Rachel's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rachel's Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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	19% 10% 3 9% 6% 5% 5% 4 5% 6% 28%	101629
	Night	18% 11% 4 8% 6% 5% 4 4 5% 6% 29%	97541
Transparency	Day	30% 48% 8% 10% 5%	105104
	Night	30% 48% 7% 10% 5%	102156
Seeing	Day	27% 18% 30% 17% 7% 1	15990
	Night	25% 15% 30% 23% 7%	23939

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	15% 14% 5% 11% 8% 7% 5% 4 5% 6% 21%	2207
	11th to 20th	Day	14% 11% 4 17% 12% 7% 5% 4 5% 5% 16%	2493
	21st to end	Day	9% 9% 4 13% 7% 8% 6% 5% 7% 6% 26%	2772
February	1st to 10th	Day	11% 16% 4 10% 7% 7% 7% 5% 6% 7% 21%	1669
	11th to 20th	Day	14% 15% 6% 11% 8% 7% 6% 3 6% 6% 18%	1672
	21st to end	Day	13% 12% 5% 10% 8% 7% 5% 4 5% 6% 25%	1570
March	1st to 10th	Day	18% 16% 4 9% 5% 4 4 3 5% 6% 26%	3164
	11th to 20th	Day	20% 21% 5% 11% 5% 4 4 3 3 5% 18%	3480
	21st to end	Day	24% 15% 4 9% 5% 4 3 2 4 5% 26%	3816
April	1st to 10th	Day	21% 15% 4 10% 5% 4 3 2 5% 6% 25%	2289
	11th to 20th	Day	20% 12% 3 7% 5% 4 4 3 5% 5% 32%	2442
	21st to end	Day	25% 11% 3 8% 4 3 4 3 4 5% 31%	2470
May	1st to 10th	Day	18% 11% 4 10% 6% 5% 5% 4 5% 6% 27%	4106
	11th to 20th	Day	22% 9% 3 8% 5% 4 4 3 5% 6% 30%	4401
	21st to end	Day	19% 8% 3 8% 5% 5% 5% 4 7% 7% 29%	5185
June	1st to 10th	Day	22% 8% 2 7% 6% 6% 5% 4 5% 7% 28%	2850
	11th to 20th	Day	21% 7% 3 7% 6% 5% 6% 3 4 7% 32%	2850
	21st to end	Day	23% 10% 3 8% 6% 5% 5% 4 6% 6% 25%	2850
July	1st to 10th	Day	17% 7% 3 8% 6% 6% 6% 5% 7% 8% 29%	4760
	11th to 20th	Day	16% 7% 3 7% 5% 6% 6% 5% 7% 8% 32%	4710
	21st to end	Day	15% 6% 2 6% 5% 4 5% 4 6% 9% 38%	4814
August	1st to 10th	Day	17% 5% 3 8% 6% 5% 6% 5% 6% 8% 31%	4152
	11th to 20th	Day	19% 8% 3 8% 5% 5% 5% 5% 7% 7% 27%	4045
	21st to end	Day	19% 7% 3 6% 4 4 4 4 6% 8% 35%	4532
September	1st to 10th	Day	18% 7% 3 6% 4 4 4 4 5% 7% 38%	2267
	11th to 20th	Day	18% 6% 2 6% 4 3 4 3 3 6% 45%	2090
	21st to end	Day	21% 7% 3 6% 3 3 4 3 6% 6% 38%	2090
October	1st to 10th	Day	19% 10% 3 7% 4 4 3 3 5% 6% 36%	1990
	11th to 20th	Day	25% 10% 2 8% 5% 5% 4 4 5% 5% 27%	1867
	21st to end	Day	27% 13% 3 8% 5% 3 4 3 4 6% 25%	1881
November	1st to 10th	Day	21% 14% 5% 9% 5% 4 5% 3 4 4 26%	1709
	11th to 20th	Day	21% 12% 4 10% 7% 5% 5% 3 4 5% 23%	1520
	21st to end	Day	22% 16% 5% 11% 6% 4 3 3 4 6% 21%	1487
December	1st to 10th	Day	23% 15% 5% 9% 8% 5% 4 3 5% 4 19%	1330
	11th to 20th	Day	19% 16% 5% 10% 7% 5% 4 4 5% 6% 21%	1668
	21st to end	Day	21% 13% 5% 10% 6% 5% 5% 3 4 4 24%	2431
January	1st to 10th	Night	11% 15% 7% 13% 12% 9% 6% 4 5% 6% 14%	4748
	11th to 20th	Night	14% 14% 5% 13% 9% 6% 5% 4 5% 5% 21%	4466
	21st to end	Night	9% 13% 5% 10% 6% 6% 5% 4 6% 7% 29%	4884
February	1st to 10th	Night	11% 14% 4 9% 6% 7% 6% 5% 6% 7% 25%	2509
