Allen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Allen Observatory clear sky chart from 2005-11-06 to 2024-03-23.

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	15% 11% 5% 8% 7% 3 4 4 5% 6% 31%	78619
	Night	15% 13% 6% 9% 7% 4 4 4 4 6% 29%	81325
Transparency	Day	53% 5% 11% 19% 10% 1	78320
	Night	56% 6% 11% 17% 9% 1	81145
Seeing	Day	21% 18% 31% 19% 8% 3	14936
	Night	21% 14% 29% 26% 9% 1	25015

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	12% 28% 14% 14% 9% 5% 3 2 2 2 8%	2527
	11th to 20th	Day	13% 18% 10% 18% 16% 8% 5% 3 2 3 5%	2530
	21st to end	Day	20% 22% 12% 13% 9% 4 4 3 2 3 8%	2783
February	1st to 10th	Day	10% 15% 9% 12% 16% 5% 6% 3 3 5% 16%	1680
	11th to 20th	Day	12% 17% 9% 13% 13% 4 5% 4 4 3 16%	1848
	21st to end	Day	13% 16% 9% 13% 8% 4 4 4 4 5% 21%	1570
March	1st to 10th	Day	13% 12% 7% 8% 8% 3 5% 4 5% 6% 28%	1900
	11th to 20th	Day	12% 11% 5% 9% 6% 3 5% 4 5% 7% 33%	2007
	21st to end	Day	18% 12% 4 8% 6% 4 4 4 5% 6% 29%	2311
April	1st to 10th	Day	18% 14% 5% 7% 6% 3 4 4 4 6% 29%	2160
	11th to 20th	Day	21% 12% 3 5% 5% 3 5% 4 5% 5% 33%	2181
	21st to end	Day	16% 11% 4 7% 5% 3 4 5% 5% 6% 34%	2340
May	1st to 10th	Day	17% 7% 3 6% 4 3 4 4 5% 8% 37%	2340
	11th to 20th	Day	17% 8% 2 5% 5% 2 4 4 5% 7% 41%	2375
	21st to end	Day	12% 5% 3 5% 5% 3 5% 6% 6% 7% 43%	2725
June	1st to 10th	Day	12% 6% 3 5% 5% 2 5% 5% 5% 7% 44%	2520
	11th to 20th	Day	13% 6% 2 5% 5% 2 5% 5% 6% 8% 43%	2520
	21st to end	Day	13% 5% 2 5% 6% 3 4 4 6% 8% 43%	2520
July	1st to 10th	Day	8% 4 2 4 4 2 3 5% 6% 10% 52%	2520
	11th to 20th	Day	10% 4 3 4 5% 3 5% 5% 6% 8% 48%	2520
	21st to end	Day	9% 4 2 4 5% 3 5% 6% 6% 9% 47%	2743
August	1st to 10th	Day	10% 5% 2 4 5% 3 5% 6% 7% 10% 43%	2413
	11th to 20th	Day	11% 4 2 5% 5% 3 5% 5% 6% 7% 47%	2303
	21st to end	Day	12% 6% 3 4 4 3 4 5% 6% 9% 44%	2404
September	1st to 10th	Day	14% 6% 3 7% 6% 3 6% 4 5% 7% 42%	2156
	11th to 20th	Day	13% 5% 2 5% 5% 3 3 5% 6% 7% 46%	1980
	21st to end	Day	13% 7% 4 7% 6% 3 5% 4 6% 7% 38%	1980
October	1st to 10th	Day	17% 8% 3 7% 5% 3 4 4 4 5% 40%	1980
	11th to 20th	Day	25% 13% 3 7% 7% 4 6% 4 4 6% 21%	1721
	21st to end	Day	25% 13% 5% 9% 6% 3 4 4 4 5% 22%	1782
November	1st to 10th	Day	15% 14% 3 6% 7% 3 4 5% 5% 6% 31%	1672
	11th to 20th	Day	15% 20% 7% 11% 6% 3 5% 5% 5% 5% 18%	1710
	21st to end	Day	21% 20% 8% 10% 6% 2 3 4 4 5% 17%	1677
December	1st to 10th	Day	15% 21% 14% 15% 6% 2 5% 3 3 4 12%	1520
	11th to 20th	Day	15% 20% 10% 15% 10% 4 3 4 4 5% 10%	1918
	21st to end	Day	21% 23% 9% 12% 9% 4 4 3 3 3 9%	2783
January	1st to 10th	Night	14% 22% 12% 14% 8% 5% 4 3 3 3 14%	4428
	11th to 20th	Night	12% 17% 9% 18% 12% 10% 4 3 2 3 9%	4440
	21st to end	Night	17% 22% 11% 15% 9% 4 5% 3 3 3 8%	4884
February	1st to 10th	Night	11% 15% 8% 13% 13% 5% 6% 4 3 4 17%	2498
	11th to 20th	Night	12% 13% 8% 12% 11% 4 6% 4 5% 5% 19%	2218
