Sterling
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sterling clear sky chart from 2005-11-06 to 2025-03-12.

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	13% 9% 3 8% 5% 4 4 4 6% 7% 38%	108906
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 39%	110303
Transparency	Day	43% 7% 12% 23% 14% 2	112446
	Night	44% 6% 11% 22% 15% 2	114904
Seeing	Day	16% 10% 28% 26% 15% 5%	16211
	Night	17% 8% 31% 31% 12% 1	26256

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	8% 12% 7% 17% 7% 6% 5% 4 5% 5% 24%	3617
	11th to 20th	Day	9% 10% 5% 12% 6% 6% 5% 6% 8% 5% 28%	3620
	21st to end	Day	14% 16% 6% 10% 5% 4 4 3 5% 7% 28%	4332
February	1st to 10th	Day	13% 13% 5% 11% 6% 5% 6% 4 4 6% 28%	1800
	11th to 20th	Day	13% 11% 3 9% 5% 6% 4 3 5% 6% 35%	1894
	21st to end	Day	12% 12% 5% 10% 4 4 5% 4 5% 7% 32%	1650
March	1st to 10th	Day	14% 12% 5% 6% 4 3 3 3 4 6% 39%	3220
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 6% 7% 43%	3416
	21st to end	Day	21% 12% 4 12% 5% 5% 4 3 5% 6% 22%	4091
April	1st to 10th	Day	17% 13% 3 8% 4 4 4 4 5% 6% 32%	2280
	11th to 20th	Day	17% 10% 2 7% 5% 5% 4 4 5% 6% 35%	2254
	21st to end	Day	22% 9% 3 7% 4 4 4 3 5% 6% 34%	2294
May	1st to 10th	Day	16% 7% 3 7% 5% 4 4 4 5% 8% 36%	4171
	11th to 20th	Day	15% 7% 2 6% 5% 4 4 3 5% 7% 42%	4399
	21st to end	Day	12% 6% 2 6% 5% 4 5% 4 7% 8% 42%	4824
June	1st to 10th	Day	9% 5% 2 6% 4 5% 4 5% 7% 8% 45%	2660
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 3 6% 9% 47%	2660
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 9% 38%	2660
July	1st to 10th	Day	10% 8% 3 8% 6% 5% 5% 5% 7% 10% 34%	4440
	11th to 20th	Day	7% 4 2 4 4 4 4 3 6% 10% 53%	4440
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 4 7% 9% 40%	4813
August	1st to 10th	Day	8% 5% 3 5% 5% 4 5% 5% 8% 9% 44%	4236
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	3789
	21st to end	Day	8% 6% 2 6% 4 4 4 3 6% 8% 49%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 8% 47%	2280
	11th to 20th	Day	12% 4 1 3 3 4 4 3 5% 6% 56%	2228
	21st to end	Day	10% 5% 2 6% 4 4 4 3 6% 8% 48%	2090
October	1st to 10th	Day	14% 6% 2 6% 4 3 4 3 4 7% 49%	1929
	11th to 20th	Day	17% 9% 3 6% 4 4 4 2 4 7% 40%	1900
	21st to end	Day	20% 10% 2 7% 3 3 3 3 4 6% 40%	1952
November	1st to 10th	Day	12% 8% 2 5% 3 4 4 4 6% 7% 44%	1762
	11th to 20th	Day	17% 12% 4 8% 5% 4 4 3 5% 6% 32%	1800
	21st to end	Day	15% 13% 4 9% 5% 4 4 3 4 5% 34%	1625
December	1st to 10th	Day	8% 10% 3 8% 5% 3 4 3 4 7% 44%	2610
	11th to 20th	Day	13% 17% 6% 11% 6% 6% 5% 4 5% 4 23%	3008
	21st to end	Day	24% 20% 8% 13% 5% 4 3 2 3 4 13%	3982
January	1st to 10th	Night	11% 13% 6% 15% 6% 4 3 3 4 4 31%	6348
	11th to 20th	Night	11% 14% 6% 12% 7% 6% 4 5% 6% 5% 23%	6360
	21st to end	Night	15% 15% 6% 8% 5% 4 3 3 4 5% 32%	6631
February	1st to 10th	Night	14% 14% 5% 8% 6% 4 5% 3 4 5% 34%	2600
	11th to 20th	Night	12% 10% 4 7% 5% 4 5% 3 4 6% 39%	2392
