Steve's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Steve's Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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	12% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	108532
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	110820
Transparency	Day	41% 4 10% 19% 19% 7%	112209
	Night	38% 4 10% 19% 20% 9%	115283
Seeing	Day	15% 9% 21% 25% 21% 9%	15356
	Night	15% 8% 23% 29% 20% 4	25213

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	16% 9% 3 8% 3 3 3 3 4 5% 43%	4077
	11th to 20th	Day	13% 5% 3 6% 4 4 4 3 4 5% 50%	4080
	21st to end	Day	22% 11% 3 6% 4 4 4 3 4 4 37%	4639
February	1st to 10th	Day	17% 10% 3 6% 4 4 4 3 4 6% 39%	2000
	11th to 20th	Day	19% 7% 3 6% 4 4 4 3 5% 6% 40%	1948
	21st to end	Day	15% 8% 4 9% 5% 4 4 4 4 6% 37%	1650
March	1st to 10th	Day	21% 11% 3 7% 5% 5% 4 5% 5% 6% 29%	3418
	11th to 20th	Day	16% 9% 3 8% 4 4 4 4 6% 7% 35%	3590
	21st to end	Day	13% 8% 3 7% 6% 5% 5% 4 4 5% 41%	4034
April	1st to 10th	Day	10% 8% 2 6% 5% 4 5% 5% 6% 7% 41%	2280
	11th to 20th	Day	16% 8% 2 7% 5% 4 5% 3 5% 7% 39%	2254
	21st to end	Day	8% 5% 2 6% 5% 4 5% 5% 6% 8% 45%	2280
May	1st to 10th	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 8% 34%	3765
	11th to 20th	Day	13% 7% 2 6% 5% 4 5% 4 5% 6% 43%	3764
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	4129
June	1st to 10th	Day	9% 6% 3 7% 6% 7% 7% 7% 8% 8% 32%	2446
	11th to 20th	Day	6% 5% 2 8% 7% 7% 7% 5% 8% 9% 37%	2535
	21st to end	Day	7% 6% 2 8% 8% 7% 7% 7% 9% 10% 30%	2470
July	1st to 10th	Day	6% 7% 3 9% 8% 9% 8% 6% 8% 10% 26%	4120
	11th to 20th	Day	8% 6% 2 8% 8% 7% 9% 7% 9% 11% 25%	4120
	21st to end	Day	11% 10% 4 11% 8% 6% 7% 5% 7% 8% 24%	4277
August	1st to 10th	Day	3 3 1 5% 4 4 5% 4 5% 8% 59%	3785
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 5% 6% 10% 46%	3729
	21st to end	Day	11% 5% 2 8% 5% 6% 5% 5% 6% 7% 39%	4180
September	1st to 10th	Day	10% 6% 3 7% 6% 6% 7% 5% 6% 8% 35%	2280
	11th to 20th	Day	12% 6% 2 7% 6% 5% 6% 4 5% 6% 42%	2247
	21st to end	Day	11% 5% 2 5% 4 4 4 3 5% 7% 51%	2090
October	1st to 10th	Day	7% 4 2 5% 4 3 4 4 6% 7% 56%	1950
	11th to 20th	Day	8% 4 2 6% 4 3 3 2 5% 7% 55%	1900
	21st to end	Day	11% 5% 2 4 3 4 4 2 3 5% 57%	2090
November	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 5% 6% 51%	1958
	11th to 20th	Day	12% 6% 2 6% 4 3 3 3 4 5% 53%	1895
	21st to end	Day	13% 7% 3 6% 4 3 4 3 4 6% 47%	1800
December	1st to 10th	Day	16% 8% 3 6% 5% 4 3 3 5% 5% 42%	2930
	11th to 20th	Day	12% 9% 3 7% 5% 4 4 3 5% 5% 43%	3356
	21st to end	Day	16% 7% 2 5% 3 4 4 3 4 5% 47%	4466
January	1st to 10th	Night	19% 8% 3 5% 3 3 2 2 4 4 46%	5888
	11th to 20th	Night	17% 7% 4 6% 3 3 3 2 4 4 48%	5900
	21st to end	Night	25% 10% 2 5% 3 3 3 2 3 4 39%	6327
February	1st to 10th	Night	15% 9% 3 6% 4 4 4 3 5% 6% 42%	2400
	11th to 20th	Night	20% 10% 3 5% 3 3 3 2 4 5% 42%	2338
