Rafes Urban Astronomy Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rafes Urban Astronomy Center clear sky chart from 2005-11-06 to 2025-11-18.

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	10% 5% 2 5% 3 3 4 3 5% 7% 54%	126923
	Night	10% 5% 2 4 3 3 3 3 4 6% 57%	123726
Transparency	Day	30% 4 10% 25% 23% 8%	130640
	Night	28% 4 9% 23% 25% 12%	128072
Seeing	Day	12% 6% 22% 28% 24% 8%	16571
	Night	11% 7% 28% 37% 16% 1	26404

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	13% 7% 2 5% 3 3 3 2 4 5% 53%	4077
	11th to 20th	Day	7% 5% 1 4 2 2 2 3 4 5% 66%	4080
	21st to end	Day	21% 8% 2 4 2 2 3 2 3 4 50%	4488
February	1st to 10th	Day	15% 7% 2 6% 4 3 4 3 4 5% 49%	1940
	11th to 20th	Day	16% 7% 2 5% 3 4 3 3 4 5% 50%	1949
	21st to end	Day	19% 6% 2 4 3 2 2 2 3 4 54%	1650
March	1st to 10th	Day	15% 5% 1 3 2 3 3 3 4 6% 53%	4933
	11th to 20th	Day	12% 4 1 4 3 3 3 3 4 5% 57%	4980
	21st to end	Day	12% 6% 2 5% 3 3 4 4 6% 7% 49%	5462
April	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 5% 51%	2200
	11th to 20th	Day	12% 7% 2 6% 4 3 3 3 5% 6% 49%	2259
	21st to end	Day	12% 6% 2 4 4 4 5% 4 6% 6% 49%	2594
May	1st to 10th	Day	16% 7% 2 6% 4 3 4 3 5% 6% 43%	5862
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 6% 47%	5862
	21st to end	Day	13% 7% 2 7% 4 5% 4 4 6% 7% 40%	6434
June	1st to 10th	Day	6% 4 2 5% 3 4 5% 4 7% 11% 49%	2600
	11th to 20th	Day	5% 4 1 4 5% 4 4 4 6% 8% 55%	2600
	21st to end	Day	4 3 1 4 4 4 4 3 7% 8% 58%	2600
July	1st to 10th	Day	7% 5% 3 8% 5% 5% 5% 4 6% 8% 43%	5900
	11th to 20th	Day	3 2 1 3 3 3 3 3 6% 10% 62%	5900
	21st to end	Day	3 3 2 4 4 4 4 4 6% 10% 58%	6422
August	1st to 10th	Day	1 2 1 3 2 3 3 3 5% 7% 70%	4234
	11th to 20th	Day	4 2 1 3 3 3 3 4 7% 9% 61%	4171
	21st to end	Day	3 2 1 4 2 3 5% 4 6% 9% 60%	4193
September	1st to 10th	Day	9% 4 2 4 4 4 4 4 4 6% 55%	2200
	11th to 20th	Day	9% 4 2 5% 5% 4 4 4 6% 7% 49%	2200
	21st to end	Day	7% 3 1 4 3 2 4 3 4 7% 60%	2200
October	1st to 10th	Day	6% 2 2 4 2 3 3 2 5% 6% 64%	2167
	11th to 20th	Day	10% 4 1 4 2 2 3 2 3 5% 65%	2000
	21st to end	Day	13% 3 2 4 2 2 2 2 3 3 63%	2200
November	1st to 10th	Day	13% 4 1 4 3 3 2 2 3 5% 59%	2058
	11th to 20th	Day	7% 3 1 3 3 3 3 2 4 5% 66%	1889
	21st to end	Day	14% 6% 2 4 3 3 2 2 3 5% 57%	1800
December	1st to 10th	Day	15% 7% 2 5% 2 3 3 2 4 4 53%	2940
	11th to 20th	Day	17% 6% 2 5% 4 3 3 3 4 4 49%	3391
	21st to end	Day	19% 6% 2 5% 3 2 2 2 3 4 51%	4488
January	1st to 10th	Night	15% 4 2 4 3 2 3 2 4 5% 56%	5888
	11th to 20th	Night	7% 4 1 3 1 2 2 2 3 5% 72%	5900
	21st to end	Night	21% 8% 2 4 2 2 2 2 3 4 49%	6490
February	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 5% 53%	2475
	11th to 20th	Night	15% 7% 2 4 3 3 3 2 4 6% 51%	2338
	21st to end	Night	16% 6% 1 4 3 2 2 2 4 4 55%	1980
