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 2024-01-12.

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	20% 10% 4 7% 6% 4 5% 5% 5% 6% 28%	74601
	Night	19% 10% 4 7% 6% 3 5% 5% 5% 6% 29%	75119
Transparency	Day	56% 7% 12% 17% 8% 1	74295
	Night	55% 7% 12% 18% 8% 1	74960
Seeing	Day	27% 18% 30% 17% 7% 1	14819
	Night	25% 15% 30% 23% 7%	22963

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	18% 15% 7% 10% 7% 4 5% 5% 5% 6% 18%	1327
	11th to 20th	Day	18% 13% 6% 11% 8% 5% 4 4 6% 6% 20%	1439
	21st to end	Day	11% 11% 5% 10% 7% 5% 6% 7% 6% 7% 26%	1584
February	1st to 10th	Day	11% 15% 6% 9% 8% 5% 7% 7% 6% 7% 20%	1581
	11th to 20th	Day	15% 15% 7% 10% 8% 5% 6% 5% 5% 6% 18%	1582
	21st to end	Day	13% 10% 6% 10% 8% 4 6% 5% 6% 6% 25%	1480
March	1st to 10th	Day	17% 12% 5% 10% 7% 4 5% 5% 5% 5% 25%	1817
	11th to 20th	Day	18% 13% 4 9% 9% 4 5% 4 4 5% 26%	1980
	21st to end	Day	18% 14% 5% 8% 6% 3 3 3 4 5% 29%	2178
April	1st to 10th	Day	22% 13% 5% 9% 6% 3 4 3 5% 6% 23%	2168
	11th to 20th	Day	20% 11% 3 6% 5% 2 4 4 5% 6% 33%	2312
	21st to end	Day	26% 10% 3 7% 5% 3 3 4 4 5% 30%	2340
May	1st to 10th	Day	25% 11% 4 6% 6% 3 4 4 4 5% 28%	2340
	11th to 20th	Day	24% 9% 3 7% 6% 3 3 3 4 6% 33%	2467
	21st to end	Day	22% 6% 3 7% 5% 4 4 4 5% 6% 33%	2919
June	1st to 10th	Day	22% 8% 3 6% 7% 5% 5% 5% 6% 7% 27%	2700
	11th to 20th	Day	22% 7% 3 7% 6% 3 6% 4 4 6% 32%	2700
	21st to end	Day	23% 9% 4 7% 7% 4 6% 5% 6% 6% 25%	2700
July	1st to 10th	Day	19% 7% 3 6% 6% 4 6% 6% 6% 9% 28%	2700
	11th to 20th	Day	16% 6% 3 6% 6% 4 6% 6% 6% 9% 31%	2678
	21st to end	Day	17% 7% 3 6% 6% 4 6% 6% 6% 9% 30%	2744
August	1st to 10th	Day	19% 6% 3 7% 7% 4 6% 7% 6% 8% 27%	2360
	11th to 20th	Day	17% 7% 3 6% 7% 4 5% 6% 6% 7% 31%	2304
	21st to end	Day	17% 7% 3 6% 5% 4 5% 5% 6% 7% 34%	2574
September	1st to 10th	Day	18% 7% 3 5% 5% 3 4 5% 5% 7% 38%	2148
	11th to 20th	Day	19% 6% 2 6% 4 2 4 3 3 6% 44%	1980
	21st to end	Day	22% 7% 3 5% 4 2 4 4 5% 6% 39%	1980
October	1st to 10th	Day	19% 10% 3 5% 5% 3 3 4 5% 6% 37%	1886
	11th to 20th	Day	26% 10% 3 7% 6% 4 5% 5% 4 6% 26%	1769
	21st to end	Day	27% 13% 4 7% 6% 2 4 4 4 5% 24%	1782
November	1st to 10th	Day	21% 13% 5% 8% 6% 3 5% 5% 4 4 27%	1646
	11th to 20th	Day	21% 12% 5% 9% 7% 4 5% 4 4 6% 23%	1520
	21st to end	Day	22% 15% 5% 10% 7% 2 4 4 4 6% 22%	1487
December	1st to 10th	Day	23% 14% 5% 9% 9% 4 5% 4 5% 5% 19%	1330
	11th to 20th	Day	19% 15% 6% 9% 8% 4 4 4 5% 6% 21%	1668
	21st to end	Day	21% 13% 5% 9% 8% 4 5% 5% 4 4 24%	2431
January	1st to 10th	Night	15% 16% 7% 10% 10% 5% 6% 5% 4 6% 17%	2838
	11th to 20th	Night	18% 14% 6% 9% 8% 4 4 5% 5% 5% 22%	2559
	21st to end	Night	11% 10% 5% 8% 7% 4 8% 6% 7% 8% 26%	2772