	21st to end	Night	20% 9% 3 7% 4 4 4 3 5% 6% 37%	1980
March	1st to 10th	Night	20% 13% 3 7% 4 4 3 3 5% 7% 31%	3670
	11th to 20th	Night	14% 8% 2 6% 5% 5% 5% 4 6% 8% 38%	3590
	21st to end	Night	16% 8% 2 6% 4 5% 5% 5% 5% 6% 40%	3733
April	1st to 10th	Night	12% 7% 2 5% 5% 4 4 4 6% 7% 44%	1900
	11th to 20th	Night	12% 7% 2 6% 5% 4 4 4 5% 7% 45%	1875
	21st to end	Night	7% 5% 2 4 4 4 4 4 5% 8% 54%	1900
May	1st to 10th	Night	11% 7% 3 7% 3 4 4 3 5% 8% 44%	3130
	11th to 20th	Night	16% 7% 3 6% 3 3 4 3 5% 8% 43%	3129
	21st to end	Night	6% 5% 2 6% 6% 5% 6% 5% 7% 11% 41%	3426
June	1st to 10th	Night	7% 7% 2 7% 5% 5% 5% 5% 7% 11% 39%	1697
	11th to 20th	Night	3 5% 1 5% 5% 5% 4 6% 7% 11% 48%	1611
	21st to end	Night	5% 5% 2 5% 5% 5% 6% 6% 10% 10% 41%	1710
July	1st to 10th	Night	6% 6% 3 8% 7% 7% 6% 5% 9% 10% 34%	2850
	11th to 20th	Night	6% 5% 2 6% 6% 8% 7% 7% 11% 12% 30%	2850
	21st to end	Night	8% 8% 4 9% 7% 6% 6% 5% 8% 11% 29%	3277
August	1st to 10th	Night	2 2 1 3 2 3 4 4 6% 11% 62%	3144
	11th to 20th	Night	4 3 1 5% 5% 5% 6% 5% 8% 13% 44%	3100
	21st to end	Night	7% 5% 3 8% 5% 5% 5% 5% 6% 8% 43%	3476
September	1st to 10th	Night	7% 5% 2 6% 4 5% 6% 6% 9% 10% 40%	1900
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 6% 9% 46%	1914
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 7% 54%	2090
October	1st to 10th	Night	6% 4 2 5% 3 4 4 3 5% 6% 59%	2225
	11th to 20th	Night	8% 5% 2 5% 4 3 3 3 4 5% 58%	2280
	21st to end	Night	12% 6% 2 4 3 2 3 2 4 4 58%	2508
November	1st to 10th	Night	13% 5% 1 4 3 3 3 3 4 5% 55%	2340
	11th to 20th	Night	12% 6% 2 4 3 3 3 2 4 6% 55%	2500
	21st to end	Night	14% 7% 2 4 4 3 4 3 5% 5% 49%	2600
December	1st to 10th	Night	20% 7% 2 5% 3 4 3 3 4 5% 46%	4250
	11th to 20th	Night	14% 10% 3 6% 4 3 4 3 4 5% 44%	4822
	21st to end	Night	20% 10% 3 5% 3 3 2 2 3 4 46%	6490

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	44% 3 3 20% 25% 4	4078
	11th to 20th	Day	37% 2 4 18% 30% 9%	4080
	21st to end	Day	51% 3 5% 11% 17% 14%	4639
February	1st to 10th	Day	48% 2 6% 15% 18% 11%	2000
	11th to 20th	Day	46% 3 6% 16% 21% 9%	1958
	21st to end	Day	48% 4 6% 15% 16% 10%	1650
March	1st to 10th	Day	56% 3 8% 14% 14% 6%	3418
	11th to 20th	Day	48% 4 8% 17% 17% 6%	3590
	21st to end	Day	47% 3 8% 15% 20% 8%	4034
April	1st to 10th	Day	41% 4 10% 19% 18% 9%	2280
	11th to 20th	Day	45% 3 8% 17% 20% 7%	2254
	21st to end	Day	35% 5% 11% 23% 22% 4	2280
May	1st to 10th	Day	45% 4 14% 21% 13% 3	3800
	11th to 20th	Day	41% 3 12% 19% 21% 3	3768
	21st to end	Day	36% 4 18% 29% 11% 2	4141
June	1st to 10th	Day	45% 5% 14% 23% 11% 2	2446
	11th to 20th	Day	40% 5% 15% 23% 14% 3	2535
	21st to end	Day	44% 8% 17% 20% 10% 2	2470
July	1st to 10th	Day	47% 10% 20% 17% 5%	4056
	11th to 20th	Day	46% 11% 25% 14% 3 1	4051
	21st to end	Day	55% 8% 13% 16% 6% 1	4141
August	1st to 10th	Day	24% 4 13% 30% 28% 1	3773
