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 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	19% 10% 4 9% 6% 5% 5% 4 5% 6% 28%	126932
	Night	18% 11% 4 9% 6% 5% 5% 4 5% 6% 28%	123585
Transparency	Day	36% 39% 8% 12% 5% 1	130389
	Night	36% 39% 8% 12% 6%	128216
Seeing	Day	27% 18% 30% 17% 7% 1	17342
	Night	25% 15% 30% 23% 7%	25282

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% 18% 8% 13% 9% 7% 4 3 3 4 15%	3157
	11th to 20th	Day	12% 12% 6% 18% 12% 7% 4 3 5% 5% 15%	3543
	21st to end	Day	6% 7% 4 16% 7% 7% 7% 6% 7% 6% 26%	3960
February	1st to 10th	Day	10% 16% 4 11% 7% 7% 6% 5% 5% 7% 21%	1757
	11th to 20th	Day	14% 16% 6% 11% 8% 6% 6% 3 5% 6% 19%	1763
	21st to end	Day	13% 12% 5% 11% 8% 7% 5% 4 5% 6% 25%	1650
March	1st to 10th	Day	19% 18% 5% 9% 5% 4 4 3 5% 5% 22%	4538
	11th to 20th	Day	18% 17% 4 10% 6% 5% 5% 3 5% 5% 22%	4980
	21st to end	Day	24% 16% 5% 9% 5% 4 3 2 4 4 24%	5466
April	1st to 10th	Day	21% 15% 4 10% 5% 4 3 3 5% 5% 25%	2410
	11th to 20th	Day	20% 12% 3 7% 5% 4 5% 3 5% 6% 30%	2572
	21st to end	Day	25% 11% 3 8% 4 3 4 3 4 5% 30%	2600
May	1st to 10th	Day	23% 11% 4 9% 6% 5% 5% 3 4 5% 26%	5886
	11th to 20th	Day	26% 8% 3 8% 5% 4 4 3 4 5% 28%	6321
	21st to end	Day	21% 8% 3 8% 5% 5% 5% 4 6% 7% 29%	7451
June	1st to 10th	Day	22% 8% 2 7% 6% 6% 5% 4 5% 7% 29%	3000
	11th to 20th	Day	21% 7% 3 7% 6% 5% 6% 3 5% 7% 31%	3000
	21st to end	Day	23% 10% 3 8% 6% 5% 5% 4 6% 6% 25%	3000
July	1st to 10th	Day	17% 7% 3 8% 6% 6% 6% 5% 8% 9% 27%	6820
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	6756
	21st to end	Day	13% 5% 2 6% 5% 4 5% 4 6% 9% 40%	6926
August	1st to 10th	Day	17% 5% 3 8% 6% 5% 6% 5% 6% 8% 33%	4284
	11th to 20th	Day	19% 8% 3 8% 5% 5% 5% 5% 6% 7% 28%	4175
	21st to end	Day	19% 7% 3 6% 4 4 5% 4 6% 8% 35%	4675
September	1st to 10th	Day	17% 7% 3 6% 4 4 4 4 5% 8% 38%	2386
	11th to 20th	Day	17% 6% 2 6% 4 3 4 3 3 6% 46%	2200
	21st to end	Day	20% 7% 3 6% 4 3 4 4 6% 6% 38%	2200
October	1st to 10th	Day	19% 10% 2 6% 4 4 3 3 5% 7% 37%	2094
	11th to 20th	Day	24% 10% 2 8% 5% 5% 4 4 5% 6% 27%	1965
	21st to end	Day	26% 12% 3 8% 5% 4 4 3 4 6% 24%	1980
November	1st to 10th	Day	21% 14% 5% 9% 5% 4 5% 4 4 5% 26%	1822
	11th to 20th	Day	21% 13% 5% 10% 7% 5% 5% 2 4 5% 23%	1656
	21st to end	Day	22% 17% 5% 10% 6% 4 3 3 4 6% 21%	1565
December	1st to 10th	Day	19% 14% 5% 14% 9% 5% 5% 3 5% 4 18%	2280
	11th to 20th	Day	19% 13% 4 11% 7% 5% 4 3 5% 6% 22%	2618
	21st to end	Day	18% 14% 5% 11% 5% 5% 4 3 4 5% 25%	3476
January	1st to 10th	Night	12% 20% 8% 16% 12% 7% 5% 3 4 4 10%	6808
	11th to 20th	Night	12% 15% 6% 15% 11% 6% 5% 4 5% 5% 17%	6341
	21st to end	Night	6% 10% 5% 10% 6% 6% 6% 5% 6% 8% 32%	6996
February	1st to 10th	Night	10% 14% 4 9% 7% 7% 6% 5% 6% 7% 25%	2641
	11th to 20th	Night	14% 16% 5% 10% 7% 6% 5% 5% 6% 5% 21%	2520