	11th to 20th	Night	13% 16% 5% 10% 7% 6% 6% 5% 6% 5% 20%	2391
	21st to end	Night	12% 14% 5% 9% 7% 5% 5% 5% 4 6% 29%	1884
March	1st to 10th	Night	17% 13% 4 9% 5% 6% 7% 4 4 5% 27%	3675
	11th to 20th	Night	17% 19% 5% 11% 6% 5% 4 3 4 4 20%	3479
	21st to end	Night	26% 14% 4 9% 4 3 3 3 4 5% 26%	3816
April	1st to 10th	Night	22% 13% 3 7% 4 3 4 3 3 5% 31%	1890
	11th to 20th	Night	20% 10% 3 6% 4 4 5% 4 4 5% 34%	1688
	21st to end	Night	25% 10% 3 6% 4 4 3 4 5% 4 32%	1710
May	1st to 10th	Night	21% 10% 3 7% 4 4 3 3 5% 6% 34%	2834
	11th to 20th	Night	21% 10% 4 8% 5% 5% 5% 4 5% 6% 27%	2524
	21st to end	Night	17% 6% 3 6% 5% 4 4 5% 5% 9% 37%	2401
June	1st to 10th	Night	24% 8% 2 7% 4 4 4 3 5% 7% 32%	1330
	11th to 20th	Night	16% 6% 3 5% 5% 5% 4 4 7% 7% 38%	1330
	21st to end	Night	24% 8% 4 8% 5% 4 4 3 4 7% 31%	1330
July	1st to 10th	Night	13% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	2210
	11th to 20th	Night	18% 6% 2 6% 4 5% 6% 4 6% 7% 36%	2254
	21st to end	Night	13% 6% 2 6% 4 5% 4 4 6% 8% 42%	2739
August	1st to 10th	Night	21% 6% 3 7% 6% 5% 5% 4 5% 6% 32%	2745
	11th to 20th	Night	15% 7% 4 9% 6% 5% 5% 4 7% 8% 30%	2782
	21st to end	Night	18% 7% 2 6% 4 3 3 2 4 6% 44%	3124
September	1st to 10th	Night	17% 6% 2 5% 4 4 4 4 5% 8% 42%	1909
	11th to 20th	Night	18% 6% 2 4 4 3 3 3 5% 6% 45%	2090
	21st to end	Night	18% 7% 2 5% 4 3 4 3 4 7% 43%	2090
October	1st to 10th	Night	21% 7% 3 6% 4 3 3 3 4 6% 41%	2190
	11th to 20th	Night	22% 9% 2 5% 4 4 5% 4 5% 7% 32%	2308
	21st to end	Night	24% 11% 3 7% 4 4 4 3 5% 7% 29%	2717
November	1st to 10th	Night	21% 11% 3 8% 6% 4 3 4 5% 5% 30%	2528
	11th to 20th	Night	21% 12% 4 10% 5% 4 5% 4 4 6% 26%	2660
	21st to end	Night	20% 15% 4 10% 6% 4 5% 3 5% 5% 22%	2689
December	1st to 10th	Night	23% 15% 5% 9% 7% 4 4 3 4 5% 21%	2850
	11th to 20th	Night	24% 11% 4 10% 8% 5% 5% 4 5% 5% 18%	3531
	21st to end	Night	19% 17% 4 9% 7% 5% 4 3 5% 5% 22%	5236

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	37% 39% 11% 10% 2	2208
	11th to 20th	Day	45% 40% 5% 7% 3	2493
	21st to end	Day	31% 45% 7% 10% 7%	2772
February	1st to 10th	Day	20% 65% 4 6% 4 1	1669
	11th to 20th	Day	21% 64% 3 8% 3	1680
	21st to end	Day	20% 65% 3 6% 5% 1	1570
March	1st to 10th	Day	35% 42% 7% 7% 7% 2	3168
	11th to 20th	Day	46% 43% 4 4 2 1	3480
	21st to end	Day	40% 40% 3 10% 6% 2	3816
April	1st to 10th	Day	20% 64% 3 7% 5% 2	2289
	11th to 20th	Day	20% 65% 4 7% 2 1	2442
	21st to end	Day	23% 65% 2 5% 4 1	2470
May	1st to 10th	Day	33% 44% 6% 12% 4	4120
	11th to 20th	Day	32% 44% 10% 9% 4	4405
	21st to end	Day	31% 44% 7% 10% 7% 1	5185
June	1st to 10th	Day	20% 66% 4 7% 4	2850
	11th to 20th	Day	20% 66% 4 7% 4	2850
	21st to end	Day	21% 64% 4 7% 3	2850
July	1st to 10th	Day	32% 38% 10% 13% 7%	4685
	11th to 20th	Day	30% 38% 13% 17% 2	4629
	21st to end	Day	25% 38% 14% 17% 6%	4661
