Rainwater Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rainwater Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	12% 6% 2 6% 5% 5% 5% 4 6% 7% 42%	100658
	Night	13% 7% 2 5% 4 4 4 3 5% 7% 46%	98374
Transparency	Day	38% 6% 10% 21% 19% 6%	104410
	Night	36% 5% 10% 20% 21% 8%	102762
Seeing	Day	16% 8% 22% 25% 20% 9%	15074
	Night	15% 8% 24% 29% 20% 4	24788

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	16% 14% 2 6% 4 2 3 3 4 6% 41%	2733
	11th to 20th	Day	18% 8% 3 7% 4 3 3 3 4 5% 44%	2850
	21st to end	Day	20% 11% 2 4 3 3 3 2 4 5% 42%	3297
February	1st to 10th	Day	17% 9% 2 6% 5% 3 4 4 6% 7% 39%	1900
	11th to 20th	Day	22% 10% 2 6% 3 2 4 3 4 4 39%	1848
	21st to end	Day	19% 9% 3 7% 5% 4 3 2 4 5% 40%	1570
March	1st to 10th	Day	21% 10% 3 8% 5% 4 4 4 5% 6% 31%	3120
	11th to 20th	Day	15% 8% 2 7% 4 4 5% 3 6% 8% 38%	3444
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 4 5% 39%	4091
April	1st to 10th	Day	12% 5% 3 6% 5% 4 4 3 6% 6% 46%	2280
	11th to 20th	Day	15% 7% 2 6% 4 4 3 4 5% 6% 44%	2254
	21st to end	Day	9% 5% 2 6% 4 4 4 3 5% 7% 49%	2280
May	1st to 10th	Day	12% 4 2 6% 5% 5% 5% 4 6% 9% 43%	3765
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 6% 7% 42%	3917
	21st to end	Day	10% 5% 2 6% 6% 6% 6% 5% 7% 9% 39%	4476
June	1st to 10th	Day	8% 5% 3 7% 6% 6% 6% 5% 6% 9% 38%	2587
	11th to 20th	Day	5% 5% 2 6% 6% 5% 6% 6% 7% 9% 44%	2660
	21st to end	Day	6% 5% 2 6% 7% 6% 5% 5% 8% 9% 42%	2660
July	1st to 10th	Day	6% 6% 2 7% 6% 7% 7% 6% 9% 10% 32%	4440
	11th to 20th	Day	10% 5% 2 7% 7% 7% 8% 7% 10% 11% 27%	4130
	21st to end	Day	13% 8% 3 9% 6% 6% 6% 6% 8% 8% 25%	4227
August	1st to 10th	Day	3 3 1 4 3 4 4 3 6% 6% 62%	3785
	11th to 20th	Day	5% 4 2 7% 5% 6% 5% 5% 7% 10% 45%	3729
	21st to end	Day	8% 4 2 7% 4 5% 5% 5% 6% 8% 46%	4180
September	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 7% 44%	2280
	11th to 20th	Day	12% 5% 2 8% 5% 5% 4 4 4 7% 45%	2186
	21st to end	Day	12% 4 2 5% 4 4 4 4 5% 7% 51%	2090
October	1st to 10th	Day	7% 4 1 3 4 4 4 3 5% 6% 59%	2052
	11th to 20th	Day	11% 4 1 4 3 3 4 4 4 7% 56%	1900
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 5% 54%	2090
November	1st to 10th	Day	10% 6% 2 5% 3 3 3 3 3 6% 56%	1775
	11th to 20th	Day	13% 7% 2 4 4 3 3 2 4 5% 53%	1710
	21st to end	Day	15% 7% 4 5% 4 4 3 3 4 6% 45%	1710
December	1st to 10th	Day	18% 8% 3 6% 4 4 3 3 5% 7% 39%	1710
	11th to 20th	Day	14% 9% 4 6% 4 4 4 3 5% 5% 43%	2149
	21st to end	Day	13% 9% 3 7% 5% 4 4 3 6% 6% 41%	2783
January	1st to 10th	Night	19% 11% 2 5% 3 3 3 3 5% 5% 42%	4204
	11th to 20th	Night	16% 9% 3 7% 4 3 3 2 4 4 46%	4120
	21st to end	Night	23% 11% 3 5% 3 2 2 2 2 3 43%	4360
February	1st to 10th	Night	17% 9% 3 6% 4 3 3 3 5% 6% 40%	2280
	11th to 20th	Night	20% 8% 2 6% 4 3 3 3 3 4 44%	2218
	21st to end	Night	21% 8% 2 7% 4 4 4 3 4 5% 40%	1884
