tents and the wind

Introduction

managing tent ventilation vs wind chill vs wind damage is one of the main goals of camping in a tent - and much comes down to tent design as well as camp site selection

very few tents will survive intact when wind gusts are over 80kph, and many will fail at over 50kph
see camping in strong winds

wind at night will always be noisy - bring ear plugs to help sleep
- it will be much more noisy if you have cheap tarps in the wind - use higher quality hiking type tarps in aerodynamic wind pitch set up or take them down

Ventilation

adequate ventilation options are a critical component to tent design as inadequate ventilation is likely to lead to internal condensation in tents or excessively warm environment which is not conducive to sleeping (this is mainly a problem in the warmer months or in the tropics) - see staying cool while camping in summer
hence nearly all tents have at least some mesh in the internal “inner tent” and some vents on the fly +/- space below the fly so air can enter from under the fly
some “4 season” tents do not have much ventilation options so they are NOT really 4 season but are really only for winter or cold nights
a true “4 season” tent will generally have two large doors each with dual layers - a fabric layer to reduce wind chill, and a mesh layer to allow much more ventilation, and you can adjust the amount of ventilation as needed, while the second door allows for cross-flow of air
a “3 season” tent will generally have a fabric section in the lower third of the inner tent to reduce wind chill at floor level and much of the upper part of the inner tent is mesh to reduce condensation
a summer tent may be all mesh inner apart from the bathtub floor

Wind chill

wind chill is one of the major enemies of being outside or camping as it can dramatically impact your ability to keep warm - especially on exposed skin areas such as your face, neck, etc
for this reason, in cool to cold seasons, inner tents should have a reasonable amount of fabric to protect from wind chill, especially at the level you will be sleeping at (if you sleep on a stretcher bed, you will need a higher level of fabric).

Sand, dust and snow drift protection

stronger winds in dusty areas will result in the air near ground level in particular, to be filled with sand or dust which will make for a very uncomfortable night
this is particularly a problem in coastal areas or in the desert regions
ANY mesh in the inner tent is likely to allow this dust to enter into your sleeping area and make the night miserable for you, even if the fly is fully closed - it will blow in from under the fly and go up to the top of your inner tent and enter inside your inner tent from there
thus, you need the ability to fully seal the inner tent and for this, a true “4 season” tent with a full fabric inner and zippable vents is highly beneficial
this is extra important in snow blizzard conditions as this may become life threatening if the inside of your tent becomes wet and cold with snow drift
you may still need an eye mask and even a face mask

