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 2024-11-03.

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	14% 11% 4 9% 5% 4 4 3 5% 6% 34%	101333
	Night	15% 13% 5% 9% 5% 5% 4 3 5% 6% 31%	97733
Transparency	Day	51% 5% 11% 20% 12% 1	101124
	Night	54% 5% 11% 19% 10% 1	97614
Seeing	Day	21% 18% 31% 19% 8% 3	15896
	Night	21% 14% 30% 26% 9% 1	25746

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% 30% 13% 16% 7% 5% 3 2 2 2 8%	2527
	11th to 20th	Day	12% 19% 9% 21% 13% 10% 5% 2 2 2 5%	2530
	21st to end	Day	19% 24% 11% 15% 7% 5% 4 2 2 3 8%	2783
February	1st to 10th	Day	10% 17% 7% 14% 15% 6% 4 2 3 5% 16%	1680
	11th to 20th	Day	12% 18% 8% 15% 11% 5% 4 3 4 3 16%	1848
	21st to end	Day	12% 19% 7% 15% 7% 4 4 3 5% 4 20%	1570
March	1st to 10th	Day	13% 14% 7% 8% 6% 4 5% 3 5% 5% 31%	3120
	11th to 20th	Day	11% 9% 9% 13% 5% 4 4 3 4 6% 32%	3345
	21st to end	Day	19% 12% 6% 12% 5% 4 3 3 4 4 27%	4022
April	1st to 10th	Day	18% 16% 4 8% 5% 4 4 3 5% 5% 30%	2280
	11th to 20th	Day	21% 12% 3 6% 4 4 4 4 5% 5% 33%	2304
	21st to end	Day	16% 12% 4 7% 4 4 4 4 5% 6% 35%	2470
May	1st to 10th	Day	15% 7% 3 6% 4 3 4 3 5% 8% 41%	4082
	11th to 20th	Day	12% 7% 2 6% 4 4 4 4 6% 7% 44%	4225
	21st to end	Day	13% 6% 2 5% 4 4 5% 4 6% 8% 43%	4824
June	1st to 10th	Day	12% 6% 2 5% 5% 4 5% 4 5% 8% 44%	2660
	11th to 20th	Day	13% 6% 2 5% 5% 3 5% 4 6% 8% 43%	2660
	21st to end	Day	14% 6% 2 6% 5% 4 4 3 6% 8% 43%	2660
July	1st to 10th	Day	15% 6% 3 6% 4 4 4 4 6% 9% 38%	4440
	11th to 20th	Day	7% 4 1 4 4 3 4 4 6% 9% 54%	4440
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 6% 8% 46%	4813
August	1st to 10th	Day	11% 5% 2 6% 4 4 6% 5% 7% 9% 41%	4245
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 6% 6% 49%	4044
	21st to end	Day	10% 6% 2 6% 4 5% 5% 4 6% 8% 43%	4222
September	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 3 5% 7% 43%	2275
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 6% 7% 47%	2090
	21st to end	Day	13% 7% 3 8% 5% 4 5% 3 6% 7% 39%	2090
October	1st to 10th	Day	16% 8% 3 7% 4 4 3 3 4 5% 42%	2090
	11th to 20th	Day	23% 13% 3 9% 6% 5% 5% 3 5% 6% 22%	1815
	21st to end	Day	24% 13% 4 10% 5% 3 4 3 5% 5% 23%	1881
November	1st to 10th	Day	15% 15% 3 7% 6% 5% 4 3 5% 6% 31%	1690
	11th to 20th	Day	14% 22% 7% 12% 5% 4 5% 4 5% 5% 18%	1710
	21st to end	Day	21% 21% 7% 11% 5% 3 3 3 4 5% 17%	1677
December	1st to 10th	Day	15% 24% 12% 15% 5% 3 4 2 3 3 12%	1520
	11th to 20th	Day	15% 22% 9% 16% 8% 5% 3 3 3 5% 10%	1918
	21st to end	Day	21% 24% 8% 14% 7% 5% 4 2 3 3 9%	2783
January	1st to 10th	Night	14% 23% 11% 15% 7% 6% 4 2 2 3 13%	4428
	11th to 20th	Night	12% 18% 8% 19% 10% 12% 3 2 3 3 8%	4440
	21st to end	Night	17% 23% 10% 17% 7% 6% 4 2 3 3 8%	4884
February	1st to 10th	Night	11% 17% 6% 15% 11% 7% 5% 3 3 4 17%	2498
	11th to 20th	Night	12% 16% 7% 13% 10% 5% 6% 3 5% 5% 18%	2218
	21st to end	Night	14% 17% 5% 12% 6% 6% 4 4 4 5% 23%	1884
March	1st to 10th	Night	16% 10% 5% 8% 5% 4 4 3 4 6% 37%	3733
	11th to 20th	Night	9% 11% 4 9% 5% 4 4 3 4 7% 38%	3614
	21st to end	Night	16% 13% 4 11% 6% 5% 4 3 5% 5% 28%	3639
April	1st to 10th	Night	17% 12% 3 8% 5% 4 4 3 5% 7% 33%	1900
