Bad Weather and Night

Map Reading and Navigation

Course 1 · Chapter 9 · Bad Weather and Night: Navigating When You Cannot See

Everything so far works best when you can see the landscape. When mist, rain, snow or darkness takes the view away, you fall back on the compass, your pacing and your timing, and you make everything smaller and slower. This chapter covers how the techniques change, what margins to build in, and the hardest skill of all: deciding to turn back.

What Changes When You Cannot See

In good visibility you navigate by looking: you pick a feature in the distance and walk to it. In poor visibility the distant features vanish and you can only trust what is in the next few metres. Three things change:

Legs get shorter

Each leg ends at a feature you can reach and recognise within the visibility you have. Long legs let small errors grow.

Instruments take over

The compass sets direction, pacing counts distance (Chapter 6), and a watch checks both. Your eyes confirm, they do not lead.

Everything is checked

Each leg is confirmed against a feature at its end: a slope change, a stream, a wall. If the feature does not appear, you stop.

Margins grow

Speed drops, so allow more time. Plan to be off the high ground earlier. Make the turn-back time earlier.

Walking a Bearing in Poor Visibility

You still take a bearing from the map (Chapter 5), but you cannot walk to a distant mark. The standard answer is a pair of people, or one person and an object, working as a team.

Leapfrogging

The navigator takes the bearing and sets the compass.
A second person walks ahead, just inside the visibility limit, and stops.
The navigator directs them left or right until they stand exactly on the bearing line.
The navigator walks up to them, passes them, and the process repeats with the roles swapped.

On your own, pick a small object on the bearing line (a rock, a tuft of grass) within sight, walk to it, and then pick the next one. Never walk looking at the compass: you drift sideways without noticing. Look up, find a mark, then walk to it.

Whiteout and flat light In snow or heavy cloud there may be no contrast at all, and the ground can seem to merge with the sky. Leapfrogging still works, but your sense of slope fails, so a step can come as a surprise. Move slowly and check the ground before each step.

Why short legs matter

A small bearing error produces a sideways drift that grows with distance. If the error is e degrees, then over distance d you drift by d times tan(e).

import math

def drift(distance, error_deg):
    return distance * math.tan(math.radians(error_deg))

for d in (100, 200, 400):
    print(d, round(drift(d, 5)))
# 100 9
# 200 17
# 400 35

A 5 degree error is 9 m off line after 100 m, but 35 m after 400 m. If you can only see 30 m, a 400 m leg could take you past your target without a glimpse of it. A sensible rule is that a leg should end at something you will certainly see, and be short enough that the likely drift is smaller than the visibility.

Timing, Pacing and Checking

Each leg has an expected number of paces and an expected time (Chapter 6). In poor visibility conditions, add a margin because you walk more slowly and your pace gets shorter on rough or wet ground. Then use them as a check, not as a trigger to keep going:

Count paces in hundreds, using pacing beads or a tally on your fingers.
Note the time at the start of each leg.
If the paces or time run out before you reach the feature, stop. Do not walk on hoping.
Go through the relocation steps from Chapter 8 if you cannot find the feature.

Choose targets that are large or linear (a stream, a wall, a path), or use catching features. A single small cairn in mist is easy to walk past; a long wall across your line is not.

Night Navigation

Darkness has many of the same effects as mist, but there are extra points.

Issue	What to do
Reading the map	Use a head torch with a red or dimmed setting, which preserves your night vision. Cover the light when looking up.
Torch beam	A beam at your feet makes everything beyond it seem darker. Walk slowly and stop to look, using the torch only when needed.
Distances	They are hard to judge, and your pace is slower, so rely on timing and counted paces.
Features	Skylines and large shapes show at night; fine detail does not. Plan using big, simple features.
Spares	Carry a spare torch and batteries. A failed light with no spare turns a hard walk into an emergency.
Route choice	Prefer paths and handrails. Avoid steep slopes, crags and rough ground you cannot judge.

Safety Margins

Good navigators do not rely on being right; they rely on having room to be wrong. Build in margins before you start:

Time margin

Work out when it gets dark and when you must set off back, and add a margin on top for delays.

Escape routes

Know the quickest way down from each part of the route (Chapter 7) before you need it.

Kit

Spare warm layers, waterproofs, food, drink, torch, whistle, map and compass. A phone is a backup, not a navigation plan.

Someone knows

Leave your route card with someone who will raise the alarm if you are late.