March	1st to 10th	Night	21% 12% 3 6% 4 3 4 3 4 6% 34%	3733
	11th to 20th	Night	14% 7% 2 5% 4 4 4 4 4 8% 44%	3544
	21st to end	Night	17% 7% 3 6% 5% 4 4 4 5% 6% 41%	3538
April	1st to 10th	Night	13% 7% 1 5% 4 3 4 4 5% 6% 48%	1900
	11th to 20th	Night	13% 6% 1 5% 3 4 4 3 5% 7% 49%	1875
	21st to end	Night	9% 5% 2 6% 4 4 4 3 5% 7% 51%	1900
May	1st to 10th	Night	12% 5% 2 6% 4 3 4 4 5% 9% 45%	3130
	11th to 20th	Night	16% 5% 1 5% 4 5% 4 3 6% 7% 43%	2974
	21st to end	Night	8% 5% 2 6% 5% 6% 5% 5% 7% 9% 42%	3079
June	1st to 10th	Night	6% 5% 2 6% 5% 6% 6% 5% 6% 10% 44%	1588
	11th to 20th	Night	4 3 2 4 4 3 4 5% 7% 10% 54%	1520
	21st to end	Night	5% 4 2 5% 5% 4 4 5% 7% 10% 51%	1520
July	1st to 10th	Night	5% 6% 2 7% 5% 4 6% 6% 9% 13% 37%	2530
	11th to 20th	Night	6% 5% 2 7% 4 5% 5% 5% 9% 14% 36%	2834
	21st to end	Night	9% 8% 4 9% 5% 6% 5% 5% 7% 9% 32%	3313
August	1st to 10th	Night	3 1 1 3 2 3 3 3 7% 8% 66%	3144
	11th to 20th	Night	3 2 1 5% 3 4 5% 4 7% 13% 52%	3100
	21st to end	Night	5% 4 2 6% 4 4 5% 5% 7% 8% 51%	3476
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 6% 7% 51%	1900
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 7% 47%	1973
	21st to end	Night	9% 4 2 4 3 3 4 3 5% 7% 56%	2090
October	1st to 10th	Night	6% 3 1 4 3 2 3 3 5% 7% 63%	2123
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 4 7% 55%	2280
	21st to end	Night	16% 5% 2 4 2 3 2 2 3 5% 56%	2508
November	1st to 10th	Night	12% 5% 1 4 3 2 4 3 4 5% 57%	2400
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 6% 54%	2470
	21st to end	Night	15% 6% 2 5% 4 3 3 4 4 6% 49%	2470
December	1st to 10th	Night	16% 9% 2 6% 3 4 4 3 5% 5% 43%	2470
	11th to 20th	Night	15% 7% 3 6% 4 4 3 3 4 5% 47%	3042
	21st to end	Night	16% 10% 3 7% 5% 4 4 4 6% 6% 35%	4884

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	45% 3 4 20% 23% 4	2734
	11th to 20th	Day	44% 4 5% 22% 22% 4	2850
	21st to end	Day	46% 2 7% 16% 21% 8%	3297
February	1st to 10th	Day	44% 4 9% 19% 18% 7%	1900
	11th to 20th	Day	49% 3 5% 17% 18% 8%	1858
	21st to end	Day	49% 4 6% 14% 19% 8%	1570
March	1st to 10th	Day	53% 5% 7% 11% 16% 8%	3120
	11th to 20th	Day	43% 5% 7% 21% 18% 6%	3444
	21st to end	Day	46% 4 7% 19% 18% 5%	4091
April	1st to 10th	Day	38% 4 9% 21% 20% 7%	2280
	11th to 20th	Day	41% 4 6% 18% 25% 7%	2254
	21st to end	Day	34% 5% 9% 25% 24% 3	2280
May	1st to 10th	Day	36% 7% 12% 29% 14% 3	3800
	11th to 20th	Day	38% 7% 12% 27% 15% 1	3921
	21st to end	Day	39% 8% 13% 27% 11% 1	4489
June	1st to 10th	Day	39% 8% 14% 20% 17% 1	2587
	11th to 20th	Day	33% 7% 16% 23% 18% 3	2660
	21st to end	Day	36% 8% 16% 23% 15% 3	2660
July	1st to 10th	Day	41% 11% 18% 21% 6% 2	4371
	11th to 20th	Day	43% 11% 21% 20% 4 1	4060
	21st to end	Day	52% 10% 13% 18% 6%	4093
August	1st to 10th	Day	21% 6% 12% 31% 28% 1	3773
	11th to 20th	Day	32% 8% 18% 27% 13% 2	3765
	21st to end	Day	33% 8% 13% 20% 22% 4	4179