February	1st to 10th	Night	11% 13% 5% 8% 7% 5% 7% 6% 6% 8% 24%	2377
	11th to 20th	Night	14% 15% 5% 9% 8% 5% 6% 6% 6% 6% 20%	2261
	21st to end	Night	12% 14% 5% 9% 8% 4 5% 6% 4 6% 29%	1776
March	1st to 10th	Night	14% 10% 5% 8% 7% 4 6% 5% 5% 6% 28%	2129
	11th to 20th	Night	16% 13% 5% 8% 7% 5% 4 4 4 5% 28%	1979
	21st to end	Night	15% 11% 6% 9% 6% 3 5% 5% 5% 6% 29%	2178
April	1st to 10th	Night	24% 13% 3 7% 5% 2 4 4 3 5% 30%	1791
	11th to 20th	Night	20% 9% 3 5% 5% 4 6% 5% 4 6% 35%	1598
	21st to end	Night	26% 9% 3 5% 5% 4 3 5% 5% 5% 32%	1620
May	1st to 10th	Night	23% 10% 3 7% 6% 3 4 4 5% 5% 31%	1620
	11th to 20th	Night	22% 9% 4 5% 5% 3 5% 4 5% 5% 33%	1438
	21st to end	Night	20% 6% 3 6% 6% 3 4 5% 4 7% 35%	1356
June	1st to 10th	Night	24% 7% 2 6% 5% 3 5% 3 5% 7% 31%	1260
	11th to 20th	Night	17% 5% 3 5% 5% 3 5% 5% 6% 7% 39%	1260
	21st to end	Night	23% 6% 4 7% 6% 3 4 4 4 7% 32%	1260
July	1st to 10th	Night	18% 7% 3 5% 5% 3 4 5% 6% 7% 37%	1260
	11th to 20th	Night	17% 5% 2 5% 4 3 6% 5% 7% 9% 37%	1278
	21st to end	Night	17% 7% 3 5% 5% 4 6% 5% 6% 9% 34%	1574
August	1st to 10th	Night	16% 6% 3 4 7% 4 6% 5% 5% 8% 37%	1559
	11th to 20th	Night	16% 7% 4 6% 7% 3 4 4 6% 7% 35%	1596
	21st to end	Night	13% 6% 2 5% 5% 4 4 5% 6% 8% 43%	1782
September	1st to 10th	Night	17% 5% 2 4 5% 3 4 5% 5% 8% 42%	1808
	11th to 20th	Night	18% 6% 2 4 4 3 3 3 5% 6% 45%	1980
	21st to end	Night	18% 6% 2 5% 4 2 4 4 4 7% 44%	1980
October	1st to 10th	Night	21% 6% 3 5% 5% 2 3 4 4 6% 41%	2074
	11th to 20th	Night	23% 9% 3 5% 5% 3 5% 5% 5% 7% 32%	2186
	21st to end	Night	25% 11% 3 7% 4 3 4 4 5% 6% 28%	2574
November	1st to 10th	Night	21% 10% 4 7% 7% 3 3 5% 5% 6% 30%	2430
	11th to 20th	Night	21% 11% 4 9% 6% 2 5% 5% 4 6% 26%	2660
	21st to end	Night	20% 14% 5% 9% 7% 3 5% 4 5% 5% 23%	2689
December	1st to 10th	Night	23% 14% 5% 8% 7% 3 4 4 4 5% 22%	2850
	11th to 20th	Night	24% 10% 5% 9% 9% 4 6% 5% 5% 5% 18%	3531
	21st to end	Night	19% 16% 4 8% 8% 4 4 5% 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	66% 7% 11% 10% 5%	1328
	11th to 20th	Day	66% 6% 9% 13% 6% 1	1439
	21st to end	Day	55% 6% 10% 16% 12% 1	1584
February	1st to 10th	Day	61% 7% 10% 13% 8% 1	1581
	11th to 20th	Day	66% 6% 8% 13% 5%	1590
	21st to end	Day	58% 6% 9% 18% 8% 2	1480
March	1st to 10th	Day	61% 5% 9% 15% 9% 2	1821
	11th to 20th	Day	61% 6% 8% 14% 8% 2	1980
	21st to end	Day	59% 4 8% 18% 10% 2	2178
April	1st to 10th	Day	62% 5% 9% 14% 7% 3	2168
	11th to 20th	Day	53% 6% 9% 19% 11% 2	2312
	21st to end	Day	59% 5% 7% 16% 11% 3	2340
May	1st to 10th	Day	59% 6% 8% 15% 12% 1	2340
	11th to 20th	Day	53% 5% 12% 18% 11% 1	2471
	21st to end	Day	51% 7% 11% 20% 11%	2919
June	1st to 10th	Day	56% 8% 11% 15% 10% 1	2700
	11th to 20th	Day	52% 7% 12% 16% 11% 2	2700