March	1st to 10th	Night	15% 6% 2 5% 3 4 3 3 5% 5% 49%	4919
	11th to 20th	Night	9% 5% 2 4 3 3 3 3 4 6% 59%	4980
	21st to end	Night	12% 5% 2 5% 3 4 4 3 4 5% 53%	5467
April	1st to 10th	Night	9% 5% 2 4 4 3 4 4 6% 7% 53%	2200
	11th to 20th	Night	11% 5% 2 5% 4 3 3 3 5% 7% 52%	2088
	21st to end	Night	10% 6% 1 4 3 3 3 3 5% 7% 54%	1801
May	1st to 10th	Night	16% 8% 3 6% 4 3 4 3 5% 6% 44%	4053
	11th to 20th	Night	8% 6% 2 6% 4 4 4 3 6% 7% 50%	4053
	21st to end	Night	10% 5% 2 5% 4 3 4 3 6% 8% 49%	4443
June	1st to 10th	Night	4 3 2 4 3 4 4 3 6% 9% 58%	1800
	11th to 20th	Night	4 3 1 3 3 3 3 3 6% 9% 63%	1800
	21st to end	Night	2 2 1 3 3 3 3 3 5% 10% 66%	1800
July	1st to 10th	Night	6% 4 2 6% 5% 4 5% 4 6% 10% 48%	4078
	11th to 20th	Night	2 1 1 3 3 3 3 3 5% 8% 68%	4077
	21st to end	Night	2 3 1 3 3 3 3 3 6% 8% 65%	4449
August	1st to 10th	Night	1 1 1 2 2 2 2 2 5% 9% 74%	2926
	11th to 20th	Night	2 2 1 2 2 2 2 2 5% 11% 68%	2888
	21st to end	Night	3 2 1 3 3 3 3 3 5% 9% 65%	3705
September	1st to 10th	Night	7% 4 2 4 3 3 3 3 5% 6% 59%	2200
	11th to 20th	Night	8% 5% 2 5% 4 4 3 3 6% 7% 53%	2200
	21st to end	Night	6% 2 1 3 2 3 4 4 5% 8% 63%	2200
October	1st to 10th	Night	5% 3 1 3 2 2 3 2 4 6% 67%	2211
	11th to 20th	Night	9% 3 2 3 2 2 2 2 4 5% 66%	2400
	21st to end	Night	14% 4 2 3 1 2 2 2 3 3 63%	2640
November	1st to 10th	Night	10% 3 2 4 2 3 3 3 3 5% 63%	2460
	11th to 20th	Night	6% 3 2 3 3 2 3 3 5% 5% 65%	2661
	21st to end	Night	14% 5% 1 4 3 3 4 3 4 4 56%	2600
December	1st to 10th	Night	12% 7% 1 4 3 3 3 3 4 5% 56%	4250
	11th to 20th	Night	16% 6% 2 5% 3 3 3 2 3 5% 52%	4816
	21st to end	Night	15% 7% 2 5% 2 2 2 2 3 5% 55%	6490

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	35% 3 7% 26% 25% 4	4078
	11th to 20th	Day	23% 3 6% 22% 38% 9%	4080
	21st to end	Day	41% 3 3 17% 22% 13%	4488
February	1st to 10th	Day	39% 4 6% 19% 22% 10%	1940
	11th to 20th	Day	39% 3 8% 21% 21% 9%	1959
	21st to end	Day	37% 3 4 16% 26% 15%	1650
March	1st to 10th	Day	32% 3 7% 15% 25% 18%	4933
	11th to 20th	Day	30% 4 8% 18% 18% 22%	4980
	21st to end	Day	34% 5% 10% 24% 21% 7%	5464
April	1st to 10th	Day	35% 5% 10% 20% 19% 11%	2200
	11th to 20th	Day	36% 5% 10% 19% 20% 10%	2259
	21st to end	Day	35% 5% 9% 22% 23% 5%	2594
May	1st to 10th	Day	41% 5% 11% 23% 16% 5%	5900
	11th to 20th	Day	39% 4 8% 21% 23% 4	5866
	21st to end	Day	42% 6% 11% 23% 13% 5%	6447
June	1st to 10th	Day	27% 8% 16% 28% 17% 3	2600
	11th to 20th	Day	26% 6% 13% 29% 21% 5%	2600
	21st to end	Day	23% 6% 14% 28% 25% 4	2600
July	1st to 10th	Day	38% 4 14% 31% 11% 1	5836
	11th to 20th	Day	18% 3 16% 38% 23% 3	5831
	21st to end	Day	21% 5% 15% 34% 24% 2	6240
August	1st to 10th	Day	13% 4 12% 30% 34% 7%	4220
	11th to 20th	Day	19% 5% 15% 39% 20% 2	4210
	21st to end	Day	20% 6% 19% 38% 15% 3	4192
September	1st to 10th	Day	29% 6% 9% 22% 23% 11%	2200