	11th to 20th	Day	33% 6% 19% 29% 12% 1	3765
	21st to end	Day	43% 6% 15% 18% 16% 3	4179
September	1st to 10th	Day	46% 5% 14% 17% 14% 5%	2280
	11th to 20th	Day	43% 5% 10% 18% 18% 7%	2247
	21st to end	Day	33% 3 10% 24% 19% 10%	2090
October	1st to 10th	Day	23% 3 6% 14% 23% 31%	3236
	11th to 20th	Day	18% 2 5% 15% 28% 31%	3140
	21st to end	Day	23% 2 3 14% 37% 20%	3428
November	1st to 10th	Day	34% 3 7% 18% 26% 12%	1958
	11th to 20th	Day	35% 3 6% 14% 29% 14%	1895
	21st to end	Day	39% 2 7% 18% 23% 11%	1800
December	1st to 10th	Day	44% 2 7% 16% 24% 6%	2930
	11th to 20th	Day	43% 4 8% 14% 21% 10%	3353
	21st to end	Day	41% 2 5% 23% 27% 3	4466
January	1st to 10th	Night	42% 3 4 19% 26% 6%	5888
	11th to 20th	Night	42% 2 4 14% 27% 10%	5900
	21st to end	Night	51% 2 4 13% 20% 10%	6327
February	1st to 10th	Night	44% 3 6% 17% 21% 10%	2400
	11th to 20th	Night	46% 3 5% 16% 20% 10%	2339
	21st to end	Night	50% 3 6% 15% 14% 12%	1980
March	1st to 10th	Night	54% 3 9% 13% 15% 6%	3670
	11th to 20th	Night	45% 4 10% 17% 18% 7%	3590
	21st to end	Night	44% 3 9% 18% 21% 5%	3733
April	1st to 10th	Night	39% 4 11% 18% 19% 10%	1900
	11th to 20th	Night	38% 3 10% 20% 19% 10%	1875
	21st to end	Night	29% 3 11% 23% 27% 7%	1900
May	1st to 10th	Night	39% 4 14% 22% 17% 4	3160
	11th to 20th	Night	40% 4 12% 24% 19% 1	3130
	21st to end	Night	35% 5% 18% 27% 13% 2	3435
June	1st to 10th	Night	38% 6% 18% 25% 13% 1	1697
	11th to 20th	Night	30% 6% 19% 28% 14% 3	1611
	21st to end	Night	32% 8% 23% 22% 13% 2	1710
July	1st to 10th	Night	41% 11% 25% 17% 5% 1	2805
	11th to 20th	Night	40% 13% 30% 14% 3 1	2796
	21st to end	Night	49% 10% 18% 17% 5% 1	3173
August	1st to 10th	Night	16% 6% 20% 31% 25% 1	3132
	11th to 20th	Night	28% 10% 21% 31% 9% 1	3130
	21st to end	Night	40% 7% 14% 18% 17% 4	3476
September	1st to 10th	Night	34% 8% 20% 18% 14% 6%	1900
	11th to 20th	Night	35% 5% 16% 17% 18% 9%	1914
	21st to end	Night	30% 3 11% 25% 21% 10%	2090
October	1st to 10th	Night	22% 3 7% 13% 24% 31%	3719
	11th to 20th	Night	19% 2 3 20% 23% 33%	3779
	21st to end	Night	21% 2 5% 13% 38% 22%	4120
November	1st to 10th	Night	31% 3 8% 18% 25% 15%	2340
	11th to 20th	Night	32% 3 5% 16% 29% 15%	2500
	21st to end	Night	38% 3 6% 19% 21% 13%	2600
December	1st to 10th	Night	43% 1 5% 18% 26% 5%	4250
	11th to 20th	Night	43% 3 7% 16% 22% 8%	4824
	21st to end	Night	44% 2 5% 19% 24% 6%	6490

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	25% 23% 34% 15% 3 1	300
	11th to 20th	Day	25% 18% 36% 16% 4 1	300
	21st to end	Day	27% 22% 36% 12% 2	330
February	1st to 10th	Day	27% 18% 36% 15% 3	300
	11th to 20th	Day	35% 15% 34% 13% 3	300
	21st to end	Day	27% 27% 35% 10% 1	249
March	1st to 10th	Day	31% 16% 35% 15% 3	300
	11th to 20th	Day	28% 17% 38% 13% 3	300
	21st to end	Day	23% 15% 33% 22% 6%	405
April	1st to 10th	Day	18% 19% 30% 22% 9% 1	460
