Introduction
Despite the hot and dry summers in Margem Sul, summer rains do happen and can cause significant problems for pools. A single storm can completely alter the chemical balance of the water and introduce contaminants that take days to resolve.
Many owners underestimate the impact of rain on pools, assuming it's "just water". In reality, rainwater brings with it a range of elements that negatively affect pool water quality.
This guide explains how rain affects your pool and what to do before, during and after summer rains to minimise problems.
Impact of Rain on the Pool
What Rain Brings
| Element | Source | Effect on Pool |
|---|
| Atmospheric dust | Air | Cloudy water |
| Pollen | Plants | Organic residue |
| Pollutants | Traffic, industry | Contaminates water |
| Nitrogen | Atmosphere | Feeds algae |
| Acids | Acid rain | Lowers pH |
| Algae spores | Air | Algae initiation |
Chemical Changes
| Parameter | Rain Effect | Consequence |
|---|
| pH | Drops (rain is acidic, pH 5-6) | Corrosion, irritation |
| Alkalinity | Diluted | Unstable pH |
| Chlorine | Diluted and consumed | Loss of disinfection |
| Stabiliser | Diluted | Chlorine more vulnerable |
| TDS | Increases | "Tired" water |
Impact by Rainfall Volume
| Rain Type | Approximate Volume | Impact |
|---|
| Light rain | <5mm | Minimal |
| Moderate rain | 5-15mm | Moderate |
| Heavy rain | 15-30mm | Significant |
| Storm | >30mm | Severe |
💡 Dica Profissional
A storm that adds 50mm of water to a 50m³ pool represents 2,500 litres of untreated water. This significantly dilutes all chemicals and introduces contaminants that need to be neutralised.
Common Problems After Rain
Cloudy Water
| Cause | Description | Solution |
|---|
| Suspended particles | Dust, pollen carried in | Flocculant + filtration |
| Microscopic algae | Activated spores | Shock + algaecide |
| Organic matter | Leaves, debris | Physical cleaning |
| Chemical imbalance | Diluted water | Rebalance |
Green Water
| Speed | Cause | Severity |
|---|
| Green in 24h | Algae already present + rain | Moderate |
| Green in 48-72h | Activated spores | Normal |
| Persistent green | Inadequate treatment | Requires strong action |
Other Problems
| Problem | Cause | Frequency |
|---|
| Dirty waterline | Floating oils and debris | Common |
| Full skimmers | Leaves and debris | Very common |
| Pump problems | Clogged basket | Common |
| Overloaded filter | Excess particles | Moderate |
| High water level | Excess water | Common |
Preparation Before Rain
If You Know Rain is Coming
| Action | Why | When |
|---|
| Raise chlorine | Compensate for dilution | Hours before |
| Check pH | Start from ideal base | Hours before |
| Clean skimmers | Prepare for debris | Before rain |
| Cover pool | Minimise impact | Before rain |
| Turn off equipment | Protect from lightning | If thunderstorm |
Ideal Pre-Rain Levels
| Parameter | Normal Level | Pre-Rain Level |
|---|
| Free chlorine | 1.5-2 ppm | 3-4 ppm |
| pH | 7.2-7.4 | 7.4-7.6 |
| Alkalinity | 80-120 ppm | 100-120 ppm |
Using a Cover
| Type | Protection | Considerations |
|---|
| Bubble cover | 50-70% | Remove if strong wind |
| Opaque cover | 70-85% | Good protection |
| Winter cover | 85-95% | Best protection |
| Automatic cover | 90-98% | Ideal |
💡 Dica Profissional
If you don't have a cover, consider at least covering the skimmers with netting or plastic to prevent massive entry of leaves and debris during the storm.