September	1st to 10th	Day	37% 7% 11% 20% 18% 6%	2280
	11th to 20th	Day	40% 6% 11% 16% 21% 6%	2186
	21st to end	Day	32% 5% 10% 24% 20% 9%	2090
October	1st to 10th	Day	17% 3 6% 16% 27% 32%	3416
	11th to 20th	Day	18% 2 3 14% 38% 25%	3140
	21st to end	Day	25% 3 4 18% 35% 15%	3428
November	1st to 10th	Day	32% 3 6% 21% 29% 10%	1775
	11th to 20th	Day	34% 3 5% 19% 26% 13%	1710
	21st to end	Day	41% 3 6% 17% 24% 9%	1710
December	1st to 10th	Day	46% 4 8% 16% 19% 7%	1710
	11th to 20th	Day	44% 3 6% 18% 23% 6%	2146
	21st to end	Day	42% 5% 7% 19% 25% 3	2783
January	1st to 10th	Night	45% 3 5% 15% 28% 4	4204
	11th to 20th	Night	44% 3 4 22% 19% 9%	4120
	21st to end	Night	49% 2 3 19% 23% 5%	4360
February	1st to 10th	Night	45% 4 6% 17% 20% 8%	2280
	11th to 20th	Night	46% 3 5% 18% 19% 10%	2219
	21st to end	Night	48% 4 6% 13% 18% 10%	1884
March	1st to 10th	Night	51% 4 7% 13% 16% 9%	3733
	11th to 20th	Night	39% 5% 8% 19% 19% 10%	3544
	21st to end	Night	45% 4 7% 18% 21% 4	3538
April	1st to 10th	Night	37% 5% 9% 21% 19% 9%	1900
	11th to 20th	Night	36% 3 8% 19% 25% 9%	1875
	21st to end	Night	32% 4 10% 25% 23% 5%	1900
May	1st to 10th	Night	35% 7% 11% 25% 18% 3	3160
	11th to 20th	Night	39% 6% 11% 29% 14% 1	2975
	21st to end	Night	34% 9% 13% 26% 15% 2	3087
June	1st to 10th	Night	35% 7% 15% 23% 19% 2	1588
	11th to 20th	Night	23% 6% 17% 31% 16% 6%	1520
	21st to end	Night	27% 7% 19% 24% 20% 3	1520
July	1st to 10th	Night	35% 12% 23% 19% 9% 2	2490
	11th to 20th	Night	33% 10% 25% 25% 5% 1	2781
	21st to end	Night	46% 9% 16% 20% 7% 1	3207
August	1st to 10th	Night	15% 5% 16% 33% 29% 2	3132
	11th to 20th	Night	23% 7% 23% 29% 15% 2	3130
	21st to end	Night	29% 8% 14% 22% 21% 5%	3476
September	1st to 10th	Night	32% 6% 14% 21% 21% 6%	1900
	11th to 20th	Night	35% 7% 11% 17% 23% 7%	1973
	21st to end	Night	28% 4 9% 27% 22% 10%	2090
October	1st to 10th	Night	14% 2 7% 14% 29% 33%	3539
	11th to 20th	Night	20% 2 4 16% 29% 29%	3779
	21st to end	Night	22% 2 5% 17% 35% 19%	4120
November	1st to 10th	Night	30% 3 8% 18% 28% 13%	2400
	11th to 20th	Night	31% 4 7% 18% 26% 13%	2470
	21st to end	Night	37% 3 6% 21% 21% 11%	2470
December	1st to 10th	Night	43% 4 9% 17% 21% 6%	2470
	11th to 20th	Night	41% 3 7% 16% 27% 6%	3044
	21st to end	Night	46% 5% 8% 15% 24% 3	4884

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% 20% 36% 15% 3	270
	11th to 20th	Day	31% 18% 33% 11% 5% 2	270
	21st to end	Day	26% 23% 36% 12% 2 1	297
February	1st to 10th	Day	25% 19% 37% 13% 6%	270
	11th to 20th	Day	37% 15% 35% 10% 2 1	270
	21st to end	Day	37% 18% 36% 8% 2	225
March	1st to 10th	Day	25% 19% 34% 18% 4	270
	11th to 20th	Day	27% 17% 40% 13% 3	270
	21st to end	Day	22% 17% 36% 16% 8%	458
April	1st to 10th	Day	22% 17% 30% 22% 8% 1	460
	11th to 20th	Day	25% 13% 32% 19% 10% 2	452
	21st to end	Day	15% 12% 31% 30% 11% 1	460
