600
December	1st to 10th	Day	37% 4 8% 35% 15% 1	2610
	11th to 20th	Day	41% 4 8% 27% 19% 1	3005
	21st to end	Day	64% 4 9% 14% 8% 1	3982
January	1st to 10th	Night	58% 3 8% 19% 11% 1	6007
	11th to 20th	Night	51% 3 7% 23% 14% 3	5900
	21st to end	Night	48% 3 8% 20% 17% 4	6490
February	1st to 10th	Night	48% 5% 11% 21% 13% 2	2600
	11th to 20th	Night	46% 4 10% 21% 16% 3	2479
	21st to end	Night	45% 3 8% 22% 16% 5%	1980
March	1st to 10th	Night	45% 4 6% 20% 21% 4	4929
	11th to 20th	Night	34% 4 8% 23% 23% 8%	4980
	21st to end	Night	46% 6% 13% 23% 11% 1	5429
April	1st to 10th	Night	34% 5% 12% 26% 20% 4	2000
	11th to 20th	Night	38% 5% 11% 24% 17% 6%	1881
	21st to end	Night	45% 5% 10% 24% 15% 1	1900
May	1st to 10th	Night	41% 7% 13% 21% 15% 2	2953
	11th to 20th	Night	40% 8% 12% 27% 12% 2	2766
	21st to end	Night	37% 8% 12% 27% 15% 1	2751
June	1st to 10th	Night	30% 8% 15% 26% 18% 3	1520
	11th to 20th	Night	24% 7% 13% 29% 22% 5%	1520
	21st to end	Night	19% 6% 18% 31% 20% 7%	1520
July	1st to 10th	Night	37% 7% 11% 25% 17% 3	2505
	11th to 20th	Night	23% 5% 11% 38% 20% 2	2796
	21st to end	Night	27% 8% 19% 34% 12% 1	2999
August	1st to 10th	Night	19% 6% 19% 31% 23% 1	2826
	11th to 20th	Night	33% 6% 14% 28% 17% 2	2965
	21st to end	Night	20% 5% 15% 39% 17% 4	3476
September	1st to 10th	Night	30% 5% 9% 26% 23% 7%	1900
	11th to 20th	Night	30% 5% 10% 23% 28% 4	1900
	21st to end	Night	28% 4 10% 25% 30% 3	2174
October	1st to 10th	Night	15% 3 5% 20% 45% 12%	3800
	11th to 20th	Night	23% 3 6% 21% 30% 17%	3779
	21st to end	Night	32% 3 6% 30% 26% 3	4120
November	1st to 10th	Night	31% 4 11% 27% 21% 6%	2370
	11th to 20th	Night	37% 3 6% 24% 24% 6%	2603
	21st to end	Night	41% 4 9% 26% 18% 2	2800
December	1st to 10th	Night	38% 4 8% 34% 15% 1	4580
	11th to 20th	Night	45% 3 9% 24% 18% 1	5201
	21st to end	Night	63% 4 11% 15% 6% 1	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	25% 25% 36% 13% 1	300
	11th to 20th	Day	32% 22% 38% 7% 1	300
	21st to end	Day	23% 25% 39% 11% 2	330
February	1st to 10th	Day	27% 26% 35% 11% 1	300
	11th to 20th	Day	30% 29% 29% 10% 2	300
	21st to end	Day	35% 20% 35% 10% 1	249
March	1st to 10th	Day	25% 22% 40% 11% 1	300
	11th to 20th	Day	36% 17% 32% 13% 1 1	300
	21st to end	Day	34% 27% 30% 8%	342
April	1st to 10th	Day	25% 19% 35% 16% 4 1	500
	11th to 20th	Day	29% 19% 29% 17% 4	452
	21st to end	Day	26% 17% 34% 18% 5%	460
May	1st to 10th	Day	22% 16% 36% 20% 5% 1	582
	11th to 20th	Day	20% 15% 28% 24% 12% 1	675
	21st to end	Day	14% 10% 37% 23% 12% 4	734
June	1st to 10th	Day	10% 5% 26% 33% 20% 5%	680
	11th to 20th	Day	9% 4 27% 41% 17% 3	680
