Lawrence Hall of Science
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lawrence Hall of Science clear sky chart from 2005-11-06 to 2025-08-05.

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	6% 5% 2 5% 3 3 3 3 5% 7% 59%	129477
	Night	8% 6% 2 6% 5% 5% 4 3 5% 6% 50%	127505
Transparency	Day	25% 5% 8% 19% 25% 18%	129193
	Night	33% 7% 9% 17% 20% 14%	127304
Seeing	Day	10% 11% 29% 33% 15% 3	16348
	Night	11% 6% 17% 36% 25% 4	25643

By Month

Percentage Cloud Cover SW (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% 7% 2 5% 3 3 4 4 6% 8% 44%	3869
	11th to 20th	Day	11% 8% 3 7% 4 3 3 2 4 5% 51%	4100
	21st to end	Day	16% 8% 3 8% 4 4 3 2 4 6% 40%	4510
February	1st to 10th	Day	17% 8% 2 5% 4 3 2 4 6% 5% 45%	1800
	11th to 20th	Day	12% 7% 3 7% 5% 4 4 4 5% 8% 40%	1835
	21st to end	Day	12% 7% 2 5% 4 4 3 3 4 7% 47%	1650
March	1st to 10th	Day	10% 10% 4 9% 5% 5% 5% 4 6% 7% 35%	4880
	11th to 20th	Day	9% 7% 3 6% 4 4 4 3 6% 7% 48%	5000
	21st to end	Day	9% 7% 2 7% 5% 5% 6% 5% 8% 8% 36%	5563
April	1st to 10th	Day	11% 8% 2 6% 4 4 3 4 6% 8% 45%	2400
	11th to 20th	Day	8% 5% 2 5% 3 4 3 3 4 7% 55%	2369
	21st to end	Day	4 5% 2 4 3 3 3 3 6% 9% 58%	2400
May	1st to 10th	Day	4 2 1 2 1 1 2 2 3 5% 76%	5652
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 7% 75%	5971
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 75%	6902
June	1st to 10th	Day	4 4 2 3 4 3 3 3 5% 8% 61%	2800
	11th to 20th	Day	1 2 1 2 2 2 2 3 5% 8% 73%	2800
	21st to end	Day	2 2 1 3 3 2 3 2 4 6% 73%	2800
July	1st to 10th	Day	1 1 2 2 2 2 2 3 5% 80%	6319
	11th to 20th	Day	1 1 3 2 2 3 2 4 7% 75%	5927
	21st to end	Day	2 2 5% 3 3 3 2 4 8% 68%	6402
August	1st to 10th	Day	1 2 1 3 3 3 4 4 6% 10% 62%	4144
	11th to 20th	Day	1 1 3 2 3 3 2 5% 6% 75%	3846
	21st to end	Day	1 2 1 4 2 2 3 2 4 7% 71%	4180
September	1st to 10th	Day	1 3 2 3 2 2 2 2 3 6% 75%	2280
	11th to 20th	Day	3 5% 2 4 4 3 3 3 5% 8% 62%	2280
	21st to end	Day	2 2 1 2 2 2 2 2 3 6% 74%	2012
October	1st to 10th	Day	2 3 1 3 2 2 3 3 4 6% 71%	3180
	11th to 20th	Day	3 4 2 5% 3 3 3 2 4 6% 65%	3161
	21st to end	Day	4 4 2 5% 2 3 3 3 6% 8% 59%	3448
November	1st to 10th	Day	7% 6% 2 4 3 3 3 3 4 8% 57%	1920
	11th to 20th	Day	9% 7% 3 7% 5% 3 4 3 5% 7% 47%	1800
