Rachel
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rachel clear sky chart from 2005-11-06 to 2024-01-17.

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	2 3 2 3 3 2 3 4 5% 7% 66%	75141
	Night	2 3 2 3 3 2 4 4 5% 7% 64%	74702
Transparency	Day	18% 5% 8% 17% 27% 26%	74856
	Night	19% 5% 8% 19% 27% 22%	74508
Seeing	Day	4 9% 20% 30% 24% 12%	13979
	Night	3 5% 18% 40% 30% 4	23813

By Month

Percentage Cloud Cover SW (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	3 4 2 4 4 2 3 4 5% 9% 58%	1519
	11th to 20th	Day	1 4 3 5% 3 3 3 4 7% 9% 58%	1579
	21st to end	Day	3 5% 2 4 4 3 4 5% 6% 9% 55%	1811
February	1st to 10th	Day	2 4 1 4 5% 2 4 4 5% 10% 60%	1800
	11th to 20th	Day	4 5% 2 4 4 3 4 4 5% 9% 57%	1750
	21st to end	Day	3 5% 2 4 4 3 4 4 5% 7% 58%	1480
March	1st to 10th	Day	3 5% 3 5% 5% 3 4 4 5% 7% 56%	1800
	11th to 20th	Day	3 4 3 4 4 3 4 4 7% 9% 55%	1938
	21st to end	Day	2 4 2 4 4 3 4 4 5% 7% 60%	2208
April	1st to 10th	Day	3 3 2 4 4 3 4 4 4 7% 61%	2160
	11th to 20th	Day	2 4 2 3 4 2 3 4 5% 6% 65%	2129
	21st to end	Day	4 4 1 3 3 2 3 3 4 6% 68%	2293
May	1st to 10th	Day	2 3 1 3 3 2 3 4 5% 8% 66%	2340
	11th to 20th	Day	2 3 2 3 3 2 3 3 5% 6% 68%	2301
	21st to end	Day	2 3 2 3 4 3 5% 5% 5% 7% 61%	2530
June	1st to 10th	Day	1 2 1 2 3 2 3 3 5% 7% 70%	2353
	11th to 20th	Day	1 2 1 2 2 1 2 3 3 5% 77%	2520
	21st to end	Day	1 1 1 2 2 1 2 2 3 5% 80%	2520
July	1st to 10th	Day	2 2 1 2 3 3 4 4 4 5% 70%	2520
	11th to 20th	Day	3 2 2 3 4 2 3 4 5% 6% 65%	2461
	21st to end	Day	3 3 2 5% 5% 4 6% 5% 6% 9% 53%	2545
August	1st to 10th	Day	3 2 1 3 3 2 4 3 4 5% 70%	2328
	11th to 20th	Day	1 1 1 2 2 3 4 5% 7% 73%	2160
	21st to end	Day	1 2 1 2 2 2 4 4 4 6% 72%	2376
September	1st to 10th	Day	1 2 1 3 3 2 4 4 3 5% 72%	2112
	11th to 20th	Day	2 1 1 2 2 1 2 2 2 4 81%	1980
	21st to end	Day	1 1 1 2 2 1 2 3 3 5% 80%	1980
October	1st to 10th	Day	2 2 1 2 3 2 3 2 4 5% 72%	1980
	11th to 20th	Day	2 2 1 1 2 1 2 2 3 6% 76%	1931
	21st to end	Day	2 2 1 2 2 1 2 3 3 5% 79%	1787
November	1st to 10th	Day	2 3 1 2 2 2 3 3 5% 7% 70%	1674
	11th to 20th	Day	1 2 1 2 3 2 2 3 4 7% 72%	1710
	21st to end	Day	2 4 1 3 4 2 3 4 5% 7% 64%	1710