Structural integrity in winds

many tents have a wind rating - usually against the Beaufort wind scale in a wind tunnel test although these may not be accurate ratings
- if set up correctly, most tents will cope with winds up to 40kph, some will cope with 50kph, much fewer will survive consistently over 60kph, and you will be lucky indeed if you manage to survive without damage in over 80kph winds ¹⁾, unless they are expedition level tents (eg. a double poled Mont Epoch tent has been wind tunnel tested to 139kph)
factors which improve structural integrity against stronger winds:
- must be adequately pegged down otherwise it will either blow away or parts of the tent not meant to be stressed become stressed
- must be adequately guyed out to reduce stresses at each point and upon the poles - it is critical that these guy outs do not become loose and they must not be excessively taut
  - for hikers, the guys generally do not need springs as nylon tents stretch a bit anyway, however, polyester fabrics do not stretch much and so may benefit from springs
  - for larger tents, reflective guy ropes with springs are advisable and ensure you have them well pegged out - use larger sand pegs in sand, use stronger steel pegs in harder ground - see tent pegs/stakes and how to stake your tent or tarp
  - for awnings or tarps with vertical poles, consider wrapping the guy rope around the top of the pole to reduce stress on the eyelet
    - avoid segmented awning poles which come apart as they can come apart if a wind gust lifts up the pole
- aerodynamic design of the tent (usually as long as it is facing the correct direction into the wind although some are aerodynamic from all directions such as tipis)
  - a large wall area face on to the wind creates a lot of wind stress (hence box style or cabin style tents can be problematic) as do overhangs or open doors which can catch the wind
- for a given design, height of the tent can be important - the taller the tent, the more wind stress it will receive
  - hence swags are generally more wind resistant than tents (but they also tend to be made from stronger, heavier fabrics as well)
- tautness of the sides of the tent is critical - if the fabric is not taut the structural integrity is severely impacted, it too taut, it may increase tearing at the seams or stress points
- thickness of the fabric is important, ultralight fabrics are more likely to tear than thicker fabrics with a higher tear rating
- strength and flexibility of the poles
  - fibreglass poles tend to break when flexed excessively and the broken ends can then tear the tent fly and you and your sleeping gear may get wet and cold
  - alloy poles are lighter, and more flexible and are generally recommended for most camping tents but they can still become irreversibly bent and require replacement or potentially a larger size “repair” pole segment to be used over the broken point
    - the thicker the pole is, the stronger it will be
  - composite poles may be less likely to become irreversibly bent like alloy poles in strong winds ²⁾
  - trekking poles are stronger again, but their lack of flex means more strain is placed upon the tent fabric peg out points specially down wind and unles thee are extra reinforced, they will reak and the tent may become unusable ³⁾
  - air pole tents are potentially the most wind resistant as they don't have metal or fibreglass poles which break, but they are very bulky and heavy
- many tents have plastic joints for the poles such as “Fast Frame” tents and these need to be protected from strong winds
- ensure poles are properly connected to each other as the joins may break when excessively flexed while not fully joined
- ensure poles are attached to the fly as per manufacturer's design - there may be velcro tabs to connect
- some tent designs have the poles passing through tunnel sleeves which distributes stresses better along the length rather than just at clip sites, and also reduces the flex stresses on the poles and maintains tent shape better in the wind - the downside is that they can take longer to set up and pack down with only one person
minimising the impact of strong winds is also very important - see camping in strong winds

a high risk period for any tent is while setting up or packing up in strong winds before it is adequately pegged and guyed out
the next high risk situation is when a guy rope comes loose or the peg is pulled from the ground - pay special attention to this if strong winds are expected
- double or triple peg each guy out such that these line up in a chain of connected pegs behind each other and each are driven vertically into the ground, and a large rock placed on top of the connecting rope
- if pegging into ground is not possible due to rocks then you will need to tie the end of the guy rope around a smaller rock and sit a boulder on top of the guy rope in front of the smaller rock - this creates an effective dead-man anchor
- in sand or snow, you will likely need to bury a dead-man anchor
- if a guy or peg comes loose due to strong winds, loss of fabric tautness combined with extra loads on other guys may result within seconds all the pegs being ripped out and the tent suddenly will blow away and be ripped