	21st to end	Day	11% 7% 1 4 3 3 3 3 6% 7% 51%	1625
December	1st to 10th	Day	8% 5% 2 4 3 2 4 4 6% 10% 54%	2620
	11th to 20th	Day	20% 10% 3 7% 4 4 5% 5% 8% 9% 25%	3028
	21st to end	Day	17% 14% 5% 8% 4 4 4 3 5% 7% 28%	4004
January	1st to 10th	Night	10% 6% 2 5% 4 3 4 3 6% 9% 47%	6069
	11th to 20th	Night	11% 7% 3 6% 5% 4 4 3 4 5% 48%	5880
	21st to end	Night	14% 10% 3 7% 4 4 4 3 4 6% 42%	6468
February	1st to 10th	Night	15% 8% 2 5% 3 3 3 3 4 5% 48%	2600
	11th to 20th	Night	11% 8% 3 7% 4 3 4 3 5% 8% 44%	2451
	21st to end	Night	11% 6% 2 5% 4 4 3 3 5% 6% 51%	1980
March	1st to 10th	Night	12% 11% 3 8% 5% 5% 4 3 5% 6% 38%	5011
	11th to 20th	Night	13% 7% 3 5% 4 3 3 3 4 6% 49%	5000
	21st to end	Night	13% 11% 3 7% 4 3 3 3 4 7% 41%	5410
April	1st to 10th	Night	8% 8% 3 5% 5% 3 4 4 6% 7% 47%	2000
	11th to 20th	Night	7% 7% 2 5% 5% 3 3 3 4 6% 55%	1979
	21st to end	Night	4 5% 2 4 4 3 4 4 5% 8% 58%	2000
May	1st to 10th	Night	3 3 1 4 2 2 2 2 3 4 72%	4334
	11th to 20th	Night	1 4 2 5% 6% 6% 4 3 4 6% 58%	3918
	21st to end	Night	2 2 1 4 3 6% 4 2 3 5% 67%	3972
June	1st to 10th	Night	3 4 2 6% 5% 5% 3 4 7% 9% 51%	1600
	11th to 20th	Night	1 2 1 4 4 3 3 3 4 8% 66%	1600
	21st to end	Night	1 4 1 4 5% 6% 4 3 4 7% 61%	1600
July	1st to 10th	Night	1 3 5% 7% 5% 3 4 5% 67%	3640
	11th to 20th	Night	1 3 2 8% 10% 7% 6% 4 5% 6% 49%	4045
	21st to end	Night	1 7% 4 14% 11% 7% 5% 3 4 5% 37%	4472
August	1st to 10th	Night	2 4 2 5% 6% 11% 10% 8% 7% 9% 35%	2884
	11th to 20th	Night	2 1 5% 5% 6% 6% 5% 5% 7% 57%	2997
	21st to end	Night	2 3 2 6% 6% 7% 6% 4 4 6% 55%	3498
September	1st to 10th	Night	1 2 1 4 4 4 4 3 5% 8% 64%	1900
	11th to 20th	Night	3 5% 2 7% 6% 6% 3 2 5% 7% 52%	1900
	21st to end	Night	1 3 1 5% 3 3 2 2 4 7% 67%	2163
October	1st to 10th	Night	3 3 1 4 3 3 3 2 4 6% 69%	3800
	11th to 20th	Night	3 7% 2 7% 5% 5% 4 3 4 5% 55%	3777
	21st to end	Night	4 4 2 5% 3 3 4 4 6% 6% 58%	4121
November	1st to 10th	Night	6% 7% 2 4 4 4 4 3 4 8% 54%	2375
	11th to 20th	Night	9% 7% 2 5% 3 3 3 3 5% 8% 51%	2600
	21st to end	Night	9% 6% 1 4 4 3 3 3 5% 7% 55%	2773
December	1st to 10th	Night	8% 4 2 5% 3 4 4 3 4 6% 57%	4560
	11th to 20th	Night	19% 11% 4 7% 4 4 4 3 7% 8% 28%	5176
	21st to end	Night	19% 10% 3 9% 5% 4 5% 3 4 5% 31%	6952

