LMSC Site Forecasts using RASP BLIPspot Graphical Forecasts For
Great Britain.
Introduction The UK RASP site provides soaring forecasts at differing
resolutions for up to a week in advance. UK RASP maps are great for
planning cross country flights; however if you are not planning to go XC a
detailed spot forecast for your intended flying site for the full day might be
more useful.
RASP BLIPspot Location Graphs use the same high resolution forecast data
as the UK RASP Map but present it on a graph rather than a map making it
fairly easy to view and interpret, a whole day on one page if you like.
What are they good for? These graphs are good for a close up site forecast
for example you might be looking for that weather window after work to grab
an hour or two soaring; or maybe you are trying to weigh up which site would
work best for the whole day given that you’ve ‘pulled a sicky’ or need to
consider travel time? Equally you might be looking for that launch pad for a
monster XC?
How It Works? By clicking on the links you will be taken to the RASP
BLIPSpot Graphical Forecast page with the Lat Lon coordinates already
entered for the selected LMSC site.
The BLIPSpot Graph page will pop up displaying a number of graphs showing
RASP weather data for the nearest RASP Blipspot for selected site location
(2-5km).
It defaults to today; however you can select any day in the week ahead using
the Select Day drop down box; if it is slow loading hit the red Get Plots button
below the Select Day drop down box. As with all forecasts the nearer the day
in question the better the accuracy.
BLIPSpot Graphs. On the graphs the horizontal axis represents the hours of
the day and the vertical axes the RASP data. Some graphs contain multiple
data where they share a common vertical axis value such as height above the
ground.
I’d suggest not getting too hung up on detail here but use the graphs to build
up a picture of the day especially wind direction, wind strength, thermal
strength, cloudbase etc.
What follows is a brief description of the 8 graphs and what they can tell you,
Graph 1 is by far the busiest but stick with it. I’ve marked those I have
personally found useful with a * and in some cases* *.
Graph 1 Main RASP Metrics. The boundary layer (BL) is the vertical region
above the surface within which air has been mixed by thermal or windshear
eddies, i.e. the region where glider pilots normally fly.
This graph displays key parameters from the RASP core data to give an idea
of the boundary layer (BL) height and cloud base throughout the day.
The vertical axis here represents thousands of feet / rate per minute so in
effect it provides a vertical cross section throughout the day.
The coloured lines represent the following RASP Parameters:
*
Red = Thermal Updraft Velocity Average dry thermal updraft
strength near mid-BL height. Subtract your glider descent rate to get
average vario reading for cloudless thermals. Note: this is shown in
feet per minute. For example 500 fpm (2.5 m/s), less your glider sink
rate of 250fpm (1.2 m/s) equals a climb rate of 250 fpm or 1.3 m/s.
Updraft strengths will be stronger than this forecast if convective clouds
are present as RASP neglects "cloudsuck"). W* depends upon both
the surface heating and the BL depth. MoreInfo
*
Green = Height of Boundary Layer Top Height of the top of the
mixing layer, which for thermal convection is the average top of a dry
thermal. Over flat terrain, maximum thermalling heights will be lower
due to the glider descent rate and other factors. In the presence of
clouds (which release additional buoyancy aloft, creating "cloudsuck")
the updraft top will be above this forecast, but the maximum thermalling
height will then be limited by the cloud base (see the "Cloud prediction
parameters" section below). MoreInfo
Blue = Thermal Height Uncertainty This parameter estimates the
uncertainty (variability) of the BL Top height prediction which can result
from meteorological variations. MoreInfo
Yellow = Height of Critical Updraft Strength (Hcrit) This parameter
estimates the height at which the average dry updraft strength
drops below 225 fpm (1ms) and is expected to give better
quantitative numbers for the maximum cloudless thermalling height
than the BL Top height given above, especially when mixing results
from vertical wind shear rather than thermals. MoreInfo
*
Grey = Cumulus Cloudbase (Sfc. LCL) This height estimates the
cloudbase for small, non-extensive "puffy" clouds in the BL, if such
exist i.e. if the Cumulus Potential parameter (above) is positive or
greater than the threshold Cumulus Potential empirically determined for
your site. The surface LCL (Lifting Condensation Level) is the level to
which humid air must ascend before it cools enough to reach a dew
point temperature based on the surface mixing ratio and is therefore
relevant only to small clouds. However, this parameter has a
theoretical difficulty (see "MoreInfo" link below) and quite possibly the
actual cloudbase will be higher than given here - so perhaps this
should be considered a minimum possible cloudbase. MoreInfo
Graph 2 Cumulus Potential.
Provides a graph of Cu Potential. This evaluates the potential for small,
non-extensive "puffy cloud" formation in the BL, being the height
difference between the surface-based LCL (see below) and the BL top. Small
cumulus clouds are (simply) predicted when the parameter positive, but it is
quite possible that the threshold value is actually greater than zero for your
location so empirical evaluation is advised. Clouds can also occur with
negative values if the air is lifted up the indicated vertical distance by flow up a
small-scale ridge not resolved by the model's smoothed topography.
MoreInfo
Graph 3 Temperature. A graph of wet/dry temperatures useful for estimating
lapse rates as the day progresses based on the difference between the two,
big gap generally = big lapse rate and higher cloudbase.
Graph 4 Wind Speed. This script provides a graph of wind speeds in Knots.
*Brown = Surface Wind Speed. The speed and direction of the wind at
10m above the ground.
**Green = Upper Wind Speed Very useful as a big difference between
surface and upper winds suggest a strong wind gradient and the
danger of getting blown over the back, on a PG anyway.
Graph 5
**Wind Direction. Wind direction in degrees.
*
Graph 6 Sun. Sun as a percentage i.e. the amount of solar radiation which
would reach the surface in a dry atmosphere in the absence of clouds and
water vapour expressed as a percentage. This parameter indicates the
degree of cloudiness, i.e. where clouds limit the sunlight reaching the
surface.
*
Graph 7 ' Star Rating' Paul Scorer came up with an experimental rating
system called Stars. The metric basically states a higher number equals
better conditions. This is an experimental metric based on using data from
other standard RASP parameters. The scale is 1 to 5 with 5 being excellent
(for sailplanes).
*
Graph 8 Rain. Forecast rain in millimetres per half hour.
Site Links
Sites RASP Graphs
RASP BLIPspot Display - Wrekin SE
RASP BLIPspot Display - Wrekin NW
RASP BLIPspot Display - Shepherds Tump
RASP BLIPspot Display - Sarn
RASP BLIPspot Display - Llandinam
RASP BLIPspot Display - Long Mynd
RASP BLIPspot Display - Long Mountain
RASP BLIPspot Display - Lawley
RASP BLIPspot Display - Wrekin
RASP BLIPspot Display - Corndon
RASP BLIPspot Display - Llangollen
RASP BLIPspot Display - Llan Fawr
RASP BLIPspot Display - Clatter
RASP BLIPspot Display - Camlo
RASP BLIPspot Display - Caer Caradoc
RASP BLIPspot Display - Bache
RASP BLIPspot Display - Clunbury