Some physics

see also wind speed and wind force
some parameters:
- common extraction forces to remove a tent peg through a guy rope angled 30deg to the ground and the tent peg placed vertically into the ground:
  - lightweight tubular stakes in average ground ~5-9kg
  - Y hiking stake in average ground ~10kg
  - V-shaped steel pegs ~15kg
  - 30cm heavy duty metal pegs ~35kg or more if in hard clay ground
  - 45cm heavy duty metal pegs ~80kg or more if in hard clay ground
  - 60cm heavy duty metal pegs ~130kg or more if in hard clay ground
  - ground consistency makes a massive difference to this - a 25 fold difference between very loose to very dense ground
  - driving a peg 30deg into ground instead of straight down generally gives 30% less extraction force required
  - placing a 2nd peg 1/3rd the peg length behind the primary peg and attached to the primary peg provides ~20% stronger hold but adding a 3rd peg in line increases hold by ~2.5x⁴⁾
- air density = 1.2kg/m³
- 80kph wind = 22.2m/sec
- wind shear:
  - wind slow down along the surface by by obstructions like buildings, trees, etc is “wind shear”
  - wind speed slowed by wind shear = wind speed at height h = v_o x (h / h_o)^α
    - v_o = wind speed (m/s) at height h_o in m
    - α = wind shear coefficient (open water = 0.1, smooth, level grass = 0.15, low bushes with few trees = 0.2, forests, hills or buildings = 0.25)
  - compared to wind at 10m above ground, wind at 0.5m above ground is generally less than half, and may be close to 1/10th or less if there are numerous bushes or obstacles, and may be 70% of the wind speed at 2m elevation ⁵⁾
    - NB. weather bureaus usually forecast wind speeds at 10m above ground
  - also see https://en.wikipedia.org/wiki/Roughness_length
- atmospheric pressure = 101.325kPa
- 1 Pascal pressure = 1 newton force / m² which is approximately 1kg / m²
- surface area of a dome = 2πr²
- drag coefficient (Cd) varies with shape of the obstruction and some very approximate values are:
  - concave surface (eg. wind blowing a tent wall inwards): Cd = 1.3-1.4
  - a flat face: Cd = 1
  - corner of cube into wind: Cd = 0.8
  - curved dome with peak facing into wind: Cd = 0.4
  - some highly streamlined tunnel tent shapes have a drag coefficient as low as 0.04
  - turbulence behind a short cylinder may result in Cd = 1.2 whereas the much less turbulence behind a long cylinder may result in Cd = 0.8
  - small tent fabric panels tension more and deform less than large ones and will may have lower Cd although there are a variety of other factors at play
  - the closer a panel is to vertical the more drag it has, and the more it will be impacted by wind, however, triangular panels with a wide base reduce this effect hence pyramid tents are quite aerodynamic
- smooth laminar air blowing on a tent is much less likely to impact a tent than gusty, turbulent air.
  - strong gusts and turbulent air shake the tent, applying sudden dynamic loads that tear corners, snap poles and tug stakes loose.
  - you may sometimes be better off being in amongst shrubs than behind an inadequate solid wall that’s too small or vertical as solid obstructions create more turbulence.
  - winds coming up a steep cliff will create a rotor turbulent area at the top of the cliff - you should set up well back from the cliff edge to reduce this - perhaps more than 200m back from the cliff!⁶⁾
equation for wind pressure:
- when moving air (eg. wind) is stopped by a surface - the dynamic energy in the wind is transformed to pressure and the pressure acting the surface transforms to a force

wind pressure in Pascals = drag coefficient x air density x (velocity in m/sec)²

hence if Cd = 1, 100kph wind exerts 924Pa = 94kg/m² pressure, 90kph wind exerts 750Pa = 76kg/m² pressure 80kph wind exerts 591Pa = 60kg/ m² pressure

wind force in Newtons = wind pressure in Pascals x surface area of obstruction in m²

Let's assume there is a well pegged down dome tent 4m in diameter with a 1sq m door opening facing the wind, how much pressure would be exerted on the opposite wall of the tent to the door?
- interior surface area of wall receiving the wind pressure (half the dome) = πr² = 4π = 12.5m²
- assume the curvature of the exterior wall deflects 70% of the wind as it is taut and the facing area is 1/6th of a dome = 4m²
- thus even a 80kph gust would exert (12.5×60) + (0.3*4×60) = 822kg of force
  - ie. opening the door to the wind will thus increased the wind force on the tent by over 10x as much, from (0.3*4×60) = 72kg to 822kg!
  - if there were 9 pegs holding it in that wind direction then the force on each peg would be over 45kg and may suffice to pull all 9 pegs out and then the rest would likely follow as the direction of pull on the remaining pegs becomes vertical which the sustained gust would easily remove
  - if one stake were somehow to remain intact, that full force would end up on that one guy rope and likely rip the guy out from the tent

The windward surface area of the tent below the guyouts closest to the ground needs to be less than half the total surface area of the wind profile.
- If you have too much surface area below the guylines, that part of the tent will become concave and collect wind turning your tent into a sail. ⁷⁾
- guylines need to be high enough to hold poles in place, but low enough to allow wind to flow over the top of the tent
guylines should be only just weaker than your tent material at the attachment points, because you want them to fail before the tent itself

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