	21st to end	Night	16% 11% 4 8% 4 3 3 3 4 5% 39%	1980
March	1st to 10th	Night	19% 9% 2 5% 4 4 4 3 4 5% 41%	3853
	11th to 20th	Night	10% 6% 3 6% 5% 4 4 3 5% 7% 48%	3604
	21st to end	Night	23% 11% 5% 10% 6% 4 4 3 4 4 25%	3539
April	1st to 10th	Night	18% 10% 3 6% 4 3 5% 3 5% 5% 39%	1900
	11th to 20th	Night	19% 7% 3 6% 4 3 4 4 4 5% 41%	1875
	21st to end	Night	17% 8% 3 7% 5% 4 4 4 6% 7% 35%	1886
May	1st to 10th	Night	17% 7% 3 8% 5% 5% 5% 4 6% 6% 34%	2723
	11th to 20th	Night	15% 5% 1 6% 5% 4 5% 5% 6% 7% 40%	2506
	21st to end	Night	9% 6% 2 5% 4 4 5% 4 5% 9% 47%	2743
June	1st to 10th	Night	8% 4 2 5% 5% 4 5% 4 6% 7% 49%	1520
	11th to 20th	Night	8% 4 2 6% 5% 4 4 4 7% 9% 47%	1520
	21st to end	Night	10% 6% 2 5% 5% 4 5% 4 7% 8% 44%	1520
July	1st to 10th	Night	14% 6% 2 7% 5% 4 4 4 6% 7% 39%	2530
	11th to 20th	Night	5% 3 2 5% 4 3 4 4 5% 7% 58%	2530
	21st to end	Night	5% 4 2 6% 5% 4 5% 5% 7% 9% 47%	2750
August	1st to 10th	Night	5% 4 2 5% 4 4 5% 4 5% 9% 53%	2700
	11th to 20th	Night	6% 4 2 6% 4 4 4 4 5% 8% 52%	3041
	21st to end	Night	8% 4 2 5% 4 4 4 3 5% 9% 52%	3476
September	1st to 10th	Night	9% 6% 1 5% 4 3 3 3 5% 6% 57%	1900
	11th to 20th	Night	11% 3 2 5% 3 4 4 4 5% 8% 52%	1933
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 8% 49%	2090
October	1st to 10th	Night	11% 5% 1 6% 4 3 3 3 5% 6% 54%	2247
	11th to 20th	Night	15% 8% 3 5% 3 4 3 3 4 6% 47%	2280
	21st to end	Night	20% 9% 3 6% 4 4 4 3 4 7% 38%	2645
November	1st to 10th	Night	12% 7% 3 5% 3 3 3 3 4 6% 51%	2536
	11th to 20th	Night	15% 10% 4 8% 4 3 4 4 5% 6% 38%	2600
	21st to end	Night	17% 10% 3 7% 5% 3 4 3 4 6% 37%	2770
December	1st to 10th	Night	10% 8% 3 8% 5% 4 4 4 5% 6% 42%	4580
	11th to 20th	Night	17% 14% 5% 9% 4 4 4 3 4 5% 32%	5199
	21st to end	Night	23% 18% 6% 11% 6% 5% 4 3 4 4 15%	6996

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	60% 6% 10% 16% 8%	3618
	11th to 20th	Day	53% 5% 10% 11% 17% 3	3620
	21st to end	Day	56% 5% 14% 17% 8%	4332
February	1st to 10th	Day	56% 5% 13% 16% 8% 1	1800
	11th to 20th	Day	49% 6% 10% 23% 10% 2	1904
	21st to end	Day	51% 7% 11% 20% 10% 1	1650
March	1st to 10th	Day	45% 7% 11% 22% 12% 3	3220
	11th to 20th	Day	36% 9% 13% 30% 11% 1	3416
	21st to end	Day	60% 10% 13% 13% 3 1	4091
April	1st to 10th	Day	51% 6% 12% 21% 9%	2280
	11th to 20th	Day	47% 7% 10% 21% 12% 2	2254
	21st to end	Day	50% 7% 12% 17% 13% 2	2294
May	1st to 10th	Day	45% 8% 12% 21% 14% 1	4209
	11th to 20th	Day	41% 7% 10% 28% 13%	4403
	21st to end	Day	36% 9% 13% 23% 15% 5%	4837
June	1st to 10th	Day	34% 8% 14% 26% 17% 2	2660
	11th to 20th	Day	33% 8% 12% 24% 21% 3	2660
	21st to end	Day	39% 10% 14% 23% 12% 2	2660
July	1st to 10th	Day	43% 8% 13% 24% 10% 2	4371
	11th to 20th	Day	27% 6% 17% 31% 17% 2	4365
	21st to end	Day	37% 9% 17% 17% 18% 2	4662
August	1st to 10th	Day	33% 9% 16% 28% 13% 1	4221