May	1st to 10th	Day	19% 7% 26% 33% 12% 3	463
	11th to 20th	Day	11% 7% 25% 32% 17% 7%	487
	21st to end	Day	9% 3 28% 29% 20% 12%	529
June	1st to 10th	Day	12% 2 19% 28% 28% 10%	607
	11th to 20th	Day	6% 2 18% 32% 31% 11%	680
	21st to end	Day	8% 3 13% 34% 28% 15%	680
July	1st to 10th	Day	8% 2 9% 33% 33% 15%	671
	11th to 20th	Day	10% 2 6% 25% 36% 21%	668
	21st to end	Day	8% 2 8% 26% 37% 18%	602
August	1st to 10th	Day	7% 1 9% 27% 33% 22%	486
	11th to 20th	Day	9% 1 9% 28% 33% 20%	479
	21st to end	Day	15% 2 7% 25% 30% 22%	539
September	1st to 10th	Day	10% 2 9% 31% 36% 11%	490
	11th to 20th	Day	13% 3 11% 26% 33% 15%	490
	21st to end	Day	12% 3 14% 31% 29% 11%	490
October	1st to 10th	Day	10% 4 21% 34% 23% 9%	452
	11th to 20th	Day	10% 9% 32% 35% 12% 3	300
	21st to end	Day	19% 9% 27% 29% 13% 3	330
November	1st to 10th	Day	16% 7% 28% 31% 13% 4	282
	11th to 20th	Day	15% 14% 40% 24% 7%	270
	21st to end	Day	18% 16% 37% 18% 10% 2	270
December	1st to 10th	Day	27% 16% 36% 17% 3 1	270
	11th to 20th	Day	24% 16% 35% 16% 9%	270
	21st to end	Day	30% 12% 38% 15% 3 1	297
January	1st to 10th	Night	24% 15% 35% 22% 4	950
	11th to 20th	Night	22% 16% 31% 24% 6%	950
	21st to end	Night	24% 14% 38% 21% 3	938
February	1st to 10th	Night	23% 14% 35% 23% 6%	760
	11th to 20th	Night	26% 16% 31% 22% 4	743
	21st to end	Night	26% 17% 35% 20% 1	628
March	1st to 10th	Night	21% 14% 31% 27% 7%	760
	11th to 20th	Night	19% 10% 36% 27% 7% 1	608
	21st to end	Night	16% 14% 34% 26% 10%	627
April	1st to 10th	Night	17% 12% 33% 26% 11% 1	570
	11th to 20th	Night	17% 12% 30% 28% 11% 2	563
	21st to end	Night	12% 9% 35% 32% 10% 1	570
May	1st to 10th	Night	13% 5% 29% 36% 16% 2	570
	11th to 20th	Night	12% 2 25% 37% 20% 4	561
	21st to end	Night	6% 2 18% 38% 30% 6%	617
June	1st to 10th	Night	7% 1 12% 39% 33% 8%	570
	11th to 20th	Night	5% 1 9% 34% 39% 11%	570
	21st to end	Night	6% 1 10% 32% 38% 12%	570
July	1st to 10th	Night	6% 1 7% 29% 41% 16%	555
	11th to 20th	Night	6% 1 6% 30% 39% 18%	552
	21st to end	Night	5% 1 6% 29% 45% 14%	624
August	1st to 10th	Night	5% 4 26% 48% 17%	567
	11th to 20th	Night	6% 1 4 31% 45% 12%	564
	21st to end	Night	10% 1 6% 27% 43% 14%	627
September	1st to 10th	Night	10% 3 10% 33% 38% 7%	570
	11th to 20th	Night	13% 2 10% 29% 39% 6%	643
	21st to end	Night	11% 2 13% 34% 35% 5%	760
October	1st to 10th	Night	8% 2 18% 35% 31% 6%	760
	11th to 20th	Night	14% 6% 21% 38% 19% 2	760
	21st to end	Night	19% 6% 22% 32% 20% 2	836
November	1st to 10th	Night	14% 6% 24% 33% 20% 4	760
	11th to 20th	Night	15% 8% 31% 34% 12% 1	760
	21st to end	Night	19% 8% 29% 29% 13% 2	760
December	1st to 10th	Night	23% 10% 31% 28% 8%	760
	11th to 20th	Night	21% 11% 33% 27% 8%	760
	21st to end	Night	25% 13% 32% 25% 5%	1045

Page generated at 2024-11-03 01:11:15. Version ac07

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