	21st to end	Night	12% 15% 5% 9% 7% 5% 5% 5% 4 6% 29%	1980
March	1st to 10th	Night	21% 14% 4 10% 5% 5% 6% 4 4 6% 23%	5301
	11th to 20th	Night	18% 15% 4 10% 6% 5% 5% 4 5% 6% 22%	4979
	21st to end	Night	26% 14% 4 8% 4 3 3 3 3 5% 27%	5466
April	1st to 10th	Night	23% 13% 3 8% 4 4 4 3 3 5% 30%	1989
	11th to 20th	Night	21% 10% 3 6% 4 4 5% 4 4 5% 34%	1778
	21st to end	Night	25% 10% 3 6% 4 5% 3 3 5% 5% 32%	1800
May	1st to 10th	Night	26% 10% 3 6% 4 4 3 3 4 6% 32%	4054
	11th to 20th	Night	26% 9% 3 8% 5% 5% 4 3 5% 6% 27%	3624
	21st to end	Night	18% 6% 2 6% 4 4 4 4 5% 8% 38%	3446
June	1st to 10th	Night	24% 8% 2 7% 4 4 4 3 5% 8% 32%	1400
	11th to 20th	Night	17% 6% 3 5% 4 5% 4 4 7% 7% 38%	1400
	21st to end	Night	24% 8% 4 8% 5% 4 4 3 4 7% 31%	1400
July	1st to 10th	Night	16% 8% 3 7% 5% 4 5% 4 6% 7% 34%	3160
	11th to 20th	Night	17% 6% 2 6% 4 5% 5% 4 6% 8% 35%	3217
	21st to end	Night	11% 6% 3 6% 5% 5% 5% 4 6% 8% 42%	3926
August	1st to 10th	Night	20% 6% 2 7% 6% 5% 5% 4 5% 6% 33%	2832
	11th to 20th	Night	15% 7% 4 9% 6% 5% 5% 4 7% 8% 31%	2872
	21st to end	Night	18% 7% 2 6% 4 4 3 2 4 6% 43%	3223
September	1st to 10th	Night	17% 6% 2 5% 4 4 4 4 5% 8% 42%	2010
	11th to 20th	Night	17% 6% 2 5% 4 3 4 3 5% 7% 46%	2200
	21st to end	Night	18% 6% 3 5% 4 3 4 3 4 7% 43%	2200
October	1st to 10th	Night	20% 7% 2 6% 4 3 3 3 4 6% 42%	2305
	11th to 20th	Night	21% 9% 3 6% 4 4 5% 4 5% 7% 33%	2430
	21st to end	Night	23% 11% 3 7% 4 4 4 3 5% 7% 29%	2860
November	1st to 10th	Night	21% 12% 3 8% 6% 4 4 4 5% 5% 29%	2694
	11th to 20th	Night	21% 12% 4 10% 5% 4 5% 4 4 6% 26%	2899
	21st to end	Night	21% 16% 4 10% 6% 4 4 3 5% 5% 21%	2831
December	1st to 10th	Night	15% 14% 5% 11% 8% 6% 5% 4 5% 6% 20%	4910
	11th to 20th	Night	24% 11% 4 11% 6% 5% 5% 3 4 6% 21%	5591
	21st to end	Night	18% 16% 5% 10% 6% 5% 4 3 5% 6% 24%	7502

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	55% 28% 8% 7% 2	3158
	11th to 20th	Day	54% 29% 6% 9% 2	3573
	21st to end	Day	39% 32% 5% 10% 13%	3960
February	1st to 10th	Day	22% 62% 4 6% 4 2	1757
	11th to 20th	Day	23% 61% 4 9% 3	1771
	21st to end	Day	22% 62% 3 7% 5% 1	1650
March	1st to 10th	Day	47% 30% 5% 6% 9% 3	4542
	11th to 20th	Day	49% 31% 9% 6% 4 1	4980
	21st to end	Day	49% 28% 3 12% 6% 2	5466
April	1st to 10th	Day	22% 61% 3 7% 6% 1	2410
	11th to 20th	Day	23% 62% 4 8% 2 1	2572
	21st to end	Day	25% 62% 3 5% 4 1	2600
May	1st to 10th	Day	44% 32% 5% 13% 6%	5897
	11th to 20th	Day	42% 32% 10% 10% 4 2	6325
	21st to end	Day	38% 31% 8% 14% 8%	7451
June	1st to 10th	Day	21% 63% 4 8% 4	3000
	11th to 20th	Day	21% 62% 4 7% 5%	3000
	21st to end	Day	22% 61% 4 8% 4 1	3000
July	1st to 10th	Day	38% 28% 11% 15% 7%	6745
	11th to 20th	Day	37% 27% 15% 18% 2	6675
	21st to end	Day	28% 27% 14% 25% 6%	6728