	11th to 20th	Day	33% 7% 11% 21% 22% 7%	2200
	21st to end	Day	24% 4 7% 24% 29% 13%	2200
October	1st to 10th	Day	13% 3 6% 15% 36% 26%	3531
	11th to 20th	Day	17% 2 5% 21% 36% 18%	3240
	21st to end	Day	23% 2 5% 30% 29% 11%	3538
November	1st to 10th	Day	30% 3 7% 21% 28% 10%	2058
	11th to 20th	Day	23% 3 9% 21% 31% 13%	1889
	21st to end	Day	33% 3 5% 21% 27% 11%	1800
December	1st to 10th	Day	37% 3 5% 29% 23% 5%	2940
	11th to 20th	Day	41% 3 5% 11% 29% 11%	3389
	21st to end	Day	39% 2 4 25% 21% 8%	4488
January	1st to 10th	Night	31% 2 7% 27% 26% 7%	5888
	11th to 20th	Night	19% 2 6% 19% 38% 16%	5900
	21st to end	Night	41% 3 4 14% 23% 14%	6490
February	1st to 10th	Night	35% 3 7% 19% 23% 13%	2475
	11th to 20th	Night	37% 3 8% 18% 24% 10%	2338
	21st to end	Night	34% 2 7% 16% 24% 17%	1980
March	1st to 10th	Night	37% 4 5% 14% 22% 18%	4919
	11th to 20th	Night	28% 4 8% 19% 18% 24%	4980
	21st to end	Night	35% 4 8% 18% 30% 6%	5467
April	1st to 10th	Night	29% 6% 10% 21% 20% 14%	2200
	11th to 20th	Night	32% 5% 10% 20% 21% 14%	2088
	21st to end	Night	30% 5% 11% 23% 26% 6%	1801
May	1st to 10th	Night	42% 5% 11% 22% 15% 5%	4080
	11th to 20th	Night	33% 5% 13% 18% 24% 7%	4054
	21st to end	Night	32% 6% 14% 27% 12% 9%	4452
June	1st to 10th	Night	22% 7% 16% 31% 20% 5%	1800
	11th to 20th	Night	18% 5% 14% 33% 23% 8%	1800
	21st to end	Night	16% 7% 15% 27% 29% 6%	1800
July	1st to 10th	Night	33% 4 15% 33% 13% 2	4035
	11th to 20th	Night	15% 3 14% 37% 27% 4	4026
	21st to end	Night	18% 4 15% 35% 27% 2	4320
August	1st to 10th	Night	10% 3 9% 31% 42% 7%	2916
	11th to 20th	Night	14% 3 13% 38% 27% 5%	2915
	21st to end	Night	17% 4 20% 35% 21% 3	3705
September	1st to 10th	Night	26% 5% 9% 23% 26% 11%	2200
	11th to 20th	Night	30% 6% 10% 22% 23% 11%	2200
	21st to end	Night	20% 4 9% 23% 28% 17%	2200
October	1st to 10th	Night	12% 2 5% 15% 32% 33%	3614
	11th to 20th	Night	14% 2 5% 20% 33% 25%	3899
	21st to end	Night	25% 2 4 24% 30% 15%	4252
November	1st to 10th	Night	26% 3 7% 21% 28% 15%	2460
	11th to 20th	Night	21% 4 8% 23% 29% 15%	2661
	21st to end	Night	33% 4 7% 17% 28% 12%	2600
December	1st to 10th	Night	33% 3 5% 23% 27% 9%	4250
	11th to 20th	Night	37% 3 5% 15% 26% 14%	4817
	21st to end	Night	35% 2 6% 21% 27% 10%	6490

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	19% 14% 33% 28% 6%	300
	11th to 20th	Day	19% 15% 33% 25% 7% 2	300
	21st to end	Day	20% 11% 38% 23% 8% 1	330
February	1st to 10th	Day	19% 12% 40% 21% 7% 1	300
	11th to 20th	Day	23% 15% 41% 17% 5%	300
	21st to end	Day	27% 10% 40% 17% 5%	249
March	1st to 10th	Day	23% 18% 33% 20% 6%	300
	11th to 20th	Day	23% 15% 40% 17% 4 1	300
	21st to end	Day	14% 16% 44% 19% 7%	330
April	1st to 10th	Day	18% 16% 37% 22% 6% 1	300
	11th to 20th	Day	17% 13% 39% 24% 6%	377