	11th to 20th	Night	21% 9% 3 6% 4 4 4 4 5% 6% 36%	1825
	21st to end	Night	18% 9% 3 7% 5% 3 4 3 5% 6% 37%	1710
May	1st to 10th	Night	14% 8% 3 7% 5% 5% 4 5% 5% 7% 37%	2813
	11th to 20th	Night	14% 7% 2 6% 5% 4 5% 4 6% 9% 39%	2671
	21st to end	Night	18% 6% 2 6% 4 4 3 3 5% 7% 43%	2743
June	1st to 10th	Night	12% 8% 2 5% 4 3 5% 4 7% 8% 43%	1520
	11th to 20th	Night	11% 6% 2 6% 4 3 4 4 6% 8% 45%	1520
	21st to end	Night	10% 6% 2 7% 3 4 4 4 7% 9% 44%	1520
July	1st to 10th	Night	13% 8% 2 5% 3 5% 5% 4 6% 7% 41%	2530
	11th to 20th	Night	6% 4 1 4 4 4 4 3 5% 6% 58%	2530
	21st to end	Night	9% 5% 2 6% 3 4 4 4 6% 7% 50%	2750
August	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 7% 8% 44%	2688
	11th to 20th	Night	13% 5% 2 4 3 4 3 3 5% 6% 52%	2785
	21st to end	Night	9% 7% 2 5% 4 5% 4 4 5% 7% 48%	3431
September	1st to 10th	Night	10% 6% 2 6% 4 3 4 3 4 6% 52%	1900
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 5% 7% 54%	2076
	21st to end	Night	14% 6% 3 7% 4 4 5% 3 5% 7% 42%	2090
October	1st to 10th	Night	17% 8% 2 5% 4 4 4 3 4 6% 44%	2090
	11th to 20th	Night	22% 11% 4 6% 4 4 4 3 5% 5% 32%	2362
	21st to end	Night	25% 12% 3 8% 3 4 4 3 4 6% 27%	2717
November	1st to 10th	Night	14% 13% 4 9% 5% 5% 4 3 4 6% 34%	2433
	11th to 20th	Night	15% 19% 6% 10% 6% 5% 6% 4 5% 5% 20%	2470
	21st to end	Night	18% 20% 6% 10% 6% 5% 4 3 3 5% 20%	2498
December	1st to 10th	Night	14% 22% 9% 15% 7% 4 4 3 3 4 14%	2660
	11th to 20th	Night	15% 22% 8% 16% 7% 4 4 3 4 4 13%	3279
	21st to end	Night	24% 24% 7% 11% 5% 4 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	54% 6% 12% 20% 8% 1	3120
	11th to 20th	Day	52% 6% 16% 18% 7% 2	3345
	21st to end	Day	60% 5% 8% 20% 7% 1	4022
April	1st to 10th	Day	56% 4 10% 20% 10%	2280
	11th to 20th	Day	52% 4 9% 19% 14% 2	2304
	21st to end	Day	50% 5% 11% 18% 14% 2	2470
May	1st to 10th	Day	41% 6% 11% 25% 16% 2	4120
	11th to 20th	Day	37% 6% 13% 24% 18% 2	4229
	21st to end	Day	36% 7% 12% 23% 20% 2	4837
June	1st to 10th	Day	37% 6% 11% 27% 17% 2	2660
	11th to 20th	Day	37% 5% 13% 23% 20% 1	2660
	21st to end	Day	39% 5% 14% 24% 17% 1	2660
July	1st to 10th	Day	44% 3 13% 24% 14% 2	4371
	11th to 20th	Day	27% 3 15% 33% 20% 2	4365
	21st to end	Day	34% 5% 13% 26% 21% 2	4662
August	1st to 10th	Day	39% 5% 12% 29% 14% 1	4230
	11th to 20th	Day	36% 4 13% 29% 17% 1	4083
	21st to end	Day	38% 3 15% 27% 16% 1	4221
September	1st to 10th	Day	41% 5% 12% 26% 14% 2	2275
	11th to 20th	Day	35% 5% 13% 31% 14% 1	2090
	21st to end	Day	44% 6% 12% 27% 12%	2090
October	1st to 10th	Day	45% 5% 9% 27% 14%	2090
	11th to 20th	Day	61% 6% 11% 16% 6%	1815
	21st to end	Day	63% 5% 12% 14% 6%	1881
November	1st to 10th	Day	52% 7% 11% 20% 9%	1690
	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	48% 5% 15% 24% 7%	3733
	11th to 20th	Night	44% 7% 14% 24% 11% 1	3614
	21st to end	Night	57% 5% 9% 20% 8% 1	3639
April	1st to 10th	Night	50% 6% 10% 20% 13% 1	1900
	11th to 20th	Night	49% 5% 8% 21% 15% 3	1825
	21st to end	Night	47% 5% 10% 20% 14% 3	1710
May	1st to 10th	Night	43% 7% 13% 19% 15% 3	2840
	11th to 20th	Night	41% 7% 10% 26% 14% 2	2672
	21st to end	Night	42% 5% 11% 21% 16% 6%	2751