	21st to end	Day	59% 8% 11% 15% 7%	2700
July	1st to 10th	Day	51% 10% 17% 16% 6%	2625
	11th to 20th	Day	47% 10% 15% 20% 8%	2597
	21st to end	Day	51% 10% 14% 19% 6%	2591
August	1st to 10th	Day	51% 11% 17% 16% 5%	2346
	11th to 20th	Day	50% 9% 16% 19% 6%	2304
	21st to end	Day	47% 9% 16% 22% 7%	2574
September	1st to 10th	Day	45% 7% 14% 24% 10%	2148
	11th to 20th	Day	44% 5% 14% 24% 13%	1980
	21st to end	Day	46% 7% 12% 25% 10%	1980
October	1st to 10th	Day	49% 5% 13% 24% 9%	1886
	11th to 20th	Day	59% 7% 12% 17% 5%	1769
	21st to end	Day	63% 6% 11% 14% 5% 1	1782
November	1st to 10th	Day	61% 7% 11% 16% 5%	1646
	11th to 20th	Day	63% 7% 11% 13% 6%	1520
	21st to end	Day	65% 7% 13% 12% 3	1487
December	1st to 10th	Day	68% 6% 10% 11% 4	1330
	11th to 20th	Day	64% 6% 9% 13% 8%	1668
	21st to end	Day	63% 8% 11% 11% 6% 1	2431
January	1st to 10th	Night	68% 7% 10% 11% 3	2838
	11th to 20th	Night	63% 6% 11% 14% 6% 1	2559
	21st to end	Night	52% 8% 10% 19% 9% 1	2772
February	1st to 10th	Night	57% 7% 10% 15% 10% 1	2377
	11th to 20th	Night	63% 7% 10% 11% 8% 1	2264
	21st to end	Night	56% 5% 9% 19% 10% 1	1776
March	1st to 10th	Night	56% 7% 8% 20% 8% 1	2138
	11th to 20th	Night	58% 6% 10% 16% 10% 1	1980
	21st to end	Night	54% 7% 11% 18% 9% 2	2178
April	1st to 10th	Night	58% 4 9% 15% 11% 2	1791
	11th to 20th	Night	50% 7% 8% 19% 15% 1	1598
	21st to end	Night	54% 5% 9% 17% 13% 2	1620
May	1st to 10th	Night	56% 4 8% 16% 13% 2	1620
	11th to 20th	Night	51% 8% 11% 19% 10% 1	1439
	21st to end	Night	49% 8% 11% 18% 14%	1357
June	1st to 10th	Night	52% 6% 11% 21% 9% 1	1260
	11th to 20th	Night	43% 8% 15% 20% 13% 2	1260
	21st to end	Night	53% 7% 15% 15% 9% 1	1260
July	1st to 10th	Night	46% 8% 18% 21% 7%	1225
	11th to 20th	Night	42% 10% 16% 24% 8% 1	1236
	21st to end	Night	46% 10% 17% 19% 8%	1486
August	1st to 10th	Night	45% 10% 17% 23% 5%	1549
	11th to 20th	Night	46% 10% 18% 19% 7%	1596
	21st to end	Night	38% 10% 15% 29% 8%	1782
September	1st to 10th	Night	41% 8% 14% 26% 10% 1	1808
	11th to 20th	Night	41% 6% 17% 23% 12% 1	1980
	21st to end	Night	41% 7% 13% 26% 14% 1	1980
October	1st to 10th	Night	45% 6% 12% 26% 10% 1	2074
	11th to 20th	Night	52% 7% 14% 22% 5%	2186
	21st to end	Night	57% 7% 14% 17% 6% 1	2574
November	1st to 10th	Night	55% 7% 14% 18% 6%	2430
	11th to 20th	Night	59% 7% 12% 15% 7%	2660
	21st to end	Night	63% 8% 13% 12% 3 1	2689
December	1st to 10th	Night	65% 7% 10% 14% 4	2850
	11th to 20th	Night	64% 9% 9% 13% 5%	3532
	21st to end	Night	62% 8% 11% 13% 6% 1	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	34% 19% 30% 10% 7%	164
	21st to end	Day	20% 27% 32% 14% 7%	176
February	1st to 10th	Day	26% 24% 32% 12% 6%	301
