RAC Summer Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the RAC Summer Star Party clear sky chart from 2005-11-06 to 2025-08-05.

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	17% 8% 3 7% 5% 5% 5% 4 6% 7% 35%	125624
	Night	17% 8% 3 7% 4 4 4 4 5% 7% 38%	122430
Transparency	Day	45% 8% 12% 21% 13% 2	129130
	Night	43% 7% 13% 21% 14% 2	127010
Seeing	Day	25% 18% 28% 18% 8% 2	15843
	Night	22% 16% 29% 24% 8% 1	24805

By Month

Percentage Cloud Cover NE (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	10% 7% 4 13% 6% 5% 5% 4 6% 7% 32%	3597
	11th to 20th	Day	12% 10% 4 10% 5% 4 5% 4 6% 7% 32%	3570
	21st to end	Day	15% 11% 5% 8% 4 4 4 3 4 5% 37%	3984
February	1st to 10th	Day	13% 9% 3 9% 7% 6% 6% 4 5% 7% 30%	1835
	11th to 20th	Day	12% 7% 2 10% 7% 5% 5% 5% 6% 7% 35%	1948
	21st to end	Day	18% 10% 3 8% 5% 4 5% 3 5% 6% 33%	1650
March	1st to 10th	Day	18% 11% 4 7% 4 4 5% 3 5% 5% 33%	4476
	11th to 20th	Day	15% 7% 3 7% 5% 6% 5% 4 6% 6% 37%	4652
	21st to end	Day	20% 11% 3 7% 4 4 3 3 5% 7% 35%	5970
April	1st to 10th	Day	17% 10% 3 8% 5% 4 5% 3 5% 6% 35%	2400
	11th to 20th	Day	17% 8% 2 7% 5% 4 5% 4 5% 6% 37%	2374
	21st to end	Day	19% 7% 3 7% 4 5% 5% 4 6% 6% 34%	2535
May	1st to 10th	Day	32% 8% 3 8% 4 4 4 4 4 7% 23%	5886
	11th to 20th	Day	21% 9% 3 8% 5% 5% 4 3 6% 7% 29%	6261
	21st to end	Day	20% 8% 2 6% 5% 4 5% 4 7% 8% 31%	6949
June	1st to 10th	Day	18% 6% 2 6% 6% 5% 5% 5% 7% 9% 31%	2800
	11th to 20th	Day	18% 6% 2 6% 4 5% 5% 4 7% 9% 35%	2800
	21st to end	Day	16% 7% 2 7% 6% 5% 5% 4 5% 8% 34%	2800
July	1st to 10th	Day	13% 6% 3 8% 5% 6% 6% 5% 7% 9% 33%	6360
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 8% 11% 37%	6360
	21st to end	Day	16% 6% 2 6% 4 4 4 4 5% 7% 42%	6926
August	1st to 10th	Day	17% 6% 2 7% 6% 5% 5% 4 6% 9% 34%	4367
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 5% 6% 9% 35%	3805
	21st to end	Day	11% 4 2 6% 4 5% 6% 5% 7% 9% 39%	4180
September	1st to 10th	Day	13% 5% 2 6% 5% 4 5% 4 5% 7% 43%	2280
	11th to 20th	Day	15% 5% 2 5% 4 4 3 3 4 8% 48%	2152
	21st to end	Day	18% 5% 2 6% 3 4 4 4 6% 7% 40%	2090
October	1st to 10th	Day	23% 7% 2 6% 5% 4 4 4 5% 5% 36%	2047
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 4 5% 7% 41%	1867
	21st to end	Day	21% 10% 3 6% 4 4 3 3 5% 8% 34%	1881
November	1st to 10th	Day	11% 7% 3 6% 4 4 4 3 5% 6% 47%	1761
	11th to 20th	Day	16% 7% 2 5% 4 4 4 3 6% 7% 42%	1800
	21st to end	Day	18% 9% 2 5% 4 5% 5% 3 5% 6% 38%	1715
December	1st to 10th	Day	15% 8% 3 9% 6% 4 5% 4 6% 7% 34%	2600
	11th to 20th	Day	19% 9% 3 6% 4 4 4 3 4 6% 37%	2986
	21st to end	Day	16% 10% 3 8% 5% 5% 4 3 5% 7% 33%	3960
January	1st to 10th	Night	13% 7% 3 11% 5% 4 3 3 6% 7% 36%	6349
	11th to 20th	Night	11% 9% 3 12% 5% 5% 6% 5% 6% 8% 31%	6315
	21st to end	Night	13% 9% 3 8% 5% 5% 4 4 5% 7% 36%	6968
February	1st to 10th	Night	13% 9% 3 7% 6% 4 5% 5% 6% 7% 35%	2564
