One of the most difficult things to judge as a new pilot is "is the weather good enough?". This is always up to the duty instructor, who will make the final decision on the morning and plan accordingly. This might result in only a limited number of aircraft being used or restrictions for early solo pilots.
Like many glider pilots, the trip organizers will judge a few days prior whether the conditions are worth getting out of bed for! Should you wish to go on your own or are interested, this page outlines some of the tools we use to make the decision.
As part of the Bronze badge, an understanding of meterology is required. You can find this in the excellent "Bronze and Beyond" book (amonst others). This page covers the various forecasts instead of the theory behind it.
Can we fly at all?
To judge the viability of the day, we can first use a general tool such as BBC weather, the MET office basic forecast etc.
Gliders can't fly in very strong winds. Into a 40kt wind, a glider could fly at cruise speed and not make any forward progress!
Wind speed increases with altitude, so be aware that most forecast sites will show speeds lower than that at 1000'.
As a benchmark value, 20mph (about 17kts) is considered strong, but the cutoff point will depend on its direction in relation to the runways as well as other conditions.
If rainwater accumultes on the wings of a glider, it will affect its perfromance by increasing its stall speed. In very cold conditions or at altitude, it can also freeze on the wings or in incovenient places. Therefore gliders avoid flying in rain. Pilots will often make the best of gaps between showers, but prolonged downpour is not flyable.
Cloud can be a lot lower than you might think! A reasonable winch launch will put you at about 1400' above the airfield, but cloud layers in typical overcast conditions can be 800' or lower!
Cloud also rises during the day as the air temperature increases, so an unflyable day can become better by midday.
Not all cloud can be easily predicted, however there are a number of tools at your disposal. These include:
Global Forecasting System (GFS) predictions - use "Cloud Base (Low)"
RASP Blipspot - For Lasham enter the turning point LAS
RASP will be mentioned in greater detail later, but for now, the grey line on the first graph will give you the cloud base above the surface during the day. Use this with caution, it can have significant error, especially more than a couple of days in advance.
Thick fog or mist can be extremely dangerous to fly in, with visibility of other aircraft (and where you took off from!) being severely limited, so is avoided. Many weather services will help predicting this, although it is far less common on its own than the last 3 issues.
Thunderstorms can be extremely dangerous, bringing with them strong winds and lightning, which isn't ideal when mile-long cables are being suspended into the air! All flying stops to let these pass, but soaring conditions are quite often favourable on such days.
Snow is not an issue as a moderate covering (and can look spectacular from the air!), however falling snow has the same issues as any other form of precipitation.
Is it a good soaring day?
Alright, so we've established that we can at least get in the air, now the question is "Can we stay up?". This is less important for pre-solo pilots, but everyone should experience soaring flight if they can.
Gliders stay up using three main sources of lift.
Thermals are formed as certain parts of the ground are heated faster than others. This causes bubbles or columns of air to rise, before they eventually condense into cumulus clouds. This leads to the classic "puffy clouds in blue sky" look. There is a lot more to how these work, and it's not as simple as just finding a hot day. They can be found at all times of year, but are stronger and most consistent during the summer months.
The Regional Atmospheric Soaring Prediction (RASP) that we looked at for cloud base serves a primary purpose of helping us pick out the soaring days.
The map can tell you what the whole country looks like at various times of day. This is great for planning cross country flights.
The Blipspot gives you the data graphed for one location throughout a specified day. Lasham's turn point code is LAS. To avoid trawling through lots of data, the "Star Rating" will give you an idea of the quality of the soaring on the day. Bear in mind that anything over 0* is thermally active, and 3* might be an extremely good day just for local soaring.
Ridge lift is formed when wind currents are pushed up an inclined piece of terrain, such as a hill or cliff. Although Lasham doesn't have a ridge of its own in the local area, several other sites that we visit do, including Parham on the South Downs, to which we take experienced pilots, and the Long Mynd, to which we take expeditions.
Predicting this is simple, just have a look at the wind direction and look for a moderately strong wind in the right direction. Parham requires a N or NE wind.
Certain sites in the country have "wave" lift, which is formed by massive air currents forming standing waves over mountain ranges. This can take gliders to 10,000s of feet. This is rare enough that we don't need to cover it here.