	11th to 20th	Day	22% 28% 30% 15% 5%	340
	21st to end	Day	25% 23% 29% 17% 5%	346
March	1st to 10th	Day	28% 22% 32% 14% 3	420
	11th to 20th	Day	27% 21% 31% 13% 7% 1	420
	21st to end	Day	27% 22% 28% 15% 6% 2	462
April	1st to 10th	Day	32% 25% 26% 13% 4 1	420
	11th to 20th	Day	28% 19% 33% 16% 4 1	412
	21st to end	Day	32% 21% 30% 12% 4 1	420
May	1st to 10th	Day	35% 17% 32% 12% 4	423
	11th to 20th	Day	30% 17% 32% 16% 5% 1	542
	21st to end	Day	29% 18% 29% 17% 6%	680
June	1st to 10th	Day	30% 17% 30% 17% 5% 1	630
	11th to 20th	Day	27% 16% 25% 22% 9% 1	630
	21st to end	Day	25% 17% 29% 22% 7% 1	630
July	1st to 10th	Day	21% 15% 32% 21% 10% 1	620
	11th to 20th	Day	19% 13% 33% 24% 10% 1	596
	21st to end	Day	18% 12% 37% 24% 8% 1	488
August	1st to 10th	Day	21% 12% 35% 21% 9% 2	446
	11th to 20th	Day	22% 11% 31% 27% 8% 1	439
	21st to end	Day	24% 15% 25% 24% 10% 3	495
September	1st to 10th	Day	20% 16% 31% 20% 11% 2	450
	11th to 20th	Day	24% 16% 29% 21% 8% 3	450
	21st to end	Day	23% 17% 32% 16% 9% 4	450
October	1st to 10th	Day	20% 19% 28% 19% 10% 3	403
	11th to 20th	Day	33% 18% 24% 14% 8% 3	360
	21st to end	Day	37% 18% 25% 14% 5%	396
November	1st to 10th	Day	34% 19% 27% 15% 4	365
	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% 19% 31% 17% 5%	733
	21st to end	Night	18% 18% 36% 22% 6%	792
February	1st to 10th	Night	19% 23% 35% 18% 4	720
	11th to 20th	Night	24% 24% 31% 17% 4 1	703
	21st to end	Night	21% 18% 37% 18% 5% 1	592
March	1st to 10th	Night	22% 16% 40% 20% 3	720
	11th to 20th	Night	26% 19% 31% 18% 6%	720
	21st to end	Night	22% 17% 32% 23% 5% 1	792
April	1st to 10th	Night	32% 18% 27% 19% 4	684
	11th to 20th	Night	24% 16% 36% 18% 6%	534
	21st to end	Night	31% 14% 30% 20% 5%	540
May	1st to 10th	Night	26% 16% 28% 24% 6%	540
	11th to 20th	Night	28% 13% 30% 23% 6%	475
	21st to end	Night	24% 13% 33% 24% 5%	389
June	1st to 10th	Night	25% 6% 30% 31% 7%	360
	11th to 20th	Night	20% 8% 28% 33% 11%	360
	21st to end	Night	24% 7% 27% 33% 9%	360
July	1st to 10th	Night	20% 6% 33% 29% 11% 1	350
	11th to 20th	Night	17% 7% 26% 38% 12%	348
	21st to end	Night	20% 5% 26% 35% 14%	394
August	1st to 10th	Night	18% 4 29% 34% 14% 1	485
	11th to 20th	Night	18% 6% 26% 36% 14%	532
	21st to end	Night	14% 10% 28% 32% 15% 1	594
September	1st to 10th	Night	20% 9% 33% 26% 11%	638
	11th to 20th	Night	22% 11% 27% 28% 11% 1	720
	21st to end	Night	22% 12% 28% 27% 10%	720
October	1st to 10th	Night	23% 13% 26% 27% 9% 1	720
	11th to 20th	Night	26% 13% 31% 19% 11%	720
	21st to end	Night	32% 17% 25% 20% 5% 1	792
November	1st to 10th	Night	28% 18% 27% 21% 6%	751
	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

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