	21st to end	Night	14% 16% 6% 11% 7% 4 5% 5% 4 5% 23%	1884
March	1st to 10th	Night	15% 10% 5% 7% 7% 3 4 5% 5% 6% 33%	2280
	11th to 20th	Night	12% 10% 4 8% 5% 3 4 5% 4 8% 37%	2172
	21st to end	Night	19% 8% 3 7% 7% 4 4 4 5% 6% 33%	2106
April	1st to 10th	Night	17% 10% 4 7% 5% 3 5% 4 5% 7% 33%	1800
	11th to 20th	Night	21% 8% 3 6% 5% 3 4 5% 5% 6% 36%	1728
	21st to end	Night	18% 9% 3 6% 5% 3 4 4 5% 6% 38%	1620
May	1st to 10th	Night	14% 7% 3 7% 7% 5% 5% 5% 5% 7% 34%	1620
	11th to 20th	Night	15% 6% 2 6% 6% 4 5% 5% 5% 8% 37%	1516
	21st to end	Night	11% 5% 2 6% 5% 3 4 4 5% 9% 45%	1551
June	1st to 10th	Night	12% 7% 2 4 4 2 5% 5% 6% 9% 44%	1440
	11th to 20th	Night	11% 6% 2 5% 5% 3 4 5% 5% 9% 46%	1440
	21st to end	Night	10% 6% 2 6% 4 3 3 5% 7% 9% 45%	1440
July	1st to 10th	Night	7% 3 1 3 3 3 5% 6% 7% 9% 54%	1440
	11th to 20th	Night	7% 4 1 4 4 3 5% 5% 6% 8% 53%	1440
	21st to end	Night	9% 4 2 6% 4 3 5% 5% 6% 8% 49%	1575
August	1st to 10th	Night	8% 4 2 4 5% 3 4 6% 8% 9% 48%	1525
	11th to 20th	Night	8% 3 2 4 4 4 4 5% 6% 8% 53%	1598
	21st to end	Night	11% 6% 2 4 4 3 4 4 5% 7% 49%	1949
September	1st to 10th	Night	11% 5% 2 6% 5% 3 4 4 4 6% 50%	1800
	11th to 20th	Night	12% 5% 2 4 3 2 3 4 5% 7% 54%	1966
	21st to end	Night	15% 6% 3 6% 5% 3 4 4 5% 8% 41%	1980
October	1st to 10th	Night	18% 8% 3 4 4 3 3 3 4 6% 43%	1980
	11th to 20th	Night	24% 10% 4 5% 6% 3 4 4 5% 5% 30%	2236
	21st to end	Night	26% 11% 4 7% 5% 3 4 4 4 6% 27%	2574
November	1st to 10th	Night	14% 13% 4 8% 6% 3 4 4 5% 6% 34%	2406
	11th to 20th	Night	16% 17% 7% 9% 7% 4 6% 5% 4 6% 20%	2470
	21st to end	Night	19% 18% 7% 9% 7% 4 4 4 3 5% 20%	2498
December	1st to 10th	Night	15% 19% 11% 14% 8% 4 4 3 3 4 14%	2660
	11th to 20th	Night	15% 21% 9% 15% 8% 3 4 4 4 4 13%	3279
	21st to end	Night	24% 23% 8% 10% 6% 3 3 3 4 4 11%	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	84% 5% 5% 5% 1	2528
	11th to 20th	Day	87% 4 4 3 1	2530
	21st to end	Day	83% 5% 6% 4 1	2783
February	1st to 10th	Day	72% 5% 7% 13% 3	1680
	11th to 20th	Day	71% 6% 8% 11% 5%	1858
	21st to end	Day	66% 5% 11% 13% 5%	1570
March	1st to 10th	Day	57% 5% 13% 17% 7% 1	1900
	11th to 20th	Day	51% 6% 14% 18% 10% 2	2007
	21st to end	Day	55% 5% 10% 20% 8% 1	2311
April	1st to 10th	Day	57% 4 10% 19% 9%	2160
	11th to 20th	Day	52% 4 10% 18% 14% 2	2181
	21st to end	Day	50% 5% 12% 18% 14% 2	2340
May	1st to 10th	Day	44% 7% 11% 24% 13% 2	2340
	11th to 20th	Day	43% 5% 13% 22% 16% 2	2379
	21st to end	Day	38% 7% 14% 26% 14% 1	2725
June	1st to 10th	Day	37% 6% 11% 27% 17% 2	2520
	11th to 20th	Day	38% 6% 13% 23% 20% 1	2520
	21st to end	Day	39% 6% 14% 24% 17% 1	2520
July	1st to 10th	Day	28% 5% 15% 32% 18% 3	2451
	11th to 20th	Day	32% 5% 14% 29% 18% 2	2445
	21st to end	Day	31% 7% 13% 28% 18% 3	2591
August	1st to 10th	Day	32% 8% 16% 30% 12% 2	2398