December	1st to 10th	Day	3 4 2 5% 4 3 3 4 4 6% 61%	1710
	11th to 20th	Day	4 7% 4 6% 5% 3 4 6% 7% 10% 45%	2169
	21st to end	Day	2 5% 2 4 4 3 6% 6% 7% 10% 50%	2977
January	1st to 10th	Night	2 5% 3 4 5% 3 4 5% 6% 11% 53%	2646
	11th to 20th	Night	2 3 2 3 3 2 3 5% 5% 11% 60%	2519
	21st to end	Night	4 5% 2 5% 5% 3 4 5% 6% 9% 52%	2542
February	1st to 10th	Night	3 4 1 3 4 3 5% 5% 6% 10% 57%	2160
	11th to 20th	Night	3 4 2 3 2 3 5% 5% 6% 9% 58%	2098
	21st to end	Night	3 6% 3 5% 4 2 3 3 5% 9% 56%	1776
March	1st to 10th	Night	2 3 3 5% 4 3 4 5% 5% 10% 55%	2160
	11th to 20th	Night	3 6% 2 4 5% 3 5% 5% 6% 8% 53%	2021
	21st to end	Night	3 5% 3 4 3 2 3 4 5% 7% 62%	2145
April	1st to 10th	Night	3 2 1 3 3 2 4 5% 7% 7% 64%	1800
	11th to 20th	Night	3 2 2 3 3 2 3 3 5% 5% 69%	1779
	21st to end	Night	4 4 2 3 3 2 3 3 5% 6% 65%	1666
May	1st to 10th	Night	3 3 1 4 4 3 3 3 5% 7% 65%	1620
	11th to 20th	Night	3 3 1 3 2 2 3 4 4 6% 69%	1594
	21st to end	Night	3 2 2 4 4 3 4 5% 4 8% 61%	1746
June	1st to 10th	Night	1 1 1 2 2 1 3 3 4 6% 74%	1601
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 4 78%	1440
	21st to end	Night	1 1 2 3 1 2 3 3 5% 80%	1440
July	1st to 10th	Night	2 2 1 3 3 2 3 3 3 5% 74%	1440
	11th to 20th	Night	2 2 2 3 3 2 4 4 4 7% 67%	1496
	21st to end	Night	2 3 2 4 5% 4 6% 5% 5% 8% 55%	1773
August	1st to 10th	Night	3 3 1 3 4 2 3 3 4 6% 69%	1611
	11th to 20th	Night	2 1 1 2 2 1 3 3 4 5% 75%	1738
	21st to end	Night	1 2 2 3 1 2 3 4 5% 76%	1980
September	1st to 10th	Night	1 1 1 2 3 2 3 3 4 5% 75%	1846
	11th to 20th	Night	1 2 1 2 2 1 2 2 3 5% 78%	1980
	21st to end	Night	1 1 1 1 2 2 2 3 2 5% 82%	1980
October	1st to 10th	Night	2 3 1 2 2 1 3 3 4 5% 73%	1980
	11th to 20th	Night	2 1 1 2 2 1 2 3 2 6% 78%	2029
	21st to end	Night	1 1 1 2 2 2 2 3 3 5% 79%	2564
November	1st to 10th	Night	2 2 1 3 2 2 2 3 3 6% 73%	2405
	11th to 20th	Night	1 2 1 2 3 1 3 3 4 8% 70%	2470
	21st to end	Night	2 3 1 3 3 2 3 4 4 7% 67%	2470
December	1st to 10th	Night	3 5% 2 5% 4 2 4 4 5% 7% 58%	2470
	11th to 20th	Night	4 6% 2 5% 5% 3 4 4 6% 11% 49%	3041
	21st to end	Night	2 6% 3 7% 6% 4 6% 6% 6% 9% 44%	4676

