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Rain

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Legend

Legend

METAR Code

-RA/RA/+RA or -SHRA/SHRA/+SHRA

Weather Symbol

HAZARDS

Rain and associated mist can act to reduce visibility and cause pooling in regions with no drainage. High rainfall rates, often associated with convection, can cause flooding, and rapid ice melts.

About

Definition

MANOBS: Precipitation of liquid particles, either in the form of drops or larger diameter than 0.5mm, or of smaller widely scattered drops.

Associated terms coming soon:

Dewpoint, lake effect, low-pressure system, orographic lift, and upslope flow are all terms associated with rain that will be coming soon to the Aviation Meteorology Reference.

Associated Terms

9

Rain

Visualization

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Dissipation

Rain can dissipate for a variety of reasons. Stratiform rain will begin to fizzle out when there is no longer adequate moisture being fed into the system to support more precipitation forming. In the case of upslope flow, when the wind shifts away from terrain, or there is a change in air mass bringing in drier air (cutting off moisture inflow), rain will taper off.

In the case of convective rain showers, when supported by frontal features (or troughs), rain showers will weaken with dissipating fronts. Convection will otherwise exhaust itself once the influx of moisture has stopped, and been precipitated out, and/or its supporting trigger dissipates.

In the case of virga, it will cease when one of two things happens. First, as the rain falls, it will continually evaporate into the dry air, saturating it from top down. Once the falling rain has reduced the depth and dryness all the way to the ground, rain can be reported, and the virga is no more. Otherwise, virga will cease when the cloud has precipitated out all available moisture, but the depth of the dry layer remains deep.

Duration

A rain event associated with a low-pressure system could last from hours up to a few days depending on the trajectory of the system. Upslope flow generated snow can persist for several hours, until wind direction shifts or there is a change in air mass.

Meanwhile, convective snow showers are relatively short and sporadic, lasting from only minutes, to several hours in duration.

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Similar to other precipitation types, forecasting rain can be challenging with temperatures near zero, as it can transition rapidly between rain and wet snow in humid conditions, and surface temperatures are also an important factor. Forecasters do not usually have real time updates on the true depth of warm air above the surface, which can render forecasting precipitation type difficult.

Virga is especially difficult to forecast as it is not often well handled by model guidance. With a lack of real time data showing the true humidity of the mid-levels, it can be quite challenging to determine the exact moment when virga will saturate the dry air enough to reach the ground. While verifying the true edge of precipitation against surface observations of rain is very useful, in data sparse regions, it can still often be difficult to determine where rain is actually reaching the ground.

Convection is often quite small compared to numerical model resolution, which makes the exact location of less organized convective showers in long term forecasts harder to predict, as models cannot resolve small features very well. This is also the case for embedded convection, which can make accumulations difficult to predict, as locally higher amounts in convective showers are often smoothed out.

Additionally, rainfall amounts are difficult to forecast, especially in convective situations. While there will always be some aerial variation in precipitation amounts with both stratiform and convective precipitation, convective precipitation amounts can vary wildly even over a short distance. In certain extreme situations, intense convective showers can produce over 50mm in a short period of time over one portion of a city whereas the other portion sees little accumulations, as an example.

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MAIN CONCERNS

Aviation hazards for rain include reduced braking action, crosswind limitations, slippery surfaces, increased runway occupation time, reduced airport capacity, and reduced ceilings/visibility.

Service Providers

We would start to be concerned when rainfall warnings are issued or the amount of rain being experienced is starting to have impacts and effects to airport operations. The overall duration, accumulation amounts including hourly would be of use to us. We rely on our usual sources of weather information including: Environment Canada, TAF, and airport authority-specific providers (IBM Weather Services, RWDI, WPRED).

The main impacts of concern are:

  • Flooding of all different surfaces on the airport environment.
  • Airport drainage system and our ability to move and flow water including pump station management.
  • Standing water, increased risk of hydro-planing vs. available friction on runways and taxiways.
  • Impacts to critical equipment such as ILS, lighting, or other airfield electrical assets.
  • How is it affecting other operators, are they impacted by the weather (rainfall) in their operations.
  • Other associated weather phenomena such as the risk of thunderstorms, freezing rain, fog, snow given our temperatures like to hover around close to the 0C mark in the winter.
  • Are there any other effects to runway operations associated with the weather being observed such as downdrafts, windshear or items that would impact the safety of aviation to be aware of – increased missed approaches, or impacts to the system.

Operations Duty Managers

Rain can impact operations when enough falls to impact breaking action and aircraft control (increase in runway occupancy time) and/or force a change in runway configuration due to crosswind thresholds for wet/contaminated runways. For example, CYYZ has a maximum crosswind limit of 20KT for wet runways while for dry it is 30KT. Note that crosswind limits for other airports differ: 15KT for wet runways and 25KT for dry. These limits can force a less favorable configuration and potentially impact the airport’s arrival rate. Hourly accumulations (HubWX), Forecaster Notes, and the visual TAF are primary strategic tools when assessing onset, accumulation, and duration - which are the primary critical factors. More tactically, CoSPA and/or radar imagery, and upstream observations become the primary tools.

CZE (Edmonton Flight Information Region): Rain is a concern when you have wet runways and gusty sideways winds. Runway changes are required when runways are at 15 knots compared to 25 when runways are dry. We do pay attention to when the rain is forecasted to end so that we have an idea when the runways may be switched back. This is important to an airport like Calgary that has only a single runway for west winds.