	11th to 20th	Night	11% 9% 2 7% 6% 6% 6% 4 6% 7% 36%	2338
	21st to end	Night	15% 9% 3 7% 5% 5% 4 4 4 6% 37%	1980
March	1st to 10th	Night	19% 9% 2 6% 4 4 4 4 6% 6% 35%	5364
	11th to 20th	Night	16% 8% 2 6% 5% 4 5% 3 6% 7% 39%	5291
	21st to end	Night	21% 10% 2 6% 4 3 4 3 5% 6% 34%	4962
April	1st to 10th	Night	17% 10% 3 6% 5% 5% 5% 3 7% 8% 33%	2000
	11th to 20th	Night	19% 8% 2 6% 6% 4 4 4 5% 7% 37%	1975
	21st to end	Night	21% 9% 2 6% 3 3 3 3 5% 6% 39%	1864
May	1st to 10th	Night	33% 9% 2 6% 3 4 4 3 5% 7% 25%	4054
	11th to 20th	Night	18% 11% 4 8% 5% 5% 5% 4 5% 7% 30%	3676
	21st to end	Night	22% 8% 3 7% 4 4 4 4 6% 8% 31%	3926
June	1st to 10th	Night	18% 8% 2 6% 5% 4 3 3 6% 5% 41%	1600
	11th to 20th	Night	17% 6% 2 5% 4 4 5% 4 6% 7% 39%	1600
	21st to end	Night	15% 7% 2 5% 4 4 3 4 6% 8% 44%	1600
July	1st to 10th	Night	13% 6% 2 6% 5% 4 5% 4 6% 8% 42%	3600
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 4 6% 9% 39%	3600
	21st to end	Night	11% 6% 2 5% 4 4 3 3 5% 6% 50%	3926
August	1st to 10th	Night	22% 5% 2 7% 4 4 4 4 5% 6% 37%	2619
	11th to 20th	Night	15% 7% 3 6% 4 4 4 4 5% 7% 40%	3006
	21st to end	Night	10% 3 1 4 3 3 3 3 6% 9% 55%	3476
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 7% 46%	1900
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 5% 7% 51%	2027
	21st to end	Night	18% 5% 2 4 4 3 4 4 4 7% 44%	2090
October	1st to 10th	Night	22% 6% 2 6% 4 3 3 3 5% 6% 40%	2129
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 5% 7% 44%	2313
	21st to end	Night	22% 8% 2 6% 4 4 4 3 5% 6% 38%	2717
November	1st to 10th	Night	13% 6% 3 5% 4 3 4 3 5% 7% 47%	2537
	11th to 20th	Night	17% 6% 3 6% 4 4 4 3 5% 6% 43%	2600
	21st to end	Night	13% 8% 2 6% 4 4 5% 4 6% 7% 42%	2680
December	1st to 10th	Night	16% 9% 3 7% 4 4 4 4 6% 8% 35%	4580
	11th to 20th	Night	22% 9% 3 5% 4 4 4 3 5% 6% 36%	5208
	21st to end	Night	17% 11% 5% 8% 5% 4 4 3 4 6% 34%	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	44% 6% 13% 25% 10% 1	3598
	11th to 20th	Day	47% 7% 12% 17% 13% 4	3600
	21st to end	Day	48% 5% 8% 21% 16% 2	3984
February	1st to 10th	Day	51% 7% 14% 17% 11% 1	1835
	11th to 20th	Day	47% 8% 11% 20% 13% 2	1957
	21st to end	Day	50% 8% 10% 18% 12% 1	1650
March	1st to 10th	Day	51% 5% 11% 15% 12% 7%	4479
	11th to 20th	Day	43% 8% 15% 22% 8% 3	4652
	21st to end	Day	49% 5% 7% 16% 19% 5%	5970
April	1st to 10th	Day	48% 7% 10% 19% 12% 3	2400
	11th to 20th	Day	44% 9% 11% 20% 15% 2	2374
	21st to end	Day	47% 9% 9% 19% 14% 3	2535
May	1st to 10th	Day	59% 9% 10% 13% 8% 1	5897
	11th to 20th	Day	50% 8% 14% 17% 10% 1	6265
	21st to end	Day	46% 9% 13% 20% 11% 1	6949
June	1st to 10th	Day	45% 9% 14% 20% 11% 1	2800
	11th to 20th	Day	43% 9% 14% 20% 13% 1	2800
	21st to end	Day	45% 9% 14% 19% 13% 1	2800
July	1st to 10th	Day	45% 9% 12% 21% 13% 1	6290
	11th to 20th	Day	37% 9% 17% 24% 12% 1	6285
	21st to end	Day	40% 7% 11% 26% 15% 1	6728
August	1st to 10th	Day	45% 9% 14% 21% 10% 1	4353