	11th to 20th	Day	35% 7% 15% 28% 14% 2	2303
	21st to end	Day	35% 5% 14% 26% 17% 2	2404
September	1st to 10th	Day	42% 6% 12% 24% 14% 2	2156
	11th to 20th	Day	37% 6% 13% 32% 11% 1	1980
	21st to end	Day	44% 6% 12% 26% 11%	1980
October	1st to 10th	Day	47% 5% 9% 25% 13%	1980
	11th to 20th	Day	63% 6% 11% 15% 5%	1721
	21st to end	Day	63% 5% 12% 13% 6%	1782
November	1st to 10th	Day	52% 7% 11% 20% 9%	1672
	11th to 20th	Day	68% 5% 11% 11% 4	1710
	21st to end	Day	71% 5% 12% 9% 3	1677
December	1st to 10th	Day	79% 4 6% 9% 2	1520
	11th to 20th	Day	78% 5% 11% 6% 1	1915
	21st to end	Day	81% 4 7% 6% 2	2783
January	1st to 10th	Night	77% 5% 7% 9% 1	4428
	11th to 20th	Night	83% 5% 6% 5% 2	4440
	21st to end	Night	82% 5% 7% 4 1	4884
February	1st to 10th	Night	70% 5% 7% 11% 5% 1	2498
	11th to 20th	Night	67% 6% 8% 14% 5%	2219
	21st to end	Night	63% 6% 11% 14% 5%	1884
March	1st to 10th	Night	51% 5% 14% 21% 8% 1	2280
	11th to 20th	Night	45% 7% 13% 20% 13% 2	2172
	21st to end	Night	52% 4 10% 23% 9% 1	2106
April	1st to 10th	Night	50% 5% 11% 20% 13% 1	1800
	11th to 20th	Night	49% 5% 8% 21% 15% 3	1728
	21st to end	Night	47% 5% 11% 20% 14% 4	1620
May	1st to 10th	Night	47% 6% 12% 21% 13% 2	1620
	11th to 20th	Night	44% 6% 9% 21% 16% 3	1517
	21st to end	Night	38% 5% 12% 28% 15% 3	1552
June	1st to 10th	Night	36% 5% 12% 29% 16% 3	1440
	11th to 20th	Night	35% 6% 13% 25% 19% 2	1440
	21st to end	Night	33% 7% 15% 28% 14% 3	1440
July	1st to 10th	Night	26% 7% 13% 30% 21% 4	1400
	11th to 20th	Night	27% 7% 17% 31% 16% 3	1392
	21st to end	Night	31% 9% 14% 26% 17% 3	1487
August	1st to 10th	Night	30% 6% 17% 31% 14% 1	1516
	11th to 20th	Night	28% 6% 15% 33% 16% 1	1598
	21st to end	Night	34% 6% 12% 26% 20% 3	1949
September	1st to 10th	Night	35% 6% 11% 28% 18% 2	1800
	11th to 20th	Night	31% 4 13% 36% 13% 2	1966
	21st to end	Night	42% 5% 14% 28% 11% 1	1980
October	1st to 10th	Night	41% 6% 9% 27% 15% 1	1980
	11th to 20th	Night	54% 6% 13% 20% 6% 1	2236
	21st to end	Night	58% 6% 12% 17% 7%	2574
November	1st to 10th	Night	51% 6% 11% 21% 10% 1	2406
	11th to 20th	Night	64% 7% 12% 11% 5%	2470
	21st to end	Night	67% 5% 14% 12% 2 1	2498
December	1st to 10th	Night	75% 5% 10% 8% 2	2660
	11th to 20th	Night	74% 6% 9% 8% 2	3281
	21st to end	Night	78% 4 10% 6% 2	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	22% 26% 32% 14% 6%	270
	11th to 20th	Day	27% 23% 39% 9% 3	270
	21st to end	Day	28% 23% 34% 14% 2	297
February	1st to 10th	Day	23% 36% 25% 10% 6% 1	270
	11th to 20th	Day	23% 39% 29% 9%	270
	21st to end	Day	32% 36% 28% 4 1	225
March	1st to 10th	Day	29% 30% 32% 7% 1	270
	11th to 20th	Day	28% 18% 31% 16% 7% 1	377
	21st to end	Day	32% 19% 30% 13% 4 1	470
April	1st to 10th	Day	28% 21% 35% 13% 3 1	420
	11th to 20th	Day	34% 22% 24% 12% 8% 1	412
	21st to end	Day	25% 22% 30% 15% 7% 1	420