	21st to end	Day	16% 12% 39% 24% 8% 1	694
May	1st to 10th	Day	14% 10% 34% 33% 8% 1	703
	11th to 20th	Day	18% 7% 28% 31% 12% 3	725
	21st to end	Day	15% 6% 28% 32% 16% 4	789
June	1st to 10th	Day	7% 3 15% 40% 28% 7%	730
	11th to 20th	Day	6% 2 15% 40% 31% 7%	730
	21st to end	Day	4 1 13% 32% 38% 12%	730
July	1st to 10th	Day	6% 1 7% 32% 38% 16%	721
	11th to 20th	Day	2 1 5% 31% 45% 16%	718
	21st to end	Day	3 1 6% 28% 45% 18%	794
August	1st to 10th	Day	2 1 5% 28% 48% 15%	726
	11th to 20th	Day	7% 1 4 27% 46% 14%	715
	21st to end	Day	4 1 5% 24% 43% 22%	516
September	1st to 10th	Day	6% 2 5% 22% 41% 24%	330
	11th to 20th	Day	5% 1 6% 25% 43% 19%	330
	21st to end	Day	12% 1 8% 27% 34% 18%	330
October	1st to 10th	Day	3 1 16% 28% 39% 13%	330
	11th to 20th	Day	9% 5% 30% 29% 22% 5%	330
	21st to end	Day	22% 8% 23% 25% 18% 4	363
November	1st to 10th	Day	19% 4 31% 26% 17% 3	330
	11th to 20th	Day	10% 7% 33% 28% 16% 5%	321
	21st to end	Day	15% 14% 36% 20% 13% 2	300
December	1st to 10th	Day	17% 13% 30% 26% 10% 4	300
	11th to 20th	Day	18% 9% 37% 28% 9%	300
	21st to end	Day	22% 12% 33% 26% 7%	330
January	1st to 10th	Night	13% 8% 37% 33% 9% 1	1000
	11th to 20th	Night	14% 13% 33% 34% 6%	1000
	21st to end	Night	15% 10% 34% 35% 7%	1100
February	1st to 10th	Night	16% 9% 39% 31% 5%	875
	11th to 20th	Night	19% 12% 37% 27% 4	784
	21st to end	Night	20% 13% 40% 24% 2	660
March	1st to 10th	Night	17% 11% 37% 31% 5%	600
	11th to 20th	Night	17% 9% 39% 32% 4	600
	21st to end	Night	11% 12% 41% 31% 4	660
April	1st to 10th	Night	11% 13% 42% 29% 5%	600
	11th to 20th	Night	14% 12% 41% 28% 5%	593
	21st to end	Night	14% 8% 41% 30% 7% 1	600
May	1st to 10th	Night	12% 6% 39% 36% 7% 1	600
	11th to 20th	Night	10% 6% 34% 36% 12% 1	591
	21st to end	Night	8% 4 31% 40% 15% 2	650
June	1st to 10th	Night	5% 3 17% 49% 25% 2	600
	11th to 20th	Night	4 2 19% 56% 20% 1	600
	21st to end	Night	2 1 16% 45% 33% 4	600
July	1st to 10th	Night	4 1 11% 43% 37% 5%	585
	11th to 20th	Night	3 8% 44% 41% 3	582
	21st to end	Night	1 1 6% 44% 46% 3	657
August	1st to 10th	Night	2 7% 45% 43% 3	597
	11th to 20th	Night	6% 1 6% 43% 40% 5%	594
	21st to end	Night	4 1 4 43% 42% 6%	660
September	1st to 10th	Night	8% 1 7% 40% 39% 5%	600
	11th to 20th	Night	10% 2 6% 42% 36% 3	600
	21st to end	Night	5% 1 15% 43% 32% 3	600
October	1st to 10th	Night	7% 1 19% 41% 30% 2	611
	11th to 20th	Night	10% 5% 25% 41% 17% 2	800
	21st to end	Night	17% 8% 27% 33% 15%	880
November	1st to 10th	Night	10% 4 30% 40% 15% 1	820
	11th to 20th	Night	8% 7% 34% 39% 12% 1	1005
	21st to end	Night	17% 9% 33% 29% 11%	1000
December	1st to 10th	Night	14% 9% 33% 37% 7%	1000
	11th to 20th	Night	18% 9% 33% 34% 6%	1000
	21st to end	Night	17% 9% 37% 31% 6%	1100

Page generated at 2025-11-14 23:55:57. Version ac07

Copyright © 2025 Attilla Danko.