	11th to 20th	Day	41% 10% 15% 26% 8% 1	3843
	21st to end	Day	35% 9% 15% 28% 13% 1	4180
September	1st to 10th	Day	38% 9% 13% 25% 15% 1	2280
	11th to 20th	Day	35% 7% 11% 24% 20% 2	2152
	21st to end	Day	40% 7% 12% 23% 15% 2	2090
October	1st to 10th	Day	43% 7% 7% 21% 20% 2	3398
	11th to 20th	Day	34% 5% 11% 27% 21% 3	3083
	21st to end	Day	35% 6% 13% 28% 14% 4	3081
November	1st to 10th	Day	36% 7% 16% 24% 14% 2	1761
	11th to 20th	Day	40% 6% 14% 19% 18% 3	1800
	21st to end	Day	45% 6% 15% 23% 9% 2	1715
December	1st to 10th	Day	46% 7% 15% 23% 8% 1	2600
	11th to 20th	Day	45% 6% 13% 19% 16% 1	2986
	21st to end	Day	49% 7% 9% 16% 14% 5%	3960
January	1st to 10th	Night	43% 6% 13% 25% 12% 2	6349
	11th to 20th	Night	47% 8% 15% 17% 10% 3	6360
	21st to end	Night	45% 6% 11% 16% 17% 4	6968
February	1st to 10th	Night	46% 8% 14% 21% 9% 2	2564
	11th to 20th	Night	44% 8% 14% 19% 13% 2	2340
	21st to end	Night	46% 7% 12% 22% 10% 3	1980
March	1st to 10th	Night	46% 7% 12% 17% 15% 4	5374
	11th to 20th	Night	42% 9% 17% 21% 9% 2	5292
	21st to end	Night	48% 6% 6% 15% 22% 3	4962
April	1st to 10th	Night	47% 8% 11% 20% 14% 2	2000
	11th to 20th	Night	46% 7% 11% 19% 16% 1	1975
	21st to end	Night	45% 7% 10% 20% 15% 4	1864
May	1st to 10th	Night	57% 8% 13% 12% 9% 1	4060
	11th to 20th	Night	51% 11% 11% 16% 9% 2	3677
	21st to end	Night	49% 9% 13% 17% 11% 1	3927
June	1st to 10th	Night	43% 7% 14% 24% 12% 1	1600
	11th to 20th	Night	42% 9% 15% 22% 11% 1	1600
	21st to end	Night	39% 9% 14% 24% 14% 1	1600
July	1st to 10th	Night	39% 8% 15% 21% 17%	3560
	11th to 20th	Night	40% 9% 14% 21% 14% 1	3552
	21st to end	Night	34% 6% 12% 30% 17% 1	3809
August	1st to 10th	Night	47% 7% 14% 21% 9% 1	2609
	11th to 20th	Night	41% 7% 17% 25% 9%	3033
	21st to end	Night	26% 7% 14% 37% 16% 1	3476
September	1st to 10th	Night	36% 10% 11% 25% 17% 1	1900
	11th to 20th	Night	32% 6% 12% 25% 22% 2	2027
	21st to end	Night	40% 7% 11% 24% 17% 2	2090
October	1st to 10th	Night	40% 7% 11% 23% 17% 1	3558
	11th to 20th	Night	29% 5% 13% 25% 23% 5%	3836
	21st to end	Night	35% 6% 14% 31% 14% 1	4467
November	1st to 10th	Night	37% 7% 13% 23% 18% 3	2537
	11th to 20th	Night	42% 7% 12% 20% 17% 3	2600
	21st to end	Night	40% 8% 17% 24% 11% 1	2680
December	1st to 10th	Night	44% 7% 12% 29% 7%	4580
	11th to 20th	Night	47% 7% 15% 14% 16% 1	5208
	21st to end	Night	49% 7% 9% 17% 13% 5%	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	31% 27% 28% 12% 4	200
	11th to 20th	Day	32% 25% 33% 8% 3 1	200
	21st to end	Day	27% 30% 30% 10% 4	226
February	1st to 10th	Day	33% 33% 26% 8% 1	335
	11th to 20th	Day	33% 27% 28% 10% 1	496
	21st to end	Day	33% 24% 28% 14% 1	414
March	1st to 10th	Day	31% 26% 28% 14% 2	500
	11th to 20th	Day	32% 24% 29% 11% 3 1	500
	21st to end	Day	33% 26% 27% 9% 4 2	550
April	1st to 10th	Day	36% 22% 25% 11% 6% 1	500
	11th to 20th	Day	33% 23% 27% 11% 4 1	492
