Robertson Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robertson Observing Site clear sky chart from 2005-11-06 to 2026-03-03.

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	9% 8% 3 9% 6% 5% 5% 4 7% 8% 36%	134209
	Night	7% 9% 4 9% 6% 6% 5% 4 6% 8% 35%	135602
Transparency	Day	43% 7% 17% 24% 9% 1	133908
	Night	45% 7% 16% 23% 9% 1	135485
Seeing	Day	13% 9% 28% 29% 17% 5%	17409
	Night	12% 7% 28% 36% 15% 2	26838

By Month

Percentage Cloud Cover NW (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	6% 13% 6% 15% 9% 7% 5% 4 6% 6% 23%	4125
	11th to 20th	Day	3 6% 3 9% 7% 8% 8% 6% 8% 8% 33%	4119
	21st to end	Day	6% 11% 5% 14% 7% 5% 5% 4 7% 8% 29%	4686
February	1st to 10th	Day	8% 12% 5% 12% 9% 7% 7% 5% 5% 8% 24%	1780
	11th to 20th	Day	5% 13% 5% 12% 6% 6% 5% 5% 8% 8% 27%	1845
	21st to end	Day	5% 9% 5% 13% 7% 7% 8% 5% 8% 7% 24%	1557
March	1st to 10th	Day	4 10% 4 12% 9% 7% 7% 6% 7% 8% 27%	4534
	11th to 20th	Day	6% 12% 5% 14% 8% 5% 5% 4 6% 7% 29%	4999
	21st to end	Day	9% 13% 6% 11% 6% 5% 5% 3 5% 6% 32%	5705
April	1st to 10th	Day	7% 11% 4 8% 5% 6% 5% 4 6% 8% 36%	2400
	11th to 20th	Day	11% 12% 5% 9% 6% 4 5% 5% 7% 7% 30%	2511
	21st to end	Day	12% 9% 3 9% 5% 5% 5% 4 7% 8% 33%	2770
May	1st to 10th	Day	12% 9% 3 6% 4 4 4 4 6% 7% 41%	6340
	11th to 20th	Day	11% 8% 3 10% 7% 6% 6% 4 7% 8% 30%	6356
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 4 6% 8% 39%	7541
June	1st to 10th	Day	13% 7% 2 7% 5% 5% 6% 6% 7% 8% 34%	3200
	11th to 20th	Day	21% 9% 4 8% 5% 5% 6% 4 6% 8% 26%	3200
	21st to end	Day	11% 7% 3 7% 5% 6% 5% 5% 8% 8% 35%	3200
July	1st to 10th	Day	7% 4 1 5% 4 4 6% 5% 8% 10% 47%	7039
	11th to 20th	Day	10% 4 2 6% 4 5% 5% 5% 8% 10% 40%	6780
	21st to end	Day	6% 4 1 4 4 4 4 5% 7% 11% 50%	7342
August	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 6% 8% 44%	4552
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 11% 45%	4366
	21st to end	Day	10% 5% 2 4 4 4 4 3 6% 8% 48%	4607
September	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 6% 8% 47%	2400
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 4 5% 8% 38%	2400
	21st to end	Day	9% 6% 2 5% 4 4 4 3 5% 7% 51%	2115
October	1st to 10th	Day	8% 8% 2 6% 4 4 4 4 5% 9% 45%	3270
	11th to 20th	Day	6% 5% 2 7% 5% 4 5% 4 8% 13% 40%	3218
	21st to end	Day	5% 9% 4 8% 5% 6% 6% 5% 7% 8% 37%	3020
November	1st to 10th	Day	14% 15% 4 10% 6% 5% 4 3 5% 7% 29%	1648
	11th to 20th	Day	9% 15% 5% 12% 7% 6% 4 5% 6% 6% 25%	1680
