Saturday, May 30, 2015

The Science of Colorful Sunrises and Sunsets

A picture perfect sunset - something that sky watchers everywhere love. So, why are some sunrises and sunsets so spectacular? Lets talk about the science behind these awe-inspiring features.

To start we'll take a step back and think about how the colors of the sky are produced, which has to do with how light is scattered. Scattering refers to the reflection or redirection of light by smaller particles. Rayleigh scattering refers to the scattering of light off of the molecules of air. The particles of air are much smaller than the wavelengths of visible light and therefore air is an excellent Rayleigh scatterer.

Visible light is composed of a spectrum of colors ranging from reds and oranges on one end of the spectrum and blues and purples on the other end. Anyone remember the acronym Roy G. Biv?

Photo courtesy of:

Each color has a different wavelength, with the wavelength of violet light closer in size to air molecules than the wavelength of red light. Therefore, pure air will scattered violet light 3-4 times more effectively than it will for reds. Now think about the blue sky we see every day. The wavelength of blue is close to that of violet, but it still is not scattered as efficiently. Human eyes are more tuned to peak in the middle part of the visible spectrum, or around the color green, which is closer in wavelength to blue. Now, if it weren't for this fact, the sky might actually appear violet to us on a day to day basis!

At sunrise and sunset, the sun is lower on the horizon and the sunlight takes a longer path to travel through the atmosphere than if the sun was directly overhead. The violet and blue hues are more efficiently scattered out through this path, leaving more reds to be seen. Here is an example of this:

This picture is Figure 1 in Stephen Corfidi's paper: "The Colors of Sunset and Twilight", which is linked below
Now, most of you have probably noticed that the best sunrises and sunsets occur with some clouds  in the sky. If you are a careful sky-watcher you may have also noticed there are certain types of clouds which are much better than others for producing incredible sunrises or sunsets. We typically look for mid to high level clouds due to the fact they are above the polluted air layer near the surface of earth, and clear air scatters light the most effectively. We also want a narrow clear strip of sky near the horizon at sunrise or sunset so that the light is able to be clearly scattered through the atmosphere and reflected off the base of the clouds. What also helps add visual interest to sunrises and sunsets are how the clouds form and differing layers and textures of cloud types. There was one such sunset on August, 29, 2014, when the remnants of a tropical storm brought an incredible assortment of cloud types to the entire west coast, reaching as far inland as Nevada. Some photos from that night, taken from the NWS Reno office, can be seen at the top of the post and below:

Colors just starting to show up - August 29, 2014

Colors intensifying a few minutes later - August 29, 2014

This post is just a brief introduction to the incredible properties of atmospheric and optical physics. For more details please check this excellent, short paper written by Stephen Corfidi here, which also links to much more information on twilight phenomena.

Here is hoping everyone gets to see some beautiful sunrises and sunsets and can now appreciate the science behind them!

A couple of sunrise pictures from the NWS Reno office, January 4, 2015. The photo on the left is looking southeast, and on the right looking southwest toward downtown Reno.

Thursday, May 28, 2015

Day in the Life of a Fire Weather Forecaster

So what exactly occurs during a fire weather forecaster shift during the fire season? We are going to break it down for you. Obviously the day could be way more/less busy if we have ongoing fires or the weather is quiet, but this is just to give you an idea of how the day is laid out!

7:00 AM - The shift officially begins with a briefing from the overnight forecaster. This is our time to ask what changes were made to the forecast, how have the models been trending, and if there were any equipment issues overnight.

7:30 AM  - Look at new incoming forecast model data, review fire reports available from the National Interagency Coordination Center (NICC) and prepare for upcoming conference calls and briefings. 

8:30 AM - 10:00 AM - We participate in a couple of conference calls with our fire partners, which include the Southern California Geographical Coordination Center (aka SoCal GACC), the Northern CA GACC, and with the Great Basin GACC. These conference calls are where we talk to the local fuels experts and fire intelligence folks to determine how bad the fuels are and what the fire potential will be for the given day. This is also our chance as weather forecasters to inform them of our weather concerns for the day or the next few days.

After we have completed coordinating with our local fire agencies and partners, we lead our own call to brief folks on the upcoming weather for the next week or so. We focus on big ticket items like strong gusty winds with low humidity and/or dry lightning events, which are both weather events that would warrant a Red Flag Warning to be issued. For more information on Red Flag Warnings check out this brief video.

10:00 - 2:00PM  - For a good part of the day, the fire weather forecaster will spend their time looking at the model data and making adjustments to the forecast which includes weather elements such as, Lightning Activity Level, Chance of Wetting Rain, Haines Index, Winds and Wind Gusts, and also Relative Humidity. These are all crucial elements to fire weather and how a fire might behave once it has started.

2:00 - 3:00 PM - During the afternoon, the fire weather forecaster may be dealing with incoming spot weather requests, which are specialized forecasts for prescribed burns, hazmat incidents, or even wildfires! These forecasts are unique to the incident and are submitted to NWS Reno by local law enforcement or the fire agencies, to get a localized forecast for the wildfire, search and rescue team, or a hazmat event.

Pine Haven Fire 2012 - Reno, Nevada

3:00 - 4:00 PM - The end of the shift consists of completing or creating drafts of all spot weather forecasts, writing a fire log to track what changes were made to the fire forecast, and also sending out the fire weather products, including: Fire Weather Planning Forecast and the Fire Weather Dispatch forecasts for northern California.

That about sums it up for the fire weather shift, without getting into too many meticulous details. Much of the information that we put out is available at the NWS Reno webpage or on the NWS Reno Fire Weather webpage (seen below). If you have any questions feel free to email us.