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	23% 5% 15% 31% 22% 3	1519
	11th to 20th	Day	21% 6% 14% 30% 21% 9%	1579
	21st to end	Day	26% 5% 12% 29% 22% 5%	1811
February	1st to 10th	Day	21% 5% 11% 26% 27% 10%	1800
	11th to 20th	Day	23% 5% 8% 24% 29% 11%	1760
	21st to end	Day	25% 6% 10% 25% 26% 8%	1480
March	1st to 10th	Day	27% 5% 8% 22% 30% 7%	1800
	11th to 20th	Day	23% 5% 8% 25% 28% 11%	1938
	21st to end	Day	23% 5% 11% 28% 25% 9%	2208
April	1st to 10th	Day	24% 5% 8% 21% 25% 17%	2160
	11th to 20th	Day	20% 4 7% 19% 27% 22%	2132
	21st to end	Day	18% 4 6% 21% 36% 14%	2293
May	1st to 10th	Day	17% 5% 8% 16% 36% 19%	2340
	11th to 20th	Day	17% 3 7% 16% 36% 21%	2304
	21st to end	Day	21% 6% 8% 14% 33% 18%	2530
June	1st to 10th	Day	13% 4 6% 11% 30% 36%	2353
	11th to 20th	Day	10% 3 4 8% 25% 50%	2520
	21st to end	Day	8% 2 3 6% 21% 60%	2520
July	1st to 10th	Day	16% 4 5% 7% 12% 57%	2451
	11th to 20th	Day	19% 4 6% 11% 18% 42%	2386
	21st to end	Day	25% 7% 8% 14% 17% 29%	2404
August	1st to 10th	Day	16% 3 5% 7% 19% 49%	2313
	11th to 20th	Day	9% 4 8% 9% 19% 51%	2160
	21st to end	Day	12% 3 6% 9% 23% 47%	2376
September	1st to 10th	Day	15% 4 6% 7% 18% 51%	2112
	11th to 20th	Day	9% 3 5% 11% 27% 46%	1980
	21st to end	Day	10% 2 4 9% 37% 38%	1980
October	1st to 10th	Day	16% 4 5% 12% 32% 30%	1980
	11th to 20th	Day	12% 2 5% 15% 35% 32%	1931
	21st to end	Day	11% 2 4 21% 40% 22%	1787
November	1st to 10th	Day	15% 3 8% 24% 34% 16%	1674
	11th to 20th	Day	13% 3 8% 26% 39% 11%	1710
	21st to end	Day	21% 4 8% 23% 31% 14%	1710
December	1st to 10th	Day	24% 6% 10% 22% 29% 9%	1710
	11th to 20th	Day	32% 9% 12% 21% 20% 6%	2168
	21st to end	Day	24% 10% 13% 25% 22% 6%	2977
January	1st to 10th	Night	25% 7% 12% 31% 22% 4	2646
	11th to 20th	Night	18% 4 14% 31% 23% 10%	2519
	21st to end	Night	27% 6% 12% 25% 23% 7%	2542
February	1st to 10th	Night	22% 5% 10% 27% 27% 8%	2160
	11th to 20th	Night	21% 5% 9% 21% 31% 13%	2099
	21st to end	Night	26% 5% 10% 23% 27% 8%	1776
March	1st to 10th	Night	24% 6% 10% 25% 27% 8%	2160
	11th to 20th	Night	27% 7% 9% 22% 23% 13%	2021
	21st to end	Night	22% 5% 11% 30% 24% 8%	2145
April	1st to 10th	Night	17% 5% 10% 27% 26% 15%	1800
	11th to 20th	Night	18% 3 7% 24% 30% 18%	1779
	21st to end	Night	20% 4 6% 19% 37% 14%	1666
May	1st to 10th	Night	19% 5% 7% 15% 37% 18%	1620
	11th to 20th	Night	16% 4 7% 16% 34% 23%	1595
	21st to end	Night	20% 6% 9% 13% 35% 17%	1746
June	1st to 10th	Night	12% 4 5% 12% 30% 37%	1601
	11th to 20th	Night	13% 2 3 10% 25% 48%	1440
	21st to end	Night	8% 3 3 7% 20% 59%	1440
July	1st to 10th	Night	15% 2 5% 7% 11% 61%	1400
	11th to 20th	Night	17% 4 6% 10% 18% 46%	1448
	21st to end	Night	26% 7% 9% 13% 16% 30%	1674
August	1st to 10th	Night	17% 3 5% 10% 18% 47%	1602
	11th to 20th	Night	10% 5% 6% 11% 20% 49%	1738
	21st to end	Night	11% 3 4 11% 24% 47%	1980
September	1st to 10th	Night	13% 4 5% 7% 18% 53%	1846
	11th to 20th	Night	11% 3 5% 10% 25% 46%	1980
	21st to end	Night	8% 2 3 13% 32% 42%	1980
October	1st to 10th	Night	15% 3 6% 14% 29% 34%	1980
	11th to 20th	Night	11% 3 4 17% 31% 34%	2029
	21st to end	Night	10% 3 6% 17% 42% 22%	2564
November	1st to 10th	Night	15% 3 7% 23% 35% 18%	2405
	11th to 20th	Night	14% 3 9% 21% 39% 14%	2470
	21st to end	Night	18% 4 6% 21% 36% 16%	2470
December	1st to 10th	Night	25% 5% 11% 22% 26% 10%	2470
	11th to 20th	Night	27% 7% 12% 25% 22% 6%	3041
	21st to end	Night	31% 10% 12% 20% 22% 5%	4676