August	1st to 10th	Day	30% 38% 13% 13% 6%	4270
	11th to 20th	Day	36% 38% 11% 11% 3	4214
	21st to end	Day	31% 37% 12% 12% 8%	4675
September	1st to 10th	Day	17% 59% 6% 13% 5%	2386
	11th to 20th	Day	18% 58% 5% 13% 5%	2200
	21st to end	Day	17% 57% 5% 15% 6%	2200
October	1st to 10th	Day	34% 37% 11% 16% 3	3406
	11th to 20th	Day	38% 36% 7% 16% 3	3181
	21st to end	Day	33% 36% 13% 13% 6%	3180
November	1st to 10th	Day	33% 54% 6% 5% 2	1822
	11th to 20th	Day	30% 56% 4 7% 3	1656
	21st to end	Day	30% 58% 5% 5% 1	1565
December	1st to 10th	Day	46% 36% 9% 7% 3	2280
	11th to 20th	Day	43% 32% 7% 13% 5% 1	2618
	21st to end	Day	45% 28% 9% 7% 10% 1	3476
January	1st to 10th	Night	59% 30% 5% 5% 1	6808
	11th to 20th	Night	50% 31% 7% 8% 4	6386
	21st to end	Night	33% 30% 7% 16% 13% 1	6996
February	1st to 10th	Night	21% 63% 4 7% 4 1	2641
	11th to 20th	Night	24% 62% 3 7% 5%	2523
	21st to end	Night	19% 62% 4 6% 8% 1	1980
March	1st to 10th	Night	45% 31% 5% 9% 8% 1	5310
	11th to 20th	Night	46% 32% 10% 8% 5%	4980
	21st to end	Night	45% 29% 3 13% 9% 1	5466
April	1st to 10th	Night	22% 61% 2 6% 7% 2	1989
	11th to 20th	Night	22% 62% 2 8% 5% 1	1778
	21st to end	Night	25% 62% 3 5% 5%	1800
May	1st to 10th	Night	40% 31% 7% 11% 10%	4060
	11th to 20th	Night	42% 33% 8% 13% 4 1	3625
	21st to end	Night	29% 34% 10% 16% 10%	3447
June	1st to 10th	Night	20% 62% 3 10% 5%	1400
	11th to 20th	Night	17% 64% 5% 8% 5% 1	1400
	21st to end	Night	20% 60% 6% 9% 4 1	1400
July	1st to 10th	Night	38% 27% 14% 17% 4	3125
	11th to 20th	Night	36% 27% 16% 19% 2	3175
	21st to end	Night	29% 28% 18% 19% 6%	3809
August	1st to 10th	Night	34% 38% 10% 14% 4	2822
	11th to 20th	Night	33% 39% 14% 9% 4	2899
	21st to end	Night	30% 37% 9% 14% 11%	3223
September	1st to 10th	Night	17% 59% 6% 14% 5%	2010
	11th to 20th	Night	19% 58% 7% 12% 4	2200
	21st to end	Night	15% 57% 4 15% 8% 1	2200
October	1st to 10th	Night	32% 36% 7% 19% 5%	3773
	11th to 20th	Night	33% 36% 8% 20% 3	3953
	21st to end	Night	29% 37% 14% 12% 7%	4610
November	1st to 10th	Night	30% 54% 7% 8% 2	2694
	11th to 20th	Night	28% 56% 5% 7% 4 1	2899
	21st to end	Night	30% 58% 5% 5% 2	2831
December	1st to 10th	Night	41% 37% 8% 12% 2	4910
	11th to 20th	Night	43% 33% 6% 14% 4	5592
	21st to end	Night	43% 29% 7% 15% 6%	7502

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	30% 25% 28% 15% 3 1	200
	11th to 20th	Day	34% 20% 27% 12% 8%	200
	21st to end	Day	20% 25% 34% 13% 8% 1	220
February	1st to 10th	Day	25% 24% 31% 13% 6% 1	357
	11th to 20th	Day	22% 28% 31% 14% 4	401
	21st to end	Day	26% 23% 30% 17% 4	414
March	1st to 10th	Day	28% 22% 32% 15% 3	500
	11th to 20th	Day	27% 23% 31% 13% 6% 1	500
	21st to end	Day	31% 20% 29% 14% 5% 1	550
April	1st to 10th	Day	34% 24% 27% 11% 4 1	500
	11th to 20th	Day	30% 19% 32% 14% 4 1	492