	11th to 20th	Day	24% 14% 30% 21% 10% 1	452
	21st to end	Day	13% 10% 32% 30% 13% 2	460
May	1st to 10th	Day	15% 11% 27% 29% 15% 3	463
	11th to 20th	Day	14% 6% 24% 29% 20% 8%	487
	21st to end	Day	7% 4 22% 32% 24% 10%	529
June	1st to 10th	Day	12% 4 12% 35% 25% 12%	612
	11th to 20th	Day	8% 3 14% 34% 32% 9%	680
	21st to end	Day	10% 3 12% 27% 30% 18%	680
July	1st to 10th	Day	7% 2 7% 28% 42% 15%	671
	11th to 20th	Day	8% 2 5% 23% 40% 22%	668
	21st to end	Day	7% 3 7% 25% 39% 19%	626
August	1st to 10th	Day	6% 2 5% 26% 34% 27%	486
	11th to 20th	Day	9% 1 4 20% 43% 23%	479
	21st to end	Day	14% 2 7% 22% 33% 21%	539
September	1st to 10th	Day	13% 1 6% 25% 35% 19%	490
	11th to 20th	Day	12% 4 12% 22% 35% 14%	490
	21st to end	Day	13% 4 11% 32% 28% 13%	490
October	1st to 10th	Day	7% 7% 23% 37% 18% 8%	350
	11th to 20th	Day	11% 5% 26% 31% 22% 5%	300
	21st to end	Day	20% 9% 20% 31% 15% 6%	330
November	1st to 10th	Day	18% 9% 25% 33% 12% 3	300
	11th to 20th	Day	19% 11% 32% 28% 9% 1	300
	21st to end	Day	15% 18% 35% 20% 9% 3	300
December	1st to 10th	Day	23% 10% 38% 22% 5% 1	300
	11th to 20th	Day	21% 17% 37% 18% 6% 1	300
	21st to end	Day	26% 16% 33% 19% 6% 1	330
January	1st to 10th	Night	25% 14% 34% 22% 5%	1000
	11th to 20th	Night	24% 15% 32% 23% 7%	1000
	21st to end	Night	24% 16% 32% 25% 4	1028
February	1st to 10th	Night	22% 15% 35% 25% 4	800
	11th to 20th	Night	29% 15% 28% 23% 5%	783
	21st to end	Night	23% 20% 35% 19% 3	660
March	1st to 10th	Night	20% 15% 34% 26% 5%	695
	11th to 20th	Night	18% 14% 34% 28% 6% 1	600
	21st to end	Night	17% 10% 37% 28% 8%	647
April	1st to 10th	Night	19% 11% 34% 26% 9% 1	570
	11th to 20th	Night	17% 12% 33% 27% 10% 1	563
	21st to end	Night	9% 9% 33% 35% 14% 1	570
May	1st to 10th	Night	11% 5% 30% 35% 17% 2	570
	11th to 20th	Night	11% 3 24% 37% 21% 3	561
	21st to end	Night	5% 4 20% 40% 26% 5%	617
June	1st to 10th	Night	10% 3 10% 40% 30% 8%	570
	11th to 20th	Night	4 2 11% 35% 41% 7%	570
	21st to end	Night	6% 2 8% 31% 38% 15%	570
July	1st to 10th	Night	5% 2 3 29% 47% 14%	555
	11th to 20th	Night	6% 1 4 25% 45% 20%	552
	21st to end	Night	5% 2 5% 28% 46% 14%	624
August	1st to 10th	Night	3 1 4 23% 49% 20%	567
	11th to 20th	Night	7% 1 4 23% 48% 17%	564
	21st to end	Night	10% 2 6% 24% 39% 19%	627
September	1st to 10th	Night	9% 2 5% 30% 42% 11%	570
	11th to 20th	Night	10% 3 9% 28% 41% 9%	584
	21st to end	Night	10% 2 11% 34% 36% 6%	760
October	1st to 10th	Night	8% 2 18% 34% 33% 5%	760
	11th to 20th	Night	10% 4 24% 36% 22% 3	760
	21st to end	Night	17% 6% 20% 32% 23% 2	836
November	1st to 10th	Night	13% 7% 22% 36% 19% 2	780
	11th to 20th	Night	15% 9% 28% 33% 15% 1	800
	21st to end	Night	17% 10% 29% 28% 14% 2	800
December	1st to 10th	Night	22% 10% 29% 30% 10%	800
	11th to 20th	Night	22% 10% 34% 26% 9%	800
	21st to end	Night	27% 12% 29% 27% 6%	1100

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