Actions During Rain
What to Do
| Situation | Action |
|---|
| Light rain | Nothing special |
| Moderate rain | Check skimmers after |
| Heavy rain | Prepare for treatment |
| Thunderstorm | Turn off equipment |
Thunderstorm Precautions
| Equipment | Action | Reason |
|---|
| Pump | Turn off | Electrical protection |
| Heating | Turn off | Electrical protection |
| Lighting | Turn off | Safety |
| Automation | Keep protected | Electrical surges |
Never Do
| Action | Risk |
|---|
| Enter pool during thunderstorm | Life risk |
| Touch equipment during thunderstorm | Electrocution |
| Add chemicals during rain | Immediate dilution |
| Ignore high water levels | Overflow, damage |
Immediate Actions After Rain
Post-Rain Checklist
| Order | Task | Importance |
|---|
| 1 | Remove cover and drain accumulated water | High |
| 2 | Collect visible leaves and debris | High |
| 3 | Clean skimmer and pre-filter baskets | High |
| 4 | Check water level | High |
| 5 | Test pH and chlorine | Critical |
| 6 | Adjust chemicals | Critical |
| 7 | Run extended filtration | High |
| 8 | Observe water for next 24h | Medium |
Physical Cleaning
| Task | Tool | Why |
|---|
| Collect leaves | Surface net | Prevent decomposition |
| Clean waterline | Sponge/brush | Remove oils |
| Vacuum floor | Vacuum | Remove sediment |
| Brush walls | Brush | Prevent algae |
Water Level Management
| Situation | Action |
|---|
| Level 5-10cm above normal | Drain a little |
| Very high level (>15cm) | Drain to normal |
| Overflow risk | Urgent drainage |
Chemical Treatment After Rain
Treatment Sequence
| Order | Action | Why |
|---|
| 1 | Test water | Know current state |
| 2 | Adjust pH first | Chlorine needs correct pH |
| 3 | Shock chlorine | Restore disinfection |
| 4 | Add algaecide | Prevent algae |
| 5 | Consider flocculant | If water cloudy |
| 6 | Retest in 24h | Confirm recovery |
Post-Rain Dosages
| Rain Type | Chlorine Shock | Algaecide |
|---|
| Light | 10 g/m³ | Normal |
| Moderate | 15 g/m³ | Normal |
| Heavy | 20 g/m³ | Increased dose |
| Storm | 25-30 g/m³ | Increased dose |
pH Adjustments
| pH Measured | Action | Product |
|---|
| <7.0 | Raise urgently | pH increaser |
| 7.0-7.2 | Raise slightly | pH increaser |
| 7.2-7.4 | Ideal | None |
| 7.4-7.6 | Acceptable | Monitor |
| >7.6 | Lower | pH reducer |
💡 Dica Profissional
After heavy rains, pH commonly drops significantly (rainwater has pH ~5.5). Always adjust pH before shock treatment, otherwise chlorine will be less effective.
Filtration After Rain
Recommended Filtration Time
| Situation | Time | Notes |
|---|
| Light rain | Normal (+2h) |
| Moderate rain | 12-14 hours |
| Heavy rain | 16-20 hours |
| Cloudy water | 24h continuous | Until clear |
| After flocculant | 24-48h | Don't interrupt |
Filter Maintenance
| Task | When |
|---|
| Check pressure | Immediately |
| Backwash | If pressure +0.3 bar |
| Clean pre-filter basket | Daily |
| Check filter condition | After storms |
Signs of Overloaded Filter
| Sign | Meaning | Action |
|---|
| Very high pressure | Saturated filter | Backwash |
| Water won't clear | Ineffective filter | Check filter media |
| Weak return | Restricted flow | Check obstructions |
Using Flocculant
When to Use
| Situation | Use Flocculant? |
|---|
| Slightly cloudy water | Optional |
| Moderately cloudy water | Recommended |
| Very cloudy water | Necessary |
| Fine particles in suspension | Necessary |
| Green-cloudy water | After shock |
Types of Flocculant
| Type | Advantage | Use |
|---|
| Liquid (clarifier) | Easy, doesn't settle | Slightly cloudy water |
| Cartridge/Tablet | Slow release | Prevention |
| Powder flocculant | Very effective | Very cloudy water |
Flocculant Procedure
| Step | Action | Time |
|---|
| 1 | Adjust pH 7.2-7.4 | Before |
| 2 | Add flocculant | - |
| 3 | Filter 2-3 hours | Distribute |
| 4 | Turn off filtration | 8-12h |
| 5 | Particles settle | Overnight |
| 6 | Vacuum to waste | Next morning |
| 7 | Restore level | After vacuuming |
Special Cases in Margem Sul
Rains with South Wind
| Characteristic | Impact | Action |
|---|
| Bring Sahara sand | Very cloudy water | Flocculant necessary |
| Much dust | Extended filtration | 24h+ |
| Brownish colour | Normal | Intensive treatment |
Summer Thunderstorms
| Characteristic | Impact |
|---|
| Short but intense | Much debris |
| Strong wind | Leaves, branches |
| Possible hail | Check equipment |
Specific Zones
| Zone | Additional Challenge | Solution |
|---|
| Aroeira/Verdizela | Pine needles | Clean skimmers urgently |
| Azeitão | Agricultural dust | Frequent flocculation |