May	1st to 10th	Day	26% 22% 31% 14% 6% 2	423
	11th to 20th	Day	21% 17% 36% 17% 7% 3	521
	21st to end	Day	18% 15% 35% 22% 8% 3	679
June	1st to 10th	Day	14% 18% 37% 22% 7% 3	630
	11th to 20th	Day	17% 16% 34% 23% 7% 3	630
	21st to end	Day	17% 14% 37% 22% 8% 2	630
July	1st to 10th	Day	9% 10% 33% 31% 12% 5%	621
	11th to 20th	Day	10% 12% 34% 29% 10% 5%	618
	21st to end	Day	10% 10% 38% 26% 12% 4	684
August	1st to 10th	Day	11% 10% 30% 26% 17% 6%	532
	11th to 20th	Day	15% 6% 24% 28% 16% 10%	439
	21st to end	Day	18% 9% 25% 27% 17% 5%	495
September	1st to 10th	Day	17% 9% 32% 22% 15% 5%	450
	11th to 20th	Day	11% 10% 27% 31% 15% 5%	450
	21st to end	Day	18% 17% 22% 25% 14% 4	450
October	1st to 10th	Day	23% 15% 27% 22% 12% 2	450
	11th to 20th	Day	33% 18% 32% 15% 1	319
	21st to end	Day	39% 20% 29% 9% 3	297
November	1st to 10th	Day	28% 21% 33% 13% 5% 1	270
	11th to 20th	Day	27% 30% 33% 8% 2	270
	21st to end	Day	32% 26% 32% 8% 1	270
December	1st to 10th	Day	34% 29% 24% 10% 3	270
	11th to 20th	Day	30% 31% 27% 10% 2	270
	21st to end	Day	34% 30% 25% 8% 1 1	297
January	1st to 10th	Night	24% 22% 30% 19% 4 1	950
	11th to 20th	Night	27% 19% 31% 18% 5%	950
	21st to end	Night	22% 15% 34% 23% 5%	1045
February	1st to 10th	Night	19% 19% 33% 23% 4	851
	11th to 20th	Night	21% 21% 33% 20% 3	743
	21st to end	Night	26% 20% 34% 17% 3	628
March	1st to 10th	Night	20% 19% 32% 20% 7% 1	760
	11th to 20th	Night	19% 19% 36% 20% 6%	760
	21st to end	Night	24% 14% 29% 26% 7% 1	706
April	1st to 10th	Night	23% 16% 32% 23% 5% 1	540
	11th to 20th	Night	27% 13% 33% 20% 5% 1	533
	21st to end	Night	22% 12% 35% 22% 9%	540
May	1st to 10th	Night	17% 14% 35% 26% 6% 1	540
	11th to 20th	Night	18% 12% 33% 27% 10%	531
	21st to end	Night	15% 6% 32% 33% 13% 1	584
June	1st to 10th	Night	12% 9% 32% 34% 12% 1	540
	11th to 20th	Night	14% 8% 31% 32% 14% 1	540
	21st to end	Night	11% 8% 29% 36% 15%	540
July	1st to 10th	Night	8% 4 22% 41% 23% 2	525
	11th to 20th	Night	9% 5% 26% 38% 20% 1	522
	21st to end	Night	10% 4 25% 40% 20% 2	591
August	1st to 10th	Night	9% 3 21% 44% 20% 2	537
	11th to 20th	Night	10% 3 19% 40% 23% 5%	534
	21st to end	Night	15% 3 22% 42% 17% 2	594
September	1st to 10th	Night	12% 8% 23% 35% 17% 4	540
	11th to 20th	Night	13% 8% 23% 34% 18% 3	706
	21st to end	Night	17% 11% 26% 27% 18% 2	720
October	1st to 10th	Night	22% 10% 25% 29% 13% 1	720
	11th to 20th	Night	30% 14% 25% 24% 7% 1	720
	21st to end	Night	34% 11% 28% 21% 6%	792
November	1st to 10th	Night	24% 16% 31% 21% 7% 1	740
	11th to 20th	Night	26% 21% 31% 18% 4	760
	21st to end	Night	29% 17% 28% 22% 4	788
December	1st to 10th	Night	28% 22% 30% 17% 3	950
	11th to 20th	Night	28% 21% 31% 16% 3 1	950
	21st to end	Night	31% 18% 31% 19% 1	1045

Page generated at 2024-03-19 23:31:03. Version ac07

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