	11th to 20th	Day	34% 7% 15% 29% 15% 1	3826
	21st to end	Day	32% 7% 12% 32% 16% 2	4179
September	1st to 10th	Day	33% 8% 11% 25% 19% 3	2280
	11th to 20th	Day	29% 6% 11% 29% 22% 3	2228
	21st to end	Day	34% 6% 12% 27% 19% 2	2090
October	1st to 10th	Day	24% 3 6% 31% 34% 1	3202
	11th to 20th	Day	31% 3 10% 24% 29% 3	3140
	21st to end	Day	42% 4 9% 28% 13% 3	3190
November	1st to 10th	Day	38% 5% 16% 25% 13% 2	1762
	11th to 20th	Day	52% 5% 12% 20% 9% 1	1800
	21st to end	Day	53% 6% 13% 20% 7%	1625
December	1st to 10th	Day	41% 4 14% 32% 8% 1	2610
	11th to 20th	Day	62% 5% 11% 15% 7% 1	3005
	21st to end	Day	78% 4 7% 7% 4	3982
January	1st to 10th	Night	55% 5% 11% 20% 9%	6348
	11th to 20th	Night	61% 5% 10% 10% 13% 1	6360
	21st to end	Night	56% 4 11% 18% 11% 1	6631
February	1st to 10th	Night	53% 5% 11% 18% 13% 1	2600
	11th to 20th	Night	46% 6% 11% 23% 13% 1	2393
	21st to end	Night	47% 5% 13% 20% 13% 2	1980
March	1st to 10th	Night	44% 6% 12% 22% 12% 3	3853
	11th to 20th	Night	34% 7% 11% 30% 16% 2	3604
	21st to end	Night	61% 8% 10% 16% 5% 1	3539
April	1st to 10th	Night	46% 6% 12% 21% 14% 2	1900
	11th to 20th	Night	44% 6% 8% 21% 17% 4	1875
	21st to end	Night	46% 7% 11% 22% 11% 2	1886
May	1st to 10th	Night	46% 9% 12% 17% 14% 2	2750
	11th to 20th	Night	38% 11% 14% 24% 12% 1	2507
	21st to end	Night	30% 10% 13% 22% 21% 3	2751
June	1st to 10th	Night	31% 8% 14% 25% 19% 3	1520
	11th to 20th	Night	31% 8% 11% 23% 24% 3	1520
	21st to end	Night	36% 10% 15% 19% 17% 3	1520
July	1st to 10th	Night	43% 5% 15% 21% 12% 3	2490
	11th to 20th	Night	25% 5% 14% 33% 21% 2	2482
	21st to end	Night	30% 10% 17% 19% 20% 5%	2662
August	1st to 10th	Night	27% 7% 16% 30% 18% 2	2691
	11th to 20th	Night	30% 6% 14% 31% 17% 1	3070
	21st to end	Night	29% 6% 13% 31% 18% 3	3476
September	1st to 10th	Night	28% 7% 10% 27% 25% 3	1900
	11th to 20th	Night	30% 6% 10% 30% 21% 4	1933
	21st to end	Night	32% 6% 11% 28% 20% 3	2090
October	1st to 10th	Night	23% 4 6% 34% 30% 2	3754
	11th to 20th	Night	30% 4 8% 23% 31% 4	3779
	21st to end	Night	42% 4 9% 25% 17% 3	4357
November	1st to 10th	Night	35% 6% 11% 27% 18% 3	2536
	11th to 20th	Night	47% 6% 12% 21% 13% 1	2600
	21st to end	Night	49% 5% 11% 23% 11% 1	2770
December	1st to 10th	Night	43% 5% 14% 29% 9% 1	4580
	11th to 20th	Night	55% 5% 12% 20% 8%	5201
	21st to end	Night	71% 6% 8% 10% 4	6996

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	20% 10% 35% 24% 10% 1	300
	11th to 20th	Day	20% 14% 40% 20% 5% 1	300
	21st to end	Day	22% 15% 35% 21% 6% 1	330
February	1st to 10th	Day	20% 18% 36% 21% 5%	300
	11th to 20th	Day	21% 18% 32% 23% 6%	300
	21st to end	Day	22% 15% 45% 14% 4	249
March	1st to 10th	Day	20% 19% 37% 17% 6%	300
	11th to 20th	Day	23% 16% 32% 20% 8% 1	273
	21st to end	Day	27% 15% 32% 20% 5% 2	458
April	1st to 10th	Day	25% 13% 33% 22% 5% 1	460
	11th to 20th	Day	21% 18% 29% 21% 10% 2	452