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	9% 11% 19% 37% 19% 4	180
	11th to 20th	Day	5% 8% 21% 34% 23% 9%	213
	21st to end	Day	4 13% 15% 30% 29% 10%	293
February	1st to 10th	Day	2 17% 23% 27% 22% 8%	420
	11th to 20th	Day	8% 13% 26% 33% 16% 5%	415
	21st to end	Day	7% 12% 31% 32% 15% 4	346
March	1st to 10th	Day	9% 17% 28% 25% 19% 3	420
	11th to 20th	Day	9% 13% 27% 29% 17% 5%	420
	21st to end	Day	8% 21% 29% 26% 14% 3	462
April	1st to 10th	Day	8% 16% 23% 30% 17% 7%	420
	11th to 20th	Day	7% 22% 26% 25% 15% 5%	412
	21st to end	Day	7% 15% 26% 25% 17% 10%	420
May	1st to 10th	Day	4 13% 31% 27% 18% 6%	423
	11th to 20th	Day	4 12% 25% 26% 23% 10%	448
	21st to end	Day	5% 10% 21% 27% 24% 12%	485
June	1st to 10th	Day	3 10% 20% 29% 25% 14%	450
	11th to 20th	Day	3 11% 23% 29% 22% 13%	450
	21st to end	Day	2 5% 18% 28% 30% 17%	450
July	1st to 10th	Day	3 1 16% 36% 27% 17%	446
	11th to 20th	Day	3 1 14% 34% 29% 20%	444
	21st to end	Day	3 2 8% 37% 30% 21%	487
August	1st to 10th	Day	4 3 20% 30% 28% 16%	447
	11th to 20th	Day	2 1 16% 30% 29% 24%	437
	21st to end	Day	2 3 19% 27% 32% 18%	495
September	1st to 10th	Day	2 4 17% 26% 26% 25%	450
	11th to 20th	Day	2 8% 23% 26% 26% 16%	450
	21st to end	Day	2 6% 16% 28% 32% 16%	450
October	1st to 10th	Day	2 13% 17% 28% 23% 16%	450
	11th to 20th	Day	2 7% 19% 25% 31% 15%	401
	21st to end	Day	4 7% 15% 31% 31% 11%	297
November	1st to 10th	Day	6% 7% 16% 27% 27% 16%	270
	11th to 20th	Day	4 6% 12% 39% 25% 14%	270
	21st to end	Day	4 7% 17% 30% 28% 14%	270
December	1st to 10th	Day	6% 5% 16% 41% 24% 9%	270
	11th to 20th	Day	4 6% 16% 40% 26% 8%	270
	21st to end	Day	8% 11% 19% 33% 24% 4	248
January	1st to 10th	Night	6% 4 19% 43% 25% 4	760
	11th to 20th	Night	3 4 20% 47% 23% 3	748
	21st to end	Night	6% 4 21% 44% 22% 3	792
February	1st to 10th	Night	4 5% 23% 43% 23% 3	720
	11th to 20th	Night	4 5% 23% 44% 21% 2	704
	21st to end	Night	5% 7% 26% 43% 17% 2	592
March	1st to 10th	Night	3 10% 23% 44% 18% 2	720
	11th to 20th	Night	4 8% 21% 38% 26% 3	720
	21st to end	Night	4 13% 27% 40% 16% 2	792
April	1st to 10th	Night	3 10% 25% 39% 22% 1	720
	11th to 20th	Night	4 11% 29% 35% 19% 2	711
	21st to end	Night	5% 8% 27% 36% 20% 4	586
May	1st to 10th	Night	4 9% 25% 44% 17% 1	540
	11th to 20th	Night	5% 6% 21% 36% 29% 2	531
	21st to end	Night	4 9% 21% 37% 27% 2	584
June	1st to 10th	Night	2 6% 16% 42% 33% 2	521
	11th to 20th	Night	3 3 18% 42% 32% 1	360
	21st to end	Night	2 12% 48% 35% 4	360
July	1st to 10th	Night	2 1 5% 40% 48% 4	350
	11th to 20th	Night	2 8% 40% 46% 3	404
	21st to end	Night	3 1 5% 39% 48% 3	591
August	1st to 10th	Night	3 2 11% 49% 33% 2	537
	11th to 20th	Night	1 1 7% 34% 52% 5%	670
	21st to end	Night	2 1 7% 36% 49% 6%	792
September	1st to 10th	Night	1 2 10% 32% 46% 9%	720
	11th to 20th	Night	1 3 13% 35% 42% 6%	720
	21st to end	Night	1 3 14% 34% 40% 8%	720
October	1st to 10th	Night	3 3 17% 41% 30% 6%	720
	11th to 20th	Night	2 3 14% 38% 37% 6%	720
	21st to end	Night	2 4 18% 36% 35% 5%	792
November	1st to 10th	Night	3 5% 15% 38% 34% 5%	740
	11th to 20th	Night	1 4 14% 47% 30% 4	760
	21st to end	Night	4 3 18% 42% 27% 6%	760
December	1st to 10th	Night	5% 4 19% 46% 21% 4	760
	11th to 20th	Night	8% 3 20% 42% 24% 3	760
	21st to end	Night	5% 6% 25% 43% 19% 2	836

Page generated at 2024-01-14 06:55:04. Version ac07

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