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% 7% 17% 20% 15% 5%	3869
	11th to 20th	Day	38% 4 8% 14% 21% 15%	4100
	21st to end	Day	46% 5% 7% 12% 13% 17%	4510
February	1st to 10th	Day	41% 5% 9% 18% 20% 8%	1800
	11th to 20th	Day	40% 6% 11% 18% 20% 6%	1844
	21st to end	Day	37% 5% 8% 24% 18% 7%	1650
March	1st to 10th	Day	46% 7% 8% 18% 19% 2	4880
	11th to 20th	Day	35% 5% 16% 27% 10% 8%	5000
	21st to end	Day	40% 10% 16% 23% 10% 1	5563
April	1st to 10th	Day	38% 6% 13% 21% 19% 4	2400
	11th to 20th	Day	30% 5% 15% 23% 21% 7%	2372
	21st to end	Day	24% 5% 13% 30% 24% 4	2400
May	1st to 10th	Day	13% 3 8% 28% 31% 17%	5652
	11th to 20th	Day	12% 4 6% 33% 36% 9%	5974
	21st to end	Day	12% 7% 7% 29% 41% 4	6904
June	1st to 10th	Day	23% 6% 10% 27% 26% 8%	2800
	11th to 20th	Day	12% 4 9% 21% 36% 18%	2800
	21st to end	Day	15% 4 4 14% 32% 30%	2800
July	1st to 10th	Day	9% 3 2 7% 23% 55%	6251
	11th to 20th	Day	12% 3 3 7% 17% 58%	5856
	21st to end	Day	17% 5% 3 17% 27% 32%	6217
August	1st to 10th	Day	18% 5% 4 10% 29% 34%	4129
	11th to 20th	Day	11% 4 3 9% 42% 30%	3885
	21st to end	Day	15% 3 3 15% 35% 28%	4179
September	1st to 10th	Day	14% 2 3 11% 30% 40%	2280
	11th to 20th	Day	23% 4 6% 22% 30% 15%	2280
	21st to end	Day	14% 2 6% 21% 33% 23%	2012
October	1st to 10th	Day	16% 3 5% 11% 28% 38%	3180
	11th to 20th	Day	22% 4 6% 23% 28% 18%	3161
	21st to end	Day	25% 4 7% 23% 27% 14%	3448
November	1st to 10th	Day	27% 5% 10% 27% 20% 12%	1920
	11th to 20th	Day	37% 4 10% 22% 20% 6%	1800
	21st to end	Day	32% 4 10% 19% 24% 11%	1625
December	1st to 10th	Day	27% 5% 6% 28% 28% 6%	2620
	11th to 20th	Day	52% 5% 9% 15% 16% 3	3028
	21st to end	Day	55% 5% 12% 17% 9% 2	4004
January	1st to 10th	Night	34% 7% 21% 21% 14% 4	6069
	11th to 20th	Night	40% 5% 9% 14% 20% 13%	5880
	21st to end	Night	44% 5% 8% 10% 18% 16%	6468
February	1st to 10th	Night	39% 4 9% 18% 20% 11%	2600
	11th to 20th	Night	39% 6% 10% 18% 19% 8%	2453
	21st to end	Night	33% 5% 11% 27% 18% 6%	1980
March	1st to 10th	Night	45% 9% 9% 18% 17% 3	5011
	11th to 20th	Night	36% 7% 11% 23% 15% 8%	5000
	21st to end	Night	42% 8% 13% 23% 11% 2	5410
April	1st to 10th	Night	35% 6% 12% 22% 20% 5%	2000
	11th to 20th	Night	32% 5% 12% 25% 18% 8%	1979
	21st to end	Night	25% 7% 11% 30% 22% 6%	2000
May	1st to 10th	Night	18% 4 7% 26% 29% 16%	4331
	11th to 20th	Night	23% 11% 9% 24% 24% 9%	3919
	21st to end	Night	16% 13% 7% 23% 38% 4	3972
June	1st to 10th	Night	29% 9% 10% 20% 26% 7%	1600
	11th to 20th	Night	19% 4 7% 18% 34% 17%	1600
	21st to end	Night	22% 9% 4 12% 27% 27%	1600
July	1st to 10th	Night	18% 8% 2 5% 23% 44%	3600
	11th to 20th	Night	35% 6% 4 5% 12% 37%	3993
	21st to end	Night	46% 7% 3 6% 16% 22%	4345
August	1st to 10th	Night	36% 16% 5% 7% 21% 16%	2875
	11th to 20th	Night	21% 8% 6% 6% 33% 26%	3024
	21st to end	Night	27% 7% 7% 9% 28% 22%	3498
September	1st to 10th	Night	20% 4 6% 6% 27% 36%	1900
	11th to 20th	Night	31% 8% 6% 18% 24% 15%	1900
	21st to end	Night	19% 4 5% 20% 29% 23%	2163
October	1st to 10th	Night	17% 4 5% 10% 20% 45%	3800
	11th to 20th	Night	29% 8% 5% 18% 18% 22%	3777
	21st to end	Night	26% 5% 5% 21% 24% 19%	4121
November	1st to 10th	Night	30% 5% 10% 25% 18% 13%	2375
	11th to 20th	Night	32% 5% 11% 25% 19% 8%	2600
	21st to end	Night	30% 5% 10% 20% 22% 13%	2773
December	1st to 10th	Night	29% 4 6% 21% 29% 11%	4560
	11th to 20th	Night	53% 4 8% 15% 16% 3	5176
	21st to end	Night	55% 6% 13% 15% 9% 3	6952

