Clear skies ahead?

Weather forecasting for astronomers

Pete Lawrence shows you how to assess the changeable atmosphere to tell how it will affect your stargazing

Earth’s atmosphere is very important and without it we obviously couldn’t survive However, observing objects through it can be challenging. The impact of weather on astronomical observing from a country prone to damp conditions such as the UK is significant and although you can’t remove its effects, there are things you can do to lessen its impact. Weather forecasting, estimation of sky quality and general sky watching can help you in your quest to see beyond our world.

The atmosphere is remarkably changeable and can go from a crystal clear, rock-steady state to an extremely wobbly, hazy or opaque state in a very short time. Observing over many years produces a personal connection with your local weather and you’ll find you can make your own assessment as to how it is likely to affect your viewing. More often than not, you’ll have to develop techniques to grab views when available and getting good at this can mean the difference between a challenging observing session and spectacular success.

Sky transparency

Unless you’re extremely unlucky or are just starting out, you have probably experienced clear skies at one time or another. They can be breathtaking and highly addictive, refuelling your desire to be out under the stars. However, spectacularly clear skies are typically few and far between. Fully opaque skies, on the other hand, are all too common from wet countries such as the UK. Yet there is a huge diversity of weather that sits in between these extreme states, and although some of the intermediate conditions are not perfect for astronomy, you can still learn how to make the most of those nights when observing is a bit trickier.

If the sky is clear, the degree of clarity is quantified by a value called transparency. This may, for example, be recorded as a number ranging from 0 (opaque cloud) to 7 (perfect). It should be noted that transparency is not an observational indicator of cloudiness, but is intended to indicate the opacity of the atmosphere; in other words, how hazy it appears and how easily objects can be seen through it. Hazy skies can occur due to excessive moisture or because there’s dust in the atmosphere. Typically, hazy skies are a combination of both. Pollen is another contributor to poor transparency and it’s important to realise that this introduces an annual cycle that affects the quality of your sky. After heavy rain the atmosphere often exhibits good transparency due to the fact that much of the dust contained within it has been washed out. So even if a downpour stops one night’s observing, it means you could be due for a clear sky the next day.

Good transparency will give the best views, objects appearing brightest and faint wispy detail around nebulae and galaxies will be easier to detect. As the sky becomes less transparent, such detail is gradually lost. Although less affected than diffuse deep-sky objects, the view of a bright planet will still be degraded by poor transparency; contrast is diminished and small features become harder to discern and image.

Transparency and how to record it

Learning to measure transparency will help you compare sky clarity under different conditions and locations, from dark-sky areas to cities

Measurement of transparency can be subjective and there are many different scales that attempt to provide standardisation. For example, a popular numeric system used by the American Association of Amateur Astronomers uses an eight-point scale (see right) ranging from 0, indicating that it’s completely cloudy, through to 7, which indicates extremely clear skies.

Another popular method is to record the faintest star visible in your sky, typically one that’s overhead. The star’s magnitude is noted as a NELM (naked-eye limiting magnitude) value. This is a less subjective method of recording transparency, especially as the recorder typically has no idea what the actual magnitude of the selected star is until after it has been noted and looked up.

Transparency varies with location. The faintest stars visible from a dark site may not typically be seen from a town or city.

Heavily built-up locations tend to have more dust and smoke particles in the atmosphere. Excessive lighting throws light up into the sky and this will ultimately illuminate these particles. The result is a bit like applying an opaque filter across the night sky. It is noticeable when the sky leading up to sunset appears clear; it’s as the day transitions into night that poor transparency becomes apparent.

Astronomical seeing

The atmosphere is not a fixed optical medium. Blobs of air of different temperatures and densities cause the light from distant objects to micro-refract. This causes small deviations in the light path and, at relatively high frequencies, causes objects to appear distorted and fuzzy. The degree of distortion is quantified under the collective term ‘seeing’.

Seeing is complicated and varies greatly with location. In essence its effects occur in three layers of the atmosphere. At high altitude, a fastmoving corridor of air known as the jet stream can cause high-frequency jitters that seriously degrade the view. The effect is often likened to trying to read newspaper print placed at the bottom of a fast-moving stream of water!

At the mid-level, seeing effects come from the movement of air above and around large-scale topographic features. These could be hills or mountains, valleys or depressions, or large expanses of water. The latter can produce what’s called ‘laminar flow’, which settles the mid-layer effects quite dramatically. Being downwind of a hill or ridge, you will experience turbulence. Similarly, heat from a town or city will create air instability. If you are downwind of a heat source like this, again you will experience turbulence.

At the bottom layer, the effects are caused by local features such as immediately adjacent houses, trees and fences. Large areas of tarmac surface will warm up during the day and release heat during the night. This may cause significant seeing issues too, depending on wind direction, so it’s best to avoid these and set up in grassier areas if possible. During the winter, home heating combined with poor insulation may create disturbed seeing due to hot domestic air mixing with cold air.