	21st to end	Day	32% 22% 31% 11% 3 1	500
May	1st to 10th	Day	33% 17% 31% 13% 5%	503
	11th to 20th	Day	29% 15% 32% 17% 6% 1	634
	21st to end	Day	28% 19% 29% 18% 5%	790
June	1st to 10th	Day	29% 17% 30% 18% 6% 1	730
	11th to 20th	Day	27% 16% 26% 21% 9% 1	730
	21st to end	Day	26% 17% 30% 20% 7% 1	730
July	1st to 10th	Day	22% 14% 32% 21% 9% 1	720
	11th to 20th	Day	19% 13% 33% 25% 9% 1	693
	21st to end	Day	18% 13% 37% 23% 8% 1	576
August	1st to 10th	Day	19% 13% 35% 22% 9% 2	526
	11th to 20th	Day	21% 11% 30% 27% 10% 1	519
	21st to end	Day	24% 14% 26% 24% 9% 3	583
September	1st to 10th	Day	20% 16% 31% 21% 10% 3	530
	11th to 20th	Day	22% 15% 27% 23% 10% 4	530
	21st to end	Day	23% 16% 31% 17% 10% 4	530
October	1st to 10th	Day	20% 19% 27% 20% 10% 4	471
	11th to 20th	Day	32% 18% 23% 14% 10% 3	420
	21st to end	Day	35% 18% 26% 15% 6%	462
November	1st to 10th	Day	36% 19% 27% 14% 4	425
	11th to 20th	Day	30% 19% 29% 16% 4	421
	21st to end	Day	38% 19% 27% 13% 3	365
December	1st to 10th	Day	32% 17% 31% 13% 8% 1	200
	11th to 20th	Day	32% 24% 33% 8% 4	200
	21st to end	Day	32% 24% 27% 12% 5%	220
January	1st to 10th	Night	25% 20% 33% 19% 3	900
	11th to 20th	Night	28% 19% 30% 17% 6%	805
	21st to end	Night	17% 18% 36% 24% 5%	880
February	1st to 10th	Night	18% 23% 35% 19% 5%	800
	11th to 20th	Night	24% 24% 32% 17% 4 1	783
	21st to end	Night	21% 18% 37% 18% 5% 1	660
March	1st to 10th	Night	23% 16% 39% 19% 3	800
	11th to 20th	Night	26% 18% 31% 19% 6%	800
	21st to end	Night	24% 17% 31% 23% 5%	880
April	1st to 10th	Night	32% 18% 27% 19% 4	760
	11th to 20th	Night	25% 16% 35% 18% 6%	594
	21st to end	Night	32% 14% 30% 20% 5%	600
May	1st to 10th	Night	27% 17% 27% 24% 5%	600
	11th to 20th	Night	27% 13% 30% 23% 6%	528
	21st to end	Night	23% 12% 33% 26% 5%	433
June	1st to 10th	Night	25% 7% 30% 31% 7%	400
	11th to 20th	Night	21% 8% 29% 32% 11%	400
	21st to end	Night	26% 7% 27% 32% 9%	400
July	1st to 10th	Night	21% 6% 32% 31% 10% 1	390
	11th to 20th	Night	18% 7% 26% 38% 11%	388
	21st to end	Night	18% 5% 26% 36% 13%	438
August	1st to 10th	Night	18% 4 29% 34% 14% 1	540
	11th to 20th	Night	17% 5% 25% 38% 14%	592
	21st to end	Night	14% 10% 28% 32% 15% 1	660
September	1st to 10th	Night	20% 9% 31% 27% 12%	710
	11th to 20th	Night	20% 11% 26% 29% 14% 2	800
	21st to end	Night	22% 12% 28% 27% 11%	800
October	1st to 10th	Night	23% 13% 26% 27% 10% 1	800
	11th to 20th	Night	26% 13% 31% 21% 11%	800
	21st to end	Night	31% 16% 26% 21% 5%	880
November	1st to 10th	Night	29% 17% 27% 21% 6%	835
	11th to 20th	Night	30% 19% 26% 19% 5% 1	936
	21st to end	Night	32% 18% 27% 19% 4	900
December	1st to 10th	Night	30% 15% 32% 17% 5% 1	900
	11th to 20th	Night	30% 20% 26% 18% 5%	900
	21st to end	Night	31% 19% 27% 18% 5% 1	990

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