June	1st to 10th	Night	37% 5% 12% 29% 15% 2	1520
	11th to 20th	Night	35% 6% 13% 25% 19% 2	1520
	21st to end	Night	35% 6% 14% 27% 15% 3	1520
July	1st to 10th	Night	42% 4 11% 24% 12% 6%	2490
	11th to 20th	Night	26% 4 14% 36% 17% 3	2482
	21st to end	Night	33% 6% 14% 22% 24% 2	2662
August	1st to 10th	Night	36% 4 13% 29% 17% 1	2679
	11th to 20th	Night	33% 4 10% 35% 17% 1	2812
	21st to end	Night	36% 4 12% 25% 21% 2	3431
September	1st to 10th	Night	34% 5% 11% 29% 19% 2	1900
	11th to 20th	Night	30% 4 13% 36% 15% 2	2076
	21st to end	Night	42% 5% 13% 28% 12% 1	2090
October	1st to 10th	Night	40% 6% 9% 28% 15% 1	2090
	11th to 20th	Night	53% 6% 13% 21% 7% 1	2362
	21st to end	Night	57% 6% 12% 18% 7%	2717
November	1st to 10th	Night	51% 6% 12% 21% 10% 1	2433
	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% 30% 16% 7% 1	377
	21st to end	Day	32% 19% 31% 12% 4 1	506
April	1st to 10th	Day	29% 21% 35% 12% 2 1	460
	11th to 20th	Day	35% 22% 24% 11% 7% 1	452
	21st to end	Day	26% 21% 30% 15% 7% 1	460
May	1st to 10th	Day	25% 21% 32% 14% 6% 2	463
	11th to 20th	Day	20% 17% 37% 18% 7% 2	566
	21st to end	Day	18% 15% 35% 22% 8% 3	734
June	1st to 10th	Day	15% 17% 38% 21% 6% 3	680
	11th to 20th	Day	17% 17% 35% 22% 6% 3	680
	21st to end	Day	18% 15% 36% 21% 8% 2	680
July	1st to 10th	Day	10% 11% 33% 30% 12% 4	671
	11th to 20th	Day	11% 12% 33% 29% 10% 5%	668
	21st to end	Day	10% 9% 37% 26% 12% 5%	739
August	1st to 10th	Day	11% 11% 29% 26% 17% 6%	576
	11th to 20th	Day	15% 5% 25% 28% 16% 10%	479
	21st to end	Day	17% 9% 26% 27% 17% 4	539
September	1st to 10th	Day	17% 10% 31% 22% 16% 5%	490
	11th to 20th	Day	11% 10% 27% 31% 17% 5%	490
	21st to end	Day	18% 16% 23% 25% 14% 4	490
October	1st to 10th	Day	22% 15% 28% 22% 11% 2	490
	11th to 20th	Day	31% 18% 34% 15% 2	351
	21st to end	Day	38% 19% 29% 10% 3	330
November	1st to 10th	Day	28% 21% 32% 13% 5% 1	276
	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% 30% 25% 6% 1	735
April	1st to 10th	Night	23% 16% 33% 22% 5% 1	570
	11th to 20th	Night	28% 14% 32% 20% 5% 1	563
	21st to end	Night	22% 13% 34% 22% 9%	570
May	1st to 10th	Night	17% 14% 35% 27% 6% 1	570
	11th to 20th	Night	18% 12% 33% 28% 10%	561
	21st to end	Night	16% 6% 31% 33% 13% 1	617
June	1st to 10th	Night	12% 10% 32% 33% 12% 1	570
	11th to 20th	Night	13% 8% 33% 31% 13% 1	570
	21st to end	Night	12% 8% 30% 35% 14%	570
July	1st to 10th	Night	9% 4 23% 41% 21% 2	555
	11th to 20th	Night	9% 5% 26% 38% 20% 1	552
	21st to end	Night	10% 3 24% 40% 21% 2	624
August	1st to 10th	Night	9% 4 21% 44% 20% 2	567
	11th to 20th	Night	11% 3 19% 40% 22% 4	564
	21st to end	Night	15% 2 22% 42% 18% 2	627
September	1st to 10th	Night	12% 8% 23% 35% 19% 4	570
	11th to 20th	Night	13% 8% 23% 35% 19% 3	746
	21st to end	Night	17% 10% 26% 27% 17% 2	760
October	1st to 10th	Night	22% 10% 26% 29% 13% 1	760
	11th to 20th	Night	28% 14% 25% 25% 7% 1	760
	21st to end	Night	33% 12% 28% 22% 5%	836
November	1st to 10th	Night	24% 16% 31% 21% 7% 1	749
	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 2024-10-31 03:39:23. Version ac07

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