	21st to end	Day	6% 13% 6% 14% 6% 6% 5% 5% 6% 6% 27%	1507
December	1st to 10th	Day	7% 12% 4 10% 8% 8% 7% 4 8% 8% 25%	2020
	11th to 20th	Day	6% 14% 5% 13% 9% 8% 7% 4 6% 7% 19%	2319
	21st to end	Day	3 8% 3 12% 7% 6% 4 4 7% 9% 37%	3058
January	1st to 10th	Night	5% 13% 5% 14% 7% 7% 6% 4 7% 8% 23%	8768
	11th to 20th	Night	3 8% 4 12% 8% 9% 8% 4 6% 8% 30%	8779
	21st to end	Night	5% 11% 4 11% 6% 5% 5% 5% 7% 8% 31%	9502
February	1st to 10th	Night	6% 13% 5% 13% 8% 6% 6% 5% 7% 8% 25%	2860
	11th to 20th	Night	6% 10% 3 10% 6% 7% 7% 6% 7% 8% 31%	2662
	21st to end	Night	2 9% 4 11% 8% 6% 6% 5% 7% 8% 33%	2249
March	1st to 10th	Night	6% 9% 4 11% 7% 6% 7% 6% 8% 10% 26%	5383
	11th to 20th	Night	7% 11% 5% 12% 6% 6% 5% 5% 7% 8% 29%	5000
	21st to end	Night	9% 11% 4 10% 6% 5% 5% 5% 7% 8% 29%	5246
April	1st to 10th	Night	5% 9% 4 7% 7% 5% 5% 5% 6% 7% 39%	2000
	11th to 20th	Night	9% 11% 4 9% 6% 5% 6% 5% 7% 7% 31%	1836
	21st to end	Night	14% 8% 3 8% 6% 5% 3 4 6% 8% 36%	1629
May	1st to 10th	Night	13% 10% 3 7% 4 4 4 3 4 7% 42%	3640
	11th to 20th	Night	12% 8% 3 8% 6% 6% 6% 5% 6% 8% 32%	3554
	21st to end	Night	12% 7% 2 7% 5% 5% 4 4 5% 8% 42%	3332
June	1st to 10th	Night	10% 5% 3 9% 4 5% 5% 5% 8% 8% 38%	1200
	11th to 20th	Night	18% 8% 2 6% 4 4 7% 5% 7% 8% 32%	1200
	21st to end	Night	9% 6% 3 5% 3 5% 4 5% 7% 10% 43%	1200
July	1st to 10th	Night	8% 4 2 5% 3 4 4 4 7% 10% 48%	2904
	11th to 20th	Night	9% 3 2 6% 5% 5% 6% 5% 7% 9% 45%	3180
	21st to end	Night	4 4 1 4 4 4 4 4 7% 9% 55%	3502
August	1st to 10th	Night	7% 4 1 7% 4 4 5% 4 6% 8% 51%	2608
	11th to 20th	Night	6% 4 2 6% 5% 6% 6% 5% 8% 9% 44%	2689
	21st to end	Night	9% 5% 2 5% 4 3 4 4 6% 7% 53%	3301
September	1st to 10th	Night	8% 4 1 5% 4 4 4 3 6% 9% 50%	2000
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 5% 8% 45%	2000
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 7% 53%	2281
October	1st to 10th	Night	9% 7% 3 6% 4 4 4 3 5% 7% 48%	3920
	11th to 20th	Night	5% 6% 2 7% 5% 3 5% 4 5% 7% 51%	3898
	21st to end	Night	8% 9% 3 8% 5% 5% 5% 4 6% 9% 40%	4767
November	1st to 10th	Night	15% 14% 5% 9% 5% 5% 5% 3 5% 6% 28%	2870
	11th to 20th	Night	9% 12% 5% 13% 7% 5% 5% 4 6% 6% 26%	2940
	21st to end	Night	6% 13% 6% 12% 6% 6% 5% 4 6% 8% 28%	3107
December	1st to 10th	Night	8% 14% 5% 12% 9% 8% 7% 5% 6% 7% 22%	5380
	11th to 20th	Night	5% 14% 6% 12% 8% 7% 6% 6% 7% 9% 20%	6093
	21st to end	Night	3 7% 3 11% 10% 6% 5% 4 6% 6% 40%	8122