ATC (Major Tower)

Major airports that only have two parallel runways (ex: CYUL) do not see as large of an impact as those that have multiple parallels (ex: CYYZ). Where there are multiple configurations possible, runway crosswind thresholds in wet or contaminated (standing water) conditions may result in a runway configuration change and impact the airport’s capacity. For example, in CYYZ, the crosswind threshold for dry runways is 30KT, for wet 20KT, and for contaminated 15KT. If crosswinds exceed these thresholds, a runway change is very likely to occur. In all cases, however, wet/contaminated runways increase runway occupancy time, which can require increased spacing between aircraft depending on throughput into the airport at the time.

Rain is always of interest to FSS, as it can support rapid changes in visibility and ceilings to the detriment of flight operations. The character of rain, be it steady or showery in nature, will be defined by the weather feature which causes the rain. Intensity of rain will influence a Pilot’s decision whether to launch or not, so it is imperative that an FSS be prepared to provide accurate, timely information to support good Pilot decision making.

Advisory specialists will be on the watch during a rain event, as the increase in moisture near to the ground can cause stratus fractus and or mist/fog formation. When the temperature/dew point spread is wider near the surface than aloft, rainfall can actually drive down the temperature because of evaporative cooling while simultaneously adding water vapour to the air and driving up the dew point. Prolonged or heavy rainfall can cause standing water on manoeuvring areas, which we would be in contact with airport maintenance Personnel to assess and mitigate as required.

Rain is not a huge concern to low-level enroute controllers. A light rain falling over an airport has minimal impact (the ceiling may still be ok for visual approaches for example). Rain begins to matter when it is falling more heavily. Landing capacity is reduced when moderate or heavy rain is falling. Visibility at the airport necessitates more mileage on final approach, which in the low-level enroute can result in airborne holding and increase in workload. If rain accumulates on the runway, aircraft may take longer to exit the runway. If because of rain the landing rate is reduced and demand exceeds the airport’s capacity, it may require a ground delay program to avoid overload, and possible enroute delays that increase our workload.

If rain is forecast to fall all day, ATC will prepare for reduced capacity and the possibility of airborne delays. The forecasting tool that we use most frequently is the TAF and/or METARs. Depending on the actual severity of the rain, we will adapt throughout the day as need be.

Users

Dispatchers consider the following when considering rain:

  • Intensity (RA & +RA more impactful than -RA)
  • Duration (Long duration events or heavy downpours causing flooding)
  • Accumulation (impact on runway conditions and performance)

In general, rain has a negative effect on aircraft performance during landing and take off:

  • Wet runways reduce friction and aircraft rollouts (during landing or in the event of a rejected takeoff) are increased. On long runways this tends to not be an issue however if the airport elevation or temperatures are high, the aircraft must be planned at lower weights. Lower weights could necessitate reduced payload and less revenue.
  • Jet aircraft tend to require longer runways for landing and are more sensitive to wet runway restrictions- a jet can land on a 6000ft long dry runway but should it be wet, it cannot.
  • Wet runways can reduce the crosswind capability of aircraft. For example, an aircraft may have a 27kt crosswind capability on dry runways but when wet the crosswind capability is reduced to 22kt.
  • Wet runways in temperatures near zero may require the use of anti-ice protection which can also reduce aircraft performance and be a limiting factor for take off or landing.

The presence of rain adds some operational considerations and risks for general aviation (GA) pilots which can largely be grouped into ground vs flight. On the ground, these are primarily relating to takeoff and landing operations. Rain can create more hazardous conditions including slippery sections of a runway, hydroplaning and loss of control. It will often increase takeoff and landing distances and need to be accounted for. A pilot who fails to account for this might find themselves having a harder time slowing down the aircraft and might apply the brakes more heavily, potentially locking the brakes and creating a risk of loss of control. They may also find themselves moving too fast at the point they had planned to exit the runway, causing a loss of control as they turn to exit the runway.

In flight, rain can sometimes change to different types of precipitation related to a front and/or changes in temperature, sometimes related to changes in altitude. If rain becomes snow or freezing rain, the associated risks will change as well (please see sections relating to those topics). That said, the biggest issue associated with rain is usually visibility. This is further expanded upon in the section relating to precipitation rate. In brief, light rain can make it harder for pilots flying visually (VFR) to navigate based on long range references (mountains, bodies of water) as well as to notice obstacles or conflicting traffic approaching. As rain becomes heavier, it can make visual navigation increasingly difficult by removing closer navigation references such as landmarks, rivers, towns. Each pilot will have a different tolerance level for flying in rain, given their level of experience with precipitation and their knowledge of the area in which they will be flying. That said, a good pilot will always have backup options and be prepared for conditions to change, in addition to making use of available navigation aids to mitigate risks when possible. These include radio navigation, GPS, Air Traffic Services, and other related equipment and tools. A pilot that is not as experienced or prepared may find themselves in a risky situation if the rain changes in severity or type (ice/freezing). As always, pilots should use all available weather products, consult a FIC for an interpretive weather briefing, and know which will provide the best picture of the overall weather that they will expect. For rain, the METAR, TAF, and GFA are good resources. When available, weather radar and airport camera imagery can add some useful local information.