August	1st to 10th	Day	30% 40% 12% 12% 5%	4138
	11th to 20th	Day	37% 40% 11% 10% 2	4084
	21st to end	Day	30% 38% 12% 11% 8%	4532
September	1st to 10th	Day	16% 61% 6% 11% 5%	2267
	11th to 20th	Day	18% 61% 4 12% 5%	2090
	21st to end	Day	16% 60% 5% 14% 6%	2090
October	1st to 10th	Day	34% 38% 10% 15% 3	3302
	11th to 20th	Day	38% 37% 7% 16% 3	3083
	21st to end	Day	31% 37% 13% 12% 6%	3081
November	1st to 10th	Day	31% 58% 5% 4 2	1709
	11th to 20th	Day	26% 61% 5% 6% 3	1520
	21st to end	Day	27% 61% 5% 5% 1	1487
December	1st to 10th	Day	28% 59% 3 6% 2	1330
	11th to 20th	Day	33% 48% 4 8% 6%	1668
	21st to end	Day	39% 39% 8% 8% 6% 1	2431
January	1st to 10th	Night	44% 42% 6% 6% 1	4748
	11th to 20th	Night	40% 42% 7% 8% 4	4466
	21st to end	Night	32% 42% 7% 11% 7% 1	4884
February	1st to 10th	Night	19% 66% 4 7% 4 1	2509
	11th to 20th	Night	22% 65% 3 6% 4	2394
	21st to end	Night	17% 65% 4 6% 8% 1	1884
March	1st to 10th	Night	34% 44% 6% 10% 6%	3684
	11th to 20th	Night	43% 43% 5% 5% 3	3480
	21st to end	Night	38% 41% 3 11% 6% 1	3816
April	1st to 10th	Night	19% 64% 2 6% 7% 2	1890
	11th to 20th	Night	20% 65% 2 7% 5% 1	1688
	21st to end	Night	23% 65% 2 5% 5%	1710
May	1st to 10th	Night	29% 44% 9% 12% 6%	2840
	11th to 20th	Night	34% 46% 8% 10% 2	2525
	21st to end	Night	23% 46% 10% 14% 7%	2402
June	1st to 10th	Night	18% 65% 3 9% 4	1330
	11th to 20th	Night	15% 67% 5% 7% 5% 1	1330
	21st to end	Night	19% 63% 6% 8% 3 1	1330
July	1st to 10th	Night	29% 37% 13% 15% 5%	2175
	11th to 20th	Night	32% 36% 14% 16% 3	2212
	21st to end	Night	23% 38% 18% 15% 6%	2651
August	1st to 10th	Night	35% 39% 10% 13% 4	2735
	11th to 20th	Night	34% 41% 14% 9% 3	2809
	21st to end	Night	29% 38% 8% 14% 11%	3124
September	1st to 10th	Night	16% 61% 5% 12% 5%	1909
	11th to 20th	Night	19% 61% 7% 10% 3	2090
	21st to end	Night	14% 60% 4 14% 7% 1	2090
October	1st to 10th	Night	33% 37% 7% 18% 5%	3658
	11th to 20th	Night	33% 37% 7% 20% 3	3831
	21st to end	Night	28% 38% 15% 12% 7%	4467
November	1st to 10th	Night	27% 57% 6% 7% 2	2528
	11th to 20th	Night	24% 61% 5% 6% 4	2660
	21st to end	Night	26% 60% 5% 5% 2	2689
December	1st to 10th	Night	26% 60% 3 7% 3	2850
	11th to 20th	Night	33% 52% 5% 7% 3	3532
	21st to end	Night	37% 40% 8% 11% 5%	5236

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	29% 26% 26% 16% 3 1	180
	11th to 20th	Day	36% 20% 29% 9% 6%	180
	21st to end	Day	20% 24% 34% 13% 8% 1	198
February	1st to 10th	Day	25% 24% 32% 12% 6% 1	329
	11th to 20th	Day	22% 28% 31% 15% 5%	370
	21st to end	Day	26% 24% 29% 16% 4	382
March	1st to 10th	Day	27% 22% 33% 15% 3	460
	11th to 20th	Day	28% 21% 31% 13% 7% 1	460
	21st to end	Day	30% 21% 28% 15% 5% 1	506
April	1st to 10th	Day	32% 24% 27% 12% 4 1	460
	11th to 20th	Day	29% 19% 32% 15% 4 1	452
	21st to end	Day	32% 22% 31% 11% 3 1	460