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	62% 5% 8% 16% 8%	4125
	11th to 20th	Day	45% 7% 15% 19% 12% 1	4140
	21st to end	Day	52% 7% 15% 18% 9%	4686
February	1st to 10th	Day	57% 6% 13% 15% 9%	1780
	11th to 20th	Day	51% 7% 13% 19% 9% 1	1854
	21st to end	Day	52% 10% 12% 19% 7%	1557
March	1st to 10th	Day	50% 8% 18% 16% 7%	4534
	11th to 20th	Day	52% 8% 16% 17% 7%	5000
	21st to end	Day	52% 6% 9% 20% 12%	5705
April	1st to 10th	Day	45% 7% 16% 20% 12%	2400
	11th to 20th	Day	50% 7% 13% 18% 11% 1	2511
	21st to end	Day	45% 7% 16% 22% 9% 1	2770
May	1st to 10th	Day	40% 5% 13% 24% 15% 2	6340
	11th to 20th	Day	48% 8% 12% 22% 9% 1	6360
	21st to end	Day	39% 10% 14% 23% 12% 2	7542
June	1st to 10th	Day	43% 8% 17% 21% 10% 1	3200
	11th to 20th	Day	54% 8% 15% 16% 6% 1	3200
	21st to end	Day	42% 11% 19% 21% 7%	3200
July	1st to 10th	Day	30% 9% 25% 29% 7%	6964
	11th to 20th	Day	36% 8% 16% 36% 4	6699
	21st to end	Day	27% 8% 23% 34% 8%	7135
August	1st to 10th	Day	36% 8% 21% 29% 6%	4536
	11th to 20th	Day	32% 7% 23% 33% 5%	4408
	21st to end	Day	34% 6% 22% 32% 7%	4606
September	1st to 10th	Day	34% 8% 17% 27% 12% 2	2400
	11th to 20th	Day	43% 7% 19% 25% 6% 1	2400
	21st to end	Day	31% 6% 18% 32% 11% 2	2115
October	1st to 10th	Day	36% 5% 25% 23% 10%	3270
	11th to 20th	Day	35% 6% 19% 33% 6%	3218
	21st to end	Day	44% 7% 19% 26% 5%	3020
November	1st to 10th	Day	56% 5% 15% 17% 7%	1648
	11th to 20th	Day	58% 10% 13% 14% 5%	1680
	21st to end	Day	57% 7% 14% 18% 4	1507
December	1st to 10th	Day	55% 9% 15% 14% 7%	2020
	11th to 20th	Day	63% 7% 14% 12% 5%	2320
	21st to end	Day	44% 6% 14% 21% 14% 2	3058
January	1st to 10th	Night	57% 5% 12% 16% 9%	8769
	11th to 20th	Night	51% 5% 14% 18% 12%	8820
	21st to end	Night	48% 7% 19% 17% 8%	9502
February	1st to 10th	Night	55% 6% 12% 18% 9%	2860
	11th to 20th	Night	47% 8% 13% 21% 10%	2663
	21st to end	Night	45% 7% 16% 23% 9%	2249
March	1st to 10th	Night	49% 8% 19% 18% 6%	5383
	11th to 20th	Night	50% 7% 16% 19% 8%	5000
	21st to end	Night	47% 7% 12% 21% 11%	5246
April	1st to 10th	Night	40% 8% 15% 21% 15% 1	2000
	11th to 20th	Night	47% 7% 14% 19% 11% 2	1836
	21st to end	Night	45% 7% 12% 23% 11% 1	1629
May	1st to 10th	Night	43% 5% 9% 21% 19% 2	3640
	11th to 20th	Night	46% 8% 12% 21% 12% 1	3554
	21st to end	Night	39% 7% 14% 24% 14% 2	3332
June	1st to 10th	Night	38% 9% 16% 26% 10% 2	1200
	11th to 20th	Night	47% 9% 15% 20% 9% 1	1200
	21st to end	Night	35% 10% 21% 25% 10%	1200
July	1st to 10th	Night	30% 8% 25% 32% 5%	2870
	11th to 20th	Night	32% 6% 17% 39% 6%	3138
	21st to end	Night	25% 7% 22% 37% 8% 1	3404
August	1st to 10th	Night	31% 7% 21% 31% 9%	2600
	11th to 20th	Night	32% 8% 21% 29% 9% 1	2713
	21st to end	Night	29% 6% 20% 34% 10%	3301
September	1st to 10th	Night	29% 7% 16% 32% 12% 4	2000
	11th to 20th	Night	37% 6% 15% 30% 10% 1	2000
	21st to end	Night	30% 5% 16% 33% 14% 2	2281
October	1st to 10th	Night	36% 4 19% 33% 7%	3920
	11th to 20th	Night	33% 5% 14% 35% 13%	3898
	21st to end	Night	41% 6% 20% 28% 5%	4767
November	1st to 10th	Night	57% 6% 11% 17% 7% 1	2870
	11th to 20th	Night	55% 9% 15% 15% 5% 1	2940
	21st to end	Night	53% 7% 16% 19% 4	3107
December	1st to 10th	Night	61% 7% 14% 13% 4 1	5380
	11th to 20th	Night	58% 8% 16% 13% 4	6091
	21st to end	Night	44% 5% 11% 24% 13% 2	8122