| Coastal zones | Salt + rain | Check corrosion |
| Palmela/Sesimbra Hills | More rain | Reinforced preparation |
Preventing Algae After Rain
Why Rain Causes Algae
| Factor | Contribution |
|---|
| Diluted chlorine | No protection |
| Nutrients (nitrogen) | Feeds algae |
| Airborne spores | Introduces algae |
| Unbalanced pH | Ineffective chlorine |
| Organic matter | Extra nutrients |
Post-Rain Anti-Algae Protocol
| Step | Action | Timing |
|---|
| 1 | Strong shock | Immediate |
| 2 | Preventive algaecide | With shock |
| 3 | 24h filtration | Continuous |
| 4 | Brush walls | 24h later |
| 5 | Maintain high chlorine | 48-72h |
| 6 | Monitor | Daily, 1 week |
Signs of Developing Algae
| Sign | Severity | Action |
|---|
| Slightly greenish water | Beginning | Double shock |
| Slippery walls | Moderate | Brush + shock |
| Corners with slime | Moderate | Brush + shock |
| Visible green | Advanced | Complete treatment |
Frequent Summer Rains
If It Rains Several Days in a Row
| Day | Action |
|---|
| Day 1 | Normal post-rain treatment |
| Day 2 | Check and adjust if needed |
| Day 3 | Preventive shock |
| Day 4+ | Maintain high chlorine, long filtration |
Maintenance During Rainy Period
| Frequency | Task |
|---|
| Daily | Test chlorine and pH |
| Daily | Clean skimmers |
| Every 2-3 days | Preventive shock |
| After each rain | Check level |
| Weekly | Backwash |
💡 Dica Profissional
During periods of frequent rain, consider maintaining chlorine level higher (3-4 ppm) permanently. It's easier to prevent problems than to solve them.
Complete Post-Rain Checklist
Immediately (0-2 hours)
- [ ] Remove cover (if applicable)
- [ ] Collect large leaves and debris
- [ ] Clean skimmer baskets
- [ ] Check water level
- [ ] Drain excess if necessary
First Hours (2-6 hours)
- [ ] Test pH, chlorine, alkalinity
- [ ] Adjust pH if needed
- [ ] Shock treatment
- [ ] Add algaecide
- [ ] Start extended filtration
- [ ] Clean pre-filter basket
Next Day
- [ ] Check water clarity
- [ ] Retest chemicals
- [ ] Vacuum floor if needed
- [ ] Check filter pressure
- [ ] Backwash if necessary
- [ ] Watch for algae signs
48-72 Hours Later
- [ ] Confirm crystal clear water
- [ ] Stable chemical levels
- [ ] No signs of algae
- [ ] Resume normal routine
Common Mistakes to Avoid
| Mistake | Consequence | Correction |
|---|
| Not treating after rain | Algae in 48-72h | Always treat |
| Adding chlorine without adjusting pH | Ineffective chlorine | pH first |
| Not cleaning debris | Decomposition, algae | Clean immediately |
| Insufficient filtration | Cloudy water persists | Minimum 12-16h |
| Ignoring high water level | Overflow, dilution | Drain excess |
| Waiting for it to "pass" | Problems worsen | Immediate action |
Frequently Asked Questions
Do I really need to treat the pool after every rain?
Yes, especially after moderate to heavy rains. Rainwater is acidic (pH ~5.5) and brings contaminants that affect water quality. Light rain (<5mm) may not require special action, but it's always good to check levels.
Can rain cause algae that quickly?
Yes. The combination of diluted chlorine, rain nutrients (nitrogen), airborne algae spores and unbalanced pH can result in green water in 24-48 hours, especially in summer heat.
Should I cover the pool whenever it's going to rain?
If possible, yes. A cover can significantly reduce the amount of contaminants and rainwater entering the pool. However, remove the cover if there's strong wind to avoid damage.
Can I use the pool right after rain?
Not recommended to use immediately after heavy rains. Treat the water first, check chemical levels, and wait until chlorine is between 1-3 ppm and pH between 7.2-7.4 before swimming.
Why does my pool always turn green after rains?
Probably because post-rain treatment isn't being done properly or in time. The key is to act quickly (within the first 2-6 hours), do adequate shock treatment, and maintain extended filtration.
Conclusion
Summer rains in Margem Sul, although infrequent, can cause significant pool problems if not treated properly:
- Prepare the pool before rain when possible
- Act quickly after rain (first hours)
- Clean debris and check levels immediately
- Treat with adequate shock and algaecide
- Filter enough time to clarify
- Monitor in the following days
With a quick and appropriate response, your pool can recover in 24-48 hours after any summer rain.
If you need professional help treating your pool after summer rains in Margem Sul, ManutençãoPiscinas is here to help. Contact us for a no-obligation quote.