	21st to end	Day	8% 8% 32% 35% 13% 4	680
July	1st to 10th	Day	14% 3 18% 29% 29% 7%	671
	11th to 20th	Day	4 4 24% 39% 24% 5%	668
	21st to end	Day	10% 3 22% 32% 28% 6%	739
August	1st to 10th	Day	9% 3 20% 39% 24% 4	676
	11th to 20th	Day	11% 4 22% 34% 24% 5%	582
	21st to end	Day	12% 2 16% 32% 28% 10%	539
September	1st to 10th	Day	13% 3 15% 37% 26% 6%	490
	11th to 20th	Day	8% 4 21% 38% 21% 8%	490
	21st to end	Day	10% 11% 26% 35% 15% 4	304
October	1st to 10th	Day	15% 9% 33% 28% 12% 3	300
	11th to 20th	Day	20% 18% 36% 20% 6% 1	300
	21st to end	Day	30% 20% 31% 17% 2	330
November	1st to 10th	Day	19% 14% 40% 20% 5% 1	300
	11th to 20th	Day	26% 16% 35% 16% 6% 1	300
	21st to end	Day	33% 15% 31% 17% 4	300
December	1st to 10th	Day	21% 23% 35% 15% 4 2	300
	11th to 20th	Day	24% 23% 36% 14% 4	300
	21st to end	Day	29% 26% 32% 11% 2	330
January	1st to 10th	Night	22% 18% 38% 20% 3	1000
	11th to 20th	Night	23% 22% 35% 16% 4	1000
	21st to end	Night	20% 20% 38% 21% 1	1100
February	1st to 10th	Night	25% 17% 40% 16% 3	1000
	11th to 20th	Night	25% 18% 36% 19% 1	924
	21st to end	Night	27% 19% 35% 17% 2	660
March	1st to 10th	Night	23% 18% 36% 21% 2	610
	11th to 20th	Night	26% 15% 35% 21% 3	600
	21st to end	Night	28% 15% 35% 18% 4	660
April	1st to 10th	Night	16% 19% 39% 22% 4	600
	11th to 20th	Night	22% 13% 36% 23% 6%	565
	21st to end	Night	22% 13% 37% 22% 6%	570
May	1st to 10th	Night	16% 9% 36% 32% 6% 1	570
	11th to 20th	Night	19% 8% 28% 29% 14% 1	561
	21st to end	Night	14% 7% 29% 30% 18% 2	617
June	1st to 10th	Night	9% 5% 20% 40% 24% 2	570
	11th to 20th	Night	6% 2 29% 40% 21% 1	570
	21st to end	Night	4 2 29% 46% 17% 2	570
July	1st to 10th	Night	12% 2 13% 37% 30% 5%	555
	11th to 20th	Night	5% 1 18% 44% 28% 3	552
	21st to end	Night	8% 1 16% 40% 32% 2	624
August	1st to 10th	Night	7% 2 18% 43% 29% 2	567
	11th to 20th	Night	9% 2 17% 43% 27% 2	564
	21st to end	Night	7% 1 16% 39% 31% 5%	627
September	1st to 10th	Night	14% 2 15% 40% 26% 3	570
	11th to 20th	Night	11% 2 19% 35% 28% 5%	570
	21st to end	Night	11% 5% 26% 36% 19% 3	662
October	1st to 10th	Night	10% 6% 28% 32% 21% 3	760
	11th to 20th	Night	18% 12% 34% 29% 7%	760
	21st to end	Night	23% 12% 33% 25% 6%	836
November	1st to 10th	Night	15% 14% 37% 26% 8%	780
	11th to 20th	Night	21% 16% 33% 23% 6%	803
	21st to end	Night	24% 16% 33% 21% 5%	1000
December	1st to 10th	Night	22% 17% 35% 21% 5%	1000
	11th to 20th	Night	25% 14% 36% 22% 4	1000
	21st to end	Night	25% 19% 34% 19% 2	1100

Page generated at 2025-04-08 08:17:27. Version ac07

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