Seeing and how to record it

Using scales to assess the atmospheric seeing will enable you to predict how much it will affect your observing

The two main seeing scales are the Antoniadi scale and the Pickering scale (above). Others exist but these are the two most commonly used by amateur astronomers. The Antoniadi scale has five values, indicated by Roman numerals, I being the steadiest, V being very unstable.

From the UK, Antoniadi seeing types III and IV are most common, but periods of II do happen, typically at times when high pressure dominates the weather. Antoniadi I conditions are rare, occurring perhaps only on a couple of nights throughout the year. It is important to understand that seeing may vary considerably over the course of a single night. Initially, poor seeing conditions may give way to much steadier conditions over the course of a night or vice versa.

The Pickering scale was devised by William H Pickering of Harvard College Observatory. It’s based on the views through a 5-inch (120mm) refractor and is a 10-point scale. Just to make things complicated, the Pickering scale denotes the worst seeing at the lower end of the scale and best at the upper end, which is the opposite of the Antoniadi scale. As a rough equivalence to the Antoniadi scale, Pickering 1-2 is considered very bad seeing, 3-4 is poor seeing, 5-6 moderate, 7-8 good and 9-10 excellent seeing.

Seeing is something that gets easier to assess over time as you gain experience, simply because you need to actually see the effects of each level of stability before you can confidently identify them. Perfect seeing is really something to behold and allows a view which is hard to forget.

As mentioned in the main text, seeing generally occurs in three discrete layers in the atmosphere. The lower and mid layers are greatly affected by wind and it’s important to keep a note of the wind direction when you get good seeing. High level seeing can be predicted to a degree, by the location of the jet stream. It is well worth using a forecast service such as NetWeather (www.netweather.tv/chartsand-data/jetstream), which is very useful for this purpose.

Forecasting strategies

There are many ways of getting predictions for astronomical weather and there isn’t one single method that will always give accurate results. If things don’t pan out as predicted, the next best option is to adapt your strategy to what you’ve been offered.

For high-resolution Solar System work, good seeing is critical. At high altitude within the atmosphere, this is at the mercy of the jet stream. Services such as NetWeather (www.netweather.tv) provide constantly updated jet stream forecasts that will help you plan your observing. If the jet stream is raging over your location, chances are the seeing will be poor.

If you’re a solar observer, the Sun introduces its own special considerations. As the Sun rises, its light bathes the ground, causing it to warm up. The heat released causes thermal instability, which stabilises as the ground and air temperatures equalise. Towards the end of the day, energy absorbed by the ground is released as heat which destabilises the view, so picking the optimal time for solar observing requires keeping a record of the best time of day to begin.

Sometimes location just works against you. If you live to the east of a hill range, with the prevailing wind coming from the west, your location to the lee, or sheltered, side of the hills will likely promote instability and this will create poor mid-level seeing. You might want to find a new observing site on the other side of the hills, or keep watch for days when the wind is coming from a more suitable direction.

Looking at synoptic charts showing pressure, fronts and wind speeds will give you an idea of the weather and airflow over your observing site. Low pressure isn’t ideal as it tends to introduce clouds and disturbed, windy conditions. High pressure can produce clear and stable conditions, but these are not hard and fast rules and exceptions do happen. High pressure with fog can lead to frustration, although observing a planet or the Moon at high altitude can still give stable views.

Cloud forecasts are difficult to get right, especially at night. usee these as a guide rather than an accurate prediction of what you’re going to get. Monitoring infrared satellite images will give you a much better idea as to the viability of such forecasts and enable you to take advantage of unexpected cloud gaps.

Reading the weather

If you’re an active observer, it’s surprising how your senses become tuned to various conditions that may affect your view. For example, you soon learn that the aftermath of an active front may leave you with good transparency, but poor seeing as the atmosphere tries to regain a degree of equilibrium.

Dry, cold conditions tend to be stable too, but add moisture into the mix and things can turn unsettled. A cloudless high-pressure system sitting over the UK will bring observational joy to many, especially highresolution Solar System imagers who require steady seeing for the best results.

If the skies aren’t perfectly clear and you’re relying on cloud gaps, a free satellite imagery service such as Sat24 (bit.ly/3HcRSp6) can be invaluable. However, bear in mind that the sky appears like a flattened dome above your head and cloud gaps near the horizon will appear foreshortened. A large circular gap will appear low down as a narrow ellipse with barely any sky visible through it. As it approaches your overhead position, you’ll experience the full glory of its clear sky area, but geometry has a cruel twist in store. As the gap in the clouds passes overhead, the distance between you and the gap minimises. This means the apparent speed of the gap across the sky reaches a maximum value. Although the area will be best presented, you’ll have it for the shortest time!

---

DENNIS H. MILLER/ISTOCK/GETTY IMAGES, KSCHULZE/ISTOCK/GETTY IMAGES, ALL CHARTS AND PICTURES: PETE LAWRENCE