Notes

Outline

Sailing Colorado Lakes ….Or….how to integrate frustration into your recreational time, ……………..and learn to enjoy it. Part I I                 by  John McGinley                    CLSC

Fact Denver….the largest city in the US farthest away from a major water body (Great Lake or Ocean).

Despite this we have a large sailing community. As a Colorado sailor you have seen some unique things…..

….two downwind boats on opposing courses

Slide 5

….every leg of a race a beat

….major sailing voyages three miles long and usually circular….

…..a tornado from your boat

….races like this!

…..never had to deal with this on your lake

…..an anchor used as a tactical tool, racing or cruising, where there is no current or tide

Colorado Winds and Sailing Weather – an attempt to explain this craziness Colorado wind regimes Lake breezes Daily wind evolution - sailing strategies Thunderstorms and lightning Waves on lakes Summary

The summer weather pattern. Winds in the Plains are dominated by the Bermuda High; mountains are dominated by monsoon and local circulations: Denver sits between.

In Eastern Colorado we get four main wind regimes

Some days are dominated by downslope or mountain wave flow

Downslope Day

Lenticular Clouds

Lenticular Clouds

Weak Summer cold fronts. Cool upslope with clouds. Banding is possible near the Front Range. Showers and thunderstorms likely.

Cloudy Upslope Day

Cloud Bands

Looking down on cloud bands

Stratus and Stratocumulus

Stratus and Stratocumulus

Mountain Upslope Dryline (red-dash) is key feature separating Bermuda High flow from mountain-disturbed flow - near the Front Range we get a light upslope mountain breeze and unpredictable winds. Winds are dominated by local effects.

Mountain Breeze

Sunny Day – thermals form

The Infamous Thermal

Looking down on a cumulus/ thermal . Moves along with steering flow.

Cumulus cloud and winds - find the edge of the cloud/thermal

Size relative to our favorite lakes …..             These things                  can dominate                  lake winds!

Cumulus clouds

Cumulus clouds – visible tracer of thermals

Dryline sometimes is right on the Front Range (late summer) - forms what meteorologists call the “DENVER CYCLONE”

Stormy Day

Formation of downbursts

Schematic of Downburst

Downburst example

Downburst example

Downburst example

LAKE BREEZES

Why Breezes Form – Land versus Water Heating

For breeze, lake must maintain a temperature deficit  to create pressure gradient Easy to do on large lake

“Gradient” wind impact on breezes: Breeze Index    wind = DP/(2rRDS) = 589 x  D P/D S     sin (lat)         Wind Speed = 22.9 kts          Breeze Index = Wind Speed2         DT DT is the air temperature difference from lake to land (F)  Over lake air temp = watertemp +10F If BI is greater than 14, there will be no breeze…. Here BI = 26, no breeze

Lake Breeze on Medium Sized Lake

Lake Breeze on Medium Sized Lake – with light background flow

Lake Breeze on Large Lake – Sometimes forms mid lake convergence zone ---------

Lake Breeze on Large Lake – with light background wind

Elevated Lake Breezes in Dammed Canyons

Limits to Breezes Size of Lake – need to maintain a deep layer temperature difference Background Gradient Wind – if it is too strong breeze will be erased

Size and configuration limits: Why Colorado lakes rarely have breezes Tough to get a breeze with a lake smaller than 2 miles wide Tough to get a breeze where lake has steep shores (canyon, surrounded by mountains, etc)

Thermal contrast  is mixed away by the thermal eddies

When breezes don’t form Lakes that are too small ( < 2 mi) Lakes dominated by gradient flow Lake winds are controlled by land effects on winds as if the lakes aren’t there

On a Colorado lake, what you see

What you see

What you see

Features that influence winds on Lakes Channeling Land heating over time

Channeled flow in high-terrain bounded lakes – air is lazy and often goes around rather than over

Concept of Air Flow

Evolution of winds thru a day -active over lakes with or without a breeze Morning – Cool layer – light winds Mid Morning – Laminar flow – steady wind Late morning  - Steady wind with beginning thermals – gusts and calms Noon – Cellular Clouds – wind areas and calms Afternoon – Rolls – oscillating flows Evening – Cellular to Laminar – steady winds return Night – Laminar – sometimes great winds Early morning – cool buffer over lake from drainage flow

Early Morning- Decoupled Flow

Late Morning- First Thermals

Morning Steady Winds and Thermals

Noon- Organized Cells

Late Morning/Noon: Hexagonal Cells

Afternoon- Deep Rolls

Afternoon – Deep Rolls

Early Evening- Last Thermals

Night- Laminar Flow

Early Morning- Decoupled flow

Waves on Lakes Fetch to attain a fully developed sea (FDS) - Ffds        Ffds (nm)  = 3.17 x Wind 4/3  (kts) On a lake waves happen sooner and grow faster Wave Height on Lake = .020 x Wind 2 x (Fetch/Ffds)1/2                                                         = .011 x  Wind 4/3  x Fetch (nm) Wind duration for fully developed waves  Time (hrs) =  H   (ft)

Wave Case Study: Lake McConaughy 60 kt WNW wind

Thunderstorm Hazards

Thunderstorms, shelf clouds and outflows

Thunderstorm wind flow

Lightning with Thunderstorms

Charge in a Thunderstorm

A mast in a thunderstorm – a target or not?

A mast on a lake – a target or not?

Summary Colorado is in a complex wind regime with 4 basic patterns Colorado lakes are minimally impacted by lake breezes. Wind evolution over lakes are dominated by processes that act over  a land surface going through a heating cycle: steady winds- imbedded thermals-hexagonal cells – longitudinal rolls- steady winds – morning calms Colorado summer hazards: thunderstorm outflows/downbursts and lightning

Some good metro Denver/ Colorado web sites