Friday, May 1, 2015

Tools of the trade (Skew-T Soundings)

I am sure that some of you have seen the entertaining videos of weather balloon releases that we have done at the NWS office, or maybe you have been lucky enough to be here in person when the balloon launch occurs. But what happens with this data and how does it assist us with predicting weather? Since we have already had thunderstorms this year (June came early), let's focus on how we utilize the weather balloon data to determine the potential for thunderstorms.

Let's start from the beginning...Everyday one of our meteorologists releases a balloon at 11Z and 23Z. You are probably thinking 'Z?' Well it is called 'Zulu' time which you might know as 'GMT' (Greenwich Mean Time).  It is a standard measure of time we utilize here at the Weather Service offices to ensure all of our data and products get issued around the same time of day worldwide. All balloon launches world-wide are released at 11Z and 23Z! Anyways, back to the balloon launch.

Not only does the data from the balloon get ingested into the forecast models around the world, but we can analyze it to get an idea of the structure of the atmosphere around Reno-Tahoe. On the day shift, we analyze the morning sounding, also known as the 12Z sounding. The morning sounding gives us an idea of what the existing state of the atmosphere looks like and then we can attempt to extrapolate what it will be in the afternoon. Let's take a look at a sounding from April 22nd which turned out to be a fairly decent thunderstorm day.

Generally, there are 3 things necessary for thunderstorm development - Moisture, Instability, Lift. We will try to focus on those primary ingredients when analyzing our sounding. Let's first see if we have sufficient moisture. There are a couple of different ways to determine the moisture in the sounding. First just by looking at the lines, what you can't tell? Let me explain.

The main lines on the chart represent the dry-bulb temperature (red) and the dewpoint temperature (green). The closer the lines are together the closer the atmosphere is (at that height) to saturation. Basically when the dewpoint temperature = the dry-bulb temperature you have 100% humidity (saturation) which could result in the formation of a cloud. So just glancing at the sounding below, it appears that we have quite a bit of moisture in the atmosphere the morning of April 22nd. Another way that we can determine the moisture content of the soundings by looking at the precipitable water value (PWAT). The sounding contains around 0.45 inches of moisture, which is a slightly above average for late April (average 0.3 inches)

 So it looks like we can check moisture off of our ingredient list. Let's see if the sounding will show enough instability. Instability occurs when a parcel of air is warmer than the environmental air and rises on its own due to positive buoyancy. Just like how a hot air balloon would rise on chilly September mornings in Reno. Instability is often expressed using positive CAPE (Convective Available Potential Energy) or negative LI (Lifted Index) values. Both of those values can be calculated from the sounding. The values off the 12z sounding that morning were -2.8 for the LI and 707 J/Kg (joules per kilogram...which is a unit of energy per unit mass). These values are impressive for around here, but nothing compared to the big severe weather outbreaks in the plains where CAPE values can exceed 4000 J/Kg and LI's can be -10 to -12!

Another thing you can look at to determine stability is the tilt of the temperature curve in relation to the dry adiabat and the moist adiabat. Now...what the heck is an adiabat? An adiabatic process makes the assumption there is no transfer of heat or matter between a system and its surroundings. In the atmosphere there is something known as the "adiabatic lapse rate". Imagine filling an invisible balloon with air, and now we will call this our air parcel. If we were to raise the parcel in the atmosphere, pressure would decrease, causing the volume to increase and the temperature to decrease to hold steady with the ideal gas law. This is known as the dry adiabatic lapse rate and assumes an unsaturated parcel. The average dry adiabatic lapse rate is 5.5 degrees F per 1000 feet of lift. Now what about when the parcel cools to the point of saturation, i.e. the temperature of the air parcel now is the same as the dew point. At this point, the moisture condenses and this process actually releases heat through the latent heat of condensation. Therefore the moist adiabatic lapse rate is only 3 degrees F per 1000 feet of lift. All those background lines on the skew-T diagram aren't just there to play tricks on your eyes...they actually all mean something. Here is how it breaks down with the dry and moist adiabats:

So, now that we can recognize the lines which represent dry and moist adiabats, we can compare the environmental lapse rate (the actual measurements from the weather balloon) to these lines to see layers of the atmosphere that may be unstable. If the environmental lapse rate is less than the moist adiabatic lapse rate, then the atmosphere is stable. On the other side of the spectrum, it is unstable if the environmental lapse rate is greater than the dry adiabatic lapse rate. If the environmental lapse rate falls between the two, then the environment is considered conditionally unstable and we have to investigate other items to access the potential instability. Here is an example:

So, looking at the features in our sounding from the morning of April 22, we can see that there is instability present. Check! 

Now onto lift! Around here we have tons of mountains which can provide the necessary lift for thunderstorm development. Other sources of lift may include cold fronts or natural convergence zones as wind flows around the topography as shown in the image below. The reality is that we have a source of lift most days around here...check!

Image courtesy of Wildfire Course S-290

So there you have it, we were able to use the data from our upper air balloon to deduce the development of thunderstorms in the area. Pretty cool, right? If you are ever around north Reno 3 am/pm PST or  4 am/pm PDT, take a look and you might just see that balloon taking flight over the city!

There are actually chances for thunderstorms in the forecast for this afternoon! Be sure to read the Forecast Discussion for more details and check out the afternoon sounding after 5pm to see what it looks like!

Note for sounding webpage: Make sure to select GIF: Skew T under Type of Plot and click on REV on the map.