	21st to end	Day	29% 22% 29% 13% 5% 1	500
May	1st to 10th	Day	34% 21% 26% 13% 4 2	503
	11th to 20th	Day	21% 21% 35% 17% 5% 1	527
	21st to end	Day	22% 14% 31% 20% 11% 2	573
June	1st to 10th	Day	19% 17% 31% 22% 10% 2	530
	11th to 20th	Day	21% 14% 31% 23% 9% 2	530
	21st to end	Day	17% 20% 31% 19% 9% 3	530
July	1st to 10th	Day	16% 9% 33% 27% 14% 2	525
	11th to 20th	Day	14% 8% 32% 30% 13% 3	524
	21st to end	Day	15% 10% 36% 23% 12% 2	576
August	1st to 10th	Day	13% 9% 28% 30% 14% 5%	502
	11th to 20th	Day	16% 11% 29% 25% 13% 6%	479
	21st to end	Day	13% 12% 28% 28% 15% 6%	539
September	1st to 10th	Day	20% 10% 25% 22% 17% 6%	490
	11th to 20th	Day	17% 13% 23% 21% 16% 10%	490
	21st to end	Day	21% 12% 27% 22% 12% 6%	490
October	1st to 10th	Day	29% 12% 23% 19% 12% 4	490
	11th to 20th	Day	21% 18% 26% 24% 9% 2	473
	21st to end	Day	31% 15% 29% 17% 7% 1	429
November	1st to 10th	Day	23% 19% 26% 23% 7% 2	395
	11th to 20th	Day	29% 24% 25% 17% 5% 1	400
	21st to end	Day	32% 22% 24% 17% 4 1	315
December	1st to 10th	Day	28% 28% 26% 16% 3	200
	11th to 20th	Day	40% 21% 29% 8% 2	200
	21st to end	Day	27% 25% 29% 14% 5%	220
January	1st to 10th	Night	23% 23% 34% 17% 3	800
	11th to 20th	Night	25% 22% 31% 19% 3	800
	21st to end	Night	19% 24% 36% 18% 3	880
February	1st to 10th	Night	22% 22% 37% 18% 1	800
	11th to 20th	Night	25% 25% 34% 15% 1	783
	21st to end	Night	25% 24% 31% 18% 2	660
March	1st to 10th	Night	26% 23% 31% 17% 3	800
	11th to 20th	Night	24% 23% 30% 20% 3	800
	21st to end	Night	25% 20% 30% 20% 5%	880
April	1st to 10th	Night	27% 19% 33% 17% 4	800
	11th to 20th	Night	24% 22% 27% 20% 6%	791
	21st to end	Night	27% 20% 28% 20% 4	664
May	1st to 10th	Night	30% 12% 28% 23% 7%	600
	11th to 20th	Night	19% 15% 35% 25% 6%	443
	21st to end	Night	21% 8% 27% 31% 12%	433
June	1st to 10th	Night	19% 8% 30% 33% 10%	400
	11th to 20th	Night	18% 9% 28% 34% 12% 1	400
	21st to end	Night	17% 7% 27% 36% 13%	400
July	1st to 10th	Night	15% 6% 25% 38% 15% 1	390
	11th to 20th	Night	16% 4 21% 42% 15% 1	388
	21st to end	Night	11% 5% 24% 43% 16%	438
August	1st to 10th	Night	14% 3 25% 39% 17% 1	428
	11th to 20th	Night	15% 5% 23% 34% 20% 2	695
	21st to end	Night	17% 7% 21% 33% 21% 1	836
September	1st to 10th	Night	15% 7% 27% 31% 19% 2	760
	11th to 20th	Night	16% 7% 22% 29% 21% 5%	760
	21st to end	Night	21% 8% 26% 29% 15% 2	760
October	1st to 10th	Night	26% 10% 23% 27% 13% 2	760
	11th to 20th	Night	20% 14% 28% 26% 11% 1	760
	21st to end	Night	28% 15% 29% 22% 6%	836
November	1st to 10th	Night	17% 18% 31% 25% 9% 1	780
	11th to 20th	Night	23% 22% 27% 21% 6% 1	800
	21st to end	Night	22% 20% 29% 24% 5% 1	800
December	1st to 10th	Night	25% 22% 31% 17% 4	800
	11th to 20th	Night	27% 20% 30% 20% 4	800
	21st to end	Night	26% 23% 30% 20% 2	880

Page generated at 2025-08-02 01:32:24. Version ac07

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