	21st to end	Day	26% 12% 33% 19% 9% 1	460
May	1st to 10th	Day	20% 12% 30% 26% 8% 3	513
	11th to 20th	Day	21% 10% 29% 24% 12% 3	675
	21st to end	Day	15% 10% 27% 23% 17% 8%	734
June	1st to 10th	Day	8% 8% 28% 32% 18% 6%	680
	11th to 20th	Day	11% 5% 31% 31% 17% 5%	680
	21st to end	Day	12% 9% 29% 28% 17% 5%	680
July	1st to 10th	Day	10% 3 16% 30% 25% 16%	671
	11th to 20th	Day	11% 3 21% 32% 23% 10%	668
	21st to end	Day	7% 2 19% 37% 28% 7%	739
August	1st to 10th	Day	7% 4 20% 32% 28% 10%	676
	11th to 20th	Day	10% 5% 17% 28% 24% 15%	515
	21st to end	Day	9% 3 17% 34% 28% 10%	539
September	1st to 10th	Day	13% 7% 20% 29% 24% 8%	490
	11th to 20th	Day	11% 2 17% 34% 26% 10%	490
	21st to end	Day	12% 9% 28% 24% 19% 7%	490
October	1st to 10th	Day	16% 7% 26% 29% 17% 4	329
	11th to 20th	Day	21% 18% 34% 18% 8% 1	300
	21st to end	Day	30% 13% 30% 23% 4 1	330
November	1st to 10th	Day	17% 11% 37% 24% 9% 3	300
	11th to 20th	Day	24% 17% 33% 17% 8% 2	300
	21st to end	Day	25% 10% 34% 26% 5%	300
December	1st to 10th	Day	18% 14% 39% 20% 7% 2	300
	11th to 20th	Day	17% 12% 38% 26% 7%	300
	21st to end	Day	28% 15% 36% 17% 4	330
January	1st to 10th	Night	19% 11% 39% 24% 7%	1000
	11th to 20th	Night	19% 14% 36% 26% 5%	1000
	21st to end	Night	21% 11% 35% 29% 4	1100
February	1st to 10th	Night	22% 11% 38% 25% 5%	1000
	11th to 20th	Night	18% 11% 38% 27% 5%	837
	21st to end	Night	20% 12% 41% 22% 4	660
March	1st to 10th	Night	21% 11% 38% 23% 7%	800
	11th to 20th	Night	16% 10% 35% 32% 7%	643
	21st to end	Night	25% 10% 36% 23% 6% 1	627
April	1st to 10th	Night	22% 14% 36% 24% 5% 1	570
	11th to 20th	Night	22% 12% 33% 26% 6%	563
	21st to end	Night	20% 10% 32% 30% 7% 1	570
May	1st to 10th	Night	16% 9% 36% 31% 7% 1	570
	11th to 20th	Night	19% 5% 30% 34% 11%	561
	21st to end	Night	14% 4 31% 35% 15% 1	617
June	1st to 10th	Night	9% 4 24% 48% 14%	570
	11th to 20th	Night	9% 4 30% 42% 14% 1	570
	21st to end	Night	11% 4 29% 41% 15% 1	570
July	1st to 10th	Night	9% 3 11% 43% 30% 4	555
	11th to 20th	Night	7% 3 18% 42% 27% 3	552
	21st to end	Night	6% 1 16% 45% 30% 1	624
August	1st to 10th	Night	4 2 16% 44% 31% 4	567
	11th to 20th	Night	8% 2 12% 41% 31% 5%	564
	21st to end	Night	10% 2 15% 42% 28% 3	627
September	1st to 10th	Night	11% 3 19% 37% 27% 4	570
	11th to 20th	Night	11% 3 20% 36% 27% 4	603
	21st to end	Night	12% 8% 26% 34% 18% 2	760
October	1st to 10th	Night	13% 5% 30% 32% 18% 2	760
	11th to 20th	Night	20% 12% 29% 27% 12%	760
	21st to end	Night	24% 11% 30% 26% 8%	836
November	1st to 10th	Night	17% 6% 41% 25% 9% 1	780
	11th to 20th	Night	19% 14% 37% 26% 5%	800
	21st to end	Night	22% 11% 34% 26% 7%	970
December	1st to 10th	Night	21% 12% 38% 25% 5%	1000
	11th to 20th	Night	23% 10% 34% 28% 6%	1000
	21st to end	Night	28% 11% 35% 23% 3	1100

Page generated at 2025-03-09 07:08:44. Version ac07

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