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	26% 12% 30% 17% 14% 1	300
	11th to 20th	Day	20% 12% 24% 32% 12% 2	400
	21st to end	Day	15% 9% 29% 31% 15% 1	440
February	1st to 10th	Day	23% 11% 21% 26% 17% 2	400
	11th to 20th	Day	21% 16% 27% 26% 11% 1	435
	21st to end	Day	14% 15% 28% 28% 14% 1	414
March	1st to 10th	Day	21% 18% 30% 20% 9% 2	500
	11th to 20th	Day	20% 18% 26% 20% 13% 3	500
	21st to end	Day	16% 19% 31% 24% 9% 1	550
April	1st to 10th	Day	16% 17% 30% 24% 9% 3	500
	11th to 20th	Day	14% 17% 34% 23% 8% 4	492
	21st to end	Day	8% 21% 29% 27% 10% 5%	500
May	1st to 10th	Day	9% 16% 34% 29% 10% 1	503
	11th to 20th	Day	10% 14% 33% 30% 11% 1	528
	21st to end	Day	7% 14% 34% 31% 12% 2	573
June	1st to 10th	Day	8% 10% 36% 32% 13% 2	530
	11th to 20th	Day	4 12% 38% 37% 7% 2	530
	21st to end	Day	2 8% 34% 42% 12% 2	530
July	1st to 10th	Day	3 5% 34% 45% 13% 1	526
	11th to 20th	Day	3 25% 53% 16% 2	524
	21st to end	Day	2 2 31% 48% 14% 4	575
August	1st to 10th	Day	3 7% 31% 44% 12% 3	504
	11th to 20th	Day	2 5% 31% 41% 17% 5%	477
	21st to end	Day	2 3 25% 47% 20% 3	539
September	1st to 10th	Day	2 4 22% 44% 21% 7%	490
	11th to 20th	Day	5% 11% 27% 37% 15% 6%	490
	21st to end	Day	3 8% 25% 33% 27% 4	490
October	1st to 10th	Day	4 7% 28% 30% 22% 8%	490
	11th to 20th	Day	7% 10% 21% 31% 25% 6%	490
	21st to end	Day	9% 9% 25% 29% 22% 6%	539
November	1st to 10th	Day	11% 10% 21% 25% 28% 5%	456
	11th to 20th	Day	10% 9% 19% 34% 23% 5%	300
	21st to end	Day	16% 8% 27% 31% 16% 1	213
December	1st to 10th	Day	15% 13% 27% 27% 16% 4	200
	11th to 20th	Day	19% 12% 25% 28% 16% 1	200
	21st to end	Day	24% 10% 31% 24% 11%	220
January	1st to 10th	Night	20% 5% 15% 34% 21% 5%	800
	11th to 20th	Night	15% 7% 16% 36% 23% 4	800
	21st to end	Night	18% 5% 14% 36% 23% 4	880
February	1st to 10th	Night	20% 7% 13% 35% 22% 3	800
	11th to 20th	Night	15% 9% 18% 33% 20% 5%	784
	21st to end	Night	14% 9% 19% 35% 21% 2	660
March	1st to 10th	Night	20% 10% 20% 28% 20% 3	800
	11th to 20th	Night	18% 9% 17% 32% 22% 3	800
	21st to end	Night	16% 10% 23% 33% 18% 2	880
April	1st to 10th	Night	13% 10% 23% 36% 17% 2	800
	11th to 20th	Night	12% 11% 22% 33% 18% 3	791
	21st to end	Night	8% 8% 28% 32% 20% 4	800
May	1st to 10th	Night	8% 6% 24% 39% 21% 2	704
	11th to 20th	Night	6% 4 20% 41% 25% 3	527
	21st to end	Night	6% 10% 26% 37% 18% 3	433
June	1st to 10th	Night	5% 6% 25% 44% 19% 2	400
	11th to 20th	Night	2 4 23% 53% 17% 2	400
	21st to end	Night	4 4 19% 43% 28% 2	400
July	1st to 10th	Night	2 2 16% 49% 29% 1	390
	11th to 20th	Night	3 1 10% 50% 30% 5%	555
	21st to end	Night	3 4 13% 42% 34% 5%	657
August	1st to 10th	Night	8% 3 17% 38% 30% 4	579
	11th to 20th	Night	3 3 16% 43% 29% 5%	671
	21st to end	Night	6% 1 12% 44% 32% 5%	836
September	1st to 10th	Night	3 1 11% 40% 37% 8%	760
	11th to 20th	Night	6% 3 16% 39% 31% 5%	760
	21st to end	Night	4 3 14% 36% 36% 8%	760
October	1st to 10th	Night	7% 3 14% 34% 34% 8%	760
	11th to 20th	Night	9% 5% 12% 32% 35% 7%	760
	21st to end	Night	9% 4 11% 33% 32% 11%	836
November	1st to 10th	Night	9% 6% 13% 31% 33% 8%	780
	11th to 20th	Night	13% 4 14% 35% 28% 7%	800
	21st to end	Night	12% 6% 14% 37% 26% 5%	800
December	1st to 10th	Night	17% 5% 15% 35% 24% 5%	800
	11th to 20th	Night	24% 6% 13% 33% 21% 3	800
	21st to end	Night	20% 9% 20% 31% 18% 1	880

Page generated at 2025-08-01 22:32:26. Version ac07

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