May	1st to 10th	Day	33% 17% 31% 14% 5%	463
	11th to 20th	Day	28% 16% 32% 17% 6% 1	588
	21st to end	Day	28% 20% 30% 17% 6%	735
June	1st to 10th	Day	29% 17% 30% 17% 6% 1	680
	11th to 20th	Day	26% 16% 26% 22% 10% 1	680
	21st to end	Day	26% 17% 29% 21% 7% 1	680
July	1st to 10th	Day	22% 14% 33% 21% 9% 1	670
	11th to 20th	Day	19% 13% 33% 25% 9% 1	644
	21st to end	Day	18% 12% 36% 25% 8% 1	532
August	1st to 10th	Day	20% 13% 36% 21% 8% 2	486
	11th to 20th	Day	22% 11% 30% 27% 9% 1	479
	21st to end	Day	24% 15% 24% 25% 10% 3	539
September	1st to 10th	Day	20% 16% 32% 20% 10% 2	490
	11th to 20th	Day	22% 15% 28% 22% 9% 3	490
	21st to end	Day	23% 16% 30% 17% 10% 4	490
October	1st to 10th	Day	21% 19% 27% 19% 11% 4	437
	11th to 20th	Day	33% 19% 23% 14% 8% 3	390
	21st to end	Day	36% 19% 25% 14% 6%	429
November	1st to 10th	Day	35% 19% 26% 16% 4	386
	11th to 20th	Day	30% 19% 30% 16% 5% 1	370
	21st to end	Day	36% 19% 28% 14% 4	337
December	1st to 10th	Day	31% 17% 31% 12% 8% 1	180
	11th to 20th	Day	31% 24% 34% 8% 3	180
	21st to end	Day	33% 25% 28% 11% 4	198
January	1st to 10th	Night	25% 19% 34% 19% 3	850
	11th to 20th	Night	28% 20% 30% 17% 4	765
	21st to end	Night	17% 18% 36% 23% 6%	836
February	1st to 10th	Night	18% 23% 35% 19% 5%	760
	11th to 20th	Night	22% 24% 32% 17% 4 1	743
	21st to end	Night	21% 18% 38% 17% 5% 1	628
March	1st to 10th	Night	22% 16% 40% 20% 3	760
	11th to 20th	Night	26% 19% 30% 19% 6%	760
	21st to end	Night	24% 18% 31% 22% 5%	836
April	1st to 10th	Night	31% 18% 27% 20% 4	722
	11th to 20th	Night	24% 16% 35% 18% 6%	564
	21st to end	Night	31% 14% 31% 19% 5%	570
May	1st to 10th	Night	26% 16% 27% 24% 5%	570
	11th to 20th	Night	27% 13% 31% 23% 6%	501
	21st to end	Night	23% 13% 34% 25% 5%	411
June	1st to 10th	Night	25% 7% 30% 31% 7%	380
	11th to 20th	Night	20% 8% 29% 33% 11%	380
	21st to end	Night	26% 6% 27% 32% 9%	380
July	1st to 10th	Night	19% 6% 33% 30% 11% 1	370
	11th to 20th	Night	18% 7% 25% 38% 12%	368
	21st to end	Night	19% 5% 26% 36% 14%	416
August	1st to 10th	Night	19% 4 29% 34% 13% 1	513
	11th to 20th	Night	18% 6% 26% 37% 14%	562
	21st to end	Night	14% 10% 28% 32% 15% 1	627
September	1st to 10th	Night	20% 9% 32% 26% 12%	674
	11th to 20th	Night	21% 11% 26% 28% 13% 2	760
	21st to end	Night	22% 12% 27% 27% 11%	760
October	1st to 10th	Night	24% 13% 26% 27% 10% 1	760
	11th to 20th	Night	26% 13% 31% 19% 10%	760
	21st to end	Night	31% 17% 26% 20% 5%	836
November	1st to 10th	Night	28% 17% 27% 21% 6%	782
	11th to 20th	Night	29% 19% 27% 19% 5% 1	850
	21st to end	Night	30% 18% 27% 20% 4	850
December	1st to 10th	Night	31% 16% 32% 16% 5% 1	850
	11th to 20th	Night	29% 21% 27% 18% 5%	850
	21st to end	Night	31% 20% 27% 17% 4 1	935

Page generated at 2024-11-05 09:15:51. Version ac07

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