The turn-back time is worked out in advance from when it gets dark, how long the return will take, and a margin.

def to_min(t):
    h, m = t.split(":")
    return int(h) * 60 + int(m)

def to_clock(mins):
    return f"{mins // 60:02d}:{mins % 60:02d}"

def turn_back_time(dark, remaining_min, margin=30):
    return to_clock(to_min(dark) - remaining_min - margin)

print(turn_back_time("16:30", 150))        # 13:30
print(turn_back_time("16:30", 150, 60))    # 13:00

When to Turn Back

Turning back is not failure; it is part of the plan. The summit, the viewpoint or the finish will still be there another day. Decide your criteria in advance, because on the hill, tired and close to the goal, judgement is at its worst. Common reasons to turn back:

You have reached your turn-back time.
The visibility has dropped below the minimum you can navigate safely.
The weather is worse than forecast, or getting worse fast (wind, cold, rain, ice).
Someone in the group is cold, exhausted, injured or unhappy.
You are unsure of your position and cannot fix it.
The route is not as you expected, and the alternatives are not clearly safe.

The group sets the pace The weakest or least confident member is the limit for the whole group, not the strongest. If anyone wants to turn back, take it seriously and listen to why.

def decide(now, turn_back, visibility, min_vis, group_ok):
    reasons = []
    if to_min(now) >= to_min(turn_back):
        reasons.append("past turn-back time")
    if visibility < min_vis:
        reasons.append("visibility below minimum")
    if not group_ok:
        reasons.append("a member is struggling")
    if reasons:
        return "TURN BACK: " + ", ".join(reasons)
    return "Continue"

print(decide("12:50", "13:20", 60, 30, True))   # Continue
print(decide("13:25", "13:20", 60, 30, True))   # TURN BACK: past turn-back time

Turning back safely uses the same skills: go back along a route you know, using the handrails and catching features you already identified. If you have to turn back in poor visibility, a reverse bearing (Chapter 8's back bearing) along your outward route is often the safest way home.

Put It Together

Imagine setting out in a forecast of cloud lifting later. By midday the cloud is down and you can see 40 m. You are on a bearing with a leg of 350 m to a stream. You halve the leg by choosing an intermediate mark, leapfrog along the bearing with a partner, and count paces. At the pace you expected the stream has not appeared; you stop, check the time against your turn-back time (not yet reached), and relocate using the slope under your feet and the direction of the wind. The stream appears 60 m to the left: you had drifted. You note the error, correct for it, and decide to shorten the next legs. Nothing dramatic happened because the margins were in place.

Common Mistakes

Mistake	Fix
Long legs in mist	Shorten legs to a feature you will certainly see and reach
Walking while staring at the compass	Take the bearing, pick a mark on it, look up and walk to the mark
Walking on past the pace or time limit	Stop and relocate when the limit is reached
Deciding the turn-back time on the hill	Set it before you start, from when it gets dark
No spare light	Carry a spare torch and batteries
Pressing on because the goal is close	Stick to the criteria you set earlier

Challenges

Exercise 1

Write the functions to_min, to_clock and turn_back_time(dark, remaining_min, margin=30). It gets dark at 16:10 and the return to the road needs 140 minutes. Print the turn-back time with a 30 minute margin, and again with a 60 minute margin.

→ Solution

Exercise 2

Write a function drift(distance, error_deg) and print a table of the drift for errors of 2, 5 and 10 degrees over 100 m and 300 m. Then work out the longest leg for which a 3 degree error stays within 30 m of visibility, and say in one line why you would still use a shorter leg.

→ Solution

Exercise 3

Write a function decide(now, turn_back, visibility, min_vis, group_ok) that returns "Continue" or "TURN BACK:" followed by every reason that applies. Test it with three cases: all fine at 12:50 (turn-back 13:20, visibility 60, minimum 30); 13:25 with otherwise fine conditions; and 12:50 with visibility 20 and a struggling group member.

→ Solution

Quick Reference

Poor visibility: short legs, instruments lead, confirm every leg, bigger margins
Leapfrogging: a partner stands on the bearing line; walk to a mark, then the next, never staring at the compass
Drift: distance times tan(error); long legs magnify small errors
Limits: count paces and time; stop and relocate when the limit passes
Night: dimmed or red light, spare torch, simple features, paths and handrails
Turn-back time: dark minus return time minus margin, set before you start
Turn back when: time is up, visibility too low, weather worse, someone struggling, or position unknown

Coming next Chapter 10 is the capstone: you will plan a full day walk using every skill from the course, from the route and the time budget to the escape routes, the route card and the turn-back criteria.