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	13% 10% 26% 30% 15% 6%	220
	11th to 20th	Day	12% 9% 33% 29% 15% 3	220
	21st to end	Day	13% 13% 28% 31% 11% 5%	270
February	1st to 10th	Day	14% 8% 32% 33% 10% 3	330
	11th to 20th	Day	11% 7% 27% 35% 16% 4	330
	21st to end	Day	5% 8% 33% 36% 16% 4	273
March	1st to 10th	Day	12% 10% 25% 35% 16% 3	309
	11th to 20th	Day	11% 11% 34% 29% 12% 4	500
	21st to end	Day	14% 6% 35% 30% 12% 3	550
April	1st to 10th	Day	11% 11% 35% 31% 9% 4	500
	11th to 20th	Day	13% 17% 29% 29% 9% 2	492
	21st to end	Day	14% 10% 32% 26% 15% 2	670
May	1st to 10th	Day	13% 10% 26% 30% 16% 5%	703
	11th to 20th	Day	10% 11% 24% 32% 18% 5%	725
	21st to end	Day	13% 10% 24% 30% 17% 6%	789
June	1st to 10th	Day	14% 12% 29% 28% 13% 3	730
	11th to 20th	Day	21% 11% 28% 22% 15% 4	730
	21st to end	Day	14% 6% 23% 26% 22% 8%	730
July	1st to 10th	Day	10% 5% 21% 33% 23% 7%	721
	11th to 20th	Day	10% 7% 24% 29% 24% 7%	718
	21st to end	Day	6% 6% 24% 29% 25% 9%	794
August	1st to 10th	Day	11% 7% 20% 28% 26% 8%	726
	11th to 20th	Day	10% 7% 23% 31% 23% 6%	650
	21st to end	Day	12% 7% 23% 31% 20% 7%	583
September	1st to 10th	Day	11% 10% 29% 28% 19% 4	530
	11th to 20th	Day	19% 7% 28% 29% 15% 3	530
	21st to end	Day	15% 9% 29% 26% 18% 2	408
October	1st to 10th	Day	16% 5% 29% 31% 15% 4	330
	11th to 20th	Day	13% 10% 31% 28% 15% 3	330
	21st to end	Day	10% 15% 36% 25% 12% 2	363
November	1st to 10th	Day	21% 7% 33% 27% 10% 2	330
	11th to 20th	Day	15% 13% 34% 27% 9% 2	330
	21st to end	Day	10% 11% 33% 34% 10% 2	313
December	1st to 10th	Day	13% 5% 27% 34% 14% 7%	220
	11th to 20th	Day	11% 10% 37% 34% 8%	220
	21st to end	Day	11% 5% 31% 24% 24% 6%	242
January	1st to 10th	Night	11% 7% 31% 36% 13% 1	1050
	11th to 20th	Night	11% 9% 34% 36% 10% 1	1050
	21st to end	Night	13% 9% 34% 33% 10% 1	1155
February	1st to 10th	Night	10% 6% 29% 37% 15% 2	970
	11th to 20th	Night	11% 4 29% 38% 16% 2	824
	21st to end	Night	5% 5% 25% 40% 22% 3	692
March	1st to 10th	Night	10% 5% 30% 38% 15% 1	808
	11th to 20th	Night	10% 6% 32% 38% 13% 1	800
	21st to end	Night	12% 5% 27% 39% 14% 3	779
April	1st to 10th	Night	8% 6% 32% 38% 14% 2	600
	11th to 20th	Night	15% 8% 29% 33% 13% 2	593
	21st to end	Night	18% 8% 29% 33% 12% 1	600
May	1st to 10th	Night	14% 7% 29% 33% 17% 1	600
	11th to 20th	Night	12% 7% 28% 34% 17% 2	567
	21st to end	Night	16% 5% 21% 36% 19% 3	433
June	1st to 10th	Night	12% 8% 26% 35% 18% 2	400
	11th to 20th	Night	20% 6% 23% 33% 16% 2	400
	21st to end	Night	10% 7% 18% 40% 24% 2	400
July	1st to 10th	Night	7% 4 19% 45% 23% 1	390
	11th to 20th	Night	11% 4 24% 42% 18% 2	388
	21st to end	Night	6% 4 22% 38% 27% 4	448
August	1st to 10th	Night	7% 4 21% 39% 26% 3	597
	11th to 20th	Night	8% 4 19% 42% 26% 2	594
	21st to end	Night	8% 6% 24% 37% 24% 1	660
September	1st to 10th	Night	10% 4 25% 40% 19% 2	600
	11th to 20th	Night	15% 5% 19% 44% 17% 1	600
	21st to end	Night	9% 8% 30% 38% 14% 1	760
October	1st to 10th	Night	15% 6% 27% 38% 13% 1	800
	11th to 20th	Night	11% 6% 31% 36% 15% 2	800
	21st to end	Night	15% 8% 33% 34% 9% 1	1100
November	1st to 10th	Night	23% 8% 28% 33% 8% 1	1025
	11th to 20th	Night	15% 8% 30% 33% 12% 2	1050
	21st to end	Night	12% 9% 34% 35% 9% 2	1050
December	1st to 10th	Night	11% 7% 29% 35% 15% 3	1050
	11th to 20th	Night	12% 9% 33% 34% 12% 1	1050
	21st to end	Night	10% 4 29% 35% 19% 3	1155

Page generated at 2026-02-28 00:40:22. Version ac07

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